Khaperkheda TPS 2 X 210 MW extn. 3 & 4

  Project approved vide M.B.R. No. 843 dated 8th Sept 1997 and zero date of the project is 10th Sept 1997. Main plant ordered on BHEL. Total estimated project cost is 1370.31 crores. Total packages ordered 61 Nos. order on BHEL for 13 packages, total cost of Rs.977.93 crores inclusive of PV. The main packages are, Boiler & Auxiliaries, Turbine Generator & Auxiliaries, ESP, CW Pumps, Transformers, HT Switchgears & Bus ducts.

 The project was scheduled for completion in 32 months for unit # 3 i.e. May 2000 and 36 months for unit # 4 i.e. Nov.2000.

 Lease finance available from IFCI, SBI, NCD.

 The Salient Features of this project (BHEL package) are as follows.

BOILER

 1.      Separate casing ESP.

2.      No common duct at ESP inlet (APH- A outlet goes to ESP A & B, APH- B outlet goes to ESP C & D)

3.      VFD for I.D.Fans

4.      ESP IOS panel & PC in main control room

5.      Superheater & Reheater spray control valves of CCI ,DRAG make

6)      Drum DWLG is of BONT LEVEL GAUGE TYPE BCI 10 , CESARE BONETTI (Italia) make & EWLI is of YARWAY make.

7)      ID fans, PA fans and Coal Mills are on spring supported foundation.

8)      Hot PA fans, each one for each Mill.

PIPING

1)      APRDS comprising 3 streams one each of High ,Intermediate, & Low capacity.

2)      P-91 material used for M.S. & HPBP lines.

 

TURBINE AND GENERATOR

1)     Fixed pedestal & spring supported TG deck

2)     IP Turbine of single flow.

3)     HP Stop valve & Control valve mounted directly on HP casing

4)     Both HP & IP valves are of 250 MW size.

5)     Hydraulic motor operated barring gear at front bearing pedestal.

6)     Turbovisory by Bently Neveda.

7)     No monitoring of HP & IP differential expansion. Only LPT differential expansion is monitored.

8)     Heat rate is lowest for turbine 1939 kcal/kwhr.

9)     Generator with solid stator winding conductor with hydrogen cooling.

10) Hydrogen pressure is only 2 kg/cm2.

11) Condenser with stainless steel tubes.

12) Only one CRH NRV.

13) No level switches for Hot well, Deaerator, LP Heaters & HP Heaters.

14) DVR for excitation.

 

All the events and activities are nearly completed as per schedule   UNIT # 3 UNIT # 4 EVENTS Contractual Actual Contractual Actual

  Boiler erection start 9/98 17/08/98 02/99 15/12/98 Drum lifting 1/99 31/12/98 07/99 31/03/98 Hydraulic test(drainable) 10/99 24/07/99 04/00 23/10/99 Full hydro test   14/02/00   26/09/00 Control room clearance   8/12/99   26/07/00 Boiler light up 1/00 28/02/00 07/00 10/10/00 T. G. erection start 5/99 17/05/99 11/99 27/01/00 Box up of T.G. 3/00 22/02/00 9/00 20/10/00 Barring gear 5/00 26/05/00 11/00 31/12/00 Rolling /Synchronisation 5/00 31/05/00 11/00 07/01/01 Trial operation 9/00 15/12/00 5/01 25/08.01

Comparative statement between unit # 3 and unit # 4 for the duration of activities is as follows.

 

Duration between activities in number of days Unit # 3 Unit # 4

  Boiler erection start to drum lifting 136 106 Drum lifting to hydraulic test(Drainable) 205 206

HT(Drainable) to Boiler Light Up 218 353 Hydro test (Full) to boiler light up 14 14 Control room clearance to boiler light up 80 74 TG erection start to barring gear 352 369 Oil flushing duration 22 16 Steam blowing duration 9 8 Boiler light up to synchronisation 93 89 Synchronisation to coal firing 81 76 Coal Firing to full load 51 122 Full load to trial operation(Start) 24 4 Trial operation completion to PG test 103  

 Some of the main factors contributed in timely completion of project are,

  

1)     Sequential and timely supply of materials from manufacturing units.

2)     Meticulous and micro level planning followed by daily internal review meeting and constant follow up.

3)     Sub contractors capability from man power, T & P, and resources point of view

 

Erection contractors :

 

For unit 3, M/s. U.B. Engg. Ltd. for Boiler and TG.

For unit 4, M/s. L & T for Boiler and M/s. Indwel for Turbine.

M/s. Bells control for C & I package of both the units.

M/s. Mahati Electricals for Electrical package of both the units.

 

4)     Highly experienced and sufficient staff from BHEL.

5)     Support from other BHEL staff on tour.

6)     Good infrastructure facility availability.

7)     Close interaction and review meetings with customer, monthly review by T.D. and daily review with seniors and constant follow up and supervision at working level.

8)     Timely major inputs from customer

 

 

In some of the areas, delay has occurred, because of following main reasons,

 

Delay in clearance from civil agencies of MSEB viz.) TG foundations of both units, ii) Deaerator floor and PRDS floor of both units. iii) M & N row columns for unit 4. iv) ID-4A foundations v)Mill foundations in unit-4 vi) Unit-4 PA fans foundations

Delay in CW pump erection & commissioning mainly due to foundation mismatch for sole plate and design deficiencies in control panels and limit switches mounting flimsiness.

M/s. Navbharat insulation agency for insulation application was not very much resourceful, the supervision and planning was comparatively poor and the local influence could not be managed by them properly. Moreover this agency was doing insulation application on thermal boiler for the first time for BHEL.

The labor strike of M/s L & T at the stage of coal firing of unit 4 caused delay of almost one month. Also the approach for mills erection was hindered because of civil works for coal reject belt by customer. This consumed more time for mill erection.

The shortage of insulation material and supply delay has considerably delayed last 3 mills and 2 passes of ESP completion.

 

In addition to this, for time cycle reduction two SIP’s were taken up for cycle time reduction.

 

TG, oil flushing activity was planned by making all preparatory works such that in between time wastage was almost minimised and activity was continued non stop.

 

Steam blowing cycle time reduction by suitable line layout with additional valves. This has reduced time for preparation between stages. Also the circuits could be changed as required for effective blowing and getting early results.

 

Cycle time was reduced in following areas by advance planning and making temporary / alternative arrangement.

 

1)     Unit # 3,Main control room civil clearance for start of control panel erection was given on 08/12/99. The total panel erection, cabling and charging for boiler light was completed in only 80 days, which normally takes 4 months time. To achieve this, panels were shifted to TG floor in advance so that immediately panel erection can be started. Parallely the cable gallery was also made ready with cable trays. So that cabling work to be started immediately on placement of panels.

 

Similarly in unit # 4 also the panel erection was completed in 74 days from control room clearance to boiler light up milestone. To avoid panel shifting problem in rainy season, all the panels were shifted before hand and cycle time reduction was achieved.

2)     ESP air in leak test is one of the test to be conducted on completion of erection. The LT control panel for the blower was not available at that time. To cut short the waiting time, the control panel of acid cleaning pumps was modified to suit the blower and test was completed with available construction supply itself.

 

3)     Similarly there was delay from customer, for ESP control room and providing LT supply for rapping motors trials. Temporary starters were arranged and with construction supply the rapping system trials were completed in unit 3 & 4.

 

4)     Also the customer LT power supply was not available for carrying out the lub oil lines flushing in Boiler & Turbine auxiliaries. All the lub oil system in Fans, APHs, Mills and BFPs were flushed with construction power from using LT starters in unit 3 & 4.

   

SARS AND CARS

 

At Khaperkheda the number of SARs and CARs raised as on 21.1.02 are as follows,

 

Total SARs raised Total CARs raised

Mfg unit Unit-3 Unit –4 Mfg. unit Unit-3 Unit-4 Trichy 165 45 Trichy 11 10 Ranipet 67 9 Ranipet 6 9 Hyderabad 54 12 Hyderabad 16 8 EDN 34 8 EDN 17 7 Bhopal 36 16 Bhopal 12 8 Hardwar 86 44 Hardwar 28 15 Jhansi 28 21 Jhansi 3 2 PEM 10 3 PEM 2 1 Piping center

66 27 Piping center

1 1

Total 546 185 Total 96 61

 

 

The major problems faced during commissioning causing anxiety are listed as below

A: BHEL Supplied Equipment

 

1)     Frequent failures of DC JOP motors.

2)     LPTDE fixed type mounting bracket arrangement and calibration difficulty.

3)     Bearing no. 3 vibration probe location problem causing non working of the instrument.

4)     Barring gear, jamming problem.

5)     Gland Seal steam supply and leak off valve unreliable operation.

6)     F.D. Fans servomotor oil leakage’s and subsequent SCAPH choking.

7)     VFD commissioning for I.D. Fans, frequent card failures.

8)     Failure/sticking of over speed device.

9)     Mix up/non clarity of absolute and relative shaft vibration and bearing vibration display in WSPOSE due to incomplete modification by Bently Neveda.

10) Failure of ESP internals due to high ash built up in hoppers.

11) Increasing trend of generator bearing vibration and high vibrations at bearing No.4.in unit-3

12) APRDS operation difficulty due to capacity selection problem at design stage.

13) UAT failure.

14) Frequent failures of MOTPRO relays and 6.6 KV switchgear problem required lot of corrections.

 

B: Customer Supplied Systems:

 

1.     Ash handling System

(a)   Dry system from ESP hoppers , Economiser hoppers & Airheater hoppers to Silo

(b)   Wet Slurry(Dense phase) through GEHO Pump ( First Time in India)

 

 

BRIEF DESCRIPTION ON SOME OF THE ABOVE LISTED PROBLEMS

 

A)      BOILER

 

1)     APRDS OPERATIONAL PROBLEM.

 

PROBLEM

 

The APRDS system provided here has three streams as per capacity i.e-High capacity H.C.), Intermediate Capacity (I.C.) and Low Capacity (L.C.), unlike normally 2 streams in earlier projects.

The HC is having one pneumatic operated steam side PRV and spray side TCV, additionally motorised bypass remote manual valve is also provided. The HC is envisaged for 18 T/Hr to 86 T/Hr.flow.

The IC is having one motor operated regulating valve on steam side PRV and one on spray side TCV having flow capacity of 8 T/Hr. to 18 T/Hr.

The LC is having pneumatic operated steam side PRV and spray side TCV having capacity of 0.7 T/Hr.

 

Full auto operation is envisaged for minimum flow to maximum flow and vice versa requirement with auto change over from one stream to another stream. The main problem faced is the HC having very high capacity than actual requirement, so it was not suitable for low flow condition at unit partial load operation. LC having very low capacity than normally required to keep the system in charged condition at unit full load operation with no system consumption. It was felt that IC will be ideal for such cases, but the motor operated regulating valve was not suitable for auto operation.

 

ACTION TAKEN

 

When problem referred to PC Chennai, it was informed that the capacity selection was based on customer inputs and approval. The IC PRDS was later addition to facilitate only for change over from HC to LC and vice-versa. After lot of correspondence PC Chennai agreed to replace the IC PRDS valves actuators from electrical to pneumatic. These are recently installed and performance is under observation.

 

2)F.D. FAN SERVOMOTOR OIL LEAKAGE.

 

PROBLEM

 

In all the four F.D. fans oil leakage developed from servomotor causing serious operational problem of SCAPH choking and uneven flow through APH and wide difference at APH outlet flue gas and air temperatures. To attend F.D. fan problem load reduction and SCAPH cleaning was requiring total unit shut down.

 

ACTION TAKEN

The problem was reported to BHEL Ranipet and the oil seals and simmer rings were changed with better quality. In one fan, bearing replacement was also was required. As per information these fans were fitted with indigenous seals and simmer rings. The performance was not satisfactory and hence changed with imported one. Another likely reason is seal erosion due to ash entry from atmosphere. This is during initial problem in ash handling system, when the ash hoppers were ground unloaded frequently caused the entire area dusty. Ranipet suggested to provide seal air connection to avoid ash entry in to oil seal area.

   

3)ASH HANDLING SYSTEM / ESP INTERNALS DAMAGES

 

The ash handling system supplied by M/s. Mahindra Ash Tech is with new technology adopted first time for 210 MW unit by MSEB. The main working principle is as below

1) The dry ash from ESP hoppers is collected by gravity flow, in vessel down below. For this a vent line is connected from vessel to ESP casing for equalising the pressure.

