SHAFT TURNING GEAR

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HYDRAULIC TURNING GEAR 1.0 Introduction:

The function of hydraulic turning gear is to rotate the Turbo-Generator shaft

system at a sufficient speed during the period before start-up and the period after

shut down. During cold or hot rolling of turbine, it is necessary to measure and

monitor the eccentricity of the rotor even before the admission of steam into the

turbine in order to ensure that no vibration is caused and no rubbing between the

moving and the stationary parts takes place. The eccentricity of the rotor is caused

when it becomes unstraight due to bending. Hydraulic Turning gear is useful to

rotate the Turbo generator shaft system at a sufficient speed during the period

before start up and the period after shut down. Mechanical Barring gear is also

available as a backup.

2.0 Necessity Of Turning Gear:

Rotor is said to be rotating with eccentricity when its axis of rotation does not

coincide with its true centerline of mass. Eccentricity of the rotor is caused during

cold or hot rolling.

Before cold rolling of turbine, a natural deflection of rotor is there due to the

non-uniform distribution of its own weight since the weight of the different stage

differ. This is an initial condition of eccentricity. More over uniform heating up of

rotor is necessitated while admitting the steam for sealing at glands on raising

vacuum. Due to the above reasons, slow rotation of turbine rotor is done with the

help of turning gear for a sufficient duration of time prior to cold rolling. As the

turbine speed is gradually increased, the rotor starts to straighten itself and the

eccentricity gets greatly reduced beyond the critical speed.

The conditions in hot rolling are different. When the turbine is tripped, the

rotor comes to rest from its rated speed. The turbine starts cooling very slowly since

it is well insulated. Due to the difference in the area exposed in different sections of

the cylinder, viz. the cylinder top and bottom, the rate of cooling varies leading to

uneven cooling. In such conditions if the rotor is allowed to be stationary, after it

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comes to rest, it would be exposed to uneven temperatures, resulting in thermal

stresses, bending and eventually eccentricity in the rotor. This problem is also

overcome by having a slow rotation of rotor by means of turning gear. The blade

ventilation during turning operation provides good heat transfer at the inner wall of

the casing, which is conducive for temperature equalization between the top and

the bottom cylinder.

3.0 Hydraulic Turning Gear:

Hydraulic turning gear is located in the bearing pedestal between IP turbine

and the HP turbine. Mechanical barring gear is available as a back up to this.

During turning gear operation, the turbo-generator shaft system is rotated by

a double row blade wheel, which is driven by oil. The oil is supplied by the auxiliary

oil pump and it flows through a gate valve gearing and a nozzle box. Gate valve

gearing is an electrically operated valve through which the high pressure oil is

supplied to turning gear thro Nozzle box. Nozzles guide the jet of oil towards the

moving blades. Nozzles increase the velocity of oil and guide the jet of oil towards

the moving blades. This flow of high velocity jet of oil through the two rows of

moving blades result in slow rotation of the Turbo generator rotor system. Speed of

rotation of TG rotor during the turning gear operation is 120 rpm without condenser

vacuum and 160 rpm with condenser vacuum.

In order to reduce the gap losses at the moving blades, sealing strips are

caulked into the nozzle boxes. After passing through the moving blades, the oil

drains into the bearing pedestal and flows along with the bearing drain oil into the

return flow piping.

To overcome the initial breakaway torque on startup and to prevent dry

friction, the bearings are relieved for a short time by jacking oil supplied below the

shaft. The shafts are thus slightly.

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4.0 Mechanical Barring Gear:

The turbo generator is also equipped with a mechanical barring gear, which

enables the combined shaft system to be rotated manually in the event of failure

of the normal hydraulic turning gear.

4.1 Construction: The mechanical barring gear consists of a gear machined on the rim of the

turning gear wheel and a pawl. The pawl engages with the ring gear and turns the

shaft system when operated by means of a bar attached to a lever. The pawl can

be engaged or disengaged by using a lever. The lever is held in position by means

of a latch.

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Fig.No.1a HYDRAULIC TURNING GEAR & MANUAL BARRING GEAR

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4.2 Operation of STG:

The following steps of operation are to be made

Remove the cover and unlatch, Attach a bar to the lever, Barring of lever will rotate the combined turbo-generator shaft system. After the barring has been completed return the lever and the pawl to

disengaged position.

Secure the lever by means of latch and replace the cover. The barring gear shall be operated only after the turbo-generator shaft

system has been lifted with jacking oil. If it is hard to start turning by means of the

mechanical barring gear, this may be due to incorrect adjustment of the jacking oil

system or due to rubbing of the shaft. Corrective action must be taken before the

steam is admitted into turbine. After shut down of turbine, turning gear should be in

operation till the maximum metal temperature reaches 120 C.

