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1
GUIDELINES
FOR
OPERATION AND MAINTENANCE OF
SPILLWAY GATES ON DAMS
M.V.S. Murthy B.E. (Mech.)
C.E. (Retd.), Govt. of Gujarat
May-2012(Revised)
2
A word from Sri.V.B. PATEL.
Ex.Chairman.C.W.C. G.O.I. Sec.to Govt. of Gujarat (Retired)
3
Comments from a few eminent engineers
From Sri.H.S.Srinivas Prasad. Secretary irrigation govt of Karnataka (retired).
Friday, October 29, 2010 8:58 PM
"H.S.Srinivas Prasad" <[email protected]>
View contact details
To:
Cc:
"Nuggehalli Harikrishna" <[email protected]>
Dear Mr.Murthy:
Thank you for your mail. I would like to compliment you upon the huge effort you have
put in preparing the e-book. I feel that it should reach the all the readers who could
benefit from the book:
I would therefore like to suggest that the e-book may be sent to the following:
1. Secretary Irrigation Govt. of Karnataka, Vidhana Soudha, Bangalore 560001
2, Central Water and Power commission
3. Central Board of Irrigation and Power
4. The President, The Institution of Engineers, Gokhale Road, Kolkata, 700 020
You may also send it the Secretary, Irrigation of various states such as AP and
Maharashtra.
You may request them to distribute the material widely.
The question of copyright is also there if one of the organisations wants to print and
distribute it.
I hope that you are enjoying your stay in Hyderabad.
With regards,
Yours sincerely
Srinivas Prasad
4
5
6
A WORD ABOUT THE AUTHOR
Mr.M.V.S.Murthy was born in Karnataka in 1928, and received his B.E. in Mechanical
Engineering from Mysore University in 1951. Upon graduation, he joined the P.W.D. of the
erstwhile Bombay State,. As Deputy Engineer, Executive engineer and Superintending engineer
(mechanical), he was engaged in the operation and maintenance of heavy earthmoving machinery
and construction equipment on major irrigation projects in Gujarat, including Mahi Canals, Ukai,
and Kadana dams. He managed the departmental workshops engaged in the fabrication and
erection of spillway gates- Radial and Vertical lift type, dam sluice gates, and canal regulator
gates for several projects in the state. He underwent special training in ‘Operation and
Maintenance of earthmoving machinery and construction equipment’ at Larsen & Toubro and in
the US under the USAID program, as well as in Welding Technology at Roorkee University (now
IIT, Roorkee).
At the Secretariat of the Gujarat Government, he was responsible for monitoring the planning,
procurement and management of equipment for irrigation projects of the state (excluding the
Narmada Dam). During this period he participated actively in several committees, seminars and
conferences at State and Central levels. His efforts in evolving Model specifications of equipment
and tender documents for procurement of machinery under World Bank financing were
commended by World Bank. Since the work of Fabrication and erecting of Gates had been
departmentally executed, he had the unique opportunity of acquiring a first hand knowledge on
the subject.
After his retirement as Chief Engineer and Jt.Secy., in1986, he settled in Mysore . Govt.of
Karnataka WRD dept. had utilized his services on several tasks such as quick disposal of
unproductive machinery and investigating failure of Gates. He was a regular Resource-person
on Gates and Construction equipment at the State Engineering Staff College. of Karnataka. He
was a KSFC nominated Director of an Engineering company. He was a Valuer of Machinery. He
was honored by Institute of Engineers-Mysore Centre for his contribution in the field of
Engineering.
He is now 84 and has withdrawn from technical activity. His desire to share his knowledge about
Gates with serving young Irrigation Engineers has resulted in this booklet
This E-Book was offered to Water Resources Departments of several states in India during 2010
and has been well received as useful to Field Engineers. Now, it is revised and updated in
Aug.2012. Son Prasan and granddaughter Anagha have been helpful in editing the book.
7
GUIDELINES FOR OPERATION AND MAINTENANCE OF
SPILLWAY GATES ON DAMS.
Preamble:
The author, who had opportunity of working on several small and large dams in Gujarat
and handling departmental fabrication and erection of several spillway, sluice, and canal
gates, during ‘60s -80s, ventures to put forth this pamphlet for the benefit of Irrigation
engineers, particularly freshers, who may be handling gates. After retirement, he had
opportunity of giving lectures to junior level irrigation engineers at Engineering Staff
College of Karnataka which were well received. This compilation draws from those
Lecture notes.
Spillway gates are dealt in greater detail looking to their importance and complexity.
Hydro power system like Trash rack, High head gates , penstock-gates, draft tube gates
etc. are not included as, generally these are attended to, by a separate setup, and also
because the author had no opportunity to handle them during his service.
The author does not claim any deep knowledge of Design, nor update on practices, yet
draws from his experience and exposure, a knowledge which is rarely documented.
Since even the present-day engineer may have to handle gates of several decades, and
also since basic and elementary knowledge does not become obsolete, this pamphlet is
expected to be relevant even today. This is vouchsafed by the fact that some of the
observations made by the author in this booklet are reflected in the ‘Extracts’ of recent
documents of repute like ICOLD, enclosed at the end of the pamphlet.
Author welcomes any constructive criticism which could be incorporated with gratitude.
His E .Mail address: [email protected],
8
CONTENTS
CHAPTER TOPICS PAGE NO. 1 Introduction 9
2 Spillway gates – Maintenance &
Operation
10 – 51
Other topics useful for gate maintenance
3 Welding 52 – 57
4 Vibration 57 – 59
5 Corrosion and Cracks 60 - 63
6 Relevant Extracts from Journals 64 – 96
7 Bibliography , Acknowledgement 97
Plates Index Page 1 General Layout of Radial Gates on Spillway crest of Dam 37
2 Radial gates:–Rope Drum Hoist. 38
3 Radial gates- Downstream Suspension, Un-bonded
Anchorage
Anchor Girder- Yoke Girder, Tie –flats
39
4 Radial Gate with Inclined Arm, Hydraulic Hoist, Stoplog 40
4 (A) Pre-stressed Anchorage, 4(B) Trunnion tie. 41
5 Radial Gates- Upstream Suspension, Downstream
suspension Access to Gate parts from Hoist Bridge-
Ladder, Railing
42
6 Rubber Seals for Gates, Musical note type, Z type, Flat
Seals, Teflon-clad Seals
43
7 Bulkhead gate failure at Naranpura Dam 48
8 General configuration of Stop log Gates. 49
9 Vertical Lift Fixed wheel Gate 50
10 Weld Joints. Butt and Fillet welds 52
9
1. INTRODUCTION:
The basic purpose of an Irrigation Project is storage, conservation of water, and its
controlled and regulated discharge. Hydraulic Equipment like Spillway gates, Dam Outlet
gates and Canal gates etc do this function. Thus, proper operation and maintenance of
these vital components needs no emphasis. These dynamic components are subjected to
movement, wear and tear, stresses and vibration, hence their vulnerability, and need for
proper operation and maintenance.
Engineers of Irrigation and Water Resources Departments are often required to handle,
operate and maintain this hydraulic equipment.
Design aspects of these are well documented, and standard practices have been
established. However, there is little literature with regard to maintenance and operation
aspect, problems involved and their tackling. This pamphlet attempts to address this
aspect to a limited extent.
Each dam has a schedule of gate operation in relation to storage, inflow, and outflow.
Topics covered in this pamphlet only supplement such standard operating procedures.
It is presumed that the engineers who use this pamphlet are familiar with gate
components, their nomenclature and their function.
10
2. SPILLWAY GATES - Maintenance and operation
Proper operation and maintenance of spillway crest gates is very essential for
conservation of water and also safety of the dam and other structures. In many cases, a
substantial part of the storage is above the crest and behind the spillway gates. Generally,
every dam head-works has a schedule of operating of gates indicating the sequence of
opening of gates, the height of opening at a time, position of gate opening which should
be avoided being harmful and checks to be carried out before commencing gate operation
etc. This Schedule should be displayed prominently at the Control room on the dam
and should be meticulously followed.
Spillway crest gates could be of several types but most common types found in our
country are:
Radial gates such as those on Hemavathy - Harangi in Karnataka and Ukai, Kadana and
Khodyiar dams in Gujarat,
We may consider ‘Radial Gates’ first as these are more common.
2.1 RADIAL GATES.
A Radial gate is a gate in the shape of a segment of a cylinder rotating about its
horizontal axis. The general layout of a radial gate is at Plate 1 (p.37)
Radial Gates are most suitable for spillways because of several advantages they have
over other types:
1) Simplicity of construction 2) Light Weight
3) Low hoist capacity 4) Reduction in height of pier
5) Smooth flow, as gate-slot is avoided 6) Larger span
7) Ease of maintenance of critical components 8) Less vibration at partial opening
However, there are a few minor disadvantages:
1. Increased cost due to extended pier to accommodate trunnion.
2. With greater mass of concrete due to this extension, Seismic resistance may be less
favorable.
11
3. End arms may encroach on the waterway and disturb the flow, particularly in case
where the down-stream water level is high.
During very heavy floods, a large flow may pass over the top of a radial gate
causing serious vibration resulting in structural damage to gate. Care has to be taken to
prevent such a situation.
There is a general belief that Radial Gates are fool-proof and need very little
maintenance and, failure of Radial Gate for want of maintenance is not possible. This is
totally wrong. Meticulous and thorough inspection of vulnerable points of gates is
very essential for proper functioning of gates and avoiding of catastrophic failures.
In order to understand the operation and appreciate the maintenance requirement,
we may familiarize ourselves with various components that go to make a Radial Gate and
their function. General layout of Radial gate on Spillway crest is at Plate 1.
Maintenance points are indicated and highlighted at appropriate places while
discussing the components.
Radial Gates are designed to Indian Standard Specification IS: 4623-Latest
revision. Other relevant IS Specifications are IS: 10096 - Recommendation for
inspection, testing and maintenance of Radial Gates & hoists. IS6938-1989 for Rope
drum Hoist and IS 10210-1993 for Hydraulic Hoist. It is desirable that Irrigation
Engineer be familiar with these specifications in particular.
2.1.1. GATE SIZE:
In designating the size of a radial gate, the width is given first, followed by height.
Height is the vertical projection of the distance from the sill to the top of the gate i.e., the
height of opening that the gate covers in the vent between piers. It is not the curved
length of gate or the height above spillway crest. Sill is generally located slightly
downstream of the spillway crest to improve flow condition and avoid cavitations on
downstream glacis.
12
2.1.2 COMPONENTS OF RADIAL GATES (Plate-1 at p.37)
1. Gate Leaf
2. Horizontal Girders
3. End arm assembly
4. Trunnion Assembly
5. Anchorage and Embedment
6 Hoist
7. Suspension of gate.
8. Seals and seal seats.
Water thrust acting on the convex surface of the gate leaf is transmitted to the
horizontal girders, then to the radial arms, to Trunnion to Anchorages and ultimately to
the piers. The gate assembly hinges at the trunnion and moves in an arc with the trunnion
pins as axis. Suitable seals are provided at sides and bottom for water- tightness. For
proper functioning of gates, both trunnions of a gate should be co-axial, and the load axis
should not fall below trunnion axis.
2.1.2.1 Gate leaf: (Plate-3 at p.39)
The gate leaf of a radial gate is made up of skin plate, bent into the shape of an arc.
Water thrust is taken on the convex face. The radius of curvature of gate is generally 1.25
H, where H is the vertical distance between the sill and the top of gate. The skin plate is
made of M.S. Plate to IS: 2062 specification, good weld able quality steel. In large gates
plates of different thickness are used at different heights depending on the location above
sill and stress due to water thrust on the segment. A 1.5mm corrosion allowance is
provided while deciding the thickness. The skin plate assembly is fabricated in segments
so that they are conveniently transported and assembled at site. Necessary match marks
and locating pins are provided. Butt joints, particularly field joints should be carefully
checked for any sign of crack during inspection.
The skin plate is provided with either vertical or horizontal stiffeners. Generally
vertical stiffeners are tees formed by cutting I sections in the middle and bent to the same
curvature as that of skin plate and welded to it. Where the gate size is small, horizontal
13
stiffeners and end box girders are provided and radial arms are fixed to vertical end-
girders.
2.1.2.2 Horizontal girders
Horizontal girders are provided to take up the water thrust from the gate leaf. These are
generally plate girders, with webs, web stiffeners and flanges .Welds should be
checked. Drain holes are provided to prevent water collection and rusting. They are
required to be kept clean.
2.1.2.3 Radial arms
Radial arms emanate from the trunnion hub and are connected to,(a) the vertical end
supports of skin plate in the case of small size gates, or (b) to horizontal girders.
There are two types of Arm assemblies:
a) Parallel arms: For smaller gates where water thrust is not much, parallel arms are
provided. These are straight and parallel to each other and are rigidly connected to
Vertical End supports of Skin plate or to the horizontal girder, at either end.
b) Inclined Arms: In the case of larger gates, inclined arms are provided. The arm
assembly is connected to the horizontal girder at about 1/5 th of span from the end,
resulting in substantial saving in weight and size of the horizontal girders. (Plate-4 at
p.40)
The complicated geometric configuration of the inclined arm assembly is required
to be properly laid out in jigs both in the shop and in the field during installation for
proper aligning of the arm assembly and trunnion hub before welding. so that the gate
movement is smooth .
The flange joints between horizontal girders and Radial arms have to be
checked thoroughly for slackness of bolts or fastenings. Holes should be drilled and
reamed and bolts used should be machined bolts of superior quality and fit snugly.
The shims should be of steel. All bolts to be tightened to correct torque as
recommended by manufacturer. Loose bolts or oblong /gas cut holes should be
avoided. The flange joints of Arms at Horizontal Grinder and Trunnion ends
14
should be fully bearing without gap. Steel shims with machine-drilled holes should
be used to fill any gap,
2.1.2.4 TRUNNION ASSEMBLY:
This consists of (a) trunnion hub connected to the arms,(b) trunnion bracket
mounted on Anchor/Yoke girders, and (3) trunnion pin acting as a hinge. If the gate is
designed with inclined arms, side thrust due to inclined arms could be tackled by suitable
anchorage or by providing (4) Tie Girder between two trunnions of a gate (See Plate-
4(B). Cracks or failure of weld joint at Tie girder is an indication of trunnion
shifting or malfunctioning of gate. Hence, if cracks are noticed during inspection,
mere strengthening of joint by welding would not be sufficient and the problem
would reappear. The underlying causes of this failure such as trunnion shift, gate
moving skew, gate binding, miss-alignment of trunnions etc. should also be checked
and rectified.
Trunnion hub is a complicated and heavy steel casting and as such, it is a critical
joint. The castings are to be thoroughly checked for soundness while accepting, blow
holes and cracks should be avoided.