2)     The ash collected is transported from vessel to silo by transportation air through M.S. pipes.

3)     Ash from silo is transported to slurry tank through dust conditioner and slurry density is maintained to have dense phase ash. The slurry tank is having stirrer for uniform ash water mixing.

4)     The thick slurry is sucked by reciprocating pump (GEHO) and pumped to ash bund located about 6 Kms. The maximum discharge pressure is 180 Kg/Cm2.

 

The total operation i.e. ash collection in vessel, transportation to silo, measuring and maintaining slurry density and transporting the dense slurry to ash pond is designed for full auto operation.

 

PROBLEM

 

During initial commissioning period due to various reasons the ash evacuation from hopper was not effective and hoppers were getting full resulting in field tripping on ash level high protection. Many times the ash collection was much above collecting rapping system. This has caused serious problem of ESP internals damage viz.,

i) Shock bar bending.

ii) Collecting electrode bending

iii) Collecting electrodes coming out of suspension hook.

iv) Emitting electrodes snapping.

v) Collecting rapping motor tripping on over load.

 

This was a routine failure observed whenever the field was checked for attending to damages after ground unloading of ash. In unit 3 ‘B-1’ field extensive damage was noticed. The emitting electrode middle and bottom tier frame got bent. The collecting electrode suspension frame bent. Almost 30 to 40 % collecting & emitting electrodes, shock bars damaged, emitting rapping insulator, deflectors in hoppers were damaged and required replacement. MSEB apprehended the design deficiency in ESP internal components and asked to study the same. BHEL stated that with higher efficiency & higher size ESP’s the design incorporated and supplied for more than 100 ESP’s had similar problems, when excessive ash built up above high level probe are allowed.

 

ACTION TAKEN

 

i)                    Ash handling system improved and high level built up to be avoided.

ii)                  In case ash evacuation is not done through system, it should be ground unloaded.

iii)                 The field to be switched off, but rapping system in service, where hopper is not getting evacuated.

iv)                Fluidising pads provided for assisting easy ash flow to vessel.

v)                 To avoid back flow from vessel to hopper, the passing intake valves to be attended immediately.

vi)                Depending on condition adjusting the field loading by changing charge ratio, current limit and rapping frequency.

 

All these were temporary measures and lots of modifications were carried out in ash handling system and now since August 2001 considerable improvement is observed and ESP internals damages also reduced/eliminated.

For BHEL, the extra work towards rectification to the order of 50 to 60 lakhs(including supply of material) was involved. The continuous operation and maintenance of ash handling system and it’s effect on ESP internals is under observation.

  

4)P.A FAN- 3E, BEARING FAILURE

 

PROBLEM

 

On two occasions total 3 nos. bearing failed in P.A.fan -3E associated with bearing failure, rubbing in impeller seal, shaft seal and shaft near protective tube was observed. All the possible reasons of bearing damages were checked and confirmed for their correctness viz. lub oil flow, cooling water flow, bearing clearances, seal clearances, journal diameter, centering, alignment, looseness etc. It was observed that the in hot condition the casing is not expanding as desired during design stage. The guide and expansion provisions provided can not control the casing movement.

 

ACTION TAKEN

On referring the problem to Ranipet, and during visit of Ranipet engineer also the reason for casing shifting could not be ascertained. The similar unpredictable casing shift was observed in all other fans also to various extents. Finally it was decided to lock the casing after ensuring the centering in cold condition. The expansion provision was removed and additional locking was provided to avoid the casing shift. This modification was carried out for all the 12 fans. After this no damage happened on account of casing shift.

 

 

5.BMCR TEST OF BOILER

 

BMCR test is one of the main boiler capability tests to be proven as part of boiler PG test. The main constraint faced is consumption of extra steam above boiler MCR capacity and duration of test. The various possibilities are as follows,

1)     The turbine can be loaded more up to its maximum capacity to consume more steam flow (measured by first stage pressure Vs steam flow curve) from boiler. The duration of such loading w.r.t. turbine limitations are not clearly specified.

2)     The excess steam to be diverted by consuming for other purpose through APRDS. At full load operation, APRDS steam consumption by same unit is not possible for prolonged duration.( only the SCAPH can be charged to consume more steam ).consumption of excess steam through APRDS to other units also poses restriction due to above reasons.

3)     Venting of steam from APRDS to atmosphere through spare tappings. The main problem faced is noise pollution. A suitable line with silencer and proper routing of this line is to be provided at design stage it self.

4)     Parallel operation of HP by pass for diverting excess steam from main steam line in to re-heater system. At turbine full load condition the parallel operation philosophy and safety of turbine is not very much clear and also the condenser capability for this condition is not clearly mentioned.

 

The main parameter to be measured for BMCR test is steam flow from boiler. The normal measurement facility and its authenticity become a question mark. Then we resort to other indirect method like feed water flow measurement, which is not convincing to customer. To overcome this problem being faced in almost every utility boiler following shall be thought of,

1)     Measurement of steam flow from boiler by some other method. e.g. the DP across super heater Vs flow( as provided at Trombay unit # 6)

2)     Provision of additional line from APRDS with silencer for steam venting.

3)     Possibility of use of start up vent and its flow measurement at full load parameter.

4)     Loading of turbine to maximum possible load and its time duration keeping in view of safety and special precautions to be taken for main turbine, generator, transformer, condenser etc.

5)     Parallel operation of HP bypass system at turbine full load.

   

B)      TURBINE

 

1)     GLAND STEAM SUPPLY AND LEAK OFF VALVES OPERATION.

 

PROBLEM

The operation of gland steam supply and leak off valves with hydraulic actuators are not satisfactory. Hardwar normally imports the hydraulic actuators from Germany. At Khaperkheda Hardwar has supplied M/s. Iyappan make (indigenous) hydraulic actuators. To attend to the various problems in these actuators and make reliable operation of these valves, number of actions were taken as follows.

ACTION TAKEN

i)                    The feed back linkages were getting disturbed frequently and it was modified.

ii)                  The filters were getting choked very frequently. The oil was replaced and filters were cleaned regularly. Still oil contamination was observed.

iii)                 It was suspected that the oil tank of aluminum casting is causing oil contamination and hence these tanks were acid cleaned. But finally it was decided to replace the aluminum tank with stainless steel tank.

iv)                Also the motor supplied was of 0.25 KW capacity and was just sufficient. Hence the motors were replaced with 0.37 KW capacity.

v)                 Relief valves were passing and it was replaced.

vi)                After all this exercise still the auto operation of these valves were not satisfactory and hence the overlap opening between two valves i.e. supply & leak off was tried to increase. Also the impulse sensing line was modified to avoid any delay in sensing.

vii)              Even after this action it was observed that the tank is getting heated up and in hot condition approximately above 60 deg. C, the valve operation was erratic. Hence temporarily tank cooling arrangement was done with compressed air and finally a separate oil cooler arrangement was supplied by vendor.

viii)             Still the operation is not satisfactory, and with so many uncertainties and frequent maintenance requirement, customer is demanding to replace the hydraulic actuator with pneumatic. The same problem was faced in Khaperkheda unit # 1 & 2 also and customer has modified the valve by providing pneumatic actuator. We have requested MSEB to carry on the work and debit the cost to BHEL.

   

2)    UNIT # 4 , BARRING GEAR

 

PROBLEM

 

On one occasion when unit tripped turbine came to barring gear and after about 5 Hrs. alarm appeared barring gear solenoid energised along with seal oil H2 differential pressure low, and barring speed started reducing and turbine came to stand still. On investigation it was observed that as per logic when turbine speed less than 15 rpm and seal oil H2 diff.pr. disturbed the barring gear solenoid will energise to close the oil supply to barring gear motor. Even though actual speed was more, some work was being done in TG panel and the card was removed hence speed less than 15 rpm was sensed and solenoid got energised. After normalising and de-energising the solenoid it was observed that turbine did not come on barring speed. Following checks were carried out.

ACTION TAKEN

1)     Rotor freeness checked by hand barring.

2)     Dial gauges were put and making JOP pump off and on checked the rotor lift. Slight difference was noticed in Brg. 1 & 4-lift value and it was adjusted. Rotor freeness checked and found to be further very free. Still M/C could not be put on barring gear hence boiler was boxed up.

3)     On cooling down of machine the barring gear motor was removed for servicing. It was observed that one of the casing bolts was sheared off and was touching the outer ring of over run clutch. The sheared bolt was removed and new bolt was fixed. Again barring was tried but M/C did not came on barring.

4)     The hydro motor removed and dismantled for servicing. Nothing abnormal was noticed, hence all parts were simply cleaned and assembled back.

5        When rotor became colder the barring gear motor was removed. It was observed that one of the casing fixing bolts got broken and fallen inside. This was rubbing with the outer ring of the overrun clutch and causing restrain for rotation of the motor. The same was removed and a new bolt was fixed. Trial was taken but still not much improvement was noticed. Barring gear was possible only with hand barring assistance along with the hydromotor. There was no repeatability.

6        Doubting some problem inside the hydrometer, the same was dismantled . All parts and gears inside were clean and have no damage. So it was assembled and fixed into position.

7        For checking the oil flow to hydro motor the solenoid down stream connection was removed and by making solenoid on off the oil flow was checked. It was observed that oil flow sufficient is not coming instantaneously and was increasing slowly. Making temporary arrangement for pressure gauge fixing checked oil pressure and it was observed that the oil pressure is building up very slowly after opening of the solenoid valve. Suspecting some problem in solenoid valve the solenoid was removed and spool piece was fixed. The sufficient oil flow and pressure was observed. Barring gear was tried in this condition and barring speed was picking up from turbine stand still condition..

8        So the solenoid valve block was dismantled for inspection. It was observed that the drain oil port from above the piston of the pilot valve was blocked with a plug. By studying the oil flow path in the solenoid valve block and it was confirmed that for proper functioning of the solenoid valve the plug should not be there. So the same was removed and the block was fitted back after through cleaning. There was instantaneous oil with full pressure as soon as the solenoid vale was de-energized. Trial and repeatability of barring was also found o.k.

9        Similar inspection was carried out in unit # 3 and the blocking plug was removed from solenoid. The feedback is given to BHEL Hardwar.

 

C) ELECTRICAL

 

PROBLEM

1)     D.C. JOP MOTOR FAILURE

 

So far the D.C. JOP motor has failed five times. The type of failures its analysis and remedial action taken is as below,

Sl. Date & Failure details Action taken

No unit 1 29/04/00,

Unit – 3 Motor jammed. Rotor end winding bulged out.

Motor replaced from unit # 4, & before starting the trimmer resistance was adjusted to have field current 20% higher than rated at cold condition.

2 05/09/00, Unit-3

Motor jammed. Rotor end winding bulged out.

Motor replaced with earlier failed repaired motor. Also the end winding was strengthened during repair. Trimmer resistance set from original 150 ohms to 50 ohms & trial taken.

3 06/09/00, Unit - 3

Motor rotor winding damaged due to one slot-winding strip coming out. Suspected poor workmanship.

Spare motor arranged from M/s.CGL. The trim resistance totally removed as per CGL. The motor was modified for rotor winding strengthening.

4 10/07/01, Unit-4

Motor burnt due to over current.

The 3TR timer malfunctioned causing 1st & 2nd step series resistance bypassing and giving direct start. The timer was replaced. Motor from Chandrapur (55KW), arranged and by temporary adjustments of resistance the motor was put in service.

5 30/12/01, Unit-3

Rotor winding found burnt. Reason could not be established, most likely the IR value may be low at the time of starting. The space heater condition and brush area cleaning is in doubt. Motor replaced with earlier failed repaired motor

A meeting was held at Hardwar between CGL, TUSHACO and BHEL on 26/9/00 wherein the failures were discussed and CGL proposed to reduce the field resistance to 50 ohms from 150 ohms. Also they proposed to strengthen the armature overhang winding by extra number of turns of resiglass tape and additional glass braiding provision. Accordingly the motors sent for repair were modified.

After 3rd failure CGL recommended to bypass the trim resistance totally. Also the various modifications carried out by CGL during motor repair are as follows,

 

1)     Armature Banding – We are ensuring adequate number of turns and tension while overhang bending. Further we are curing the bending at 140 degrees before impregnation

2)     We are doing vacuum pressure impregnation for stator as well as armature.