5.0 Hydraulic Lifting Device:

When the turbine is started up or shut down, the hydraulic lifting device is

used to maintain the oil film between rotor and bearings. The necessary torque for

rotation is reduced in this way when the hydraulic device or manual turning device

is in service.

The turbo-generator bearings are supplied with high-pressure oil delivered by

a jacking oil pump. The high pressure oil lifts the rotor when it is forced under the

journal of the bearings. To avoid damage to the bearings, the jacking oil pump

must be switched ON at turbine speed below 510 RPM. (approximately) during shut

down and it should be switched OFF at turbine speed above 540 RPM.

(approximately) during start-up.

5.1 Need Of The Jacking Oil For Lifting: The way in which liquids lubricate can be explained by considering the

example of a plain journal bearing as shown in fig no.4.

As the shaft (journal) rotates in the bearing, lubricating oil is dragged into the

loaded zone. Since the loaded zone will be the point at which the shaft and the

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bearing surfaces are closet together, the entry into this zone is tapered, like a

curved wedge. As the oil is forced to move into the narrower part of the wedge, its

pressure increases, and it is this hydrodynamic pressure which supports the shaft

load.

Increasing load reduces the oil film thickness while increasing hydrodynamic

pressure increases the oil film thickness. The hydro dynamic pressure, in turn, is

determined by the viscosity of the oil and the speed at which it is squeezed into the

wedge shaped entry zone. Thus the rise in hydrodynamic pressure and therefore

the thickness of the film will depend on the shaft speed and the lubricant viscosity.

The relationship between speed, viscosity, load, film thickness and friction

can be understood by considering a graph shown in fig no.4. In this graph, the co-

efficient of friction is plotted against expression V/P where, V/P = (Oil viscosity *

Shaft speed)/ Bearing pressure.

There are three different zones in the graph, separated by the points A & B.

At B, the co-efficient friction is at its minimum, and this is the point at which

the oil film is just thick enough to ensure that there is no contact between the shaft

and the bearing surfaces. The zone 3, to the right of B, the oil film thickness is

increasing and the co-efficient of friction also increases (as the film thickness

increases). This increase in the oil film thickness is because of increasing viscosity or

increasing shaft speed or reducing the bearing load. Zone 3 is the zone of

hydrodynamic lubrication or Full film lubrication.

As the conditions change from B towards A, the oil film thickness reduces

and hence the shaft and the bearing rub against each other, the amount of

rubbing, and the friction increases as the oil film thickness decreases, zone 2,

between A & B, is known as the zone of mixed lubrication or partial lubrication. The

shaft load was supported by a mixture of oil pressure and surface contacts,

At A, the oil film thickness has been reduced to nil and the load between

shaft and bearing is carried entirely on surface contact. In zone1, the co-efficient of

friction is almost independent of load, viscosity and shaft speed. Zone1 is the zone

of boundary lubrication.

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The different lubrication zones also have an influence on wear, the amount

of wear which takes place depends on the severity with which two surfaces rub

against each other. In zone 3, there is no contact between the surfaces and

therefore the wear is minimum. As the oil film thickness becomes thinner in zones 2

and 1, there is a greater tendency to wear.

When the turbine is on Turning Gear during start-up or shut down, the shaft

speed is much less compared to its normal operating speed. Hence the shaft

rotates in the region of boundary lubrication. Since the oil film thickness is minimum

during low shaft speed, there is increasingly severe contact between the shaft and

the bearing surface resulting in increased wear and reduction in life of the bearing.

To avoid this, a jacking oil system also known as hydraulic lifting device is

necessitated to supply high pressure oil called as jacking oil under the journal of the

bearing thereby slightly lifting the journal. Slow rotation of turbine rotor during

turning operation is thus done in slightly lifted condition so as to avoid damage to

the bearings. Hence the shaft rotates now in the region of Hydrodynamic

lubrication.

5.2 Jacking Oil System: To supply the high-pressure oil for the lifting device, two jacking oil

pumps of each 100% capacity are provided on the main oil tank. When one pump

is intended to be in service, the other one is stand by.

5.2.1 Jacking Oil Pump: The jacking oil pump is a self-priming screw spindle pump with three spindles

and internal bearings. The screw spindle pump is connected vertically to the cover

plate (2) of the Main oil tank via a support (5) and immerses with the suction casing

(15) into the oil. The drive is an electric motor that is bolted to the cover plate. The

oil flows into the suction branch of the suction casing from underneath and is

supplied to the jacking oil system by the pump via the pressure pipe (3).