The joints between arm assembly and trunnion hub on one side and horizontal
girder on the other, have to be checked carefully, for cracks, slackened bolts and
loose shims
The shims should be of good steel sheets of suitable thickness with machine-drilled
holes. Shims are intended, not only to fill the gap between two mating parts ,but also
to transmit the load from one to other.
There had been instance of lead sheets used to fill the gap between mating parts. Though
a gap-free joint was achieved, during operation, lead sheet got extruded under load and
there was a loose joint.
15
Sometimes, instead of steel casting, fabricated trunnions are adopted for ease of
manufacture, but in that case, very carefully controlled welding and subsequent stress -
relieving are essential.
Generally Phosphor-bronze bearing metal bushings are shrink-fitted to trunnion
hub. Trunnion pins are of cast steel or forged carbon steel with hard Chrome or Nickel
plating to reduce rusting, friction and wear. Greasing of Trunnion hub bushings and
checking for wear are essential maintenance points. Trunnion pins are locked to the
Trunnion bracket to prevent them from rotating when gate is lifted. Check the lock.
Use a high pressure grease gun, and grease until old grease is expelled and fresh
grease appears.
On most of the dams, the need for operating gates for full-open condition may
rarely occur. Hence, during maintenance Radial gates might not have been tested
for full open condition. Consequently, trunnion pin might not get lubricated all
around. In the event of unexpected high flood, the gate may fail to open full because
of lack of lubrication. Such failures have occurred..Hence it is desirable to operate
the gate to full opening after greasing during maintenance. During pre-monsoon
inspection, if the lake level is above crest, stop log gates will have to be lowered, to
facilitate inspection and servicing of gates. Checking the gates for full opening will
also be possible.
Where arms are welded to the trunnions hubs, this being a very heavy and critical
weld, great care should be taken in selecting the electrodes, sequence, checking etc.,
during fabrication or repairs. Low hydrogen electrodes should be used and radio-graphic
test of weld is to be carried out to detect any blow hole, porosity or slag intrusion, which
may develop into crack and fail under load. Trunnion joint is one of the key points for
careful periodic inspection during the life of the gates. If riveting or bolts are used, the
holes should be accurately drilled and reamed and only tight fit bolts fitted to ensure
rigidity of connection. Bolts should be tightened by torque wrenches. Sometimes sealed
roller bearings are used in the trunnion in place of bushings. But bearings of the size and
capacity suitable for use are very expensive and difficult to procure; the indigenous
manufacturers of gates provide conventional greased bush bearing, though use of roller
16
bearing may result in less hoist capacity due to lower friction. Graphite impregnated
bush bearing are desirable for reducing friction.
In the case of gates where water- thrust is transmitted to pier by anchor rods
(bonded anchorage)(plate.4 at p.40) the trunnion assembly is mounted on anchor girder
which bears against the pier face. In the case of un-bonded anchorage,(Plate 3 at p.39)
Anchor Girder - Yoke girder system of Anchorage is used, the trunnion rests on a 'rest
beam' and bears against yoke girder. Brass pads are provided between trunnion bracket
and thrust block as also trunnion bracket and rest beam so as to permit sliding movements
due to elongation of ties under load. These surfaces should be cleaned and greased
every season.
Trunnion ties:
In the case of gates with inclined arms, a Tie Girder is provided, connecting the two arms
of the gate (Plate 4B at p.4 1). The Joint between Tie-girder and Arm assembly should be
checked during maintenance. Causes for any crack in weld, loose joint etc. should be
investigated and rectified.
2.1.2.5. ANCHORAGE AND EMBEDMENTS
Trunnion Anchorage could be of three types: (a) Bonded, (b) Un-bonded and (c) Pre-
stressed
(a) Bonded Anchorage System (Plate – 4 at p.40): The Trunnion rests on an Anchor
girder which is held on to the concrete of the spillway piers by a set of Anchor Rods.
Here the total water thrust exerted on the gate is transmitted to the piers as bond stress
between anchor-rod and concrete of the pier along the length of the anchor rods. Hence
the anchor bars are in full contact with concrete. All anchor rods are pre-tensioned
uniformly, to ensure proportionate sharing of load by the rods and also to avoid
developing tension in the pier concrete.
It is customary to insulate the rods from concrete for a length of 0.5m at the D/S
end of the rods to avoid development of cracks in the pier face. This insulating is
achieved by covering the rod in that portion only with Cork Mastic or polythene sheets
17
during erection and concreting. Generally a simple box type girder spanning the pier
width is provided and the trunnions are mounted at either end. Thus each anchor girder
holds trunnion of 2 adjacent gates. The anchor girder is held in position and connected to
the pier by anchor bars of suitable diameter and length of about 2/3 gate-radius embedded
in pier-concrete. The bars are splayed to achieve distribution of load over larger area in
the pier. Shorter anchor girders are used at abutments.
Often during installation of anchor girder, it is found that the alignment of anchor
rods has been disturbed during pier-concreting and anchor girder cannot be fitted easily.
The erecting staff may widen the holes in the girder by gas cut in the field to allow the
rods to pass through the girder. This is a bad practice and may be cause for
malfunctioning of gate. One way of minimizing the problem is to get additional plates
with machine –cut holes and weld them carefully to the anchor girder after fixing the
girder in position. The gate manufacturer should provide a ‘Dummy Girder’ to hold the
anchor bars in correct alignment during pier concreting.
During maintenance, the nuts holding the anchor rods to the girder should be
checked for looseness and tightened uniformly for same torque. Any relative
movement of girder while operating gates or crack in the holes should be checked
and Trunnion alignment checked and adjusted to ensure that the trunnion axis of
gates is true.
(b) Un-bonded Anchorage (Anchor Girder, Yoke Girder)( Plate-3 at p.39)
In larger size gates, the length and size of bars needed to transmit the water thrust to
the piers by bond stress alone would be too large. Hence Un-bonded Anchorage is used.
Anchor girder is embedded in the concrete of the pier and connected to a Yoke Girder on
the downstream face by means of un-bonded tie bars. The trunnion is mounted to bear
against the Yoke Girder and the gate load is transmitted to piers through the yoke girder,
tie bars and anchor girder. Here the load is transmitted by 'Bearing Stress' of anchor
girder which is embedded in pier concrete, and the tie bars passing through the piers are
insulated by wrapping the entire length with cork mastic or polythene sheets to isolate
them from the concrete. Care will have to be taken to avoid damage to this covering
while concreting of pier. In order to avoid a cascading effect in the event of failure of
one of the gate anchorages, independent yoke, and anchor girder are provided for each
18
gate instead of a common girder for 2 adjacent gates as done in case of bonded anchorage
in smaller gates.. The anchor and yoke girders are heavy plate - girders involving a lot of
welding. Hence, after fabrication these are stress relieved in a stress relieving furnace.
Unless proper arrangements are made to handle these heavy girders and correct sequence
of heating and cooling are adopted and temperature control is adequate, more harm than
good may result from the ‘stress relieving’ process
(c) Pre-stressed anchorage.(Plate 4 A at p.41)
This system is being newly adopted for radial gates. This system is especially
advantageous in case of large gates transmitting high loads to the piers and where the
above two systems are not convenient. In pre-stressed anchorage, post-tensioned steel
bars/cables are used to pre-stress concrete. These post-tensioned cables, when subjected
to water thrust relieve stress from concrete. The use of pre-stressed anchorage involves
high quality of workmanship, special equipment, and technique, and hence, is of limited
usage as at present.
Components of Post-tensioned anchorage: (a) High strength low-alloy steel bars or
strands called ‘tendons’ which hold the trunnion girder to the pier, (b) Anchorage device
or’ bearing plates’ at both ends of tendons-one at girder-end and another embedded in
the pier concrete.(c)Ducts through which tendons are placed to isolate them from
concrete,(d)end-caps, (e)grout tubes,(f)and Special compressible material pad to fill the
gap between Trunnion girder and pier face to ensure even distribution of stress on to the
pier concrete.
There are instances where, during inspection it was noticed that some of the tendons
had got corroded or had lost tension. If such condition is noticed, it is a serious
matter as the remaining trunnion anchors that share the gate load are overstressed
and tend to fail. This calls for a thorough investigation of all other post-tensioned
anchors also. Whenever the tendons are post-tensioned, a record of loading of each such
tendon is to be maintained. (A case study in Google Book- ‘Dams 2000 page 108 –
Deterioration of Pre-tensioned bars… A case study’ would be of interest.)
Each of these exposed components need checking for deterioration and to ensure
secured fastening,
19
As a matter of routine, weld joints between ties and yoke girder as well as welds in
plates of Trunnion/ yoke girder should be checked for cracks during maintenance.
2.1.2.6 HOIST:
Gate Lifting Hoist could be (A) Mechanical winch type (plate-2 at p.38) or
(B) Hydraulic Hoist (Plate-4 at p.40)
Mechanical winch-type hoists are very common in India as they are simple in
construction and easy to maintain, though they are bulky and heavy and rely on gate
weight for self-closing. Hydraulic Hoists are sophisticated with precision components but
have the advantage of positive closing effort and do not need hoist bridge.
(A) Mechanical Winch Hoist and its Components (Plate-2 at p.38):
( a ) Central Drive Unit:
The central drive unit consists of a High Torque low speed induction motor coupled
to a worm reducer from which drive shafts are taken to the two end- reduction -
units. An electromagnetic brake is provided on the motor shaft so that as soon as
the motor is switched off, the brakes are applied automatically and the gate is held
in that position. A manual operation arrangement is also provided so that the
gate can be raised or lowered by operating a lever manually without power supply,
but this is very slow process needing a lot of manual effort and hence, cannot
be relied upon for raising the gates in times of emergency or when power
supply fails. Manual operation is basically useful for adjusting and testing of gate.
.
At the Hoist Bridge a gate position indicator coupled to the hoist gives the position
of the gate from ‘closed’ to ‘maximum’ opening. Limit switches are provided
which automatically cut off power as soon as the gate reaches the fully closed or
fully open position. Limit switches are also provided for intermediate positions so
NOTE: Normal speed of operating gate is about 1 ft. per minute.
20
that when the operator starts the gate motor, it automatically stops after a travel of a
specific height. This is to ensure uniform opening of all spillway gates for
regulated flood discharge, particularly where spillway has large number of gates.
Motors are generally TEFC (Totally Enclosed Fan Cooled). High-torque, low
speed motors for direct-on-line starting. Yet, since spillway area is highly damp
and these motors are only occasionally run, there are instances when motor
windings may get damp and cause overheating when switched on. Hence, during
pre monsoon inspection resistivity and insulation of motor winding must be
checked. Lubricating oil level in Speed Reduction unit should be checked,
mechanical brakes are to be checked, brake shoes are to be cleaned and
adjusted to ensure that gate comes to a stop as soon as motor is switched off
and gate does not creep.
( b ) End Gear Box:
Generally these are large open spur gears and pinions with shafts and keys and
pedestal bearings. Though these are very slow moving, they transmit heavy loads.
Generally large size cast steel spur gears are provided for hoist winch.
Spur gear hoist is a simple mechanism. The gears are to be manufactured by
generating tooth profile on a Hobbing Machine. Though this process is much costly
compared to gears from a Slotting Machine, this would ensure correct tooth-profile
and smooth working. Otherwise, during operation, hoist may vibrate and chatter or
point load on gear may wear or even break the teeth. Since, high quality alloy steels
are available now-a- days, it should be possible to have compact Gear units and thus
reduce the size and weight of hoist mechanism.
During Inspection soundness of gear teeth and key slots have to be checked.
Gear tooth contact profile and alignment are to be checked. Bearing pedestals
must be checked for slackness or breakage and cordium compound (a special
type of thick grease) applied to open gears.
21
( c ) Line shafts and supports: Line shafts transmit the power from central drive
unit to the 2 End Gear Boxes which are far apart. They are in segments and are
supported by support brackets, joints, couplings, keys etc., which have to be checked
and adjusted to ensure smooth operation and minimum vibration while
operating. Since Gear boxes are subjected to horizontal component of rope-pull, they
should be properly anchored.
( d ) Wire Rope Pulleys and Fastenings: Wire Ropes carry the weight of gate leaf
and also a component of the water thrust. They are subjected to stretching and
slackening, rubbing against gate leaf and pulleys, and get crushed and frayed due to
overlap, during operation of gates.
While procuring wire ropes for gates, quality cannot be compromised.
The Size, Lang, Lay, Core, Construction, Tensile strength etc. of wire rope,
should be as specified by the gate manufacturer. Correct lengths of wire rope as
per this specification should be procured and kept properly stored. During
maintenance, wire rope in use should be checked for broken strands. Wire rope
in full length is replaced on both sides and splicing is generally avoided. It is
desirable to replace wire ropes of both sides of a gate at a time, to avoid unequal
elongation under load.
One to one and a half winding of wire rope should be on the drum in the fully
closed position of gate to avoid tension on wire rope clamps on drum and wire
rope slipping.
Cast steel pulleys and pins and bushing should be checked to see if wire rope
does not rub the sides of pulley. Equalizer Bars, Turn buckles, wire rope sockets
and wedges, brackets etc., should be checked for wear. If the gate is of upstream
lifting type, these parts are not readily accessible for inspection. If it is suspected
that gate is not functioning properly and lake level is above sill , Stoplog gates
may have to be lowered to gain access to the Radial gates to check gate parts and
rectify any defect and carry out maintenance On upstream lifting type gates, an
extra plate is generally provided where the rope rubs on the gate leaf, to take up
any wear due to movement of rope and save the skin plate. Excessive wear or
22
scratches on the plate should be checked and cause located. Lack of lubricant
and worn–out strands of wire rope need checking.
(B) Hydraulic Hoists: Consideration of gate size and lifting height, skill in maintenance
and repair etc restrict their use, though they are sophisticated and with precision
components, and number of gates could be operated with one power unit,
Components of Hydraulic Hoist (Plate-4 at p.40) are:
(a) Hydraulic Power pack with Hydraulic tank, motor driven Hydraulic Pump unit
valves filters gauges, and piping.
(b) Two synchronized Hydraulic cylinders each mounted on adjacent pier, piston
connected to either side of gate on the downstream
(c) Trunnion to mount the cylinder on to the pier with anchorage
(d) Control panel and gate position indicator.
The system demands clean and dust free environment, moisture-free oil of
specified grade, meticulous calibrating and servicing of Hydraulic components
such as hydraulic pump, pressure regulating valves and filters and rectifying leaky
seals and pipe fittings.
On projects with 20 to 30 gates, particularly with upstream suspension, a phased
program of lowering stop log gates and taking up inspection and testing of radial
gates- 5 to 6 numbers a year, may have to be planed and implemented so that all
radial gates are covered in a cycle of not more than 5 years.