3)     Soldering of armature winding to commutator is done on an automatic dip soldering m/c.

4)     We have changed over to permanently greased sealed ball bearing.

5)     We have introduced additional surge testing of armature before assembly

6)     Though the specification calls for class ‘F’ insulation, we have provided class ‘H’ winding wires and strips with temperature class 210 degree C

7)     An additional coat of Epoxy gel coat is applied on winding overhangs.

 

2)GENERATOR TRANSFORMER COOLING ARRANGEMENT

 

PROBLEM

 

In unit 3 transformer high temperature problem was encountered after some day’s operation. The first doubt raised was insufficient cooling water flow due to fouling in cooler. Heavy blocking was noticed due to contamination/ dirty cooling water.

 

 

ACTION TAKEN

 

The coolers were cleaned thoroughly, even then ensuring sufficient flow was in doubt.

One vital point emerged out in this issue was, the cooler downstream pipeline layout and its sizing. BHEL Bhopal has envisaged free flow after cooler but not given any recommendations for line lay out or size. For upstream and down stream pipeline, the end connection size provision is for 100-mm dia pipe connection for individual cooler with common inlet and outlet pipe connection of same size. This was in customer scope and accordingly customer also has provided only 100-mm dia. pipe for upstream and down stream matching to end connection provided along with cooler. But due to inlet pressure design restrictions and locally decided pipeline layout, the flow was not sufficient.

To increase the flow, the down stream pipe size was changed to 150-mm size and layout was modified with minimum bends, to have sufficient flow without higher resistance. BHEL Bhopal shall suggest the pipe sizing and layout also to avoid such problems.

D) CONTROL AND INSTRUMENTATION

1) LPTDE CALIBRATION

PROBLEM-1

For this turbine the HP & IP turbine differential expansion measurement provision is not envisaged. Only LP turbine differential expansion measurement provision is there and hence is most important.

1)     The LPTDE measurement works on the principle of dual ramp differential pick-up measurement. The range of the instrument is -10mm to +32mm. It was observed during calibration that the actual displacement of pickup and distance shown in the monitor is not matching.

ACTION TAKEN

On investigation it was found that the extension lead of the probe that connects the probe to the proximeter was wrongly used. The colour and size of the extension cable for the vibration pickups, key phasor and that of LPTDE pickup are same although each had a different part number. But it had got mixed up and the extension cable of key phasor pickup and LPDE pickup had got interchanged. By rectifying the mistake, the reading became reasonably accurate. Edn Banglore/Bently Nevada has been requested :

 

i)                   To use different colour for different application extension cable and/or

ii)                 To avoid use of extension cable altogether.

 

PROBLEM-2

 

The LPTDE reading had an error of 3 to 4mm in the range of 20 to 30mm expansions.

ANALYSIS/ACTION TAKEN

This is due to three possible problems out of which two have been resolved and one remains unresolved.

i)                    The bracket that holds the pickups need be slanted in such a way that it should be perfectly parallel to the shaft surface for setting the gap required for calibration. It was found that the face of the pickups is not parallel to that of the shaft surface and there is a gap of nearly 2.5mm between the top and bottom of the pickup. It was found out that the slant in the bracket should be 10degrees but the actual slant provided was five degrees. The bracket was changed.

 

ii)                  The bracket supplied was of fixed type and for calibration shifting of the bracket again and again was required to cover the entire range of 42mm. This introduces an error in the calibration of the pickup as each time the bracket is shifted and no proper fixing can be ensured, this leads to an error in maintaining parallelity. For solving the above, site suggested a sliding type arrangement such that the bracket can be slided in a guide and parallelity maintained over entire range during calibration. Such a sliding arrangement was done in SLPP a Mangrol site and was convenient to use. So the bracket was again modified to sliding type and has been used for Unit #4 LPDE.

iii)                 The use of proper extension cable, correct bracket slant and sliding arrangement for calibration has still not solved the problem of accuracy entirely and there is an error of 2 to 3mm in the operating range of the instrument.

This in our opinion is mainly due to the type of monitor selected for the purpose. As per the present configuration and the monitor selected, both the probes, probe A and probes are active in the entire range of measurement. As a result in the operating range of the instrument one probe is faraway from the shaft surface and so the output from that probe is inaccurate. This inaccuracy is contributing to the final reading of the monitor.

This problem can be resolved by using a complimentary type of monitor where one pickup contributes only for half the instrument where the air gaps is less and accuracy of the reading is more. As the shaft expands, the air gap in the active probe side will increase and will decrease in the passive probe side. Once the air gap in the passive probe site comes to the required linear zone, it will take over the active probe and will be active for further expansion range. This type of monitor is in use at SLPP Mangrol.

Another problem related with LPDE is the alarm is set at 30mm, where as the range of the instrument is 32mm. Since the negative expansion of the shaft is very rare the range of instrument can be modified.

   

2) Vibration MEASUREMENT SYSTEM:-

 

Two problems were faced in vibration measurement system. One has been solved and one remains unresolved as a today.

PROBLEM

1) Shaft vibration measurement problem in bearing-3.

After completion of calibration of all the vibration pickups, all the vibration signals were healthy in controls. But after putting the machine on barring gear, bearing No.-3, bearing and shaft vibration readings were becoming invalid. For calibration, in turbine stand still condition, the probe is set with a gap of 2.5mm between the shaft surface and probe face and voltage of 10v is adjusted corresponding to this gap,. This was done for bearing No. 3 also, as it was done for bearing 1 to 4. But it was observed that as soon as the rotor was put on barring gear, both the channels for shaft vibration measurement became faulty and whenever the rotor became stand still, the channels became healthy. This was really mysterious and extremely difficult to analyze because checking is possible on rotor stand still condition and in rotor stand still position the fault was disappearing. After doing all routine checks like changing the cable, changing the monitor, changing the pickup etc., the problem could not be resolved and it remained as it is.

 

ACTION TAKEN

 

As all the possibility of finding a defect in the measurement system was exhausted, it was decided to check for any problem in the shaft surface and the problem was right there!

On inspection of the shaft surface by a torch light with the pickups removed and hand barring, it was found that there were four big holes on

shaft circumference at equidistant just facing the probe. Whenever the probes were facing these holes, the voltage level was jumping and creating a fault. These holes are provided for individual shaft rotation facility. The matter was taken up with BHEL Hardwar and to resolve this problem there was no other solution than to shift the probe location. The pickup was relocated by shifting toward generator by about 50-mm, such that the probe did not face the hole on the shaft. By shifting the probe location the problem got resolved. Later on it was learnt that in Kayamkulam steam turbine, similar problem was faced but the feed back was not available with us.

 

 

WS POSE indication of bearing/shaft vibration

 

PROBLEM

While comparing the vibration reading of control room and local readings taken with portable vibrometer, it was observed that there was wide difference in two bearings and shaft vibrations. The reason could not be found out and it was left as it is. Subsequently a detailed study was done & following facts was surfaced out as follows.

 

ANALYSIS/ACTION TAKEN

 

For shaft vibration of shaft 1 to 4 the pickups are mounted on to the respective bearing vibration pickups. The pickup out put indicates bearing vibration and relative shaft vibration. For getting absolute shaft vibration, the two values are added by vector addition method. BHEL has asked for modification in Bently Neveda panel, for indicating as well as generating signals for indication in two possible combinations in panel monitor as well as WSPOSE indication. The combinations were bearing vibration + relative shaft vibration or absolute shaft vibration + relative shaft vibration. Accordingly Bently Neveda has modified the panel but halfway i.e. first combination of bearing vibration + absolute shaft vibration was indicated on panel monitor only. Whereas the output signal to WSPOSE was not modified and the provided jumper selection was giving combination of absolute shaft vibration + relative shaft vibration only. The matter was referred to EDN, since for normal monitoring the bearing vibration + absolute shaft vibration measurements are required for operator monitoring. M/s Bently Nevada were requested to modify the system, so that the combination of bearing vibration + absolute shaft vibration is available for indication in the monitor as well as indication in WS POSE. Accordingly M/s. Bently Neveda has done the modification and the confusion was avoided.

But unfortunately, M/s Bently Nevada has done the half modification. They have modified the indication in the monitor (which is rarely used by the operator) and left the signal for WS POSE is unmodified and we have missed this point. To compound the confusion further, the jumper selection for WS POSE indication was selected in such a way that the output signal was for absolute shaft + relative shaft. But since Edn Banglore has no knowledge about this half modification, they have alone the engineering and internal wiring of the panel for indication bearing vibration + absolute shaft vibration so the final picture is the monitor indication was bearing vibration + absolute shaft vibration. In the WS POSE what were showing as bearing

vibration was actually absolute shaft vibration reduced to a scale of 100microns, that is one fourth of absolute shaft vibration. What we were showing as absolute shaft vibration was actually relative shaft vibration.

As soon as the problem come to our notice, the jumper setting was changed so that now the monitor indication is bearing vibration + absolute shaft vibration. In WS POSE the indication is bearing vibration + relative shaft vibration.

M/s Benthly Nevada had sent their representative on 30th August 2001 to site to carry out the necessary modifications and the modifications have been carried out in Unit #3 and #4. The necessary changes in the documentation have also been carried out. Now, the indication in the Monitor as well as WOPOSE is Bearing Vibration + relative shaft Vibration.

SITE DEVIATION REPORT

 

During commissioning it was found necessary to change some of the logic/provisions for improving operation facility and avoid unnecessary tripping of auxiliaries. As per the directives, for any change from designer’s recommendations, concurrence is to be sought from respective units engineering before carrying out any change. As per new system to overcome the problem and to avoid delay in activity, SDR(Site Deviation Report) is to made and clearance to be obtained from Construction Manager and same to be reported to H.O. Afterwards it has to be regularised through CAR/SAR etc. Any deviation concerned with safety of equipment, then it is necessary to obtain units clearance before implementation. Only one SDR was of this nature was raised and so far no disposition has been received from unit.

 

Some of SDR’s are described here as below,

 

1)Details of Deviation: PRDS normal operating temp. is 220 deg. C, & temp. very high to close steam pr. Control valve is 270 deg. C. On many occasions it is observed that temp. is exceeding more than 270 deg.C and PRDS is getting isolated. This is leading to unit tripping very frequently. The temp. control loop is having only PI control & original control circuit is not workable at all, because of full feed forward of pressure control valve and there in jump while putting in auto.

 

Action Proposed: To avoid frequent tripping it is proposed to increase the very high set point to 300 deg. C Also for fine tuning & to improve temp auto control loop, present auto control loop having only PI control, it will be changed to PID control. The control loop is modified to cater system requirement & better control. The feed forward of PCV to TCV has been provided which an adjustable factor for tuning. Accordingly suitable modification has been made to avoid jerks while putting PCV from manual to auto and vice-versa

 

2)Details of Deviation: For starting additional pump one condition is steam flow shall be >50 % as per EDN logic whereas as per PEM instead of steam flow it is given as feed water flow > 50 %

 

Action Proposed: IT is decided to change EDN logic from steam flow to feed flow as per PEM logic, because BFP loading is decided by feed flow rather than steam flow.

 

3)Details of Deviation: As per EDN logic for stand by pump tracking of scoop is not given whereas as per HYD. Manual for stand by pump the scoop shall track as per running pump. PEM scheme also does not call for tracking, but for stand by pump auto check back is required as per PEM.

 

Action Proposed: Tracking of the scoop of stand by BFP is very useful for maintaining drum level in case of takeover by stand by pump. The average of two running pumps will be tacked by the stand by pump. Also after staring stand by pump the scoop will be automatically put on auto.

 

4)Details of Deviation: For CST level Hi/ Low alarms no window is provided on back up panel audio visual alarm. Customer wants alarm to be provided for operator’s convenience.

 

Action Proposed: The necessary modifications are carried out for providing the audiovisual alarm on back up panel.

 

5)Details of Deviation: In BFP start sequence logic ,ON command for main BFP motor & its cooling water valve goes simultaneously &only after getting CW valve open feedback further command goes for discharge bypass valve & then main discharge valve. Since CW valve takes quite some time to open, BFP loading gets delayed and may cause drum level control problem. Hence instead of waiting for getting CW valve open status, not closed status is preferred to avoid delay in pump loading

 

Action Proposed: Instead of the CW valve open status not closed status will be used to proceed further commands for BFP loading.

