SUBJECT
:
500 MW BOILER DRUM CBD LINE MODIFICATIONS CARRIED OUT TO ACHIEVE PROPER BOILER WATER SAMPLING.
PROJECT
:
TALCHER UNIT-4
Continuous blow down system – It is very essential to maintain perfect drum water quality to achieve required super heater steam purity for high-pressure utility boilers (0.02 ppm of silica and conductivity 0.2 microS/cm). This is being done by the regulated use of continuous blow down control valve, intermittently as per the instruction of chemists concerned. Sometimes, the regulating valves may have to be closed fully since 500 MW units are provided with 100% condenser polishing units. PROBLEM: Whenever CBD regulating regulating valve is closed fully, the sampling received at sample cooler quality started improving towards saturated steam condition. The problem was studied by opening the drum manhole door and inspecting internal arrangement of CBD line. It was noticed that the CBD line was taken above the CBD header and also weld defects noticed in the bent portion of the line, entering into the dish end of the drum. Through these defects, steam might have entered into the CBD line and came to the drum water sample cooler, resulting in the improvement of sample received especially when flow through the line is limited.
Modifications carried out: 1)
CBD line was taken from bottom of the CBD header. This resulted in complete immersion of the line, during entire operating range of the drum level, as shown in the sketch enclosed.
2)
The sampling line will be taken before the root valve of the CBD line at the drum level. This will facilitate continuous representative sample flow into the drum water sampling cooler at all times, providing accurate monitoring of chemical regime.
CONCLUSION: This modification resulted in getting correct chemical regimes through drum water sampling cooler for measuring parameters such as pH, conductivity, TDS, alkalinity and residual phosphate. The same modification may be followed at other 500 MW boiler units, as well. Relevant drawing titled “Talcher unit-4/ 500 MW Boiler drum CBD line modification carried out” is enclosed.
Modifications carried out: 1)
CBD line was taken from bottom of the CBD header. This resulted in complete immersion of the line, during entire operating range of the drum level, as shown in the sketch enclosed.
2)
The sampling line will be taken before the root valve of the CBD line at the drum level. This will facilitate continuous representative sample flow into the drum water sampling cooler at all times, providing accurate monitoring of chemical regime.
CONCLUSION: This modification resulted in getting correct chemical regimes through drum water sampling cooler for measuring parameters such as pH, conductivity, TDS, alkalinity and residual phosphate. The same modification may be followed at other 500 MW boiler units, as well. Relevant drawing titled “Talcher unit-4/ 500 MW Boiler drum CBD line modification carried out” is enclosed.
FEED BACK – 1 PROJECT
: RAICHUR TPS (210 MW) – UNIT-2
PROBLEM
: HIGH VIBRATION IN HP FRONT AND REAR SHAFT AND IN BEARINGS 1,2 AND 3.
Raichur Unit – II , 210 MW Turbine was experiencing high HP front and rear shaft vibrations along with high pedestal vibrations in bearings 1,2 and 3. Vibration analysis during September 2003 indicated predominance of running speed component in both shaft and bearing vibrations. Table 1 gives the measured vibration levels on bearing pedestals An attempt was made to reduce HP turbine vibration levels by trim balancing LP rotor at site. Balance correction weights of 540 grams were added in each of LP front and rear planes for bringing down vibration levels of the total shaft system. Table 2 gives the pedestal vibration levels after balancing at 3000 RPM. Table 3 gives the shaft and bearing vibration levels at control room before and after balancing. There is a good reduction in vibration levels at all locations except at HP Rear shaft. Figues 1 and 2 show the coast up plots of HP rear shaft vibrations before and after balance correction. The vibration amplitudes in fig.1 show continuous increase in levels whereas in fig.2 ( coast up plot after balancing ) the amplitude after attaining 2000 RPM remains almost same. It is suspected that HP rotor at rear could have developed bend which can be ascertained during next overhaul of HP Turbine.