2.1.2.7 SUSPENSION OF GATE
Suspension of gates can be of ‘Upstream’ or ‘Downstream’ type. (Plate 5 at p.42)
(a) Upstream suspension of Gate
In this method, the wire rope is connected to bottom corner of the gate leaf on the
upstream. This has the advantage of providing maximum leverage (equal to gate
radius) to the hoist while lifting the gate, so that hoist capacity requirement is
23
minimized. Also, during gate travel, the wire rope does not change the inclination
so that hoist has a uniform unidirectional lifting force. Hence the hoist bridge is
not subject to varying load or direction of pull.
But the disadvantages are:
1) Wire rope and its linkage to gate leaf are always under water and are likely
to deteriorate faster. Corrosion of core strands of wire rope may remain unnoticed
and could cause failure of wire rope under load.
2) The components are not readily accessible for inspection as they remain
submerged. Any twisting or overlap of wire rope during operation is not detected
which may result in malfunctioning.
3) Gate suspension components such as equalizer bar and turnbuckles are not
accessible for close inspection.
4) The gate suspension linkages like turn buckle, equalizer bar etc., come
under the influence of vortex formed during partial opening of gate which may
result in vibrations or chattering of gate
(b) Downstream suspension:
The downstream suspension has a positive advantage of easy accessibility for
inspection and maintenance even when lake level is high above sill. The argument
that’ the hoist force required is more due to smaller lever arm and consequently
may increase the cost’, may be countered by suitably modifying the spur gear train
and pulley system or examining hydraulic hoist
We may generalize that in case of smaller size gates and where upstream levels
would normally go below crest during fair season, upstream suspension could be
adopted while in the case of larger gates and where substantial storage is against the
gates during lean season, Downstream -suspension would be preferred.
24
2.1.2.8. SEAL SEATS AND SEALS:
During construction, ‘Block-outs’ are left in the concrete of the pier for embedding
the seal travel plate or seal track. Stainless steel flats, in segments, are laid out in a
perfect plane for seal travel. Usually ,Radial gate itself is used as a template and
moved over full range of travel so as to ensure that the seal track, once fixed in
concrete, will provide a reasonably leak-proof contact with seal mounted on the gate
over its full travel. The adjacent portion serves as track for the guide roller which
limits side-sway movement of gate during travel... Similarly a block out is left in the
spillway crest for embedding the sill beam. This is also provided with stainless steel
flat for bottom seal seat. Unless great care is taken to ensure verticality of plane of
seal seat and full contact of seal on both sides over full travel of gate, leakage and
vibration of gate or gate jamming cannot be avoided.
Rubber seals are mounted on the gate leaf for effective sealing and water tightness
when the gate is closed. Two types of seals are used for the sides:
(1) Musical note type
(2) ‘Z’ type or ‘L’ type (Plate-6 at p.43)
The musical note type seal has a stem with a bulb at the end. The bulb may be solid or
hollow. The stem is clamped on to the gate leaf such that the bulb bears against the
seal- seat and thus prevents leakage. Some pre-compression of seal is given during
installation so that the sealing is effective. When L or Z type seal is used, one leg is
clamped to the gate while the other bears against the seal seat.
Bottom seals are generally Flat seals. They bear against the sill.
Seal clamp bars are flats to clamp down the seals onto the skin plate with bolts. Holes
in the Rubber seals must be punched such that seal sits snugly without scope for
chattering.
25
2.1.2.9. CONTROL GEAR:
The winch type gate hoists are driven by slow speed high torque induction motors. They
are generally provided with direct-on-line starters so that high starting torque is available
to overcome the initial inertia and static friction while crack opening of gate. The upward
or downward movement of the gate is achieved by interchanging the phases in the starter
which changes the direction of rotation of motor The starter has 3 positions, Forward,
Stop and Reverse. The starters are provided at the motor itself in the central drive unit so
that the operator has to go to each motor to operate the gate.
2.1.2.10. REMOTE CONTROL:
Where a large number of gates are to be manipulated in a particular sequence to control
the downstream flow during high floods, it is customary now-a-days to provide Remote
Control operating panel to work in parallel with the conventional controls so that the gate
could be operated either from the motor on the spillway Hoist Bridge or from a Remote
Control station, according to convenience. While the gate position indicator on the central
drive unit is a mechanical device reading directly, the dial indicator on Remote Control
Panel is operated by a signal generator mounted on the hoist shaft or such device. The
reliability and accuracy of the remote gate position indicator depends on the electronic
circuit. and its functioning Indicator lamps indicate the direction of movement of the
gate, up or down. On many projects these remote controls had been more ornamental than
functional. There is a need to develop this remote operation to be positive and
dependable. With the advent of micro-processors, it is now possible to make remote
control operation of gates more reliable. It may even be possible to program the sequence
of operating of spillway gates. Yet, personal supervision by a responsible officer during
flood regulation cannot be over emphasized.
2.1.3 ACESSIBILITY: (Plate 5 at p.42 Plate 8at p.49)
Accessibility to gate components like Trunnion, Horizontal girder, Gate lifting parts, is
essential for proper inspection and maintenance of gates. Hence approach ladders and
guard rails are to be provided from the Hoist platform to Trunnion Anchorage,
26
along Arm Assembly and to Horizontal Girders up to gate-lifting arrangement,
though they may not form part of design drawings. This arrangement should be
suitable for use even by senior officers.
2.1.4 STANDBY POWER:
Though Electric grid supply is provided to the Hoists after investing huge amount, power
failure is common on a project site when it is most needed. During storm when floods are
immanent, and gates are to be lifted in a hurry, grid power may not be available because
of faults in the feeder line. Such a situation is not uncommon. In Gujarat, the practice is
to provide stand-by Generators of adequate capacity, often two of them, one on either
bank so that in event of power failure, there is the stand-by Generator to supply power for
gates. Power cables are so provided that in the event of a cable fault, at least there is an
alternative route available by another cable to operate a set of gates (often alternate
gates). As soon as the need for operating gates arises, both the generators are started up
and kept idling. In case of Grid- power failure, a throw-over switch will connect the
Generator to gates so that gate operation is not hindered. There are spare switches,
fuses and even a spare motor for immediate replacement in the event of a failure. It
should be noted that the Generator capacity must be sufficiently large to operate at least
30 % . of gates at a time. Generator capacity is worked out taking into consideration the
minimum number of gates required to be operated at a time with full load current of
motor. Induction motors draw 6 times the maximum full load current at the time of
starting though it is for only a fraction of a minute. Motors already running may trip
as soon as the last motor is switched on, because of surge in current at the time of starting
of the last motor, which may overshoot the generator capacity. Hence the standby
generator has to be of adequate additional capacity to take the surge.
Generally a siren of adequate capacity is provided for warning public in the vicinity
whenever spillway gates are opened and downstream discharge is to increase. Similarly
wireless network is provided to monitor the inflow during floods and to maintain
communication with Flood Monitoring Cell.
27
2.1.5 PAINTING OF GATES:
One of the important maintenance points of a radial gate is painting. Generally the gate
supplier and erector has to ensure that all parts are thoroughly cleaned, 2 coats of primer
applied soon after fabrication and necessary coat of paint applied before the gate is
handed over to the department. The interval at which repainting is required or the extent
of repainting i.e., whether surface is to be cleaned by sand blasting and then painted or
only additional coats of paint is to be given, depend on the present condition of paint,
care with which initial painting is done, weather and atmospheric condition, damage or
peeling off of paint etc. Thus as a routine, all gate parts, specially those which are in
contact with water or subject to spray should be inspected every season and all parts
painted with finish coat at least once in 2-3 years. If once pitting and rusting starts, due to
humid environment, it spreads very quickly and paint gets peeled off. Under such
condition, after all repair welding etc., are done, the gate should be sand blasted, primer
coated and the finish paint to be applied. It should be noted that unless sand blasting is
done, the rust and scale particularly in corners do not get dislodged. Application of new
coat of paint without proper cleaning will be a waste of effort and money. It should also
be noted that once sand blasting is done, the surface will have lost even the mill scale
and will be susceptible to rust very quickly. Hence without delay, the surface should
be cleaned and primer coating applied before rusting sets in.
Indian standards for painting of Hydraulic gates should be followed.
It is not out of place to mention here that suitable holes should be provided in the
horizontal girder, arm assembly etc., to allow water to drain off without affecting the
structural strength. Wire rope of the correct size strands lay and core as used by the
manufacturer of gate should be kept in store for use when wire rope replacement becomes
necessary due to frayed strands or broken wires. Galvanized cables can be used. Cardium
compound should be applied for protecting and lubricating wire ropes Wire ropes of
correct length should be measured out by steel tape, cut properly by wire rope cutter.
The wire rope sockets should be filled with molten lead after securely fitting wire rope in
the socket.
28
Rubber seals in lengths and the molded corner seals should be preserved in wooden crates
dusted with chalk powder, They should not be exposed to sun and rain or stored near
electrical appliances where ozone is high due to sparking Likewise,. Parts like spare
motor, electrical cables, switches and nuts, bolts etc., should be stored with care.
2.1.6 INSPECTION & TESTING:
Importance of inspection and testing during fabrication and erection as also before
acceptance cannot be over emphasized. The Indian Standards for gates does not clearly
lay down these tests. Unless the fabricator is fully experienced and knowledgeable of the
method of manufacture and quality of inspection for satisfactory performance of the
gates, and is conscientious, the onus falls on the buyer. Though normally the
specification may lay down radiographic examination of a minimum of 10% of all butt-
welds during fabrication, we should ensure that all critical joints such as Trunnion hub to
Arm., field and shop joints of Skin plate, butt welds of Tie flats etc., are radiographed in
the shop or at site and the film is interpreted by an experienced officer before acceptance.
Defects noticed must be thoroughly rectified and rechecked before acceptance. The
radiographs are the property of the buyer and are to be preserved properly for future
reference in case of a failure. Similarly heavy casting should not possess blow holes,
porosity or cracks as discontinuity may cause stress concentration and early failure of
parts. Cracks due to defective fabrication or overstress may be minute at the time of
fabrication, but have a tendency to progressively increase over a period of even years to
result in sudden failure. Failure of bulkhead gates of Narayanapur dam spillway in
Karnataka is a classic example of this type of failure occurring unexpectedly about
10 years after installation. (See Plate 7 at p.48)
Welding should be checked for cracks. Cracking or peeling off of paint near weld
is an indication of overstress or hidden crack. Die penetration test or magnetic
particle test can be carried out insitu to check the weld for cracks. Proper procedure
for rectifying defective welds should be adopted.
29
2.1.7 MAINTENANCE OF RADIAL GATES:
As most of the rainfall is confined to monsoon, the gates must be in full serviceable
condition before monsoon. Hence the gates are to be subjected to pre-monsoon and post
monsoon inspection. It is desirable to provide adequate personnel and implements for
this purpose. A detailed check list is made and the inspection party, - generally an
Executive Engineer (Mech.) with a crew experienced in maintenance and repair of gates
with tools and tackles, visit each project and examine the gates and other related parts
like generators, wiring etc., and rectify any defect and give a certificate of inspection to
the local Civil Engineer in charge of the Dam. Gate operating crew on the project is also
involved in this inspection, so that they can operate the gates effectively when called
upon. During post-monsoon inspection, problems encountered during operation, leakage
and damages observed are reviewed and corrective action is planned and executed.
Low water level in the lake prior to monsoon is taken advantage of, for checking
submerged gate parts and raising and lowering the gates several times under no-load.
Several maintenance points are already highlighted above while discussing the
components. .Wire rope, Hoist gears and shafts, Trunnion hub etc., are checked and
greased, Tie Rods, Horizontal girder joints to Arms etc., are all checked and adjusted.
Electrical system checked, to mention a few. Maintenance of gates like sand blasting,
painting of gates, checking and replacement of wire ropes, seals and other worn out parts
etc., are planned in advance and carried out by the field staff generally departmentally to
ensure good quality of work. It is desirable to set up a Gate Maintenance Organization to
attend to both pre- and post -monsoon inspection and rectify defects of Dam gates of a
region. Evolving a Gates Maintenance proforma and checklists for each project is
necessary. Maintenance of records of observations and actions taken needs no
emphasis.
WHILE RADIAL GATES ARE ALMOST TROUBLE FREE IN OPERATION,
UNLESS PROPER INSPECTION AND MAINTENANCE ARE ATTENDED TO,
THEY COULD BE CAUSE FOR CATASTROPHIES.
30
Attention is drawn to Chapter 6.2(c). Operation of gated spillways’ an extract enclosed
(www.hydrocoop.org. Question 79 ICOLD 20th
congress.) regarding recommendations
for maintenance of gates.
Inspection of all the welds and components of all gates on a project would neither be
feasible nor necessary. Critical and vulnerable parts must be selected for inspection .This
would require close and careful examining by a responsible person. Visual inspection of
all critical elements is the first step. Wear and tear of parts must be checked and causes
located.
Cracks in weld Joints: This involves cleaning, prior to inspection. During visual
inspection, if there is slightest doubt, like rust, peeled off paint, etc. a closer examination
with magnifying glass should be adopted. Adequate lighting of the work area is a
must. If cracks are suspected, further tests like die-penetration, magnetic particles or
ultrasonic methods of detecting extent of crack should be carried out and corrective
action taken as the situation calls for. Where cracks had been welded previously, such
joints should be inspected carefully. Similarly, weld –intersections, Fracture-critical parts
such as Trunnion girder, Trunnion assembly, should be inspected carefully.
Rusting: Potential areas like corners, clogged drain holes, shims, bolt and nut joints,
worn out paint, etc. need attention.
Corrosion and pitting: Components involved should be critically examined, type, extent,
and severity, should be assessed. A sketch of the location, area and extent of damage
should be recorded. Depth gauge, filler gauge etc must be used to assess loss in thickness
of metal and reduction in strength of component. Camera may be used where possible.
Wear and tear of part due to rubbing, bending or breaking. Extent of loss of strength of
the component due to reduction of thickness or size, functionality and reliability of part
etc. should be assessed , recorded, and corrective action taken. Wear plate on skin plate, ,
pulleys, Equalizer bars, and seal travel plates lifting brackets lifting cables etc. are some
of such parts.
31
2.1.8 SPECIFICATION FOR RUBBER SEALS FOR GATES:
Material: Rubber for seal may be of:
(1) Natural Rubber (NR)
(2) Styrene Butadiene compound (SBR) or
(3) Chloroprene compound (CR)
Or any combination of above containing not less than 1% by weight of copper inhibitor,
provided a durable good seal is achieved.
Manufacture of seals should be by ‘molding’ process and not ‘extrusion’ as tolerance in
dimension cannot be ensured in extrusion process.