 

6)Details of Deviation: As per EDN logic for CEP starting hot well level adequate permissive is taken as +100mm from level transmitter level signal. Whereas as per BHEL HYD. O&M manual the permissive for CEP starting is mentioned as hot well level not low which is set at - 340 mm. The normal operating level is set at zero and in auto it maintain about ± 50 to 100 mm. In case of the running pumps trips as per EDN logic the same pump or stand by pump can not be started unless level is raised to + 100 mm. If we wait till level raised to + 100 mm the unit may trip

 

Action Proposed: It is proposed to change the EDN logic and introduce level adequate permissive as level not less than -340 mm as per BHEL Hyderabad recommendation. The level very low trip set point is at -1300 mm hence it is safe to run the pump with level more than -340 mm. Necessary changes in logic modification are carried out.

 

7)Details of Deviation: As per BHEL Trichy recommendation super heater spray can not be given till total steam flow is >20% and burner tilt can not be operated till total steam flow is >25%. It is observed that after synchronisation of the unit and starting of coal firing the super heater and re-heater outlet temps. are exceeding 540 0C. This happens especially at low load upto 40MW. Because of above logic temp. can not be controlled since spray can not be given and tilt can not be operated.

 

Action Proposed: To avoid temp. exceeding >540 0C, it is proposed to lower the steam flow limit interlock to >5%. This is necessary because other measures to control the temp., like increasing feed water temp. or reducing excess air quantity is not possible during cold start up and low load condition

 

DETAIL DESCRIPTION OF VARIOUS PROBLEMS FACED AND FEW SUGGESTIONS SUARE AS FOLLOWS,

 

A) BOILER

 

 

1.DRUM LEVEL MEASUREMENT AND INDICATION

 

PROBLEM FACED

 

Drum level display at control room is the most critical requirement for boiler light up.

First boiler light up for all utility boilers is for alkali boil out. During alkali boil out it is prohibited to charge level transmitters, hydrastep and even regular DWLG. Earlier projects a temporary gauge glass was being provided for initial light up. Now a days no such gauge glass is given. Hence we have to charge the regular DWLG.

At Khaperkheda unit # 3 & 4, for the first time we have seen BONT LEVEL GAUGE TYPE BCI 10 , CESARE BONETTI (Italia)make DWLG. The main differences/problems observed in this were as follows,

1)Unlike earlier Yarway make DWLG this gauge glass due to orientation of given piping assembled along with this gauge glass, has to be erected in straight line (with out giving 90 degree elbow) from the stubs provided for this on drum.

2) For the reflector bulbs provided (assembled along with gauge glass), the lighting supply is given through a transformer of 220/20 volts, originally supplied along with this package. Accordingly the supply was given from 220volt source. It was observed that no. of bulbs were getting fused within short duration. On inspection it was found that the bulbs supplied are of 12 volts rating. The supply was then given from 110volt source so that 10volt is applied to these bulbs through the same transformer. With this the problem of bulbs fusing was resolved.

3) During initial operation to verify the level, the line & gauge flushing is required frequently. During flushing no water was coming from waterside tapping in spite of water level well above this tapping (as confirmed from spare open tapping). But after charging and waiting for some time the gauge glass was showing some level. This was leading to confusion and hence continuing the boiler light up was risky. To further analyze, the dummy plug of gauge glass waterside block was removed and one steel ball was found inside. On checking the O & M manual, no details about construction details and the purpose of providing this ball was clear. After removal of this ball the flushing could be done and level could be ascertained immediately while charging the gauge.

 

SUGGESTION

1) While supplying the new type of product the detail documents must be supplied directly to commissioning engineer. At least when ever new product is introduced some information must be given in advance to site engineer, so that some study and more care can be taken while erection and commissioning or more details can be asked for from units.

1)     The hydrastep and at least one level transmitter must be allowed to be charged even for first light up for alkali boil out. A written permission must be given for this, because as per manual use of this accessory is prohibited during alkali boil out. Because of this customer also objects for the same. But drum level monitoring being very vital for main boiler safety point of view this has to done even by sacrificing some accessories.(if damage happens to this instruments)

2)     Additional transmitter and EWLI spares shall be supplied for this purpose, since local operator and local gauge glass can not be relied.

 2.EWLI FOR DRUM LEVEL MEASUREMENT

 Electronic water level indicator, popularly known as Hydrastep being used widely in almost all power stations was imported & supplied by SOLARTRON, YARWAY OR BHELVISION. The various other instrument used are Level Transmitters, Direct Water Level Gauge, Remote Water Level Indicator. In operation all these are compared with each other. Especially the level transmitter and hydrastep indications, which are available in main control room are compared by operator. Any difference noticed between these, leads to controversy because it is visible to operator and mostly used in combination for tripping circuit. The main problem faced is level transmitter and EWLI indications exactly matches in cold condition but with increase in boiler pressure the difference increases to the extent of 100 to 150 mmwc or even more. The queries on EWLI supplied earlier and the various changes in design and installation are not explained clearly either by supplier or designer. The various differences noticed are as under.

 

1)     At Korba NTPC 500 MW unit the pressure vessel and isolating valves were imported. As per installation drawing an offset was given w.r.t. NWL, stating that it is for temperature compensation in hot/normal operating condition.

 

2)     At Chandrapur 500 MW unit 5 & 6, the same thing was supplied but offset was not asked for installation. The isolating valves used were BHEL make high pressure valves. When difference was noticed the solution suggested was, the valves shall be mounted in stem horizontal position to reduce the resistance/pressure drop as compared to valve mounted in stem vertical position. The exact effect is unknown.

 

3)     At Dahanu the pressure vessel design was changed and sleek/thin wall vessel was supplied stating that it will reduce the sub cooling effect and in turn no temperature compensation is required. But in practice the level difference was so high that at operating pressure it was not indicating any water level. Where as in cold condition the transmitter and EWLI levels were exactly matching. On further investigation it was found that the insertion tube length of water side tapping was more and the down comer suction effect was causing the whole problem. Even now the design/required insertion length is not known. Even after reducing the insertion length the levels did not match and problem was left as it is.

 

4)     At Gandhinager for the BHEL make BHELVISION EWLI the same problem repeated and same solution was applied but problem could not be resolved fully. Same is the case for Wankabori.

 

5)     At Khaperkheda unit # 3 & 4, the difference between left and right side EWLI is main issue discussed and exact solution is not yet known. Also in all earlier mentioned projects for mounting the vessel, use of bend was necessity for matching with NWL. Where as at Khaperkheda the vessel height is such that bend can not be used. The earlier theory of steam side bend shall slope toward vessel and water side bend shall slope toward drum is not applicable. Reason unknown.

 

Also refer the drawings enclosed, Drum dished end instrument tapping details and Yarway drg. No. 114182-43/0. The drum tapping points shown for EWLI as per the drg. ( 3.97.287.90972 / 01 ) is pair no.4 with a distance of 1183 mm. Also the vessel supplied is of the size 1183 mm and hence no slope can be maintained as per the Yarway drawing.

 

6)     At Sikka the difference between left and right side level was reported and probable reason investigated was one of the bend used in down comer was of higher thickness. Further report not known.

 

With all the above the problem is existing in almost all the units and the solution has to be evolved on technical/design base. Some of the questions are to be answered.

 

1)     Inclusion of EWLI for drum level protection reliable or not.

2)     What is the basis of allocation of tapping points for different instruments.

3)     How much amount of circulation of steam condensate back to drum takes place at different operating pressure.

4)     How much is the suction effect of down comer or any near by tapping (EBD) to cause level difference. The exact design solution for reducing the same.

5)     Can the level transmitter reading which has the provision of on line temperature compensation and used for level control, is to be treated as master for comparison. Other instruments like DWLG and RWLI can not be compared.

 3.CBD TANK

 

Normally the CBD line is terminated to CBD tank and the tank vent steam is reused, by connecting back to de- aerator for recovery of DM water. At Khaperkheda the steam is not recovered and left open to atmosphere. Under such condition the CBD tank itself could have been avoided and CBD line could have been terminated in IBD tank. On inquiry it was informed that, this is as per customer requirement. Reason not known.

 5.UNIT FLASH TANK

 

The unit flash tank is normally used for warming up of main lines before the drains are diverted to condenser. Mainly the MS line drain and HP-bypass upstream drains are connected to this tank and vent is led open to atmosphere, since these are high temperature drains, a quenching line is a must to reduce the tank temperature and condense the steam to some extent. At Khaperkheda the quenching line was not provided. This resulted in high tank temperature and also heavy noise at exhaust of this tank. A tap off was taken from near by ACW line and quenching was provided. This has helped in reducing the tank temperature and noise also.

 

6.MOTORISED VENT VALVES IN MS, CRH AND HRH LINES

 

The vents on these lines are required to be operated only during hydraulic test of these lines. During normal operation and charging of this lines the drains are required to be opened and vents are not used as normal practice. Providing of motor operated vent valves is not the necessity and moreover these vent line valves are to be located to an accessible location. But in schematic drawing it is shown just on the line itself and are erected as per drawing. Always these lines are required to be extended so that the valves are in accessible location. It is opined that only manual operated valves at

appropriate location are more than sufficient. This will reduce actuators, electrical modules and connected control equipment’s.

 

7.DRUM LEVEL CONTROL

 

Normally it is observed that about 90% tripping of unit are on account of drum level protection. Especially during cold start up, it is very difficult to control the level either in manual or auto mode. This is mainly because a continuous/sufficient controllable flow of feed water and steam can not be established. The control logic and the valves provided for this is not adequate and this becomes a debate every time. Following are few suggestions to be looked into.

1)     The low load control valve,(normally of 30% capacity )shall be of higher class so that passing is minimum and it should have fine flow control.

2)     Since the differential pressure across this valve varies depending on up stream and down stream pressure conditions, it is felt that the isolation valve also should be control valve for controlling the DP.

3)     The EBD valve normally interlocked with drum level are not effective for fast draining at lesser boiler pressure, instead the IBD valves shall be interlocked with drum level up to boiler pressure of about 40 kg/cm’2.

4)     The flow measurement device used in APRDS and HP bypass line shall be selected for accurate small flow measurement also.

 

8.BMCR TEST OF BOILER

 

BMCR test is one of the main boiler capability test to be proven as part of boiler PG test. The main constraint faced is consumption of extra steam above boiler MCR capacity and duration of test. The various possibilities are as follows,

5)     The turbine can be loaded more up to its maximum capacity to consume more steam flow (measured by first stage pressure Vs steam flow curve) from boiler. The duration of such loading w.r.t. turbine limitations are not clearly specified.

6)     The excess steam to be diverted by consuming for other purpose through APRDS. At full load operation, APRDS steam consumption by same unit is not possible for prolonged duration.( only the SCAPH can be charged to consume more steam ).consumption of excess steam through APRDS to other units also poses restriction due to above reasons.

7)     Venting of steam from APRDS to atmosphere through spare tappings. The main problem faced is noise pollution. A suitable line with silencer and proper routing of this line is to be provided at design stage it self.

8)     Parallel operation of HP by pass for diverting excess steam from main steam line in to re-heater system. At turbine full load condition the parallel operation philosophy

and safety of turbine is not very much clear and also the condenser capability for this condition is not clearly mentioned.

 

The main parameter to be measured for BMCR test is steam flow from boiler. The normal measurement facility and its authenticity become a question mark. Then we resort to other indirect method like feed water flow measurement, which is not convincing to customer. To overcome this problem being faced in almost every utility boiler following shall be thought of,

6)     Measurement of steam flow from boiler by some other method. e.g. the DP across super heater Vs flow( as provided at Trombay unit # 6)

7)     Provision of additional line from APRDS with silencer for steam venting.

8)     Possibility of use of start up vent and its flow measurement at full load parameter.

9)     Loading of turbine to maximum possible load and its time duration keeping in view of safety and special precautions to be taken for main turbine, generator, transformer, condenser etc.

10) Parallel operation of HP bypass system at turbine full load.

 

9.DRAIN VALVES AND CONTROL VALVES PASSING PROBLEM

 

Passing of boiler drain valves mainly EBD, IBD, MS line drains, CRH & HRH line drains and Turbine drain MAL valves is a perennial problem in almost all the plants. All these valves are of 50 mm and below size. The one of the solution suggested is torque closing of these valves. Even after this after no. of operations passing problem starts at one time or other. This problem becomes aggravated especially at high-pressure operation. It may be because of very high differential pressure subjected for these valves. These valves shall be of special design like Y type to have more pressure drop within valve. At Khaperkheda # 3 & 4 the CCI DRAG valves are asked by customer and provided for SH & RH spray and BFP minimum re-circulation application.