Table 1 Vibration amplitudes in displacement – microns (Peak-Peak) 3000 RPM – Prior To balancing 11-09-2003 , 09:15 hours
HP Front
HP Rear
LP Front
LP Rear
1X
Phase
2X
Overall Overall
Horz
43
258
43
Vert
30
322
32
Axial
37
327
43
Horz
49
69
49
Vert
40
145
40
Axial
28
324
31
Horz
63
260
63
Vert
49
108
48
Axial
69
152
69
Horz
39
28
35
62
Vert
39
145
15
43
Axial
76
127
12
74
Table 2
Vibration amplitudes in displacement – microns (Peak-Peak) 3000 RPM – After balancing – LP Turbine 15-09-2003 , 16:15 hours
HP Front
HP Rear
LP Front
LP Rear
1X
Phase
2X
Overall
Horz
10
237
10
Vert
10
284
12
Axial
16
260
17
Horz
24
49
24
Vert
32
130
30
Axial
32
302
32
Horz
10
217
14
Vert
28
139
28
Axial
45
119
42
Horz
11
6
26
34
Vert
14
160
12
21
Axial
25
151
8
30
Table 3
Displacement levels in microns - Peak
Shaft Before Vibrations Balancing 2608-03 After Balancing 16-08-03 Bearing Before Vibrations Balancing 2608-03 After Balancing 1608-03
HP Front 135
HP Rear
IP Rear
LP Rear
145
64
69
63
109
23
22
27
22
34
17
5
10
10
7
Bode plot HP Rear Shaft Coast up , Raichur Unit- II, before balancing
36 0 30 0 s e 24 0 e r g e d 18 0 e s a h P 12 0
60 0 0
300
600
900
1200
1500
1800
2100
2400
2700
3000
3300
180 160 140 p - 120 k p s 100 n o r c i 80 m
60 40 20 0 0
300
600
900
1200 1500 1800 2100 2400 2700 3000 3300 RPM
Bode plot HP Rear Shaft Coast up , Raichur Unit- II, after balancing
360 300 s 240 e e r g e d 180 e s a h P 120
60 0 0
300
600
900
1200 1500 1800 2100 2400 2700 3000 3300
FEED BACK - 2 PROJECT
:
RAYALASEEMA TPS (210 MW) UNIT-2
PROBLEM
:
VIBRATION ANALYSIS
Rayalaseema TPS unit II TG unit was rolled on 14 th Sept 03 after replacement of generator rotor. Vibration analysis was carried out on generator during rolling and unit on load since during replacement of generator rotor it was observed that the rotorstator airgap was non uniform on top and bottom air gaps. The difference in radial air gap on Turbine end and Generator end are given below. Radial Air gap in mm Top Turbine end 86.81 Generator en 86.75
Bottom
Left
Right
83.96 83.86
85.31 85.11
85.46 85.50
Difference in radial Air gap Turbine end : 2.88 mm Exciter end : 2.89 mm
Turbo generator Engineering at BHEL, Hardwar was contacted in this regard. TGE Hardwar communicated that differences in air gap is permitted up to 3 % of design radial gaps and the unit will not experience any vibration problem since the measured air gap is within permissible limits. Figures 1& 2 give the coast up vibration for generator front vertical and rear vertical respectively. It is seen that at 3000 RPM the unit is very smooth and the vibration levels are around 5 microns. The sharp peak around 1100 RPM is at generator first critical speed where the rotor has translational , bow mode . Unequal air gap could be the reason for vibrations to be high at this speed in addition to being a critical speed. The unit behaviour at rated speed and part load and full load conditions were very smooth. The air gap variation does not have any effect on smooth behavior of unit.
Figure 1 Bode plot , Generator Front Vertical coast up , Muddanur Unit II 360 300 s 240 e e r g e d 180 e s a h P120
60 0 0
300
600
900
1200 1500 1800 2100 2400 2700 3000 3300 RPM
75
p - 5 0 k p s n o r c i m 2 5
0 0
300
600
900
1200
1500
1800
RP M
2100
2400
2700
3000
3300
FEED BACK - 3 PROJECT
: SIMHADRI (500 MW) UNIT-1
PROBLEM
: STRING INSULATOR FAILURE
Unit 1 of Simhadri ( 2x 500MW ) tripped at 14:52 hours of 03/10/03 on the following class A faults. 1. Differential Relay 87TL(RY) protection,and 2. Differential Relay 87HV protection for Y phase. At Simhadri 87TL relay protection is derived using CTs located at 1. inbetween generator circuit breaker and the generator, 2. two CTs located between GCB and UT 1A , GCB and UT 1B,and 3. phase CT at switch yard before the earth switch.
Similarly 87 HV Relay senses current through CTs located at the Neutral side of generator transformer high voltage winding and the phase CT before the earth switch at 400Kv overhead line at switchyard. This provides protection for Generator transformer HV winding,400 KV bushing and the overhead conductor.