Physical Characteristics:
1. Ultimate Tensile Strength -Min 1.48 kg/mm/mm ( 145N/mm/mm)
2. Elongation at Break 450 %
3. Durometer Hardness (Shore ‘A’ Type) 60 – 70.
4. Water Absorption – not more than 10 % by weight in 7 days test.
5. Compression set. Not more than 30 %
6. Tensile strength after Oxygen Bombardment Aging test. >80%UTS.
Shape: (Ref. Plate no. 6)The shape of seal generally used are:
For side seals; for radial gates L or Z type.
For other types of gates- Musical note type (hollow or solid)
For bottom seals Flat Type.
32
General Information regarding Seals:
Friction characteristics of materials used
Mating parts Co-efficient of Friction
Static Sliding
1. Rubber Seal on Steel 1.5 1.2
2. Brass on Bronze 0.4 0.25
3. Brass on Steel 0.5 0.3
4. Steel on Steel 0.6 0.4
5. Stainless steel on Steel 0.5 0.3
6. Wood on Steel 1.0 0.7
7. Fluro Carbon seal on St. Steel 0.2 0.15
(Teflon seals)
Type of Gates Type of sealing commonly adopted
1. Low head stoplogs and needles on wiers Wooden seal (this is almost obsolete)
2. Outlet gates of Tanks Bronze to Bronze
(C.I. Slide gates)
3. Low head outlet gates of Dams and Canals Musical note type Hollow bulb Rubber
On bronze or stainless steel seal seat
4. Small canal outlet (distribution system) Steel – Steel
5. Medium / High Head Outlet gates Brass clad / Teflon clad seals on
and Penstock gates. Stainless seal seat
6. Radial gates Z type Rubber Seal on S.S. Seal seat.
7. High Head gate Double stem center bulb seals on S.S
Indian Standard Specifications No. 11855 for Rubber Seals and IS 14177 for
painting need also to be referred
NOTE: Do not mount seal directly on the skin plate where a musical note type seal
is used. A seal base plate is welded to skin plate, so that Seal is clear of the skin
plate, and lake pressure is behind the bulb to deflect it for better sealing.
Permissible leaking from seal may be considered as 15 lit / min / meter of seal length.
33
TO CONCLUDE:
Radial Gates are simple to operate and trouble free provided they are designed well,
fabricated to best standards, erected properly, with good inspection of parts and field
joints and well maintained and tested. Skilled and experienced staff who are
conscientious of their responsibility should be entrusted with this job as the Gates are
dynamic components of the Civil works of Dam and their failure could be
catastrophic.
2.1.9 MALFUNCTIONING OF RADIAL CREST GATES – their problems Causes and
Remedy (NOTE: The List is not exhaustive but indicative of causes most likely to
occur)
Defect Cause Remedies
1. Gate not lifting 1. Power failure
2. Starter contacts or electric circuit
faulty
3. Limit switch stuck
4. Power supply cable failure.
Start standby generator
Check wiring, switches, fuse,
starter contacts limit switches.
Etc., check and rectify, do not oil
limit switch. Check cable and
cable joint systematically after
disconnecting supply. Replace
defective cable, fill cable
compound in junction boxes.
5. Wire rope broken or loose
from socket or clamp
Check and rectify
6. Drive shaft Check Hoist couplings, shaft
keys, gears etc.
7. Gate touching pier side Check pier, chip-off bulges on
pier face. Check guide rollers.
2. Motor getting hot 1. Defective power supply low
voltage
Check all 3 phases for voltage.
2. Over load Gate may be stuck and not free.
3. Moisture in winding. Check resistivity in winding,
Dismantle, varnish and heat coils.
4. Clogged ventilation Check and clean the fan.
5. Loose bearings
6. Solenoid brake of central drive unit
not released when motor starts.
Check and replace
Check and adjust brake
2A) Motor direction Phase connection at the starter might Starter phase Contact might have
34
is changed have got interchanged got interchanged check.
3. Gate travel not
smooth
1. Uneven contact of side seals Check alignment and adjust seal.
If embedded parts are not
properly erected or seal not
smooth, this may occur.
2. Drive shaft alignment or coupling
defective.
Check drive shaft for bends,
slack coupling, keys, pedestal
bolts.
3. Gears not meshing smoothly Check for defective gear profile,
gear alignment, Plummer block
loose.
4. Trunnion binding. Check for debris clean and grease
Trunnion bearings.
5. Unequal wire ropes tension. Check wire rope in gate closed
position -adjust
Wire rope riding on the ridge of
groove
Check alignment of drum and
gate lifting bracket
Improper alignment of Trunnion axis Check and try to adjust by shims.
This defect is difficult to
eliminate.
4. Wire rope Snaps. 1.a) Wire rope strands broken or
rusted/corroded.
b) Wire rope out of pulley groove
c) Broken Pulleys
d) Worn out bushing of pulley.
Check and replace worn parts.
2. Wire rope length uneven Adjust turn buckle
3. Turn buckle threads worn Replace turn buckle by one of
forged steel.
4. Wire rope slipping from socket or
clamp.
Check wire rope clamp on Drum.
Check socket, wedge wire rope
clamps etc., and tighten. Ensure
more than one idle winding of
Rope on the drum in closed
position of gate.
5. Motor does not stop at extremes of
travel
Adjust limit switches.
5. Hoist platform
vibrating.
1. Lose bolts of deck plate or broken
weld.
Check and tighten or weld where
possible.
2. Wire rope rubbing against hoist
parts.
Check and rectify alignment.
3. Wire rope not moving in the groove
on drum.
Check alignment.
4. Gears not meshing properly Check alignment. Pedestal blocks
35
keys, Grease open gears.
5. Defective gear profile. If gears are not manufacture by
‘Hobbing’ accurate profile is not
achieved and gears do not run
smoothly. This manufacturing
defect is difficult to rectify.
6. Hoist platform foundation bolts
loose.
Check and rectify.
-do-while water is
released
7. Gate is vibrating. At small gate opening flow is
unsteady. Change position.
6. Seals of side and
bottom leaking.
1. Debris Check and remove
2. Worn out / damaged seals Check and replace during
maintenance.
3. Missing SS seal travel plate. Sometimes, due to defective
manufacture SS seal seat gets
pealed off during operation.
Rectify, find cause and set right.
4. Defective erection of embedded
parts.
Rectify after proper planning
5. Loose or broken clamp bolts. Check and rectify
6. Side sway of gate during travel Trunnion alignment to be
checked.
7. Side sway of Gates 1. Guide rollers missing Check and replace
2. Trunnion alignment not correct. Check and adjust by shims to the
extent possible.
3. Unequal length of wire rope. A new wire rope stretches under
load. So replace both wire ropes
or adjust turn buckle.
4. Shifting of yoke girder Check Anchorage bolts nuts tie
bars etc.
5. Inaccurate alignment of embedded
parts while erecting.
Plan rectification, consult expert.
6. Excessive torsional twist of line
shaft in the case of CDU located on
pier.
Check and use larger dia. shaft or
hollow shaft.
8. Structural failure of
components or cracks.
1. Defective design Check design and reinforce.
2. Defective manufacture, excessive
weld, stress concentration.
Rectify and stress-relieve by
heating peening etc.
3. Rust, wear and tear Clean and paint after rectification
of defect.
4. Loose bolts, uneven gas- cut holes Rectify.
36
9. Gate vibrating 1. Turbulent flow below the gate
occurring at small openings.
Do not keep gate open for long
at opening less than 10%
travel.
2. Imperfect anchorage. Check Trunnion, anchor bars
nuts, tie bars etc., for looseness
and rectify.
10. Hoist noisy while
operating.
1. Dry gears
2. No oil in reduction unit
(Radicon)
Rectify. Grease
Check oil.
11. Gate Creeps 1. Hoist brake not proper Adjust, brake shoe, Clean with
petrol. Replace worn parts.
Check spring and solenoid.
2. Gear profile not accurate Stub spur gear accurately hobbed
are self locking.
12. Manual operating
device not working.
1. Clutch not engaging or slipping Check and rectify clutch
linkages, Brake shoe
13. Dial reading not
accurate
Dial disc loose on shaft
Drive chain loose
.
Tighten screw. Adjust drive
chain, links
,Calibrate and correct Zero-error,
37
PLATES - 1 TO 6
.
PLATE-1
GENERAL LAYOUT OF RADIAL GATE
ON SPILLWAY CREST OF DAM
38
PLATE 2. RADIAL GATE – ROPE DRUM HOIST
39
PLATE 3
DOWNSTREAM SUSPENSION
UNBONDED ANCHORAGES
ANCHOR GIRDER – YOKE GIRDER –TIE FLATS
40
a
g
f
ed
cb
h
i
j
k
l
m
n
f
a
g
f
ed
cb
h
i
j
k
l
m
n
f
PLATE_4
RADIAL GATE WITH INCLINED ARMS - HYDRAULIC HOIST
STOPLOG ELEMENTS AND GOLIATH CRANE
PLAN indicates :(a) Stoplog, (b)radial gate-leaf, (c)Inclined Arms (d) Hydraulic Hoist cylinders (e) hoist
trunnion (f) power pack for Hydraulic Hoist
ELEVATION indicates: (g) Crane for stoplog handling,(h) Stoplog elements in slot (i) Radial gate in
closed position, (j)Cylinder-Hydraulic Hoist, (k) Piston connected to Gate bottom, (l) walkway, for
maintenance, (m) Bonded anchor rods in pier concrete (f) power pack (n) Trunnion
41
PLATE 4A
POST TENSIONED EMBEDMENTS - PRESTRESSED ANCHORAGE
Note; Gap in the trunnion bushing is to be
ensured to allow for expansion or
contraction of trunnion tie. Otherwise, the
joint between trunnion and Tie may get over
stressed and result in developing crack.
PLATE 4B TRUNNION TIE
(Inclined arms and Trunnion Tie connection)
42
PLATE. 5
RADIAL GATE AND HOIST
UPSTREAM SUSPENSION DOWNSTREAM SUSPENSION
Access to gate components from Hoist Bridge
Ladders and railings Gates with up stream suspension are generally provided
with wear plate where wire rope hugs the gate-leaf.
Hoist Wire rope direction does not change much, over gate travel.
So, the horizontal component of wire rope tension is minimum.
43
PLATE 6
RUBBER SEALS FOR GATES
MUSICAL NOTE TYPE, Z TYPE, FLAT SEAL, TEFLON-CLAD TYPE
WITH MOUNTINGS Note the pre-compression of musical note and ‘z’ type seals for effective water tightness.
44
2.2 VERTICAL LIFT GATES ON DAM SPILLWAY :
Spillway Gates of some of the old dams like Tungabhadra Dam in Karnataka are
of this type.
Now-a-days, with the advent of Radial Gates, Vertical Lift type gates are not
generally used for storage above Spillway crest. But they are often used as 'Stop-Log
Gates', a back- up to Radial Gates. Since they play a very important role in trouble-free
functioning of Spillway Gates, it is desirable to know briefly about these Vertical lift type
stop log gates.
2.2.1 STOP LOG GATES (Plate 8 at p.49)
Usually, Grooves are left in the piers of Spillway upstream of Radial Gate, near Spillway
Crest. Stop log gates are lowered into these grooves as Emergency Stop during an
emergency when Radial Gate cannot be closed properly .They are useful while repairs to
Radial Gate is undertaken without loosing storage. A diagram at (a) on Plate 4at p.40.
gives a general configuration of stop log gate. Though a spillway may contain several
radial gates, provision is made for only one or two such Emergency Gates, and they are
lowered into the slot only where necessary. For ease of handling, these gates are
manufactured, not as a single integral unit of vent size, but as number of tiers which
could be lowered one above the other where needed and removed and stored on top of
Dam when not in use. Each tier of Stop log covers the entire width of Vent, but height is
only about one meter. Each of these tiers is provided on either side with Rollers or
wheels to transmit water load to the piers and seals are provided at sides and bottom to
minimize leakage. At Narayanpur Dam on river Krishna in Karnataka the Vent size in 15
45
m x 12 m. Stop log for each vent has 7 elements; the weight of each element ranges
between10-20 Tons, total weight of a set is 116 MT. This gives an idea of the size and
weight of stop log generally used.
Cast steel Wheels or Rollers are fitted in End box-girders. Roller Track is provided in
Gate Groove on which these rollers move. Guides in the form of rollers or lugs, are
provided on the side-girders of stop log to limit sway. A typical configuration of a
vertical lift gate and embedment’s is at Plate9 at p.-50. Though this diagram illustrates a
typical dam outlet gate, main features of spillway stop log gates are similar to these. It is
an upstream skin plate gate with wheels mounted in end-girders. These wheels or rollers
move on a track on the downstream face of gate groove, and transmit water load to pier.
Side seals move on a seal plate on the u/s face of gate groove. The sectional plan in the
sketch shows a gate groove, original pier concrete and block-out concrete (at ‘A’) in
which gate embedment is aligned and fixed during erection of gate. Normally, pier
concrete is completed leaving a block-out for gate embedment. Subsequently, the gate
supplier comes to install and erect the embedment in the block out in small lifts very
accurately, often using gate as a template. Unless the parts are installed within
tolerance specified in relevant I.S. specification, the field staff may not be able to
lower the gate in emergency! The matter gets more complicated in case of stop log
gate elements, as each of these stop log elements should be able to be lowered into
every one of the vents of the spillway. The field staff should check this aspect. In case a
major deviation in erection of embedment has occurred, the stop log would not function
and rectification is difficult and time-consuming. An Expert agency will have to be
engaged, job has to be well planned and carefully executed. There are instances where,
46
Stop logs and emergency gates fail in their function at times of emergency because
they were not checked and maintained, or gate groove is not checked.
While these gates are of self-closing type i.e. can be lowered in flow condition, by
gravity and their own weight, lifting is generally under balanced head condition. Each
element will have necessary hooks and tackles, so that they can be handled and lowered
or lifted from under water. A Gantry Crane of adequate capacity and reach is provided
with rail track on the spill way bridge so that the stop log elements can be stored at a
convenient place on spillway bridge (at one end of spillway) or at the top of vent, and
moved to the vent where needed and lowered into the groove . Gantry Crane uses a
Launching Beam and hooks to handle the elements.
All the gate elements are of the same overall dimension ,but plate thickness, girder
size, weight etc may vary for each tier, as they are designed according to water pressure,
that tier is subjected to, corresponding to its position in the slot. Hence, the gate elements
should be numbered corresponding to their position in the vent and prominently marked
so that correct sequence is adhered to while stacking them or, picking them up for
lowering them into the vent .Care should be taken while stacking the elements to see that
rubber seals are protected and water does not get collected on gate parts and corrode.