Similar problem is faced for soot blower pressure control valve very frequently.

Selection of these special duty high-pressure differential application valves is to be thought of.

Also torque-closing recommendations shall be further studied and implemented in detail. The various improvements/points suggested for positive implementation of torque closing of valves will be as under,

1)     Separate list of valves with Tag nos. shall be mentioned in each PID ‘s with torque values specific to individual projects.

2)     The full load current and TOL settings to be provided.

3)     The control circuits of different make actuators and their hook up with control system shall be clearly given in separate drawing specific to project, instead of general drawing. This is a must to have clear inter- unit co ordination accordingly.

 

10.VALVE IDENTIFICATION NAME PLATES

 

For boiler side valves nameplates are supplied with bracket to be welded for identification of various valves. These nameplates can not mounted directly on valves and require to be welded to line near to valve. Normally this is done after full erection of various systems and by that time insulation of these lines also gets completed. Welding of these nameplates becomes difficult and also not gives aesthetic look. It is normally observed that only some plates are used for namesake and most of left out plates goes to scrap. Finally customer asks painting of tag numbers with description on individual valves. The same practice is adopted for the valves supplied on turbine side. We suggest stopping supplying these nameplates and asking for painting of it OR supply small size nameplates which can be riveted directly on to valve itself.

 

11.FUEL OIL FIRING SYSTEM

 

The fuel oil pump house for unit 3 & 4 is located near the existing pump house. The suction is taken from the available storage tank. The auxiliary steam for HFO heaters is tapped off from the earlier steam line available in unit 1 & 2.

 i)-Many times it was observed that, the steam line from APRDS of 1 & 2 to pump house was required to be isolated for attending to works in unit 1 & 2 pump house, during this period steam to unit 3 & 4 also was getting cut off. MSEB O & M wants a separate line to be laid for unit 3 & 4 from APRDS station of unit 3 & 4. The matter was referred to PC Chennai and theoretically there was no need for separate line and with the existing system itself steam can be supplied from any one running unit to fuel oil pump house. But since due to some reasons the isolation of the line to pump house is required, the necessity of separate line was felt. The matter remains pending and O & M has referred the same to P & P Mumbai.

 ii) The 2 Nos.LDO pumps supplied on one skid with common discharge header with separate line going to unit 3 & 4.

It was observed that one pump can cater the requirement of maximum 6 guns with normal operating pressure against the designed one elevation i.e. 4 guns, which is theoretically correct. But with present supplied 2 pumps if both the units are to be lighted up simultaneously with LDO, both the pumps are required to run and no standby pump is available. MSEB has compared the system with unit 1 & 2 and demanded one more pump to be installed , which was agreed by BHEL Trichy.

 

iii) The oil gun frequent choking was observed after unit stabilisation when oil support is not required very frequently. It is suspected that required oil temperature is not maintained with oil in re-circulation. The entire HFO line is having electrical heat tracing from pump house to corner station. In corner station the heat tracing is provided up to

HFO nozzle valve only. After this point about 2 meters line and flexible hose length is there, which does not have any heat tracing. It is suspected that the portion ahead of nozzle valve remains cold, which affects the oil flow leading to gun not proving. It was suggested by Trichy to provide heat tracing for the remaining portion, but since scavenge steam connection is immediately after oil nozzle valve, it will damage the electrical heat tracer hence it was not provided. For this portion steam tracing can be provided.

At present we have suggested to the operator to do scavenging before taking oil gun in service for warming up the line.

There can be one more reason of gun choking that the oil strainer after pump discharge is located in pump house and after that a line length of about 500 meters is there up to firing floor. Such a lengthy line will cause oil contamination and lead to gun tip choking. Stating this logic at Gandhinagar units 5, the strainers were located at firing floor which can eliminate the contamination due to lengthy line between pump house and firing floor. It shall be provided in future projects.

 

iv) The various set points for interlock and protections given in drawing were not suitable for the system provided at Khaperkheda. After correspondence and clarifications with Trichy the same were corrected. The reason for this mistake was in some earlier projects the oil firing pressure was controlled at firing floor with flow control valve after HOTV, but in recent projects the firing pressure control is done by pump discharge pressure control valve itself. The set points in these two systems are different.

 

v) The oil flow meters are provided in HFO supply and return lines are not working satisfactory. The first problem is failure of mechanical component mainly the gear-locking pin and gears itself. The reason analysed by supplier is excessive flow than design through this flow meter. The probable mal operation due to line layout was eliminated by removing the by pass line interconnection to flow meter. In spite of this the failures were observed due to higher flow with the orifice size (9 mm) suggested by Trichy. To restrict the oil flow the orifice was changed to 6-mm size. But with this, pressure control hunting was observed which is causing flow fluctuation exceeding the design flow.

The return flow meter capacity selected is just marginal i.e. 10 % of supply flow, which is not sufficient at various operating conditions. If mechanical failure is only due to excessive flow then higher capacity flow meter shall be provided to avoid damage.

The oil flow meters are required to know the oil consumption for which supply flow meter is sufficient. The return oil flow measurement is not required for any control/measurement and hence it was deleted in some projects. At khaperkheda the logic of return trip valve remaining always open and also return oil flow meter given, which calls for accurate working of both supply and return flow meters all the time irrespective of oil firing in service or not. Since the supply flow meter is of 30 T/Hr range and return flow meter is of 3 T/Hr range, matching of these meters is not possible. This gives wrong flow consumption when oil guns are not in service.

The measurement of return oil flow is of not much use and hence for Dahanu project the return flow meter is not provided and also in many other project the return oil trip valve gets closed the moment oil gun is taken in service. Oil flow computation starts with

supply oil flow meter only when oil gun is in service. By this provision of return oil flow meter is deleted and all other problems related to return oil flow meter and computation error is eliminated.

 

vi) For HEA igniters purge air connection is envisaged from plant service air. An orifice (3 mm ) is provided near each igniter to control the air consumption. But with service air pressure of 7 Kg/Cm2 the airflow is quiet high and it gives noise and some times gives problem for oil gun proving. In all the sites it was required to provide an isolation valve to regulate the air flow to reduce air consumption and related problem. So far the valves are managed with other surplus valves available at site. On raising the CAR for supply of valves Trichy did not agree, but these valves are required to be provided to overcome the above-mentioned problem.

 

vii) Installations of burner tilt power cylinders as received with the accessories mounted are not suitable due to space restraint. The accessories are required to be relocated as per the space to avoid fauling. This happens in every site and in spite of repeated corresponds no corrective action is taken by Trichy while ordering these cylinders. This causes unnecessary time loss and extra expenditure at site.

 

12.AIR AND FLUE GAS SYSTEM DUCTING

 

F.D. FAN SERVOMOTOR SEAL FAILURE AND BEARING DAMAGE

 

At Khaperkheda in all the four F.D. fans oil leakage from servomotor happened and unit forced outage was required to attend the same. The subsequent/ secondary effect was SCAPH choking. The reason was failure of oil seals and simmer rings. Most likely it was due to poor quality of indigenous seals assembled at shop. Subsequently Ranipet has replaced this with imported better quality seals. One more reason pointed out was erosion of seals due to ash erosion. With dusty atmosphere in thermal plant especially in boiler area, mainly due to initial problems in ash handling system, ash entry can not be avoided. Ranipet suggested providing seal air arrangement around shaft on suction side.

 

 

Suggestion

Choking of SCAPH due to oil leakage from FD fan is experienced at Khaperkheda in all the four SCAPHS. This has caused severe operation problem and unit shut down was required for attending to SCAPH. In some of the projects the SCAPH is installed in the bypass duct. SCAPH is used only during cold startup and for normal operation SCAPH is not required.

Where ever SCAPH bypass is not provided and in case the SCAPH gets choked due to the reasons mentioned above, it poses a big problem during normal operation and invariably

unit shut down of minimum 3-4 days is required for SCAPH cleaning alone. This has happened for Khaperkheda in both the units. Also it is required to attend to FD fan problem immediately to avoid this in further running. Attending to each FD fan requires necessary spares and fan shut down of minimum one week. If bypass duct provision is available normal operation with SCAPH choked condition does not pose immediate problem and unit running for some days by close watch on FD fan problem is possible. This helps & gives enough time for arranging spares and plan for shut down to attend the FD fan problem.

 

13. P.A FAN BEARING FAILURE AND SHAFT DAMAGE

 

On 9.10.00, in unit 3 P.A.Fan 3E the fan tripped on bearing temperature high protection. On inspection of bearing it was found that the DE side bearing babbit metal came out in flakes. No other abnormalities observed. Lub oil quality (no contamination), oil flow/pressure, cooling water flow etc. was normal. All bearing clearances were checked & found in order. The bearing liner was changed from unit 4.

Again on 1.11.00, P.A. Fan 3E tripped on bearing temperature high protection and this time both the bearings i.e. DE & NDE found damaged. Some rubbing marks were noticed on shaft near protection tube area also the clearances at shaft seal were not uniform. Also slight rubbing was noticed in impeller seal ring. This indicates casing disturbance/twisting. As per the drawing the casing is not fixed along the boiler axis and provisions are made for free expansions as per instructions of BHEL Raipet other checks like journal diameters, run out, alignment etc were checked and found OK. The journal area was having lot of pitting marks. Hence it was decided to change the shaft.

In all other fans also it was observed that the fan casing is not moving uniformly and hence getting shifted. The amount of shifting/twisting varies from fan to fan. This feed back was given to Ranipet. The Ranipet engineer visited site and studied the same. Even though expansion provisions are given as per design, the casing/duct expansion in hot condition does not happen as predicted, hence it was jointly decided to lock the casing after centering. With this the rubbing and further problems could be avoided. After this P.A. fan 3 E behavior was normal. Similar modification was carried out in all the 12 fans.

 

It was observed that, 2 shafts of P.A. fan & 1 shaft of I.D. fan pitted heavily in journal area and declared not fit for installation. Same were sent to Ranipet for rectification. The most likely cause for such damage is long storage without proper preservation. Even the shafts are provided with preservative coating from factory, as per Ranipet the shafts must be stored in closed shed and the coating provided at factory can last for 3 to 6 months only and needs regular check and further preservation. Site pointed out that the packing procedure of shaft should be improved like motor shafts from Bhopal. The journal and coupling area shall be packed by wooden strips tied firmly.

 

 

14. SOOT BLOWER PRV

 

The soot blower pressure control valve ( M/s. MIL make ) performance was not satisfactory and various problems like, gland leakage, valve inoperative, heavy passing, set pressure not maintaining etc. were faced right from commissioning. The valve was attended no. of times but problem could not be resolved. Similar problems were faced in earlier projects viz. Gandhinagar and Wankabori. On referring the problem to Trichy with previous feed back, now the actuator has been changed to higher size to have more closing thrust and is under observation.

 

15. H.P. BYPASS VALVE STEM & COUPLING DAMAGE

 

In unit –4, both the H P Bypass valves stem & coupling threads got partially damaged when valve operated on fast opening mode. These valves were in service for more than 6 months with normal operating mode. After failure since no spares available, the same stem & coupling threads were cleaned and re coupled. Similar problem happened at Bakareshwar site also. Trichy has analysed the failures and confirmed there is no design/manufacturing problem. Most probable reason can be the coupling got loosened in operation or not fully tightened initially. Now the same stem & coupling is in operation, however customer has demanded the replacement. The replacement material is received and being handed over to customer.

16) MILL LUB OIL FILTERS FREQUENT CHOKING :

 

PROBLEM : The bowl Mills supplied by BHEL Hyderabad are having gear box to store approx. 2000 liters of lub oil with two set of coolers and one heater. The bowl mill was preserved with oil as per the procedure given by BHEL Hyderabad during the period between supply to site and start of commissioning. The LOP skid was erected and the piping was completed with the Mill. Oil lines flushing were completed and oil is changed for running the mill. During the mill operation on load, the LOP filters were choking frequently and in some mills, the filters got damaged causing mill outages. The filters supplied as commissioning spares were consumed in unit 3 itself.