The disturbance recorder located to monitor grid side parameters had recorded a substantial dip in Y phase voltage( to 14.13KV) for about 70 milliseconds just before the tripping. The disturbance recorder at Generator end had recorded substantial voltage drop in R and Y phases and also over current in R phase and Y phase currents during the fault duration of 70 milliseconds. Because of the typical delta formation of the Generator transformers, fault in Y phase of HT side has reflected in R and Y phases in the 21KV side. Immediately after the trip out, “Y” phase Generator Transformer was inspected physically for abnormal temp. rise, operation of pressure relief valves, gas collection in buckholtz relay and cracks / black spots on HT bushing. No abnormality observed. Then Y phase Generator Transformer was isolated from HV side and megger values were found normal. Tan delta of HT bushing was also found well below .0007. Subsequently inspection of over head 400KV conductor, lightning arrestors and surge counters was taken up. Finally slight discolouring was observed on 3nos string insulators at the first gantry after Generator Transformer Y phase connection towards switchyard.( Each phase conductor is held in tension by two parallel insulator strings , each string consisting of 35 insulators).
Further close examination revealed black spots and carbon deposits on the back side of these insulators. As a precautionary measure the entire 35x2 nos. insulators were replaced. Because of continuous rains and difficult nature of the work the shutdown got extended. Unit was finally synchronised at around 0900 hrs on 06/10/03. It is suspected that the insulators are deposited with salts since they are near sea cooling water system and rain water collected on them might have caused deterioration of insulation and failure. In order to avoid such trippings, customer has introduced on line cleaning of insulators with DM water.
FEED BACK – 4
PROJECT
: NALCO CPP UNIT-7 (120 MW)
PROBLEM
: AIR PREHEATER GUIDE BEARING FAILURE
PROBLEM FACED: Nalco CPP Unit-7 Boiler was stopped on 31.08.03 for Annual overhauling activity. To carry out PG Test after the overhauling, as a routine check up for PG Test , Air Pre-heater Seal setting was inspected. During this course of above work, the following were observed:?
?
? ?
?
?
Guide bearing housing inside was looking dry and ash was found in the form of cake in both the APHs. The bearings were fully damaged with lot of pittings and abrasions marks on rollers and races. The failure of Guide Bearing was observed in both APH A&B. All 3 Nos. of locking cap holding bolts (M24x90) Gr.A 10.9 were found to be in sheared condition in APH A and found to be loose in APH B. Locking of cap plate bolts was not proper. Bearing adopter sleeve found dislocated in APH A by 90mm from its position and was in position in APH B. Trunion shaft tapered potion got worn out in both the APHs.
?
?
? ?
The seal tube thinned out vertically creating a big hole for a width of 30-40 mm and more at the middle in APH A. Similar failure noticed in APH B as well. The protection sleeve provided in the trunion shaft (Trunion Sleeve) got thinned out and formed two grooves in both the APH. The adopter sleeve worn out from inside. Both the thermostats for oil temperature measurement have gone defective due to ash entry.
FAILURE ANALYSIS: There was heavy deposit of ash over and below the guide bearing. The fine ash fallen from the Eco hopper cover the entire trunion shaft air seal housing arrangement touching upto the bottom of Guide bearing housing. The seal air was unable to escape to atmosphere and might have forced the fine ash between protective sleeve & trunion shaft and entered the bearing. The ash entered the bearing had contaminated the lub oil and jammed the bearing. This had resulted in free rotation of adaptor sleeve over tapered trunion and damaged the trunion as well. This has caused the loss of verticality of the trunion shaft and other associated problems. This has been depicted in the attached Sketch.
REMEDIAL PROBLEM:
?
MEASURES
TO
AVOID
RECURRENCE
OF
THE
Ash falling from Eco hopper around the APH guide bearing should be totally avoided. Before every light up, proper functioning of Eco hopper sealing/flushing system is to be ensured. The area has to be kept cleaned always.
During erection, all erection checks should be thoroughly done jointly and protocols are to be made by incorporating a log sheet for tightening of locking cap with locking cap holding bolts and locking the bolts. (Eventhough, the above event is to be checked as per Field Quality Plan, there is no log sheet for the above purpose). This has been incorporated in the Field Quality Plan of Erection/Commissioning of APH in Log sheet L-08 with a special note to avoid recurrence of such problem.