On large dams where 20 to 30 radial gates are provided, it may be necessary to
provide two to three sets of stop logs to ensure that maintenance of all radial gates can be
attended to, even with lake level above sill..
Though use of stop logs may not arise frequently, they need maintenance and
proper storage when not in use. Hence, it is wise to attend to these, during pre-
47
monsoon check of radial gates. Greasing of Track rollers, checking of seals, nuts and
bolts, painting, etc. should be attended to.
An incident of failure of bulkhead gate at Narayanapura dam on river Krishna, in
Karnataka, may be recalled, where lack of inspection and maintenance was one of the
causes of failure. The dam has a storage capacity of 37.86 TMC with30 gates of 15M
x12M size. Bulk-head gates had been installed on 5 vents of additional spillway, to store
water before regular radial gates could be completed. One of the bulkheads of size of a
vent, failed about 10 years after installation. The bulkhead of size 15Mx12M, broke in
the middle and damaged the radial gate in front. The picture at Plate no.7 at p.48 below
shows how a 20mm plate of a member was torn and twisted due to water pressure on
failure. This illustrates how a massive structure could fail without warning.
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PLATE 7
BULKHEAD FAILURE AT NARAYANPUR DAM 3.11.1992.
A) Top; a part of bulkhead damaging radial gate in front.
B) Middle; A twisted plate girder.
C) A plate girder web splice torn.
49
PLATE 8
General Configuration of Stop log gate
Six tiers in slot, seventh under Gantry Crane
Access to Radial gate parts-Walkway, railing
50
Upstream skin plate
PLATE 9
VERTICAL LIFT FIXED -WHEEL GATE.
Showing gate leaf, embedment and block out in pier concrete
A. Black Out
B. Side seal seat on
upstream face of gate
groove
C. Track girder for
Wheel travel
D. Lug mounted on gate
to limit side sway
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3. WELDING
3.1. Welding is an important process used often during gate maintenance and
repair. As improper method and procedure would do more harm than good,
a basic knowledge of defects in weld and their rectification, is necessary to an
engineer handling gates. This is brought out briefly in the following
paragraphs and the article appended at ‘Extracts’ (6.1) at p. 60.
3.2. What is Welding? Welding is a process of permanently joining two pieces of
metal by fusion. The process generally used in the manufacture of gates is ‘metal-
arc welding,’ where an arc is struck from an electrode at the joint between two
steel parts to be connected .The heat of arc melts the parent metals at the seam.
The filler metal, generally, the electrode striking the arc also melts and fuses with
parent metals to give a joint as strong as the parent metal itself. Any stress
induced in the member gets transmitted across the weld, provided all precautions
are taken during welding to get a good joint.
3.3. A defective weld joint may superficially look like a good joint but may fail
during use and result in catastrophe. It is therefore necessary to know the
defects that could occur, their causes, way to detect and rectify them and
restore the joint.
3.4. Construction welds or ‘shop welds ‘can be done in near-ideal conditions, but
‘field welds’ which have to be carried out at site, under unfavorable conditions,
need extra care to meet the required strength and hence need greater attention
both during construction and maintenance.
3.5. There are basically two types of weld joints 1) Butt welds (2) Fillet welds. (See
Plate 11 at p.51) In Butt weld joints, both the plates to be welded are in one plane
abutting, and the joint is welded. This is called Butt joint. If full welding over the
thickness is not possible, (because of reasons like inaccessibility), a backup plate
is kept on the joint and this plate is welded all-round to the base plate. It is called
‘Lap joint’ First run of weld joining two plates is called a root run. Where the
52
thickness of plate is more, successive runs of weld, bridge the gap. Any weld
metal deposited above the thickness of plate does not make the joint any stronger;
On the other hand, heat input due to redundant weld may cause harm.
.
PLATE No.10
COMMON TYPES OF WELD JOINTS
FILLET JOINTS BUTT JOINTS
3.6. FILLET WELDS
In a fillet weld, the two plates to be joined are kept (1) at right angles to each-other
(T Joint or Corner joint) (2) over lapping each-other (Lap Joint) and the edges of contact
are welded .The weld deposit is in the shape of triangle, the effective thickness of weld is
the throat thickness at the joint and not the length of leg of weld.
53
3.7. Pre requisites of good weld are:
(a) A Clean Joint: Joint must be cleaned and made free from dust, moisture,
oil etc. before welding so that fusion is good and weld metal is free of
defects.
(b) Edge Preparation: In the case of joining thick plates where a single run
cannot cover the joint, Single - Vee. Or Double Vee joint will have to be
formed by gas cutting or machining.(See plate no.11 page 48) Cutting
the groove by electrode using high current is not desirable as excess
heat may leave ‘Heat-affected Zone’ which is harmful.
(c) Dry electrode: Moisture in the weld area or electrode used may result in
porosity or crack in weld
(d) Correct size electrode. Too small a size will mean more number of
passes and excess heat input.
(e) Correct current setting: Too low current may cause lack of fusion of
weld metal, Too high current may cause undercut.
(f) Fixing and positioning weld members. While the members should not get
disturbed during welding, stress may develop if they are constrained from
expansion due to weld heat input.
(g) Wire Brushing: After each weld run, it should be thoroughly cleaned by
wire brush to avoid slag or dirt getting trapped in next weld bead.
(h) Peening and Stress-relieving. As the weld metal cools, stress may
develop in weld. This can be relieved by gently peening with a hammer. In
the case of excessive welding or critical joint, Stress relieving may be
achieved by using a blow torch or a stress-relieving furnace.
(g) Position of welding: In the case of manual arc welding Down-hand welding
is adopted so that weld metal spreads well and good bead is formed. But at
54
times overhead welding becomes necessary. Extra care has to be taken
then.
(3.9) Inspection of welds: During Gates Maintenance work, existing weld needs to be
examined to detect any defect or crack. Several steps are followed:
(a) Visual Inspection. After thoroughly cleaning the weld joint, weld is
inspected with a magnifying glass to see if there are any signs of crack in
weld or parent metal, a light tapping with a peening hammer will dislodge
any scale or rust, paint etc .If any crack is suspected (pealing off of paint,
discoloration, etc) but visual inspection does not reveal any, further tests
are carried out.
(b) Dye penetration test .This was first developed by railroad workers! They
noticed that when some oil fell on the rail and then some ash, the crack in
the rail showed up!; oil penetrated into the crack, ash absorbed oil in
the crack, and the crack showed up.
In Dye penetration test a dye is applied on the weld and crack, if any ,is
detected. The test-kit has two components; 1.a penetrator, 2. a developer.
Steps involved are -
(1) First, clean the weld with wire brush to remove dirt, slag or paint..
(2) Then apply Penetrator by brush or spray. The colored liquid penetrates into the
crack and fills up the gap.
(3) Allow some 10 minutes and wipe out extra penetrator with cotton.
(4) Then apply developer. The crack shows up, against the background of developer.
In the case of critical parts like pressure vessels, a fluorescent penetrator is used and
viewed under ultra-violet light to detect even minute cracks.
(c) Magnetic particle test. Here, fine iron particles are spread on weld and a
powerful magnet is attached to one of the members. On gently tapping the
55
area, magnetized iron filings align themselves and show up, because ,
,polarity on either side of crack changes due to discontinuity
(d) Ultra-sonic test :Here, Ultra-sonic waves are produced in a portable
machine and conducted into the component and by moving a probe,
presence of discontinuity in weld due to crack ,porosity etc. could be
observed on a screen and their size, depth and location could be estimated.
(e) Radiography Test: If the component is very critical or repair extensive,
an X-ray film is kept behind the weld and weld is subjected to x-ray. This
is done if the component can be taken to shop-floor. If in-situ testing is
necessary, instead of x-ray, gamma ray from a portable machine with a
radio isotope is used. An expert can interpret the film to estimate the
soundness of weld or ascertain the extent of defects present. This is one
test where a record can be obtained and preserved for future use.
(f) Physical testing of weld: When an extensive repair by welding is
carried out, it may be necessary to check the quality of weld as done.
The weld joint is extended beyond the component so that the welder
continues the weld on this extended plate of joint. Subsequently a
‘Coupon’ is cut out and sent to a physical laboratory for test of tensile
strength; shear test, etc on the specimen to see if the weld as done
meets the designed criteria.
(g) Metallurgical testing: During investigation of failure of weld, it is
necessary to know if there was any stress concentration before failure. A
crystallography test is conducted. A piece of weld joint with parent-metal
is cut out and polished to mirror-finish. When this specimen is viewed
under microscope, the crystallography is observed. If there is stress
concentration, the grain size is different in that area.
56
(3.10) Some Misconceptions:-
If a crack is detected, welding a thick plate over it is not a solution similarly adding
additional weld runs over a defective weld is not useful, because, welds are designed
to take load and any crack, though hidden, could develop to cause failure. A crack
in the weld of a load-bearing member will have to be chipped or gouged out, and, re-
welded carefully and rechecked.
If a crack is noticed in the parent metal or a casting, and if strength is not
compromised, one can arrest its growth by drilling a small hole at the tip of the
crack. When the sharp end of a crack is removed by drilling a hole, its progress is
checked.
Above information, together with the enclosed ‘EXTRACTS-FAULTS IN ARC
WELDS,’ though not exhaustive, may help the engineer to approach the problem
objectively and with confidence.
All the above, emphasize the importance, complexity and skill and thoroughness
needed in gates manufacture and maintenance which the Engineer has to be aware
of.
4. VIBRATION
4.1 Basic knowledge of Vibration, its causes, and its effect on gate components
and gate structures, is desirable for a person involved in operating and
maintaining Hydraulic Gates
4.2 Vibration can result from either mechanical or fluid flow related causes.
Whenever water flows around a structural member like a trash rack, gate or pier, a
change in direction of flow occurs resulting in turbulence and vibration. When a
gate is lifted from closed position, and flow commences from below the gate,
vibration occurs, this is a case of flow-induced vibration.
Vibration of a component is potentially harmful as it may lead to serious damage
or failure .Vibration can be conducted from one member to another by contact.
57
Hence the root has to be located and corrective action taken. Vibration can cause
loosening of bolts nuts or screws, initiate and develop cracks in welds. Vibration
can cause reversal of stresses in the component resulting in fatigue failure even
when the magnitude of stress is low. These are apart from the perceptible effects
such as discomfort to persons working in that area, loud high pitched noise
originating from inaccessible places like bottom of a gate-well etc.
4.3 An adequately designed configuration may minimize the scope of vibration
reaching dangerous levels, but, improper fabrication or lack of maintenance could
be a cause for vibration resulting in failure. A flat surface or an uninterrupted
contact of two sliding surfaces (like top musical note type seal in contact with
breast wall seal seat of dam outlet) can be easily achieved on the drawing board of
a designer, but, a fabricator, unless knowledgeable and conscientious, could leave
errors which may go undetected, and be a cause for vibration when the component
is put to use.
Most serious vibration could occur when gate is kept with small openings. When
gate is discharging into a conduit, vibration may induce a pulsating pressure wave
which may cause damage to structures.
In a situation where a service gate in the dam is stuck and an Emergency Gate is
lowered, problem of severe vibration could occur at a stage when gate-openings
of both gates are the same. Emergency gate should pass through this stage as
quickly as possible.
Vibration induced failures are generally Fatigue-failures because, though stresses
may be low, because of high frequency of reversal of stress, fatigue sets in.
Hence, while operating gates, conditions of prolonged vibration or unsteady flow
condition should be avoided.
4.4 Some of the dam outlet gates ( of early 1960) had been designed with gate lip
protruding below bottom girder level or with a curved plate welded to bottom
girder flange presumably for smooth flow. When such gates are raised from
58
closed position, and flow commences, vibrations of dangerous amplitudes could
result.
(Para 2..3 General Design Criteria Concerning Flow-induced Vibrations ,in
ICOLD 1996 Bulletin 102 Vibration of Hydraulic Equipment for Dams .may be
perused)
4.5 While replacing rubber seals, seal clamps are fixed such that the stem of seal is
long, thinking that it helps in better water tightness. This is wrong and
dangerous, because, when gate is opened, and water starts flowing across the
seal, a long and unsupported stem would vibrate. When the gate is lifted, flow
commences, the long stem flexes in the direction of flow. Because of lower
pressure on downstream, the stem tends to become straight causing momentary
interruption in flow. This intermittent flow may cause serious vibration problems.
4.6 Similarly, if holes in seal or seal-clamp are oblong, seal may chatter and cause
vibration.
4.7 A leaking side seal is a potential source of vibration.
4.8 Any vibration of gate observed during routine operation should be recorded
and reported by the gate operating staff. If the vibration is only transitory
during gate travel, and is confined to small opening, the gate should be
moved out of the vibrating zone and reported after getting confirmation that
vibration has stopped.
4.9 The bulletin no 102 of ICOLD 1996 ‘Vibration of Hydraulic Equipment for
dams’ gives useful information on causes of Vibration of Gates, Trash racks
etc.
59
5. CORROSION & CRACKS: Their generation & propagation:
5.1. CORROSION:
5.1.1 Corrosion is a chemical or electro-chemical reaction on metal, resulting in its
deterioration, or weakening. Impurities in the metal also induce corrosion. When
iron or steel which is not protected, is in contact with moist air, or dampness,
corrosion in the form of rust is generated. ’Mill scale’, a thin layer of oxide, is
formed on the surface of steel plate during its manufacture; this protects the plate
from rusting. But this scale gets removed during welding, machining, abrasion
etc. exposing the raw plate to rusting. Hence, after such operations, steel has to be
protected by a suitable coating.
A common form of corrosion, we come across, in gates is Rust. Rust, once
formed, allows the metal below it to rust further. The part gets pitted or crumbles.
Its strength gradually reduces, and ultimately the part fails, all this happening
without coming to our notice. When one part fails, other parts which are
otherwise sound, would also fail due to over-stress, resulting in a disaster! The
process may take years, but if it goes undetected and uncorrected, would result in
a catastrophe! Plate 7 shows one such incident, a bulkhead gate on Narayanapura
dam on Krishna in Karnataka, failing 10 years after installation. The failure was
almost instantaneous, twisting and mangling 25mm plates and ripping open web
from flange of a plate girder. This is the reason why, simple things like drain
holes in girders, their cleaning during maintenance, etc. are repeatedly mentioned.
Rusting may occur and progress undetected under a layer of paint, if surface
cleaning before painting was not attended to properly
Weld joints are more resistant to corrosion than Bolted joints as moisture trapped
below the fastener may start corrosion,
60
Even Stainless steel wire ropes are found to corrode at the core leading to a
broken wire rope and a gate failure. Regular applying of Cardium compound
could avoid it.
5.1.2. Stray Electric currents in the vicinity of steel parts could induce corrosion. This
can be treated by ‘Cathodic Protection.’(C.P.)