 

ACTION TAKEN : On pump suction side settling tank is provided to trap the coal powder. The partition plate provided in this tank was fixed to the top cover plate. It was felt that the partition plate should have been welded at the bottom of the tank, so that the coal dust can be trapped. Also it was observed that immediately after mill running the filter frequency choking is very high. Since all the pipe lines are acid cleaned, the only possibility of oil contamination was from gear box itself. Even though the gearbox is supposed to be cleaned at works during assembly, satisfactory cleaning was not done. Hence it was decided to do the trial run of mill with flushing oil for about 30 mins. prior to 8 hrs. trial run with fresh oil.

   

B)TURBINE AND AUXILIARIES.

 

1)     GLAND STEAM SUPPLY AND LEAK OFF VALVES OPERATION.

 

PROBLEM

The operation of gland steam supply and leak off valves with hydraulic actuators are not satisfactory. Hardwar normally imports the hydraulic actuators from Germany. At Khaperkheda Hardwar has supplied M/s. Iyappan make (indigenous) hydraulic actuators. To attend to the various problems in these actuators and make reliable operation of these valves, number of actions were taken as follows.

ACTION TAKEN

ix)                The feed back linkages were getting disturbed frequently and it was modified.

x)                  The filters were getting choked very frequently. The oil was replaced and filters were cleaned regularly. Still oil contamination was observed.

xi)                It was suspected that the oil tank of aluminum casting is causing oil contamination and hence these tanks were acid cleaned. But finally it was decided to replace the aluminum tank with stainless steel tank.

xii)               Also the motor supplied was of 0.25 KW capacity and was just sufficient. Hence the motors were replaced with 0.37 KW capacity.

xiii)             Relief valves were passing and it was replaced.

xiv)            After all this exercise still the auto operation of these valves were not satisfactory and hence the overlap opening between two valves i.e. supply & leak off was tried to increase. Also the impulse sensing line was modified to avoid any delay in sensing.

xv)              Even after this action it was observed that the tank is getting heated up and in hot condition approximately above 60 deg. C, the valve operation was erratic. Hence temporarily tank cooling arrangement was done with compressed air and finally a separate oil cooler arrangement was supplied by vendor.

xvi)            Still the operation is not satisfactory, and with so many uncertainties and frequent maintenance requirement, customer is demanding to replace the hydraulic actuator with pneumatic. The same problem was faced in Khaperkheda unit # 1 & 2 also and customer has modified the valve by providing pneumatic actuator. We have requested MSEB to carry on the work and debit the cost to BHEL.

 

   

2)    UNIT # 4 , BARRING GEAR

 

PROBLEM

 

On one occasion when unit tripped turbine came to barring gear and after about 5 Hrs. alarm appeared barring gear solenoid energised along with seal oil H2 differential pressure low, and barring speed started reducing and turbine came to stand still. On investigation it was observed that as per logic when turbine speed less than 15 rpm and seal oil H2 diff.pr. disturbed the barring gear solenoid will energise to close the oil supply to barring gear motor. Even though actual speed was more, some work was being done in TG panel and the card was removed hence speed less than 15 rpm was sensed and solenoid got energised. After normalising and de-energising the solenoid it was observed that turbine did not come on barring speed. Following checks were carried out.

ACTION TAKEN

5)     Rotor freeness checked by hand barring.

6)     Dial gauges were put and making JOP pump off and on checked the rotor lift. Slight difference was noticed in Brg. 1 & 4-lift value and it was adjusted. Rotor freeness checked and found to be further very free. Still M/C could not be put on barring gear hence boiler was boxed up.

7)     On cooling down of machine the barring gear motor was removed for servicing. It was observed that one of the casing bolts was sheared off and was touching the outer ring of over run clutch. The sheared bolt was removed and new bolt was fixed. Again barring was tried but M/C did not came on barring.

8)     The hydro motor removed and dismantled for servicing. Nothing abnormal was noticed, hence all parts were simply cleaned and assembled back.

3)     When rotor became colder the barring gear motor was removed. It was observed that one of the casing fixing bolts got broken and fallen inside. This was rubbing with the outer ring of the overrun clutch and causing restrain for rotation of the motor. The same was removed and a new bolt was fixed. Trial was taken but still not much improvement was noticed. Barring gear was possible only with hand barring assistance along with the hydromotor. There was no repeatability.

4)     Doubting some problem inside the hydrometer, the same was dismantled . All parts and gears inside were clean and have no damage. So it was assembled and fixed into position.

5)     For checking the oil flow to hydro motor the solenoid down stream connection was removed and by making solenoid on off the oil flow was checked. It was observed that oil flow sufficient is not coming instantaneously and was increasing slowly. Making temporary arrangement for pressure gauge fixing checked oil pressure and it was observed that the oil pressure is building up very slowly after opening of the solenoid valve. Suspecting some problem in solenoid valve the solenoid was removed and spool piece was fixed. The sufficient oil flow and pressure was observed. Barring gear was tried in this condition and barring speed was picking up from turbine stand still condition..

6)     So the solenoid valve block was dismantled for inspection. It was observed that the drain oil port from above the piston of the pilot valve was blocked with a plug. By studying the oil flow path in the solenoid valve block and it was confirmed that for proper functioning of the solenoid valve the plug should not be there. So the same was removed and the block was fitted back after through cleaning. There was instantaneous oil with full pressure as soon as the solenoid vale was de-energized. Trial and repeatability of barring was also found o.k.

7)     Similar inspection was carried out in unit # 3 and the blocking plug was removed from solenoid. The feedback is given to BHEL Hardwar.

 

3) CW Pumps and it’s discharge butterfly valve.

 

i)The CW Pumps, provided at Khaperkheda unit # 3 & 4 does not have provision of reverse rotation locking ratchet mechanism as per pump design. Also there is no NRV provision at pump discharge. When ever pump trips the pump reverse rotation takes place due to back flow till discharge butterfly valve gets fully closed. This requires positive closing of discharge valve on pump tripping. If by any reason the discharge valve fails to close there is a danger of pump continue to run in reverse direction till operator does not take necessary action. Provision of reverse rotation locking arrangement or NRV shall be incorporated in design for all the pumps to avoid any emergency conditions.

 

ii) For hydraulic operated discharge butterfly valve the limit switch mounting and its actuating levers provided are of very poor quality and flimsy. Any mal operation of the limit switch causes valve operation and hydraulic pump on/off logical problem and CW pump sequential start up problem. The switches must be of heavy-duty type and the actuating levers shall be sturdy and requires easy adjustment facility (preferably CAM arrangement / non-contact type switches to be developed).

In one of the pump during initial commissioning of the discharge valve, the hydraulic power cylinder got damaged and the butterfly valve got over traveled. The exact reason could not be ascertained but most likely cause may be mal functioning of limit switch and weak component failed. It is suggested that the butterfly valve shall be provided with stopper arrangement to avoid over travel.

 

iii) The hydraulic system provided is from M/s. Yuken Company and BHEL Bhopal has not forwarded any of the drawing or manual for the same until we asked for the same. In absence of the document lot of time was wasted in studying and commissioning the system with general experience. Also the logic given and incorporated in control panel for the butterfly valve operation was not correct as required for the pump start up sequence. Providing additional timers and contactors required suitable modification, which has consumed time and money. This should have been checked at factory itself to avoid the same.

 

iv) The temperature scanner supplied by M/s. Procon in the control panel, was not reliable and caused number of spurious pump tripping. A modified version is now provided by M/s. Procon but due to initial experience customer has decided to bypass some of the trip protection and instead only alarm provisions are kept in service.

 

iv)                For each pump, one number pump control panel and one number valve control panel is provided. The valve operation logic and required commands and status are duplicated in pump control panel. With slight additions in

pump control panel the valve control panel deletion was possible. This would have saved space and money. BHEL Bhopal shall explore the possibility.

 

 

C)      ELECTRICAL PACKAGE.

 

1. S.P. BUS DUCT PRESSURISATION SYSTEM

 

BHEL Jhansi has provided the M/s. ELEMECH make bus duct pressurization system having M/s. ELGI make oil cooled compressor. This system requires close watch and routine frequent maintenance unlike unit 1 & 2.

Once in unit 3 a fire incidence occurred in this system due to oil carry over from the compressor and unit hand tripping was done to avoid further fire spreading. Due to this customer is strongly demanding replacement of oil cooled compressor with air-cooled compressor. BHEL Jhansi has expressed their inability to provide the same. Customer suggested removing the compressor and using the plant instrument air as per unit 1 & 2 scheme.

Still Jhansi has not conveyed their final decision and this issue requires commercial settlement only.

 

 

 

 

2.GENERATOR TRANSFORMER COOLING ARRANGEMENT

 

In unit 3 transformer high temperature problem was encountered after some day’s operation. The first doubt raised was insufficient cooling water flow due to fouling in cooler. Heavy blocking was noticed due to contamination/ dirty cooling water and same was cleaned thoroughly. One vital point emerged out in this issue was the cooler downstream pipe line layout and its sizing. BHEL Bhopal has envisaged free flow after cooler but not given any recommendations for line lay out or size. The upstream and down stream pipe line size provision is for 100 mm dia pipe connection for individual cooler with common inlet and outlet pipe connection. Accordingly customer also has provided only 100-mm dia. pipe for upstream and down stream connection. But due to inlet pressure design restrictions and locally decided pipeline layout and matching pipeline size by customer the flow was not sufficient. To over come the problem the down stream line layout and pipe size was changed to 150-mm size to have sufficient flow without higher resistance. BHEL Bhopal shall suggest the pipe sizing and layout also to avoid such problems.

 

3.EXTRANEOUS ITEMS FOUND INSIDE THE TRANSFORMERS

 

At Khaperkheda in both the generator transformer and station transformers while general inspection few extraneous items like nut bolts, gas cut plate pieces, washers etc. were found from inside the transformer. This has created an anxiety/apprehension and adverse impression about Quality Control at BHEL Works during testing and prior to dispatch. It was embarrassing to explain customer as to how such serious lapses(sabotages) can occur at Works. The frequent failures of BHEL transformers already occurred at MSEB Chandrapur, customer was critical on BHEL product and its quality.

 

4.POOR QUALITY OF COPPER FLEXIBLES:

 

Similar quality lapses were faced for the copper flexible links supplied by BHEL Jhansi. The links received were totally corroded and became unusable. Customer was very sore on such issues and was even threatening to report such matters even to BVQI.

 

5. H. T. MOTORS FAILURES.

 

In one of the CW pump motor, the motor tripped on differential protection during initial motor commissioning trials. On inspection it was found that, one of the lead has rubbed and got shorted with casing. The reason was the loose leads are not tied up properly causing rubbing with rotor. The Bhopal supervisor pointed out that such feed backs are given to shop floor but no corrective actions are taken. Another fact pointed out was, the vertical motors are dispatched/transported in horizontal condition against the instructions from factory.

In another CW motor while installation it was notice that the mounting flange bolt holes were not matching with the pedestal flange holes. To rectify the same it was required to send the motor back to Bhopal factory.

 

One of the BFP motor tripped while in operation after few days, on winding temperature protection. Subsequently it was observed that all the winding temperature RTD’s were showing open. On dismantling it was found that the RTD cables bunched together was not tied properly and caused rubbing with rotating fan and got damaged. The same was rectified locally. The time and amount spent for such small mistake costs heavily.

 

I.D. Fan 3 A, VFD motor also failed due to poor quality work of excitation cable dressing and end winding insulation finish.

 

I.D.Fan 4 B, motor bearing found damaged during initial commissioning it self. The most likely reason is no proper preservation at site and lapse in periodical rotation of rotor during long storage period.

 

6. D.C. JOP MOTOR FAILURE

 

 

The D.C. JOP motor failed 5 times and details are as follows

 

Failure No

Motor Sl.No Location Date Remarks / Action taken

1 DASM2017 U # 3 29/4/00 During first commissioning circutory checked and direction trial taken & same was corrected. After correction motor started and motor got over speeded and jammed with in short time. On dismantling and inspection it was observed that rotor end winding bulged out and rotor jammed. Motor from unit 4 was cannibalized and tested in presence of M/s. CGL & Tushaco rep. The shunt field current was set by adjusting trimmer resistance.

2 DASM2016 U # 3 5/9/00 Whenever Motor start was given, it was drawing high current and failing to take full start. On checking the bus bar links in DCDB found melted. Even though motor was free by hand rotation, after dismantling the rotor end winding found bulged and damaged. The motor was replaced with the earlier repaired motor (DASM2017) from M/s. S.G. Engg. Nagpur, authorized repair center of CGL.