?
The lub oil level in bearings are to be regularly monitored and oil samples may be collected and analysed in case of increase in mechanical impurities, the bearing oil is to be replaced with fresh lub oil after thoroughly flushing out the contaminants.
CONCLUSION: The other reported lose of verticality of trunion and other associated problems can be avoided by maintaining very clean surroundings free from ash and other foreign materials around the guide bearing and properly ensuring all erection checks along with locking of lock plate will avoid recurrence of such problem. Periodic physical inspection of guide bearing lub oil and bearing and proper closing of inspection cover will further aid to improve the availability and reliability to the equipment.
FEED BACK NO.1 PROBLEM
:WATER INGRESS INTO THE GAS SYSTEM
PROJECT
:KOVIL KALAPPAL COMBINED CYCLE POWER PROJECT
PROBLEM: TNEB customer from Kovilkalappal Combined Cycle project reported that on 27/09/03 after two hours of starting of the Gas Booster Compressor No.1, when GT load was about 30 MW, a loud noise was heard from the Gas Booster Compressor (GBC) and motor current was hunting between 90 and 290 amp (normal full load current is 253 amps). The compressor was stopped immediately. On checking the system, water stagnation was found in the gas side of gas cooler, which was drained, indicating possible leakage of cooling water into the gas system. Hydraulic test of the cooler was carried out to locate the leaks. The test revealed no leaks except sweating around few tubes expanded area, at a pressure of 45 kgs/cm2 (design pressure is 33 kg/cm2). M/s. TNEB could not identify the cause and hence requested BHEL’s assistance.
OBSERVATION: BHEL Commissioning Engineer was deputed and inspected the system. There was not much of condensate or water when the drains of gas line, scrubber and gas conditioning skid were opened. However, lot of condensate was seen in the cyclone separator space of scrubber as shown in the drawing. But there was no water from the drain point provided. Suspected to be due to chocking of the drain line from the cyclone chamber to the bottom of the scrubber. An atmospheric drain with valve was provided from the hand hole plate and a valve was introduced in the place of dummy as shown to drain condensate from the separator space. Although the system was checked from the low point drains, on casual examination, the drain line of gas inlet header which is at 4 mtr. elevation was opened and lot of water came out from the header. The header is almost in the level of scrubber outlet flange and the water collected has probably remaining there, since the inlet line to compressor line was tapped off on the top of the header as shown in the figure. From the discussions with TNEB officials, it is understood that GBC No.2 cooler had experienced leakage problem and arrangements are being made for rectification. With the above observation, it is suspected that the water leakage from GBC-2 has gradually caused water ingress into the gas system of GBC-1, during planned shut down and caused disturbance while restarting. Probably the gas system of GBC-2 was not completely isolated.
CONCLUSION: The cooling water quality in Kovilkalappal deteriorated over a period of time and frequent cooler chocking was experienced. It is understood that these coolers were cleaned with acid two months back for the problem of chocking. The above operation might have caused leakage problem. Customer engineers were educated to prevent such abnormality and advised to attend to cooler leakage at the earliest.
FEEDBACK NO.2 PROBLEM
:
HYDROGEN LEAKAGE IN GENERATOR
PROJECT
:
NORTH CHENNAI TPS UNIT-1 (210 MW)
--------------------------------------------------------------------------
PROBLEM: Heavy Hydrogen consumption was observed because of hydrogen leakage in Unit-1 Generator, Russian design. Specialist was deputed from PSSR, as requested by TNEB for analyzing. OBSERVATIONS: 1.
Heavy Hydrogen leakage observed at Turbine and Exciter end shield parting plane and liquid detector indicator pipe flanges.
2.
Cold and hot gas temperature was observed high beyond acceptable limits.
3.
Cooling water inlet temperature to hydrogen gas coolers observed high beyond limits. Stator cooling water inlet temperature observed high.
4. 5.
Seal oil system differential pressure was observed to be below permissible limits. The seal oil pump discharge pressure was around 7 Ksc, it should be 8 Ksc for gas pressure in Generator of 3.5 Ksc.