The driving spindle (16) and the two moving spindles (20) run in the inner

casing (13). Due to the special profile given by the sides of the threads, the three

spindles form-sealed chambers, the contents of which are continuously being

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moved axially from the suction side to the pressure side of the pump as the spindles

rotate.

There is a balancing piston in the form of a shrunk on sleeve in the main drive

spindle just above the screwed portion of the main drive spindle and this runs inside

the throttle bushing (11). Pressure oil of a small quantity flows in a very small gap

between the throttle bushing (11) and the driving spindle (16) in an upward

direction. This gap is known as throttling gap since the pressure of oil which is

coming out of this gap is very much reduced. The oil that leaves the throttle gap

flows via the grooved ball bearing (7) and lubricates it. This bearing serves as both

support and thrust bearing. There after the oil flows through the support to the main

oil tank itself via an opening in the support. The driving spindle is fixed by means of

the grooved ball bearing in the bearing carrier (9) that is bolted to the pressure

casing (12) of the pump. The drive main spindle is a solid one. The cumulative axial

thrust generated by the main drive spindle screw is countered by the balancing

piston in the form of the shrunk on sleeve. The p across the balance piston and the annular area of it are so designed to match with the cumulative axial thrust

generated by the main drive spindle.

There are two idler screw spindles, which are hollow and are driven by the

main drive spindle whose continuous helical screw is in mesh with the continuous

helical screws of them. Pressure oil in a very small quantity flows via gaps in the top

of the screwed portion of the main drive spindle though the hollow spaces of the

two idler screw spindles in a downward direction.

The two idler screw spindles also exert a cumulative axial thrust in a

downward direction, which are to be balanced, to perform this task, each idler

screw spindle is having a balancing bushing (21) in its bottom. These are fixed to the

support plate (18), which also supports the inner casing (13) by means of distance

pipes (17) attached to it. The balancing bushing has a small piston with a guide pin

in its bottom and can move only in a vertical direction in a small cylinder, which is

open in its top. The piston is located just below the ending point of the continuous

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Fig. No. 5 Jacking oil pump

helical screw of the idler spindle. Pressure oil is supplied in the bottom of the piston

via hollow space of the idler spindle through a small opening. The top of the piston

is exposed to pressure less oil in the tank. The upward counter thrust provided by the

balance piston in the balancing bushing encounters the cumulative axial thrust

exerted by the continuous helical screw of each one of the idler spindle screws in

the downward direction. There is provision for leakage oil to escape to the main oil

tank from the balancing bushing on the pressure side.

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5.2.2 Jacking Oil Supply: Ref fig no.6. The discharge pressure oil piping of both the jacking oil pumps is connected

in parallel to supply high-pressure oil to the common jacking header. In each

discharge line, a check valve is provided to prevent the jacking oil returning from

the header to the oil tank, if the pump concerned is not in operation. A spring

loaded safety relief valve is provided between the jacking oil pump and the check

valve. This is to prevent any damage to the jacking oil pumps discharge piping in

case that the concerned jacking oil pump is in operation and the check valve

continues to remain in closed position.

The pressure in the common jacking oil header is maintained at a constant

value (approximately 120 bar) by means of a pressure-limiting valve. The pressure-

limiting valve can be relieved by a bypass valve. The superfluous flow from the

pump is conducted into the main oil tank.

The jacking oil required for each bearing is supplied from the common

header as detailed below.

Bearing No of lines

Hp front One

Hp Rear journal cum thrust One

IP Rear Two

LP Rear Two

Generator Front Two

Generator Rear One

In each supply line, a fine control valve and a check valve are provided. The

necessary jacking oil pressure sufficient to lift the shaft varies with respect to bearing

load. The lift will be of 0.03 to 0.05 mm. The required jacking oil pressure is set for

each bearing by means of a finer control valve. The pressure gauges mounted in

the downstream pipes of these finer control valves indicate the jacking oil pressure

required for lifting. A check valve provided in the jacking oil supply pipings prevent

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Fig.No.6 Jacking oil system

the lub oil from flowing out of the bearings into the header during the normal

operation of turbine since the jacking oil pumps are out of service.

The finer control valve, the check valve and the pressure gauge for each line

are arranged in boxes, which are connected laterally to the bearings. At the

generator free end bearing alone, they are arranged in the hacking oil piping

outside the bearing housing.

The lift in mm of the shaft at the bearings is about 0.04 to 0.08 mm.

Bearing Jack oil pressure (in ksc) where the shaft lifts.