3 DASM2017 U # 3 6/9/00 The motor de coupled trial taken in presence of M/s. S.G.Engg. rep. After 45 minutes run, motor again tripped. On inspection, the rotor winding from one of the slot came out and rubbed with stator. Spare motor was arranged from M/s. CGL on 9/9/00.

  4 DASM2017 U # 4 10/7/01 The motor fully repaired at CGL, with

strengthened end winding binding

was in service since 27/12/00 as per requirement. On 9/7/01 during starting the motor failed to take full start. On inspection it was found that the back up fuse of 315 A has blown off. On further investigation it was found that the motor IR was zero and rotor insulation failed due to over heating. The reason is first and second step of series armature resistances got bypassed and as per control circuit through 3 TR timer direct start command went, causing high current flow and over heating. Since no spare motor available the motor from Chandrapur (55 KW) having same dimensions for mounting was arranged by MSEB. After required checking and adjustment of resistance the motor and pump was put in to service.

 

5 DASM2016 U # 3 30/12/01

Immediately on starting the motor the back up fuses blown off. On checking the IR value of armature was found zero. On opening the rotor winding found burnt. The strips from inter pole winding came out and rubbed. The repaired motor from CGL available was replaced and after checking and improving the IR, motor was put in to service.

 ANALYSIS / ACTION TAKEN

 

1)     During initial commissioning after ensuring the circuit function, direction trial was taken and same was corrected. Subsequently pump was started and speed measurement was about to be taken, but before that the motor oversped and tripped. Pump & motor was found jam. Motor was de coupled and pump was found to be slightly hard to rotate. M/s. CGL & Tushaco rep. were called for inspection and pump was serviced. From the nature of failure i.e. rotor winding getting bulging out M/s. CGL was of the opinion that over speeding has caused such damage. To avoid the same he adjusted the trimmer resistance such that at ambient temperature the field current is 20 % higher than rated (as per name plate ) i.e. 1.9 Amps. So that in hot condition the field current will be reduced but still the speed will not increase more than 3000 rpm. The original field resistance was 150 ohms and it was reduced to 50 ohms. The speed at cold start was 2800 rpm & increased to 3050 rpm after 2 Hrs. running.

 

Based on the observation of motor over speeding, M/s. CGL adjusted the trimmer resistance and recommended that, the trimmer resistance shall be adjusted such that

the field current is set 20 % higher than the rated as per name plate. Accordingly the resistance was adjusted and trial of new motor taken.

 

2)     At the time of second failure it was observed that, whenever the DC pump start command was given as per requirement, the motor was tripping on over load. The pump and motor was free by hand operation. On inspection at DCDB, the negative bus bar link was found melted. Same was rectified, in the mean time motor was opened for inspection and like earlier instance the rotor end winding bulging was noticed. The local agency M/s. S.G. Engg. representative was called for inspection and since the failed motor was unusable the earlier repaired motor was installed and tested in his presence. During motor trial, after about 45 minuets run, again motor tripped.

 

A meeting was held at Hardwar between CGL, TUSHACO and BHEL on 26/9/00 wherein the failures were discussed and CGL proposed to reduce the field resistance to 50 ohms from 150 ohms. Also they proposed to strengthen the armature overhang winding by extra number of turns of resiglass tape and additional glass braiding provision. Accordingly the motors sent for repair were modified.

 

3)     The third time failure within 45 minutes of no load (de coupled) trial was occurred in spite of the end winding strengthening. But this time, one of the rotor winding came out from the slot and reason may be poor workmanship of repair. Same was taken up with M/s. CGL. The spare motor arranged from M/s CGL was started in presence of M/s. CGL rep. and he was apprehending for no load trial. MSEB insisted for same and asked the reasons in writing for not allowing the no load trial. M/s. CGL initially told that the motor received on 9th is tested in factory just before despatch and hence decoupled trial is not required. Finally on insistance of customer motor decoupled trial was taken for one hour. Before that the trimming resistance was kept bypassed since it is not required as per M/s. CGL. Subsequently on load trial for 6 Hrs. was taken various reading were taken.

 

TIME FIELD AMPS. SPEED FRAME TEMP.

 

18.30 1.8 2548 32

 

02.30 1.5 2819 50

 

Since the temp. was stabilised, trial was concluded and released for normal operation.

 

As per CGL, during no load motor trial speed may rise to 3150 or higher even at stabilised temp. condition and running the motor for longer time may damage the motor.

 

A team of MSEB and BHEL higher officers visited M/s. CGL Ahamadnagar works and studied the modification works and witnessed the trial run and also insisted for over speed test and collected all the data sheets.

M/s. CGL informed the various improvements carried out vide letter ref. No. M3 dg/MSEB-KK dated 29/07.01 are as follows

 

8)     Armature Banding – We are ensuring adequate number of turns and tension while overhang bending. Further we are curing the bending at 140 degrees before impregnation

9)     We are doing vacuum pressure impregnation for stator as well as armature.

10) Soldering of armature winding to commutator is done on an automatic dip soldering m/c.

11) We have changed over to permanently greased sealed ball bearing.

12) We have introduced additional surge testing of armature before assembly

13) Though the specification calls for class ‘F’ insulation, we have provided class ‘H’ winding wires and strips with temperature class 210 degree C

14) An additional coat of Epoxy gel coat is applied on winding overhangs.

 

4)     The failure in unit 4 is of different nature than earlier. Motor failed to take full start and the step 1 & 2 resistance in series with armature circuit got bypassed and motor started through 3 TR direct start command. This has caused higher current for longer time leading to overheating and burning of rotor winding. The 3TR timer (4 second time delay timer) malfunctioned, and picked up instantaneously instead of 4 seconds delay. The faulty timer was replaced and motor arranged from Chandrapur (55 KW) was installed and trials taken with necessary checks and resistance adjustments.

 

As per data sheet the motor is not designed for direct start and only controlled starting through resistance is envisaged. Also number of starts shall be 2 to 3 from cold condition. This aspect requires further total review of control panel starting circuit and matching motor specification from BHEL Hardwar in consultation with TUSHACO and CGL as per the ordering specification.

 

5)     The recent failure in unit 3 also indicates over heating and burning of rotor winding.

 

Even though exact reason could not be found out, it is felt that the DC pump requirement arises very rarely and checking of healthiness of stand still motor for long period e.g. IR value status, space heater on/off status, cleanliness of carbon brush area etc. are not warranted and hence periodical checks are not done.

 

The failed motor is already sent to CGL Ahamadnagar, detail report shall be submitted by CGL. Also to ensure 100 % availability of such vital DC auxiliary as required for emergency operation, CGL shall give their recommendations on periodical maintenance aspects and procedure for how to ensure the same.

 

D) CONTROL AND INSTRUMENTATION

 

1) LPTDE CALIBRATION

 

PROBLEM-1

 

For this turbine the HP & IP turbine differential expansion measurement provision is not envisaged. Only LP turbine differential expansion measurement provision is there and hence is most important.

 

 

2)     The LPTDE measurement works on the principle of dual ramp differential pick-up measurement. The range of the instrument is -10mm to +32mm. It was observed during calibration that the actual displacement of pickup and distance shown in the monitor is not matching.

 

 

ACTION TAKEN

 

On investigation it was found that the extension lead of the probe that connects the probe to the proximeter was wrongly used. The colour and size of the extension cable for the vibration pickups, key phasor and that of LPTDE pickup are same although each had a different part number. But it had got mixed up and the extension cable of key phasor pickup and LPDE pickup had got interchanged. By rectifying the mistake, the reading became reasonably accurate. Edn Banglore/Bently Nevada has been requested :

 

iii)               To use different colour for different application extension cable and/or

iv)              To avoid use of extension cable altogether.

 

PROBLEM-2

 

The LPTDE reading had an error of 3 to 4mm in the range of 20 to 30mm expansions.

ANALYSIS/ACTION TAKEN

This is due to three possible problems out of which two have been resolved and one remains unresolved.

v)                 The bracket that holds the pickups need be slanted in such a way that it should be perfectly parallel to the shaft surface for setting the gap required for calibration. It was found that the face of the pickups is not parallel to that of the shaft surface and there is a gap of nearly 2.5mm between the top and bottom of the pickup. It was found out that the slant in the bracket should be 10degrees but the actual slant provided was five degrees. The bracket was changed.

 

vi)                The bracket supplied was of fixed type and for calibration shifting of the bracket again and again was required to cover the entire range of 42mm. This introduces an error in the calibration of the pickup as each time the bracket is shifted and no proper fixing can be ensured, this leads to an error in maintaining parallelity. For solving the above, site suggested a sliding type arrangement such that the bracket can be slided in a guide and parallelity maintained over entire range during calibration. Such a sliding arrangement was done in SLPP a Mangrol site and was convenient to use. So the bracket was again modified to sliding type and has been used for Unit #4 LPDE.

vii)              The use of proper extension cable, correct bracket slant and sliding arrangement for calibration has still not solved the problem of accuracy entirely and there is an error of 2 to 3mm in the operating range of the instrument.

This in our opinion is mainly due to the type of monitor selected for the purpose. As per the present configuration and the monitor selected, both the probes, probe A and probes are active in the entire range of measurement. As a result in the operating range of the instrument one probe is faraway from the shaft surface and so the output from that probe is inaccurate. This inaccuracy is contributing to the final reading of the monitor.

This problem can be resolved by using a complimentary type of monitor where one pickup contributes only for half the instrument where the air gaps is less and accuracy of the reading is more. As the shaft expands, the air gap in the active probe side will increase and will decrease

in the passive probe side. Once the air gap in the passive probe site comes to the required linear zone, it will take over the active probe and will be active for further expansion range. This type of monitor is in use at SLPP Mangrol.

Another problem related with LPDE is the alarm is set at 30mm, where as the range of the instrument is 32mm. Since the negative expansion of the shaft is very rare the range of instrument can be modified.

 

 

 

2) Vibration MEASUREMENT SYSTEM:-

 

Two problems were faced in vibration measurement system. One has been solved and one remains unresolved as a today.

PROBLEM

1) Shaft vibration measurement problem in bearing-3.

After completion of calibration of all the vibration pickups, all the vibration signals were healthy in controls. But after putting the machine on barring gear, bearing No.-3, bearing and shaft vibration readings were becoming invalid. For calibration, in turbine stand still condition, the probe is set with a gap of 2.5mm between the shaft surface and probe face and voltage of 10v is adjusted corresponding to this gap,. This was done for bearing No. 3 also, as it was done for bearing 1 to 4. But it was observed that as soon as the rotor was put on barring gear, both the channels for shaft vibration measurement became faulty and whenever the rotor became stand still, the channels became healthy. This was really mysterious and extremely difficult to analyze because checking is possible on rotor stand still condition and in rotor stand still position the fault was disappearing. After doing all routine checks like changing the cable, changing the monitor, changing the pickup etc., the problem could not be resolved and it remained as it is.

 

ACTION TAKEN

 

As all the possibility of finding a defect in the measurement system was exhausted, it was decided to check for any problem in the shaft surface and the problem was right there!

On inspection of the shaft surface by a torch light with the pickups removed and hand barring, it was found that there were four big holes on shaft circumference at equidistant just facing the probe. Whenever the probes were facing these holes, the voltage level was jumping and creating a fault. These holes are provided for individual shaft rotation facility. The matter was taken up with BHEL Hardwar and to resolve this problem there was no other solution than to shift the probe location. The pickup was relocated by shifting toward generator by about 50-mm, such that the probe did not face the hole on the shaft. By shifting the probe

location the problem got resolved. Later on it was learnt that in Kayamkulam steam turbine, similar problem was faced but the feed back was not available with us.

 

 

WS POSE indication of bearing/shaft vibration

 

PROBLEM

While comparing the vibration reading of control room and local readings taken with portable vibrometer, it was observed that there was wide difference in two bearings and shaft vibrations. The reason could not be found out and it was left as it is. Subsequently a detailed study was done & following facts was surfaced out as follows.