CONCLUSION: High temperature conditions found in Generator had weakened all rubber chords at both end shields and rubber gaskets at all casing connections. This weakening and failure of chords and gaskets had lead to gas leakage at end shield parting plane and flange joints. ACTION TAKEN: 1. All rubber chords and gaskets replaced with original ones from BHEL/Hardwar. 2.Seal oil pump pressure relief valve inspected and serviced. Differential Pressure Valve A serviced. 3. Recent environment regulations have put restriction in allowing hot cooling water being discharged back in to the sea in open cycle if the temperature of hot water greater than 5 Deg.C. North Chennai Power Station incidentally has a 2 Km long open channel for cooling water flow back to sea for cooling down purposes. Since this was not effective closed re-circulation of sea water was adhered to cater environment norms. This has contributed to the increase in inlet cooling water temperature. The water inlet to Ennore Creek also got isolated because of sand bar formation, hence, no fresh water replenishment from sea took place. Cooling water temperature was normalized by customer by carrying out dredging
operation for removal of sand bar and opening the Ennore creek for fresh sea water entry. Unit was put back on service to rated load. Hydrogen gas consumption in Generator is within acceptable limits. The cold and hot gas, cooling water temperatures at the Generator are also within acceptable limits.
FEEDBACK NO.3 PROBLEM:
HIGH BEARING HOUSING VIBRATION IN
PROJECT:
VALUTHUR GTPP – PERUGULAM
PROBLEM: During operation of steam turbine, it was observed that the bearing housing vibration for the steam turbine and generator were high (though not reaching alarm/alert level). During operation at higher frequencies (greater than 50 Hz), it was observed that the generator front bearing housing vibration used to reach alarm/alert level. ANALYSIS: During start up, the vibration readings were taken at local and the readings were compared with those displayed in the TSI monitor in the control room. It was observed that the readings were nearly same upto full speed. On excitation, vibration measurements carried out by portable vibration analyzer on generaor and turbine bearings indicated normal levels whereas at the TSI monitor, the readings displayed were high. During loading of the turbine, the values were observed to be increasing marginally but steadily in the TSI monitor with load.
During the above two operations, the shaft vibrations were found to be matching at the local and TSI monitor. Suspecting electrical interference in the vibration transducer cable, a new cable was temporarily laid and routed from the probe to the TSI monitor on the STG Floor without going through any cable trays. During the next start up, excitation, synchronizing and loading operations, it was seen that both the local readings and TSI monitor readings were close. After making thorough inspection of the transducer cable routing and the wiring, the following observations were made: a)
The connector cable from the vibration probe has three leads coloured white, black and green. Two vibration probes are connected to one junction box and from the junction box one cable is used upto the TSI monitor. The JB is closer to the non drive end of the generator pedestal.
b)
The connector cable from the drive end pedestal vibration probe is extended upto the JB using another cable joined and taped in between. It was seen that only 2 cores were connected to the connector cable of the probe and extended to the JB. The third wire (labeled shield green) from the probe was left unconnected.
c)
In one of the JB, the tape around the shield wire was found damaged and it is suspected that the shield could have been grounded at both ends.
For sorting out the problem mentioned in (a), two new intermediate JBs were installed to do away with the joining of the cable and resulting taping. The 3 cores of the extension cable from the transducer were connected to the JB and the signal was connected to the main JB using another cable. The shield wire from the extension cable of the probe was extended upto the TSI monitoring a spare core in the outgoing cable.
For sorting out the problem (b), the cable was re-glanded and the shield was properly taped and isolated at JB. Presently, Steam Turbine and Generator housing vibration are well within the operating limits
FEED BACK NO.1 PROJECT
: TALCHER STPP 4x500 MW, UNIT4
PROBLEM
: PA FAN VIBRATION PROBLEM
Unit – 4 PA Fan – B was experiencing high vibrations at the order of 10 mm/sec., overall levels in axial direction since commissioning. This was causing lot of concern in view of the fact that there was a major failure in PA Fan motor bearing for Unit-3, earlier at Talcher. Alignment correction and other checks were carried out but there was no reduction in vibration levels. Vibration measurements and analysis was carried out for Fan – 4B. Vibration spectrum indicated predominant frequencies corresponding to 2X, 3X and blade passing frequencies. (Refer to enclosed Figure). Analysis was also carried out from flow induced vibrations point of view, since there was a marked increase in vibration levels when the fan outlet header pressure increased by 50 mmwc from standard operating conditions. Since the vibration levels were high in axial direction and the vibration spectrum showed predominant presence of blade passing frequencies, it was decided to inspect the PA fan rotor. It was observed that one of the blades was struck up and was found in fully closed position without responding to the required angle as per command. While commissioning, all the blade movements were checked for full range and
found satisfactory. assembled
The activating mechanism – lever,
and tested at works could have failed during initial trials, resulting in the present problem of one of the blades getting struck up. This was creating flow disturbances during operation and giving rise to high vibrations in axial direction. Decision was taken to change the PA fan rotor from Unit – 5. PA Fan – 4B with new rotor was trial run on 28/11/03. The vibration values ( in microns peak to peak/ mm/sec. peak) were measured and recorded for a fan load of 20.5%. FAN NDE FAN DE MOTOR DE MOTOR NDE
HORIZONTAL 11/1.7 5/1.8 4/0.6 5/1.0
VERTICAL 4/1.2 12/1.6 4/0.6 5/0.8
AXIAL 24/2.3 22/2.3 4/0.6 5/0.8
The above vibration levels are satisfactory and PA Fan – 4B was cleared for regular operation.