C.P. is a technique to control corrosion of a metal surface by making it the
‘cathode’ of an electro-chemical cell. This is achieved by connecting the metal to
be protected, with another, more-easily corrodible metal that will act as ‘Anode’
This ‘Anode’ metal gets consumed, thus saving steel component from corrosion.
Cathodic protection helps prevent stress-corrosion as well.
5.1.3 Chemicals in the water flowing into the lake or effluents led into it, could induce
corrosion, particularly, of submerged parts of gate. Humid and salty atmosphere
or sea breeze is other causes. A periodic Inspection and cleaning of surface and
application of a good Primer and Finish coat paint, check the attack of corrosion.
5.1.4 Cavitation and lack of aeration in flowing water cause erosion and pitting, and
induce fatigue failure in the member.
5.1.5 Stress corrosion induces cracks in metals. Stress concentration due to improper
design, bad fabrication resulting in residual stress in components, constraints
during welding etc. are some of the causes which may induce microscopic cracks
which may remain dormant and undetected,. Under some situations, they progress
rapidly and result in sudden unexpected failures. Residual stress could be
minimized by proper planning, sequencing of fabrication, stress relieving
components of heavy welds, correct choice of welding rod and current etc.
A word of caution! Unless the C.P. is carried out by a specialist , improperly applied
CP could help evolve Hydrogen ions, and the welds in the parent metal may absorb
Hydrogen causing ‘Hydrogen Embrittlement’ and cause a weak weld
joint.(Ref:Cathodic protection from Wikipedia on web)
61
Even alloy steel components such as stainless steel core of wire-rope, piston rod
of hydraulic cylinder, are susceptible to this type of failure.
5.2. FATIGUE
Fatigue is a progressive and localized structural damage to the metal, which occurs when
it is subjected to cyclic loading. The stress in the metal much below UTS could cause
failure, due to fatigue. When the frequency of cyclic loading crosses the threshold limit,
microscopic cracks are formed on the surface. Eventually, the crack will reach a critical
size and the component suddenly fails.-like the proverbial ‘last straw on the camel’s
back’!
All these factors could initiate and accelerate deterioration of parts and, if they go
unnoticed over a long period, would show up all in a sudden as a failure of the
component. Hence, casual or superficial attention to maintenance is dangerous.
5.3 CRACKS
The origin and propagation of a crack may be illustrated by a simple example. Take a
thick sheet of rubber and try to cut it. Cutting with scissors is difficult. Even cutting by
knife is difficult. Now, fold this sheet, the rubber is bent to U shape. With a small cut at
the outer side of the U, a small crack is formed which will open out, along the entire
cross-section. When we bent the rubber, it was deformed. The fibers were in tension. A
small cut. relieved tension of the few fibers that got cut, but others had to share the stress
and so, got overstressed and yielded, with little effort. As more and more fibers yielded,
stress on remaining fibers increased, causing the crack to progress much faster inversely
proportional to square root of radius of cut. Stress concentration area is the seat of
fracture the mechanics of crack formation in a metal alloy like steel is very complex,
and it initially occurs at grain-level.
In the case of metals like steel, plastic deformation takes place, before a fracture is
formed. When a component is loaded, stress gets distributed in it uniformly. But if in its
path, some constraint or interruption is there, like change in section, a cut with sharp
corners, structural change due to uneven heating and cooling, etc., the stress would get
62
concentrated at those local areas, causing plastic deformation of metal. This could further
lead to formation of crack. Once a crack is formed the stress gets re-distributed or
relieved. And the crack lies dormant. Generally at both ends of a crack, an area of
concentration of stress, in the shape of ‘ear’ is formed, and lies dormant. If several such
cracks are located in an area ,even with moderate increase in stress in the member, the
‘ears’ communicate amongst themselves and jump to meet each-other causing crack to
grow in magnitude at high speed. The component which had looked sound, will fail all-
in-a-sudden, a disaster takes place. The speed of travel of a crack is said to be of the
order of half the speed of sound! .
This answers how a massive gate could fail all in a sudden.
Stress concentration leads to plastic deformation of metal at crystal level which in
turn results in subcritical micro-crack. There are always a great number of micro-cracks
at crystal level, but they are still in dormant stage and the structure is seemingly healthy
and continues doing its job. Micro cracks grow slowly and this stage can last for years!
This is the period of subcritical growth of crack when the structure still retains its ability
to resist external stress. As the component is doing its job, stress is induced in the
component, which keeps feeding the plastic deformation causing either fresh subcritical
cracks or aiding the growth of existing crack. A stage is reached when criticality sets in,
the dormant stage of crack is over. Plastic deformation proceeds at the tip of the slowly
creeping crack. A new process sets in, the crack is now active and starts devouring
surrounding small cracks, jumps from one crack to another at great speed, and the
component which was doing its function suddenly fails! The ductile metal behaves
‘seemingly brittle’. Speed of propagation of crack in steel is estimated at 1600-2000 M/s.
Another interesting phenomenon is that the crack branches out as it progresses, yet
the main crack continues in its original path.
Let us consider one of the major sources of failure-Welding .In the process of
welding, considerable amount of heat is poured into the small volume of metal in the
joint, while the rest of the parent metal remains at room-temperature. This causes
great internal stress within the structure. Added to this, if the parts are clamped
63
such that their expansion due to heat input is constrained, it adds further stress into
the system. If the choice of Electrode size and Voltage setting are wrong, the
problem is further compounded. Cracks are bound to occur, paving way to failure.
Even if breakdown does not occur initially, external stresses induced during use
(though, much below permissible stress) could add up to the locked up internal
stress and cause cracks leading to a catastrophe.
Another process which induces formation of cracks is Heat-treatment of steel.
During such treatment, crystalline structure of steel gets transformed, from Austenite to
Marten site .Austenite consists of a close-packed system of atoms arranged in the form of
cubes where the ferric atoms are situated at the vertex and in the centre of its sides. In the
marten site which is brittle, the system is less compact. During the process of heat
treatment, the steel is heated to a high temperature ( at which it is in austenite form), and
suddenly quenched to a low temperature(when it is converted to martinite).This
conversion is not uniform throughout the cross section of the part because of rapid
cooling. Outer layers get marten site structure while the core is still austenite. But as the
inner layers of the part cool, the austenite structure tends to change to marten site .As a
result, lot of internal stresses get locked –in, which lead to formation of cracks. This risk
can be minimized by proper control of heating and cooling and selecting the coolant. Oil-
Quenching will result in slower rate of cooling than in water, and is beneficial. A reliable
method of removing internal stresses is by tempering, after hardening .During this
process, the metal and its structural constituents adapt themselves to each-other. Hence it
is important to do tempering immediately after hardening
Yet another cause for crack formation is Fatigue. Fatigue fracture involves plastic
deformation. Failure takes place, when the stress-reversal cycle reaches the endurance
limit, though the stress involved is much less compared to yield point,
All these indicate that, unless sufficient care is taken in the design fabrication and
maintenance of hydraulic equipment, failure would be unpredictable and inevitable.
64
`
EXTRACTS FROM RELEVANT LITERATURE
6.1 Welding
Faults in arc welding - Their causes and Remedies
British Welding research association
6.2 Gates Operation
6.2 (a) Purpose and function of Spillways - Hydrocoop.org Question no.79
6.2 (b) Hydro-mechanical Equipment - ICOLD 20th
Congress
6.2 (c) Operation of Gated Spillways - ICOLD 20th
Congress
6.3 Vibration
Vibration of Hydraulic Eqpt. On Dams ICOLD 1996 Bulletin- #102
65
6.1 Welding (Extracts from British Welding Research Association)
66
67
68
69
70
71
72
73
74
75
6.2:GATE-OPERATION. (Extracts from ICOLD 20th congress question no.79)
3. PURPOSE AND FUNCTION OF SPILLWAYS
3.1. FLOOD CONTROL
The spillway must be able to pass the design flood without endangering the dam
and without generating greater downstream danger than would occur without the
dam. Gated and un-gated spillways do this in different manners and with different
degrees of reliability.
3.2. FUNCTION OF UNGATED SPILLWAYS
The un-gated spillway passes flow in accordance with the elevation of the
reservoir water surface above the spillway crest (the spillway head). In some
ways the un-gated spillway provides a disadvantage in that it provides little or no
control over the rate at which water is released to the river downstream. The
hydrograph of the inflowing flood, and the initial contents of the reservoir, govern
the rate at which water is released to the river downstream of the dam. If, at the
start of a flood, a significant volume is available in the reservoir for temporary
flood storage, the peak rate of flow released downstream will be much less than
that of the incoming flood. However, if little volume is available for temporary
flood storage at the beginning of a flood, the peak inflow rate may not be
significantly reduced below the peak of the incoming hydrograph. The process of
proportioning the capacity of an un-gated spillway involves routing the computed
design flood hydrograph through the reservoir and assessing the maximum
reservoir water-surface elevation that occurs. In general the starting reservoir
water-surface elevation for the routing process should be at the crest of the
spillway in order to allow for the possibility of an antecedent flood. The top of
dam elevation is then set above the maximum water-surface elevation by the
required amount of freeboard for safety. The required freeboard will depend upon
the design of the dam and the need to be conservative. In general freeboard
should never be less than one meter, and for embankment dams should be in the
76
range of 3 to 4 meters. If the spillway is being proportioned to safely pass the
probable maximum flood, there is less need to be conservative in setting the
minimum required freeboard.
Once the capacity of the un-gated spillway has been set, the designer tacitly
assumes that the spillway will be capable of safely passing all future extreme
floods. Such a project has little need for a real-time forecasting system to
forecast inflows to the dam. The exceptions are dams where programmed flood
storage in the reservoir is variable depending upon season and conditions in the
upstream watershed. Operating rule curves are typically developed for large
storage dams in California where snow pack in the Sierra Nevada Mountains is
significant and does not begin to melt until spring. In those cases the volume of
storage reserved for flood control will depend upon the date and the total
forecasted runoff (including snowmelt). A description of such rule curves and
their development has been provided in a General Report in the proceedings of
the International Symposium on Dams and Extreme Floods [7]. For those
projects, water is released as fast as allowable any time that the reservoir
contents exceed that given by the established rule curve.
3.3 FUNCTION OF GATED SPILLWAYS
3.3.1. Flexibility of operation
Gated spillways provide a flexibility of operation that is not provided by
ungated spillways. For a gated spillway it is possible to incorporate strategy in
passing extreme floods (R.5, R.17). If the area downstream of the dam is
inhabited it will be desirable to limit downstream releases to that flow rate at
which flooding along the river would begin. That maximum desirable rate of
downstream release can be used in development of the operation strategy for the
spillway. When an inflow flood begins, the decision can be made to release either
the incoming flow rate or the maximum desirable downstream rate, whichever is
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less. If the release is less than the inflow rate, the reservoir contents will
increase. If the release rate is more than the flood inflow rate, the contents of the
reservoir will decrease. Basically such simple operational policy can be followed
until the gates are fully open. No further changes can be made until the reservoir
begins to fall again and the operator can begin to close gates again. If stream
gages which measure and communicate the inflow rate to the dam operator are
not available, the inflow rate can be judged by the rate of rise or fall of the
reservoir surface provided the reservoir surface elevation is being measured, the
reservoir surface area is known as a function of elevation, and the outflow rate
from the dam are known. Following such simple operational rules will insure that
the available storage space in the reservoir is used to the extent possible while
preventing unacceptable flooding. R. Pike and G. Grant (South Africa) describe
the development of simple operational rules for operation of gate spillways and
clearly state the benefits of their simplicity (R.17) regarding dam safety during
passage of extreme floods.
3.3.2. Operating rules
The operation of gated spillways can benefit from the development of
sophisticated operating programs. The successful development and use of such
programs require dependable data acquisition programs which, as a minimum,
measure and transmit real-time data on precipitation in the drainage catchment
and flow rate in the river upstream. If real-time forecasts of future inflow rates are
to be made, other data such as temperature, humidity, and snow pack much also
be taken, recorded and transmitted to the dam control center. The result is a
program which provides real time information to the dam operator which, if
everything is working properly, make it easy for the operator to make safe
decisions on the operation of the projects spillway gates. Such programs have
been developed and installed in South Africa (R. 17) and are in the process of
being developed and installed in Spain (R.25, R.5). Morocco has also
implemented some systems to provide real-time forecasting (R.4).
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In the extreme, such programs can be automated to the extent that the
program will operate the spillway gates based on existing and predicted flood
inflow and current contents of the reservoir. However, the equipment required to
obtain and transmit data to the dam control center requires significant
maintenance, inspection, and testing and is most likely to cease operating during
severe storms when the data is most needed. In addition, operation of spillway
gates cannot be guaranteed during times of extreme floods. Diligent regular
maintenance is required to assure the operation of gates when needed. Q. Shaw
and W. Haken of France point out that the reliability of gates varies with the level
of development of the country (R.18). Particularly in countries that are and have
been subject to war, the infrastructure may have deteriorated to the point where
the reliability of gate spillways is very uncertain.
R. Pyke and G. Grant point out that pitfalls sometimes arise when
sophisticated (black-box) programs for dam operation are used (R.17). In South
Africa, where such black box programs have been use for some time, a backup
system for operation has been found to be very important. The operator is often
totally unaware of how the black-box program makes decisions on operation
because the algorithm for operation has been developed others. As a result the
operator has no means for confirming that the black-box program is performing
as intended. If the black box system fails, and no back-up strategy is in place, the
operator has no means for making decisions on gate operation. As a back up,
South Africa has developed simple operating rules which the operator can follow
as necessary in making decisions as to if and when to open spillway gates. Pyke
and Grant point out that black-box operating systems, are easier for everyone to
understand if the algorithm the program uses is based on the same simple rules.
If the operator can understand the simple system, he has the means to check on
the operation of the black box program, and ascertain that the program is
operating correctly (R.17)
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4. HYDROMECHANICAL EQUIPMENT FOR GATED SPILLWAYS
4.1. TYPES OF GATES AND THEIR DESIGN
A wide variety of spillway gates have been developed and used through
the history of dam design and construction. The most common mechanical types
are (R.10, R19):
4.1.1. Vertical-lift gates
These gates generally are wheel gates (except for very small dams when
they may be slide gates). For surface spillways on large dams they require a
substantial overhead structure and usually a gantry crane for their operation and
removal for maintenance. Maximum sizes range to 20m by 20 m. provided the
gate lip is properly designed, they are reliable for closure under full flow. They
require a bar seal across the bottom and music-note seals on the sides. Gate
slots are required and flow is disturbed by them which makes them somewhat
less efficient in passing flow than a radial gate. When vertical-lift gates are used
on tunnel spillways a bar seal is required on the bottom and music-note seals are
required on the top and edges. Two music-note seals are required on the top of
the gate in order to prevent severe vibration caused by cavitation from leakage at
the top of conduit when the gate is opened (R.1).