 

ANALYSIS/ACTION TAKEN

 

For shaft vibration of shaft 1 to 4 the pickups are mounted on to the respective bearing vibration pickups. The pickup out put indicates bearing vibration and relative shaft vibration. For getting absolute shaft vibration, the two values are added by vector addition method. BHEL has asked for modification in Bently Neveda panel, for indicating as well as generating signals for indication in two possible combinations in panel monitor as well as WSPOSE indication. The combinations were bearing vibration + relative shaft vibration or absolute shaft vibration + relative shaft vibration. Accordingly Bently Neveda has modified the panel but halfway i.e. first combination of bearing vibration + absolute shaft vibration was indicated on panel monitor only. Whereas the output signal to WSPOSE was not modified and the provided jumper selection was giving combination of absolute shaft vibration + relative shaft vibration only. The matter was referred to EDN, since for normal monitoring the bearing vibration + absolute shaft vibration measurements are required for operator monitoring. M/s Bently Nevada were requested to modify the system, so that the combination of bearing vibration + absolute shaft vibration is available for indication in the monitor as well as indication in WS POSE. Accordingly M/s. Bently Neveda has done the modification and the confusion was avoided.

But unfortunately, M/s Bently Nevada has done the half modification. They have modified the indication in the monitor (which is rarely used by the operator) and left the signal for WS POSE is unmodified and we have missed this point. To compound the confusion further, the jumper selection for WS POSE indication was selected in such a way that the output signal was for absolute shaft + relative shaft. But since Edn Banglore has no knowledge about this half modification, they have alone the engineering and internal wiring of the panel for indication bearing vibration + absolute shaft vibration so the final picture is the monitor indication was bearing vibration + absolute shaft vibration. In the WS POSE what were showing as bearing vibration was actually absolute shaft vibration reduced to a scale of 100microns, that is one fourth of absolute shaft vibration. What we were showing as absolute shaft vibration was actually relative shaft vibration.

As soon as the problem come to our notice, the jumper setting was changed so that now the monitor indication is bearing vibration + absolute shaft vibration. In WS POSE the indication is bearing vibration + relative shaft vibration.

M/s Benthly Nevada had sent their representative on 30th August 2001 to site to carry out the necessary modifications and the modifications have been carried out in Unit #3 and #4. The necessary changes in the documentation have also been carried out. Now, the indication in the Monitor as well as WOPOSE is Bearing Vibration + relative shaft Vibration.

Modifications done in HPBP System at Khaperkheda Unit #3 and 4

INTRODUCTION

HPBP (High-Pressure By Pass) system comes into service during plant start-ups, turbine/generator trip-outs and house load operation when the unit is disconnected from Grid. It is used to maintain main steam pressure and downstream CRH temperature at required set values during steam dumping and unit trip-out conditions. The scheme at Khaperkheda envisages two main BP valves for pressure control and associated two spray valves BPE for temperature control.

 

PROBLEMS FACED AND REMEDIAL ACTIONS TAKEN:

Problem 1: Drifting of HPBP Valves in auto during boiler on-load condition

Description of problem: During unit operating at significant load with HPBP system in auto where the pressure set point is kept higher than actual pressure, control system outputs zero demand. But it has been observed that the BP valves sometimes lift by themselves may be because of decrease in oil pressure in actuator due to excessive leak off from individual servo valve. However when in manual, this phenomenon has not been observed. This is because, in manual mode, blocking element is de-energized and does not allow oil to drain causing valve opening and also there is a priority closing command when valve is open not more than 2%.

 

Remedial action taken: To overcome above problem, the control scheme has been modified as below:

 

1.      Even control is in auto, priority close command will persist as long as control demand does not exceed 2%. This will keep the valve in closed position.

2.      When control scheme is in auto, the blocking will remain de-energized as long as control demand does not exceed 2%. As demand during normal condition remains 0%, the blocking element will help in keeping valve in closed position.

 

Problem 2: Sustaining boiler during fast opening of BP1 and BP2

 

Fast opening of BP1 and BP2 occurs under certain conditions viz. turbine trip , generator breaker open, load shedding relay operation during sudden load throw off, house load operation etc. Under such conditions, generally boiler also trips due to system disturbances. If boiler is saved in such drastic conditions and the set is brought to operation in minimal time, It will result in increased power availability. In case of house load operation, it means saving the total set itself. In present scenario, the customer insists upon it and it becomes a pending point in BHEL account.

 

         Constant fast opening pulse during fast opening of BP1 and BP2

 

Description: The fast opening time is generally set at 3 seconds mechanically. A provision is made in the control system wherein a pulse of the requisite width can be given to fast open the HP Bypass valve upto certain percentage and continuing thereafter in the normal mode. For example, if the pulse width is set at 2 seconds, then for a valve adjusted for full opening at 3 seconds, the fast opening will be limited to 66%.

 

The above scheme suffers from the drawback that this adjusted pulse width is a constant value and hence the amount of fast opening is the same under all conditions. Evidently, one does not want the HP Bypass valves to fast open by the same amount when the generator load had been 100% and also when the generator load had been say, 30%. In the latter case the opening would be too much and main steam pressure could drop drastically.

Suggested scheme: Please refer attached Fig. 1.

 

 

   

 

 

    In the suggested scheme, the generator load is memorised with a dead time lag element so that at the output of the lag element the exact trend of the load is continuously available, but with a 3 seconds delay. This data will be utilised to derive a percentage value through a function generator (represented by a block f(x)), which will be utilised to modify the pulse width for fast opening. For example, when the load is at 60%, the output of the function generator could be say, 70% and so the pulse width will be 70% of 3 seconds, that is 2.1 seconds. Thus the pulse width can be very finely adjusted for the whole range and this will prevent over/under operation of the valve.

 

Also this modification will prevent fast opening at low loads. At low loads, when the total steam flow is quite less, the normal control circuit would be sufficient to cater to the amount of steam to be diverted via HP Bypass and fast opening would not be necessary.

 

       Feed forward for Spray valves i.e. BPE

 

Description: Spray control valves operate to maintain downstream temperature at set valves. Under no conditions, downstream temperature should not exceed 370 degree centigrade. If the temperature exceeds this value, BP1 and BP2 valves close and boiler trips on reheat protection. In case of fast opening, temperature shoots up because of large openings of BP1 and BP2 and normal control circuitry is not tuned for such transient conditions.

 

Suggested scheme: When BP valves open fully due to fast opening, it is necessary to adjust the feed forwards to the BPE so that these open sufficiently before there is any rise in temperature This feed forward can be a function of BP valve position and main steam pressure as suggested in fig. 2.

 

 

       Pressure set point biasing

  

Description: HP bypass pressure setpoint is derived from turbine set pressure in coordinated master control and biased by a fixed valve above the pressure set point for turbine. It takes some seconds for pressure rise to occur and enabling the BP1 and BP2 valve to open.

 

Suggested scheme: During fast opening conditions, this bias could be removed so that HP Bypass can take faster action.

 

       HPBP fast opening on pressure deviation

 

Description: If the main steam pressure exceeds the pressure set point by certain amount (i.e. 5%) the HPBP system goes into auto and at the same it generates a fast opening pulse. This fast opening pulse creates system disturbance and sometimes leads to unit tripping also. Under such conditions normal control loops are sufficient to take care of the situation.

 

Suggested Scheme: Initiation of fast opening pulse on pressure deviation has been removed. Under pressure deviation exceeding the value, the system will go into auto and same will be annunciated using visual annunciation.

 

       Fast opening and auto operation of valves during fast opening

 

Description: As fast opening pulse is initiated, two independent operations occur independently. The HPBP valves go for full opening and it puts the pressure control in auto mode. Seeing the difference between set point and actual pressure, the control circuit outputs demand, which rises slowly. After ceasure of opening pulse, the valves act as per control output, which is very, less and valves close. This results in oscillations and system does not stabilise early. This generally leads to unit tripping.

 

Suggested Scheme: Under normal conditions, whenever the system is working in manual mode the output demand tracks the valve position, the moment it is put into auto, it starts controlling from the that valve position itself thus ensuring smooth changeover from manual to auto. The control circuit has been modified such that after the ceasure of fast opening pulse, the system goes into auto and takes control from its last held opening.

 

If the system is already in auto, the modified scheme puts the system into manual for a very small duration and after ceasure of fast opening pulse, system goes into auto mode.

   

 

SITE DEVIATION REPORT

 

During commissioning it was found necessary to change some of the logic/provisions for improving operation facility and avoid unnecessary tripping of auxiliaries. As per the directives, for any change from designer’s recommendations, concurrence is to be sought from respective units engineering before carrying out any change. As per new system to overcome the problem and to avoid delay in activity, SDR(Site Deviation Report) is to made and clearance to be obtained from Construction Manager and same to

be reported to H.O. Afterwards it has to be regularised through CAR/SAR etc. Any deviation concerned with safety of equipment, then it is necessary to obtain units clearance before implementation. Only one SDR was of this nature was raised and so far no disposition has been received from unit.

 

Some of SDR’s are described here as below,

 

1)Details of Deviation: PRDS normal operating temp. is 220 deg. C, & temp. very high to close steam pr. Control valve is 270 deg. C. On many occasions it is observed that temp. is exceeding more than 270 deg.C and PRDS is getting isolated. This is leading to unit tripping very frequently. The temp. control loop is having only PI control & original control circuit is not workable at all, because of full feed forward of pressure control valve and there in jump while putting in auto.

 

Action Proposed: To avoid frequent tripping it is proposed to increase the very high set point to 300 deg. C Also for fine tuning & to improve temp auto control loop, present auto control loop having only PI control, it will be changed to PID control. The control loop is modified to cater system requirement & better control. The feed forward of PCV to TCV has been provided which an adjustable factor for tuning. Accordingly suitable modification has been made to avoid jerks while putting PCV from manual to auto and vice-versa

 

2)Details of Deviation: For starting additional pump one condition is steam flow shall be >50 % as per EDN logic whereas as per PEM instead of steam flow it is given as feed water flow > 50 %

 

Action Proposed: IT is decided to change EDN logic from steam flow to feed flow as per PEM logic, because BFP loading is decided by feed flow rather than steam flow.

 

3)Details of Deviation: As per EDN logic for stand by pump tracking of scoop is not given whereas as per HYD. Manual for stand by pump the scoop shall track as per running pump. PEM scheme also does not call for tracking, but for stand by pump auto check back is required as per PEM.

 

Action Proposed: Tracking of the scoop of stand by BFP is very useful for maintaining drum level in case of takeover by stand by pump. The average of two running pumps will be tacked by the stand by pump. Also after staring stand by pump the scoop will be automatically put on auto.

 

4)Details of Deviation: For CST level Hi/ Low alarms no window is provided on back up panel audio visual alarm. Customer wants alarm to be provided for operator’s convenience.

 

Action Proposed: The necessary modifications are carried out for providing the audiovisual alarm on back up panel.

 

5)Details of Deviation: In BFP start sequence logic ,ON command for main BFP motor & its cooling water valve goes simultaneously &only after getting CW valve open feedback further command goes for discharge bypass valve & then main discharge valve. Since CW valve takes quite some time to open, BFP loading gets delayed and may cause drum level control problem. Hence instead of waiting for getting CW valve open status, not closed status is preferred to avoid delay in pump loading

 

Action Proposed: Instead of the CW valve open status not closed status will be used to proceed further commands for BFP loading.

 

6)Details of Deviation: As per EDN logic for CEP starting hot well level adequate permissive is taken as +100mm from level transmitter level signal. Whereas as per BHEL HYD. O&M manual the permissive for CEP starting is mentioned as hot well level not low which is set at - 340 mm. The normal operating level is set at zero and in auto it maintain about ± 50 to 100 mm. In case of the running pumps trips as per EDN logic the same pump or stand by pump can not be started unless level is raised to + 100 mm. If we wait till level raised to + 100 mm the unit may trip

 

Action Proposed: It is proposed to change the EDN logic and introduce level adequate permissive as level not less than -340 mm as per BHEL Hyderabad recommendation. The level very low trip set point is at -1300 mm hence it is safe to run the pump with level more than -340 mm. Necessary changes in logic modification are carried out.

 

7)Details of Deviation: As per BHEL Trichy recommendation super heater spray can not be given till total steam flow is >20% and burner tilt can not be operated till total steam flow is >25%. It is observed that after synchronisation of the unit and starting of coal firing the super heater and re-heater outlet temps. are exceeding 540 0C. This happens especially at low load upto 40MW. Because of above logic temp. can not be controlled since spray can not be given and tilt can not be operated.

 

Action Proposed: To avoid temp. exceeding >540 0C, it is proposed to lower the steam flow limit interlock to >5%. This is necessary because other measures to control the temp., like increasing feed water temp. or reducing excess air quantity is not possible during cold start up and low load condition