FEEDBACK NO.2 PROJECT
:
TALCHER STPP 4x500 MW, UNIT4
PROBLEM
:
CONDENSER TUBE LEAKAGE
Talcher Unit-4 was shutdown on 22/11/03 due to sudden increase in silica and hardness at Condensate extraction pump discharge. Since the condensate water system was in closed cycle and steam purity was satisfactory, condenser tube leakage was suspected. Water box of condenser was drained and condenser fill test was carried out. It was observed that 3 numbers of condenser tubes were leaking and large quantities of stones, debris were found in all sections of condenser water box. Some of the stones found were having sharp edges (Enclosed Figures). The leaking cooling water tubes were plugged. Entrapped stones and debris found in condenser tubes were removed with high pressure water jet. It is observed that the failure of condenser tubes is due to damages caused by stones and debris carried with high velocity of cooling water through the tubes.
NTPC/Talcher has been advised to inspect cooling water piping from CW pump discharge up to condenser, clean and to remove all foreign materials to avoid damage to condenser internals.
To prevent such carry over and to effectively clean before normalizing the CW system, flushing is carried out by looping the inlet/outlet piping near condenser. This has been practiced in some of the Projects and Site may suggest to NTPC, the above method for Talcher Unit-5.
FEEDBACK NO.3 PROJECT
:
SIMHADRI – 2x500 MW POWER STATION
PROBLEM
:
ABNORMAL OPERATION EXPERIENCED IN CW SYSTEM
Closed Cycle cooling water system with cooling towers using sea water has been engineered at NTPC Simhadri Power Station- first of its kind in India. The cooling water pump house is situated very near to the Power Station to assist close monitoring and control. CW pumps play a vital role in maintaining cooling water availability for the turbine condenser and other auxiliaries. At Simhadri, the CW system reliability was satisfactory since commissioning, but on 21 st Sept., 03 all CW pumps tripped due to certain system disturbance . This led to tripping of both 500 MW TG Units one after another due to cooling water non-availability. The pump house is provided with 5 CW pumps meeting the requirement of 2 Units of 500 MW in configuration of 4 pumps in operation and 1 available as standby. During failure of any one of the 4 pumps, the 5 th pump can be started. The design capacity of the pump is 31000 M3/hr. with a discharge head of 28 meter water column. Similar pump installed at NTPC/Talcher-unit no.3 is shown in the figure enclosed. The pump impeller is of mixed flow semi open type. To prevent reverse rotation of the pump and to have better pump control
discharge butterfly valve is provided with hydraulic power pack with counter weight
(enclosed figure to help in emergency closure. To ensure valve operation during power failure, an accumulator for supply of control oil is incorporated. When pump is operating, to ensure full opening of discharge valve, additional controls with limit switches are incorporated to take care of any possible creep from open position, due to leakages in the hydraulic system. Although the system is designed to take care of any possible disturbance , on 21 st Sept. all the CW pumps tripped leading to an emergency situation . The following probable reasons for tripping were concluded based on analysis:1.
One of the CW pumps in operation got tripped coinciding with certain delay in closing of discharge valve. This resulted in reversal of high pressure water into the pump casing leading to (Reverse run away speed) failure of flange joints and flooding of pump house basement.
2.
One of the CW pumps discharge valve creeped and closed to certain degree resulting in increased discharge pressure and failure of flange joints and flooding of pump house basement.