(When JO header pressure is 120ksc)

Hp front 40

Hp Rear journal cum thrust 60

IP Rear a) 62 b) 82

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LP Rear a) 50 b) 36

Generator Front a) 80 b) 70

Generator Rear 40

The values are given above for the purpose of indication only.

6.0 Logics:

6.1 Hydraulic Turning Gear:

6.1.1 Sub Loop Control Of Turning Gear: a) Bringing SLC of Turning gear to ON.

i. SLC of Turning gear can be made ON by giving manual command

from the control desk.

ii. When Sub Group Control (SGC) Oil supply is ON and the start up

programme is at STEP 4, SLC of Turning gear is made ON

automatically.

b) Bringing SLC of Turning gear to OFF.

i. SLC of Turning gear can be made OFF by giving manual command

from the control desk.

ii. SLC of Turning gear gets switched OFF automatically when any one

of the following conditions appears.

1. Fire Protection 2 Channel 1 has operated. (OR),

2. Fire Protection 2 Channel 2 has operated. (OR),

3. When SGC Oil supply is ON and the shut down programme at

STEP 51, is executed.

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FIG.NO.7 HYDRAULICTURNINGGEAR&JACKINGOILSYSTEMCONTROLDESK

ON/OFF

OFFON FAULT

SHUTDOWN ON/OFF STARTUP

PUSHBUTTON INDICATION

JACKOILPUMP1

JACKOILPUMP2

GATEVALVEGEARING

SLCJ.O.P1 SLCJ.O.P2 SLCTURNINGGEAR

SUBGROUPSHUTDOWN

SUBGROUPSTARTUP

SGC OILSUPPLY

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6.1.2 Gate Valve Gearing: 1. Protection close:

When the lub oil pressure before the thrust bearing becomes less than 1.2

ksc (MAV 43 CP 012), the gate valve gearing will protection close.

2. Permissives for manual and auto operation of gate valve gearing are

as under.

Differential pressure between the generator seal oil and hydrogen gas should be greater than 0.9 KSc. (as sensed by PS MKW 01 CP 003)

Generator seal oil pressure (turbine side) should be greater than 3.8 KSc (as sensed by pressure switch MKW 01 CP 001).

Generator seal oil pressure (Excitation side) should be greater than 3.8 KSc (as sensed by the pressure switch MKW 01 CP 002).

(OR)

Turbine speed should be greater than 15 RPM. (as sensed by MYA 01 FS 001)

3. Manual open

The gate valve gearing can be opened manually from the control

desk (after keeping SLC of Turning Gear in OFF position) provided that the

permissives detailed in Sec.2. are available.

4. Automatic Open

When SLC of Turning Gear is ON and the turbine speed is less than 200

rpm (MYA 01 FS 001), the gate valve gearing will open automatically

provided that the permissives detailed in Sec.2 are available.

5. Manual close

The gate valve gearing can be closed manually from the control desk

after keeping in SLC of Turning Gear in off position.

6. Automatic close

When SLC of Turning Gear is on and the turbine speed is greater than

250 rpm (MYA 01 FS 001), the gate valve gearing will close automatically.

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6.2 Jacking Oil Pumps:

6.2.1 SLC of JOP-A: a. Bringing SLC of JOP A to ON

(i) SLC of JOP A can be made ON by giving manual command from the

control desk (or)

(ii) SLC of JOP A is made ON automatically when SGC oil supply is ON

and the start-up programme is at STEP 5.

b. Bringing SLC of JOP A to OFF

(i) SLC of JOP A can be made ON by giving manual command from the

control desk (or)

(ii) SLC of JOP A gets switched OFF automatically when any one of the

following conditions exits.

1. Fire Protection 2 Channel 1 has operated. (or)

2. Fire Protection 2 - Channel 2 has operated. (or)

3. When SGC oil supply is ON and the shut down programme is at STEP

54.

(or)

4. Auto start command for JOP B exists.

6.2.2 SLC OF JOP B: a. Bringing SLC of JOP B to ON:

(i) SLC of JOP B can be made ON by giving manual command from the

control desk. (or)

(ii) SLC of JOP B is made ON automatically when SGC oil supply is ON

and the start-up programme is at STEP 5.

b. Bringing SLC of JOP B to OFF

(i) SLC of J.O.P. B can be made OFF by giving manual command from

the control desk. (or)

(ii) SLC of J.O.P. B gets switched off automatically when any one of the

following conditions exists.

1. Fire protection 2 Channel 1 has operated (or)

2. Fire protection 2 Channel 2 has operated (or)

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3. When SGC oil supply is ON and that shut down programme is at

STEP 54.