4.1.2. Radial gates
Radial gates are the most popular because they are in general cheaper to
construct, their lifting load is smaller than vertical-lift gates, and they are
structurally strong. For surface spillways maximum size ranges to 40m wide by
10m high and 20m by 20 m. Radial gates have better discharge characteristics
than vertical lift gates because they do not require gate slots which disturb flow.
Stiff bar seals are required across the bottom and music-note rubbing seals
along the vertical sides. Top sealing radial gates are frequently used for flow
control in tunnel spillways (R.1, R.39). In general it is not a good practice to allow
flow to overtop large radial gates.
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If large flows pass over the gate, serious vibrations can occur that may
structurally damage the gate (R.10, R.34, and R.37). Where a significant amount
of debris accumulates upstream of the gates, a movable flap is sometimes
provided at the top of the radial gate in order to provide a capability for passing
trash (R.6, R.34). B. Sagar of the USA recommends that the top of radial gates
be fitted with a shaped overflow crest, because the hinges of these flaps require
continual maintenance to insure that they will operated when needed (R.34).
A radial gate must be structurally able to resist any forces tending to rack
the gate when it is being raised or lowered. In some cases side wheels are
provided on each side of the gate in order to minimize the possibility of the gate
becoming jammed (R.36). In July 1994 the rod of a hydraulic cylinder failed while
one of the radial gates on Itaipu dam was being lowered. Bulkheads had been
installed upstream of the gate to allow for maintenance of the gate in the dry.
Detailed investigations show that racking of the gate caused the gate to vibrate in
turn causing the loads on the hydraulic cylinders to increase to the point where
one rod (in weakened condition due to corrosion) fractured (R.43).
4.1.3. Flap gates
Flap gates are used on surface spillways when close control of a reservoir
water surface is required and the maximum operating head will be relatively
small. Maximum sizes range to 70m wide but because they incur heavy lifting
loads they have been limited in height to approximately 3m. Seals are continuous
across the bottom of the gate with rubbing seals on each side. They are subject
to severe vibrations unless splitters are installed along the crest of the gate to
provide aeration to the cavity below the nappe (R.20, R.34 and R4O). Flap gates
are excellent for use where a great deal of trash accumulates in the reservoir
since they can readily pass trash over the gate. In Mozambique studies are
underway to remove and replace 38 flap gates with radial gates in order to allow
increased reservoir storage (R.10).
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4.1.4. Rubber dams
These inflatable dams are actually used as a gate primarily to control the
upstream water surface. Maximum sizes range to 100m long by 3m high. They
do not control discharge as well as a flap gate because flow concentrates at the
center of the bag instead of distributing evenly across the dam. It is impossible to
develop an accurate rating for these rubber dams. They have the advantage that
they are economical to install in cases where it is desirable to increase storage in
the reservoir without spillway capacity.
4.1.5. Segment gates
Segment gates are composed of two independently-operated gate leafs.
Generally the leafs are essentially wheeled vertical-hoist gates. The gates
provide much operational flexibility and are generally used where long spans
must be covered. Maximum sizes range to 70m in length. The leafs and the
hoisting system is designed for a variety of operational conditions:
-Flow over the top leaf only
-Flow under the bottom leaf only
-Flow over the top leaf and flow under the bottom leaf
They are ideal where large flows must be controlled and where a good deal of
floating debris must be handled. For this reason they are more commonly used
on dams controlling flow in a run-of-the-river fashion. M. Bubenik states that 43
such gates are in use in the Czech Republic (R.20). Nine of these gates are in
use in Guatemala (R. 10).
4.2. HOISTING EQUIPMENT
In general two types of hoisting equipment are available for operating
gates: cable hoists and hydraulic cylinders. The choice between these two is
generally a matter of choice. There are certain specific situations for which only
one type can be used. For example, only hydraulic cylinders can be used to
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operate a top-sealing radial gate operating in a tunnel spillway. Often a second
hydraulic cylinder and a cam are included to force tight sealing of the gate when
closed. The operating cam allows the gate to be pulled slightly away from its
sealed position; the gate can then be moved without damaging the seals.
Similarly, operating requirements of a vertical-hoist gate, operating at the bottom
of a tall control shaft, are best met by a cable hoist. In the latter case a hydraulic
cylinder could be used, but it would require a very long hoisting rod and special
equipment to lift the gate completely out of the tower for maintenance, whereas a
cable hoist can be arranged to lift the gate completely out of the tower without
special equipment.
Slide gates are sometimes raised and lowered by means of a motor driven
rotating screw stem. However, such cases are limited to relatively small gates. In
some cases, a gantry crane, or mobile crane, is provided to raise and lower
vertical-lift gates. ln such cases the gantry crane is also required to handle other
equipment su ch as trashracks and bulkheads. Since the crane can only handle
one gate at a time, its use is inconvenient and the process may be too slow at
times when extreme floods need to be passed and many gates must be
controlled (R.26).
Having only one hoist for several gates introduces additional questions of
reliability since if the single hoist fails, none of the gates can be operated.
4.2.1. Cable hoists
In general, cable hoists may be used for capacities up to 50 tons (R.19). If
well maintained they provide reliable service with a long service life. Cable hoists
have been widely used throughout the world and are manufactured in most
developed countries. Thus, in many cases it may be easier to obtain replacement
parts for cable hoists than for equivalent hydraulic systems. Gates controlled with
cable hoists must be designed to close under their own weight since the cable
hoist can provide only lifting force. Vertical-lift gates, suspended in tall towers,
have experienced serious vibration problems when the gate was lowered into
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high-velocity flow due to the elasticity of a long cable [5]. A brake must be
incorporated on the hoist in order to control the speed at which the gate is
lowered. When used on radial gates, a cable is required on each side of the gate
in order to avoid any tendency of the gate to rack. For large radial gates, two
cable drums (one for each side of the gate) may be operated by independent
electrical motors or by a single motor with a shaft connecting the two drums. If
two electrical motors are used, electrical synchronizing equipment is required to
maintain the same lifting force and speed on both sides of the gate. Lifting cables
are usually fastened to the bottom of the upstream side of the gate since this
arrangement provides for the maximum leverage and the smallest cable force
required to lift the gates. Either ordinary steel or stainless wire rope can be used
on the cable drums. Since the cable hoists are generally mounted on a platform
above the gates and are not sheltered, the cables are subject to the weather and
consequent corrosion (R.26, R.34).
4.2.2. Hydraulic cylinders
Hydraulic cylinders are now used widely throughout the world for
operation of all types of gates. They can provide a larger lifting force than cable
hoists and if fabricated as double-acting cylinders and can provide both lifting
and pushing forces. Hydraulic cylinders are preferred for gates with potential for
vibrations since the action of the hydraulic cylinder serves as an additional
damping force on the gate [5]. In Portugal most cable hoists have been replaced
by hydraulic cylinders in order to make maintenance easier (R.12).
4.3. AUTOMATIC GATES
In attempts to improve reliability and dam safety, a number of schemes
have been developed for automating the operation of gates. In most cases the
automatic features have been provided by a system of floats and counterweights.
In those cases it is intended that the spillway gate automatically open if the
reservoir water surface rises. Although such automatic operation is theoretically
interesting, the float and counterweight systems have in general proven to be
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unreliable and to require stringent maintenance. There are 15 projects in
Portugal in which automatic gates are incorporated. The largest of the gates is a
27m wide by 5.3m high radial gate. In 1987 a cable broke on a float for an
automatic gate at Moule Nova Dam in Portugal (R.12). As a result the gates were
overtopped. Recently introduced dam safety regulations require that all automatic
gates in Portugal be replaced by gates having positive control (R.12).
Other types of automatic gates include erodible fuse plugs which have
often been used particularly for emergency spillways. Systematic studies of the
design of fuse plugs have been carried out by many entities [9]. However, their
use is declining because of a concern that the fuse-plug embankment may not
erode as planned.
The most recent development in automatic gates is the Hydroplus Fuse
Gate which is constructed of steel and placed on a modified spillway crest.
Individual fuse gates fail at predetermined reservoir elevations in order to prevent
overtopping of a dam during passage of an extreme flood and to avoid sudden
large spills. These fuse gates provide an economical means of increasing
reservoir storage while maintaining the maximum spillway capacity (R.28). New
developments in fuse-gate design are described for a spillway modification at
Mont. Salvens Dam (R.16). At Mont. Salvens Dam the fuse gates are mounted in
a narrow channel where the velocity of approach would be increased significantly
by the tipping of one gate. The increased velocity of approach would lower the
water surface in the channel below that of the reservoir. To counteract this effect,
the fuse gates were designed with piping connections to the main reservoir
(R.16).
A special case of gate automation is discussed in (R.15) for the Pueblo
Viejo Dam in Guatemala where an increase in reservoir storage was desired. In
this case ungated spillway crests were modified and radial gates were installed.
The existing spillway crest was lowered in order to provide sufficient capacity to
pass the 10,000year flood (the original design flood had been the 1,000-year).
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The radial gates were controlled with hydraulic cylinders and were
counterweighted in order that the gate would open automatically in case the
hydraulic operation failed (R.15). For this particular project a number of
redundancies in the hydraulic power system were incorporated in order to insure
the reliability of the hydraulic system. The counter weight was a fail-safe
measure.
6.2.(C) GATES OPERATION: (Extracts From ICOLD 20th Congress Question no. 79.)
5. OPERATION OF GATED SPILLWAYS
Reliability of gate operation is mandatory to assure dam safety when
passing extreme floods. As noted earlier in this text, a gated spillway is inherently
less reliable than an un-gated spillway. Thus, it is necessary to take special note
of the reliability requirement when designing gated spillways and the equipment
used to operate the gates. In general, there are two major reasons for which a
gated spillway may fail to meet its necessary operating requirements.
-The operator opens the gates too late or fails to open the gates at all, even
though the gates are operable.
-The operation of the gates fails because of structural failure of the gate itself or
failure of one or more of the various components of the operational equipment.
5.1. FAILURE OF THE OPERATOR TO OPEN THE GATES
In general, failure of the operator to open the gates when necessary even
though the gates are operable is a result of one or more of the following:
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-The operator is not present at the control point when needed. Severe storms
that produce extreme floods may also make access roads impassable at exactly
the time when access to the dam and control facilities is most critical. Advance
planning regarding access to the spillway site is critical to insure reliable
operation of gated spillways during passage of extreme floods. This aspect is
particularly important since many dams are now operated remotely and
frequently no operator present at the dam when a flood begins.
.
-The operator depends upon instructions from a higher authority and
communication has been cut off. Storms that produce extreme floods often do
devastating damage to communication facilities and to the data collection and
transmission facilities of a real-time forecasting system. Unfortunately, it is at
exactly this time that communications to the dam operator are most important
(R.17).
-The operator has not received sufficient training in order to understand what is
required of him when an extreme flood is entering the reservoir, or may not have
the necessary tools which can help him make a decision about when or if to start
opening gates. Q. Shaw and W. Hakin point out this item can seriously reduce
reliability in a country where the national economy has suffered or internal unrest
has occurred (R.18). Total reliance on black-box programs for safe operation of
spillway gates without backup training of the dam operator could jeopardize the
safety of the dam if no back-up system is in place for the operator to use if the
black-box program system fails (R.17).
5.2. FAILURE OF THE GATES TO OPERATE
There are many reasons why gates may not be operable when their
operation is most critical. This concern was addressed by more responses than
any other item (R.1, 3, 9, 10, 11, 12, 14, 18, 20, 26, 33, 34, 35, 36, 37, 40,
43)This large response is clearly indicative that the problem of gate reliability is
one of concern to all those who design, operate, or own dams with gated
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spillways. There are two broad reasons for which spillway gates are inoperable
when needed (R.20, R.33):
-Design Deficiencies
-Damage Induced by Aging
5.2.1. Design deficiencies
In most cases design deficiencies are due to failure to recognize loading
possibilities for which the gates and their structures should have been designed.
Seismic Loads
Lack of consideration of possible seismic loading may be the most subtle
of these deficiencies. Attention is drawn to seismic design considerations in
responses R.3 and (R.32). The need for consideration of seismic loading was
pointed out by W. Daniell and C. Taylor (R.9) who noted that failure to analyze
seismic loading is most likely to occur for dams in zones of low seismicity. First
consideration of seismic loadings frequently occurs when the design of gated
spillways is revisited during a process of rehabilitation of the spillway (R.3). For
older dams seismic loads may have been considered to be unimportant at the
time of their original design, but more recent seismic studies have shown that the
probability of severe earthquakes is greater than first thought. Frequently the
rehabilitation is the result of mandates by regulatory authorities or the
development of more stringent standards for design.
Earthquake damage to gates is not unusual due to the large added-mass
forces that can act on the gates during an earthquake. In 1990 a magnitude 7.3
earthquake in Iran caused serious damage to the radial gates of the intermediate
spillway of the 106-meter high Sefidrud buttress dam but did not damage the
bottom outlet gates or the irrigation-outlet gates (R.37). The epicenter of the
earthquake was only 10 km from the dam. Accelerations of 6.0 9 were estimated
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to have been experienced which was significantly greater than that used in the
design. The dynamic load exerted on the radial gates caused trunnion arms to
buckle. The spillway gates were damaged even though the water level in the
reservoir was 6 meters below maximum normal level.
Vibrations
The second most common case of design errors is failure to consider the
possibility of severe vibrations of a gate (R.1). Although guidelines have been
developed which, if followed, will lead to a trouble-free design for the most
common gate designs, there will always be special cases in which more detailed
considerations should be made. Those special cases usually involve unusual
approach flow conditions which may not seem important to the average designer.
Radial gates may be subject to vibration at small gate openings due to instability
that occurs as a result of seal problems; for that reason R. Berridge and M.
MacDonald recommend that the angle between the upstream gate face and the
spillway be made as large as possible (R.40).
It should be pointed out there is some speculation that vibrations played
an part in the 1995 failure of the radial spillway gate at Foisom Dam in California
[8]. If so the vibrations would have occurred with agate opening of 0.73 meters
and a head on the gate of 12.2 meters. Stiffening of the remaining gates was
increased significantly and the replacement gate was designed to be much stiffer
than the failed gate. Field tests conducted in 1998 showed no tendency for
vibration of the stiffened gates.