Since the pump casing discharge line and the valves are situated at basement of the pump, the total basement got flooded. The valve position feedback limit switches used for controls for all the pumps are provided in the basement . The
flooding of basement area simulated false electrical feedback signals corresponding to all discharge valves
closed condition. Hence all the pumps got tripped on interlock, leading to tripping of both 500 MW TG Units. One of the Units tripped brought back after a gap of 12 hours after dewatering the pump house basement and re-establishing the cooling water system. Station authorities are planning to ensure trouble free operation of discharge valve and shift the limit switches above basement to improve the operating conditions. For a similar CW pump house at Talcher and other power stations, the discharge line and valves are above the ground level, hence such emergency may not occur.
FEED BACK - 1 PROJECT : PROBLEM :
SIMHADRI 1 & TALCHER-4 IMPORTANT PRECAUTIONSTO BE OBSERVED DURING INSTALLATION OF SAFETY VALVE EXHAUST PIPE.
During safety valve floating at unit 1 of Simhadri STPP, the discharge pipe of one of the safety valves moved upward due to vertical reaction force, since the exhaust pipe had two bends. On analysis, it was noticed that the discharge pipe had not been solidly anchored in the square bracket beam provided for the above purpose with adequate welding. After sufficiently welding the stoppers in one of the square brackets, safety valve subsequently was floated successfully. (Fig.1). At Talcher unit-4, on hand popping of one of the drum safety valves, almost all popped up. Safety valve exhaust steam came out of the drip pan of the valve violently. As adequate precautions were taken while safety valve floating, there was no injury caused to working personnel. Later on, the unit was boxed up and silencer was inspected. It was noticed that the discharge pipe from the safety valve was inserted into the silencer to full length beyond the requirement, as shown in DETAIL–A1 of FIG-2, which has blocked the steam passage.
This resulted in rejection of steam through the drip pan during hand popping. This was corrected as per the requirement shown in DETAIL– A of FIG - 2. Subsequently, the safety valve was floated successfully. Hence it is obligatory on the part of concerned, to follow the drawing strictly and understand its implications before taking any deviation. It is preferable to add safety features also in regular procedure/system check list in future to avoid such problems.
FEED BACK - 2 PROJECT
:
PROBLEM
:
VALUTHUR GTPP FAILURE OF RUPTURE DISCS OF COOLED CONDENSER OF STG
AIR
The steam turbine tripped on 28/1/03 at 1558 hrs. due to grid disturbance. Many equipments like CEP, IA/PA compressor, ACC fans (5 nos.) and steam turbine lube oil pump went off due to under voltage. The HP, IP & LP bypass valves opened fully on interlock to release the pressure in the respective system. The vacuum breaker valve opened on low lube oil pressure protection. The HP & IP bypass valves closed as per existing logic and LP bypass valve closed by the operator. Tripping of CEP, ACC fans and opening of vacuum breaker valve caused decrease in vacuum. The other vacuum pump started on auto. The plant operation normalized by starting CEP, ACC fans, IA/PA compressor and closing the vacuum breaker valve. With two vacuum pumps in service, vacuum was improving. The existing turbine trip signal and the improvement in vacuum (better than 0.4 Ksc) resulted in full opening of bypass valve again as per logic. Since the vacuum was in the marginal value and sudden opening of the bypass valve resulted in decrease in vacuum and caused the failure of rupture disc in both passes of ACC.
After analyzing the sequence and consequences, in order to prevent the re-occurrence of auto full opening of HP/IP bypass valve, the following modifications in the logic have been carried out by site. 1.
Auto open of HP & IP bypass control valve occurs under the following condition. Vacuum better than –0.65 Ksc and Turbine trip signal pulse for duration of 10 seconds will open the bypass valve to 100% for duration of 5 seconds. After 5 seconds, the valve goes back into auto (control mode) operation.
2.
Auto close of HP & IP bypass control valve occurs under the following condition. Vacuum falls to –0.5 Ksc or Turbine trip on high axial shift and when lube oil protection acts. Normal operation of the bypass control valve will resume only if vacuum improves to <-0.6 Ksc.
3.
The solenoid operated LP bypass valve will open on turbine trip as mentioned in point no.1. Operator to intervene to close the bypass valve after auto operation. LP bypass valve will close on auto as per point no.2.