6.2.3 JACKING OIL PUMP A a. Protection.

JOP A will trip on actuation of any one of the following protections

i. Fire protection 2 Channel 1 has operated (or)

ii. Fire protection 2 Channel 2 has operated

b. Manual Starting.

JOP A can be started manually from the control desk provided that

JOPB is off (irrespective of SLC of JOPA position).

c. Automatic starting.

JOP A gets started automatically when all the following conditions exist.

i. SLC of JOP A is ON.

ii. Turbine speed is less than 510 rpm (MYA 01 FS 001) (OR) (MYA 01 DS

001) and

iii. JOP B is off.

d. Manual Stopping.

JOP A can be stopped manually from the control desk after keeping

SLC of JOP A in off position.

e. Automatic stopping.

JOP A gets stopped automatically when any one of the following

conditions exists.

i. SGC oil supply is ON and shut down programme STEP 54 is

executed.

ii. SLC of JOP A is ON and

Turbine speed is greater that 540 rpm (MYA 01 FS 001) or (MYA 01 DS

001) (OR)

Auto start command for JOP B exists.

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6.2.4 JACKING OIL PUMP B a. Protection starting

JOP B will be protection started on actuation of any one of the following

protections.

i. Fire protection 2 Channel 1 has operated

ii. Fire protection 2 Channel 2 has operated

b. Manual starting

JOP B can be started manually from the control desk provided that JOP

A is off. (irrespective of SLC of JOP B position)

c. Automatic starting of JOP B along with stopping of JOP A

On occurrence of any one of the following conditions.

1. JOP B gets started automatically.

2. JOP A gets stopped automatically &

3. SLC of JOP A is made off.

Conditions:

1. (i) SLC of JOP B is ON.

(ii) JOP A is off.

(iii) JOP A Discrepancy.

(or)


(ii) Turbine speed is less than 510 rpm.

(MYA 01 FS 001) (OR) (MYA 01 DS 001).

(iii) Jacking oil pressure is less than 100 Ksc. (Time delay 5 Sec) (MAV 35 CP

001)

(OR)


(ii) Turbine speed is less than 2800 rpm (MYA 01 FS 001).

(iii) A.C. Voltage for JOP A failed.

d. Manual stopping

JOP B can be stopped manually from the control desk after keeping SLC

of JOP B in off position.

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6.3. Alarms: 1. SLC Turning Gear System Not on alarm will appear when SLC gate valve

gearing is off and temperature of HP casing top (50%) is greater than

1200C (MAA 50 CT 053A)

2. Gate Valve Gearing Not closed alarm appears if the turbine speed is

greater than 540 rpm and the valve remains open.

3. SLC Jacking System not ON alarm will appear when SLC of JOP A is

off SLC of JOP B is off and when the turbine speed is greater than 15

rpm. (MYA 01 FS 001)

4. Jacking oil pressure low alarm appears when the jacking oil header

pressure drops to a value less than 100 Kg/Cm2.

7.0 Guide Lines For Turning Gear Operation:

1. After shut down of turbine, turning gear should be kept in operation till the

maximum metal temperature comes down to 120 C.

2. The mechanical barring gear shall be operated only after the turbo-

generator shaft system has been lifted with the jacking oil. If it is hard to

start turning by means of the mechanical barring gear, this may be due to

incorrect adjustment of the jacking oil system or due to rubbing of shaft.

Corrective action must be taken before steam is admitted into the turbine.

3. Emergency operation of Hydraulic Turning Gear.

On admission of the oil for driving the hydraulic turning gear (after opening

of Gate Valve Gearing), if the Turbo-generator rotor fails to rotate, Manual

barring of the rotor should be immediately started.

After manual rotation of TG rotor for a short interval, if the rotor begins

to rotate due to hydraulic turning gear, manual barring gear can be

stopped.

In case that the rotor does not rotate at all, due to hydraulic turning

gear, even after manual barring for some time, manual barring has to be

continued such that the rotor is rotated by 180 for every five minutes. This

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has to be continued until the rotor becomes straight due to its own cooling

and begins to rotate due to hydraulic turning gear.

8.0 Technical Data:

8.1 Jacking Oil Pump: Number of pumps per unit : 2

Type :SDF 40 R54

Manufactured by :M/S AllWeiler

Capacity : 1.26 dm cubes/sec

Discharge Pressure :120 Bar

Speed : 49.16/sec.

8.2 Motor Of Jacking Oil Pump: 1. Rated Voltage : 415 V

2. Rated Power : 30 KW

3. Rated Current : 57 A

4. Starting Current : 365 A

5. Manufactured by : M/S Siemens.

Bearing No of lines

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SHAFT TURNING GEAR