Design Details
Another common error in the design of gates is in failing to recognize
opportunities for corrosion in the design of the gates or to detect serious
corrosion (R.20, 21). It is of particular importance to recognize that ail parts of the
gate must permit inspection and that care should be taken not build in
opportunities for water to accumulate and accelerate corrosion. Corrosion is one
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of the chief problems encountered as the project ages (R.29. 34, 35). It is
particularly important for gates which will operate in cold climates not to have any
pockets where water can accumulate (R.20). Often, inspection fails to detect
corrosion as quickly as desirable. When the radial gate at Foisom dam failed in
1995, inspection of the failed gate showed that several of the rivet heads placed
during the initial fabrication of the gate had corroded to the point where they no
longer had any structural strength [10]. In the period immediately after the failure
and before the more detailed forensic studies were made, this structural
corrosion was thought to be the root cause of the failure. The fact that this
structural corrosion problem went undetected in a national organization with an
established maintenance program is testimony to the fact that special attention
needs to be given to inspection and maintenance if the reliability of gated
spillway operation is to be assured.
Fabrication and/or construction errors apparently were the cause of
failure of a large radial gate (15m by 13.5m) on Singur Dam in India. The
lapse was thought to be due to a lack of inspection at all levels (R.33).
Obviously the choice of qualified contractors and sub contractors is vital in
the fabrication and construction of all projects. The need for nothing less
than complete quality in the construction of large dams was addressed
extensively in F.M. Budweg's "General Report for Question 75" at the Nineteenth
Congress [11]. It should be of considerable concern to the dam building
profession that, at least in the United States, there has been a growing tendency
to make inspection a responsibility of the contractor. Although this practice may
offer administrative advantages, it places the guarantee of quality control in the
hands of the contractor who will almost always have conflicts of interest because
of his financial involvement. The practice is not conducive to good quality control
5.2.2. Damage induced by aging
As was pointed out in many of the responses, no single element is more
important in insuring reliable operation of gated spillways than quality
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maintenance performed on a regular basis. Several of the responses discussed
the fact that problems affecting operation of gated spillways often occur as a
result of aging and should be expected. The importance of maintenance was
discussed in several responses (R.20, 29, 35, 40, 43). These responses listed
items of maintenance that were particularly important and described problems
that have arisen with gate operation as a result of aging. Regular detailed
inspections of the gated spillway after construction are needed in order that aging
problems can be detected and corrected in order to assure reliable operation.
Problems due to aging
Many problems which occur as result of aging occur as a result of
corrosion. These problems include deterioration of structural steel from which the
gates have been fabricated and corrosion of moving parts. R. Berridge and M.
MacDonald describe a failure due to corrosion of structural members of the
18.3m long vertical lift gates on a barrage across the Indus River (R.40).
Corrosion was so severe that 56 of the original 66 gates on the barrage were
replaced. It was also necessary to replace 55 of the gates on the canal head
works.
Both cables and chains used in gate hoists are subject to corrosion as a
result of being exposed to the elements of weather. Sometimes the deterioration
of hoisting cables is not detectable from surface inspection and is only
discovered during operation when reliable operation is of the most concern. Such
problems were discovered by the U.S. Army Corps of Engineers during flood
operations in the spring of 1993. Hoisting cables broke at several dams in the
Mississippi River Basin when the gates were being operated under load.
Inspection of the failed cables showed that damaging corrosion had
attacked the interior strands of the wire rope reducing the strength of the
cable to the point where it was inadequate for the hoisting loads
encountered. B. Sagar particularly recommends that stainless steel wire rope be
used for cable hoists (R.34). However, corrosion can occur rather rapidly on
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stainless steel if oxygen-depletion cells are formed. Since the corrosion of wire
rope occurs on the center strands of hoisting cables, it is unlikely that the use of
stainless steel wire rope will eliminate the need for regular inspection of hoisting
cables.
Accidents involving hoists are not unusual and where chains are used
they require special inspection to detect corrosion and points of potential failure.
A chain on the hoist for the spillway at Picote Dam in Portugal broke in February
1966 and dropped the gate. The dynamic load caused the gate to fail and it was
then swept away by the flow (R.11). The chains on the gate hoists at Foisom
Dam were found to be corroded and stiff after the failure of the radial spillway
gate in 1995 [10]. Although none of the chains operating the gates had failed,
rotation around the chain pins was restricted by corrosion and caused jerking of
the gate when it was raised or lowered. The jerking was originally thought to
have been a possible triggering mechanism for gate vibrations which might have
contributed to the failure of the gate.
Corrosion of trunnion pins is another concern that was brought to the dam
engineering community by the failure of the radial gate at Foisom Dam in 1995.
In that case lubrication did not reach the entire surface area of the pin and
coefficients of friction ranging from 0.270 to 0.672 were computed from tests
performed on two of the remaining gates and on trunnions of the failed gate [12].
A finite element structural analysis showed that a coefficient of friction of 0.25
would have been sufficient to cause buckling of a critical member in the gate.
Recommendations have been made that spherical bail bearings be used on the
trunnions of radial gates (RAO). Such spherical trunnion bearings would be
expensive and would certainly require regular maintenance. However, they would
be an aid in moving gates where a small misalignment of the trunnion pins had
occurred as a result of movement of one or both of the piers.
A unique problem involving the 1994 failure of the operating rod for a
hydraulic cylinder occurred on one of the large radial gates for Itaipu Dam (R.43).
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The rod had fractured during lowering of the gate when upstream bulkheads
were in place and the gate was not under load. Close inspection showed that
almost imperceptible corrosion of the small end of this rod had resulted in a
condition which produced small cracks in the cylinder. The cracks could only be
detected with dye penetrant. A finite-element analysis of the rod, proved that the
strength of the rod had been significantly reduced. However, static analysis
indicated that the hoisting load was not sufficient to cause the rod to fail even
with the minute cracks. Field tests done on the gate showed that the gate did not
move smoothly when it was lowered or raised when not subject to load.
Apparently the rubbing side seals caused vibrations and racking of the gate
which in turn generated a dynamic overload that was sufficient to stress the
corroded rod to the point of fracture. Since 22 of the cylinder rods on the
remaining gates were similarly corroded, special rehabilitation and maintenance
measures were instituted in an attempt to remove the danger to reliable
operation. The 22 rods were machined and polished to remove corrosion and
cracks. Future measures will include coating the rods with a corrosion protection
chemical on a regular basis. Restrictions include not moving the gates when
bulkheads are in place without spraying a mixture of water and liquid Vaseline on
the seals. Apparently, a very similar failure occurred one of the spillway gates at
Tucurui Dam in 1995. The engineers involved in the rehabilitation of the hydraulic
cylinders at Itaipu Dam recommend that hydraulic cylinder rods should have a
ceramic coating in order to eliminate the possibility of damaging corrosion (R.43).
Unusual loading
Spillway gates are apt to incur unusual loading when operated to pass
extreme floods. The experience of Hydro Quebec during the flood on the
Saguenay River during July 1996 brought to light an array of operational
problems that can occur when the largest flood of record must be passed. The
Saguenay flood surpassed the 1000-year exceedance value and, when the
actual 1996 flood flows were included in the peak-flow data base, estimates of
the 10,000-year flood were tripled [13]. Hydro Quebec found that many of their
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problems with gate operation were the result of inadequate maintenance (R.26).
Problems encountered with gate operation included:
-Hoists broke or had insufficient power to operate gates. Enormous amounts of
debris lodged against the gates and stop logs and caused loading well above
that for which the hoists had been designed. In many cases the gates were
overtopped.
-Stop logs jammed in their slots and could not be removed.
-Power failed. In several cases the failure was due to saturation of power cables
that were submerged by the flood.
-Access to the gates was cut off and in several cases key operational people
were not available or could not get to the dam site.
Legislation
The Saguenay experience has resulted in recommendations for dam
safety legislation being made to the provincial government of Quebec, Canada.
Prior to the Saguenay flood, the Province had no dam safety laws. Legislation
recommended by the Canadian Dam Safety Association includes:
-Spillway capacity for all dams must be reviewed.
-All wooden stop logs must be replaced with metal or inflatable gates.
-Alternative power supplies must be provided for all gated spillways.
-Special consideration must be given to the effects of floating debris and sunken
logs.
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-Because of the need to pass large quantities of debris during an extreme flood,
a minimum gate width of 4 meters is to be used.
-Access to gate structures must be available at all times.
Similar guidelines for the improvement of dam safety in France have been
developed by the French National Committee of ICOLD. Their recommendations
include (R.29):
-Backup power must be properly maintained.
-Hand cranks for the raising of gates should be eliminated since they are
too slow and require more effort than one operator can exert.
-Primary and backup power cables leading to hoist motors should be
routed along separate paths.
-Gate controls should be available in at least two locations.
-Instruments for data acquisition should be established and carefully maintained.
They should be inspected, tested, and maintained on a regular basis.
-Inspection must be done on a regular basis and should include observations for
local subsidence, erosion of moving gate parts by friction, erosion due to
cavitation and energy dissipation, and obstruction of gates by bed load.
-Both gate slots and the gates themselves should be inspected to detect and
remove any debris lodged in them which could prevent operation of the gate and
accelerate corrosion of structural members or operating equipment.
-Evidence of floating debris.
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6. INSPECTION AND MAINTENANCE
Several points in the text of this General Report have mentioned
inspection and maintenance of spillways as a vital part of assuring that spillways
will operate reliably and safely. Inspections are required to insure that the gates
and the spillway are in good operating condition and to detect any conditions that
may be or may become a threat to reliable operation. Different levels of
inspections should be held regularly (at least annually) by operators and
engineers familiar with the operation and the operating history of the
project. In addition, it is wise to have outside experts inspect the spillway
and appurtenant equipment at least every five years. The outside experts
provide experience from other projects and may observe items that the
owner's operators and engineers may not be aware of. The inspection
should always review the maintenance program as well as the spillway and
related equipment themselves.
Maintenance of spillway gates and their operating equipment is of
critical importance. Recent incidents involving corrosion of trunnion pins
on radial gates has shown the possibility that maintenance may have been
deficient on such projects. In the United States, the Federal Energy Regulatory
Commission, which regulates all hydropower dams, requires that all spillway
gates be periodically operated under load to assure that they can be operated
when necessary. It is of course quite important that the gates be able to operate
under load. However, it may be equally important that the gates be periodically
raised fully to insure that lubricant applied to the trunnion pins covers the entire
bearing surface (R.34, 20).
It is frequently inconvenient and expensive to test gates through
their full range particularly if devices for use of stop logs have not been
installed on the spillway. However, the critical importance of assuring that
operation of the gates must be reliable is overwhelming and measures
should be devised to do the required testing for all high-hazard dams.
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After the failure of the Foisom spillway gate in 1995, the U.S. Army Corps
of Engineers devised a laser device that can be used to measure the deflection
of radial gates when they are opened under load. Using this deflection
measurement they can estimate stresses on structural members of gates which
are tested. The device is easily attached to the trunnion of a radial gate and
provides the means for a quick assessment of overall conditions affecting the
operation of the gate. The Foisom accident also triggered the Division of Safety
of Dams in California to call for inspection of all spillway gates on California
dams. That inspection is still under way.
7. CONCLUSIONS
1. The use of uncontrolled spillways will always be the safest design.
However, even it will not adequately protect a dam if the design flood is
not carefully chosen.
2. Gated spillways because of their flexibility in operation and economy in
fabrication and construction will continue to be widely used.
3. Radial gates, because of wide experience ln their use, and their economy
of fabrication, will continue to be the gate of choice in spillway design.
4. The importance of careful regular inspections and maintenance of
spillways, gates, and operating equipment cannot be overemphasized.
5. Regular maintenance is absolutely essential to assure reliability in the
operation of spillways.
6. Periodic inspection of spillways and related equipment by outside experts
is highly desirable. Intervals between these inspections should not exceed
five years.
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6.3 VIBRATION (Extracts from Bulletin # 102 of ICOLD 1996 )
98
BIBLIOGRAPHY
1. USBR Manual on valves, gates, and steel conduits.
2. IIT Kharagpur Gates and Valves for Flow control
3. ICOLD 20th
Congress Question No. 79 Hydro Coop.org
4. ICOLD Bulletin 102, Vibration of Hydraulic Equipment
5. Lincon Arc Welding Foundation: Weld Cracking
6. US Army Corps of Engineers: Design of Spillway Tainter Gates
7. USBR Maintenance Scheduling for Mechanical Equipment(2009)
8. Water Control Structures: Selected Design Guidelines
9. V. Finkel. Mir Publications Moscow: The Portrait of a Crack
.
*********
Acknowledgement.
The author is indebted to Sri V.B.Patel Ex-Chairman,C.W.C. New Delhi.
for his encouragement in bringing out this booklet
The Author acknowledges with gratitude :-
Sri N.K.Naidu of Balaji Hydro Mech Experts Hyd. For providing gate
drawings at Plates 3,4,6,and 8 for adapting into this booklet.
IIT Kharagpur, for the Gate illustration on the Cover Page
CE CW-ET Engg.Manual for Plates 4A,Pre –stressed Anchorage , 4B
Trunnion Tie. And 9 Vertical Lift gate
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PREVIEW
Guidelines for Operation and Maintenance of Spillway Gates on Dams
(Revised: May-2012)
By M.V.S.Murthy (Retired Chief Engineer Mechl.)
Spillway Gates on Dams are essential Hydro-mechanical Equipment extensively used in
Water storage and Distribution System round the world. While design aspects of Gates
are well documented, and standard practices are established, maintenance and trouble-
shooting aspect have been less documented. These dynamic components are subjected to
wear and tear and stress and vibration etc and their maintenance and troubleshooting are
vital for safe operation, as failures are catastrophic. This booklet attempts to serve as a
guide to Maintenance Engineers handling Dam gates in particular and useful to Designer
and fabricator also.
The Author worked on a number of dams in Gujarat state in India during 1960-80and had
opportunity to get ‘hands-on’ experience in fabrication, installation and repairs of Gates
.This booklet is based on knowledge and experience gained, which he wants to share with
his fellow- engineers.
. The Gate components, their function and criticality, points of maintenance and hints on
problems and solutions are explained with adequate illustrations. A few related topics
such as Welding, Vibration, Corrosion, are also discussed .Extracts of useful literature is
also provided. It is in E-Book format for ease of transmission .Author welcomes
corrections and constructive criticism. His e-mail is: [email protected]
The E book was circulated to Secretaries/ Chief Engineers of Water resources
departments of several states in 2011 on e-mail and was well received. It has now been
revised and updated and being presented on web for wider circulation.
![Stone Lake Dam Repair [Read-Only]conservationengineers.org/conferences/2006presentations/StoneLake... · Small Dam Repair The Stone Lake Dam Story ... size dam with leaking gate](https://img.pdfslide.us/doc/110x75/5b2687fa7f8b9a502f8b46bc/stone-lake-dam-repair-read-onlyc-small-dam-repair-the-stone-lake-dam-story.jpg)
