Common C&I issues in a thermal power plant and few case studies , first thing when a problem arises due to signal problem but physically everything is fine, which can shut the unit then first return to normalcy then troubleshoot
1) Cable of instruments may be damaged.
2) Improper connectivity in the Juntion box.
3) Termination problems at the field element.
4) Feedback may be got disturbed due to vibrations.
5) Element setting may be varied need to be adjusted.
6) Element itself be damaged, need to have stock and replace it.
7) Communication network may be damaged.
8) Card may get failed.
9) Card slot may be damaged and not able to supply enough voltage or current
10) Communication cable problem due to cuts.
11) In a loop a single element may cause system to halt need to verify for that element.
12) Due to latching issues in the instruments.
13) Internal logic may got modified.
Case study 1 :
The thermal plant was successfully running for three months and one day in the spray station of the feed water cycle, the control valve present in the actuator was settled to a position and is not responding, the feeding to deaerator is decreased and the level started to fall down.
Solution:
It was found that the manual command to increment the position of the control valve is blocked, first this condition is bypassed and normalcy is restored, later when started troubleshooting it was found that the condition occurred due to the CEP ( Condensate extraction pump) common discharge pressure transmitter value reduced by half that internally is related to the increment condition. As the value is below the set point provided the valve increment is blocked. When verified at the transmitter, local reading is found to be fine. Then the channel in the card present in the DCS side is changed and now the value is normalised.
Case study 2 :
The hpbypass valves are opened due to high pressure maintained and the valves opened and closed and during this time the spray valves also got opened and closed but still the temperature kept on raising and the spray valve did not open in auto mode on temperature limit
Solution :
Physically ensured if there is any passage in the hpbypass valve and found the valve to be almost full closed. Then took the spray valves into manual mode from auto and tried to open the valve but still the valve is not opened. Checked for the logic and found the increment option is blocked, bypassed the condition and opened the spray valves and the normalcy is restored. Later checked for condition and found the spray valve did not open as the condition to open has position feedback of hpbypass main valve to be greater than certain value, as the valve is fully closed and the feedback position is less than that set point valve did not open.
case study 3:
Suddenly there is rise in the feed water flow and the level of drum started to increase very fast and there is no manual intervention and the Hpbypass valve is fully closed.
Solution:
As the hpbypass valve is fully closed, first isolated the hpbypass flow condition from the feedwater loop by making its value zero. Then went to the location, the flows are generally calculated using the Differential pressure transmitter (DP) , found out that there is a leak in impulse line of the DP transmitter beause of which there is high pressure maintained on one side and low pressure on the other which transmitter assumed to be flow and raised the feedwater flow. Closed both the impulse line and rectified the leak and working fine.
Case study 4:
LP bypass oil pumps are put off due to leakage of oil.
Solution :
This is one of the important condition and need to act as fast as possible. If oil is not available then the pump cannot supply oil to valve and if necessity arises. Make sure the oil is leakage is arrested and fill the oil in the tank. Then get the pump started and wait until sufficient oil pressure is developed and then put the pump in service. If oil pressure is not sufficient then the Lpbypass valve operation will be slow.
Case study 5:
Primary water tank level found to be faulty and varying.
Solution:
The primary water level value of the tank is not an interruption for the operation of the plant until there physically exists the level of water in the tank. But due to variation of the level transmitter value there will not be surety in the physical level and always need to check the value. Checked at the transmitter side the cable is found to be healthy, No loose contact of cables found at transmitter or DCS (Distributed control system) side. Then checked at the sensor and transmitter contact there seems to be minute disturbance in the adjustment between then. aligned it properly then it seems fine.
Case study 4:
LP bypass oil pumps are put off due to leakage of oil.
Solution :
This is one of the important condition and need to act as fast as possible. If oil is not available then the pump cannot supply oil to valve and if necessity arises. Make sure the oil is leakage is arrested and fill the oil in the tank. Then get the pump started and wait until sufficient oil pressure is developed and then put the pump in service. If oil pressure is not sufficient then the Lpbypass valve operation will be slow.
Case study 5:
Primary water tank level found to be faulty and varying.
Solution:
The primary water level value of the tank is not an interruption for the operation of the plant until there physically exists the level of water in the tank. But due to variation of the level transmitter value there will not be surety in the physical level and always need to check the value. Checked at the transmitter side the cable is found to be healthy, No loose contact of cables found at transmitter or DCS (Distributed control system) side. Then checked at the sensor and transmitter contact there seems to be minute disturbance in the adjustment between then. aligned it properly then it seems fine.
Case study 6 :
Vaccum suddenly getting dropped?
Solution :
A thermal power plant suddenly gets tripped if proper vaccum is not maintained because the steam will condense in the turbine and results in deformation if proper vaccum is not maintained.
1) Once there was sudden vacuum drop happened and first of all restore normalcy, the drop occurred due to seal steam supply was cutoff, first the seal steam supply was restored and the normalcy retained, later when found out, an RTD present in the leakoff loop was checked below a certain value and the leakoff valve of seal seam fully opened and seal steam was cutoff. Later it was checked and rectified. As it is of less importance u can also bypass this condition
2) If any valve at the vacuum pumps side is opened and is open to air, the unit gets tripped off make sure vaccum pump valves are closed.
Case study 7:
Vaccum transmitter showing the wrong value during the running condition but normal during the shutdown
solution:
A vaccum transmitter during the normal working condition of the plant seems to deviate from the normal value but when try to attend the complaint during the shutdown condition, the value seems to be normal along with the other vaccum transmitters. This happened for around six to 7 months and the condition persists. Later changed the root valve after which the transmitter is connected in the pipe. It worked for few hours, again the same problem persisted. Later changed the transmitter point to another spare location and the value seems to be fine,concluded that the problem exists in the impulse line and not in the transmitter. It is suspected to be line choke.
Vaccum suddenly getting dropped?
Solution :
A thermal power plant suddenly gets tripped if proper vaccum is not maintained because the steam will condense in the turbine and results in deformation if proper vaccum is not maintained.
1) Once there was sudden vacuum drop happened and first of all restore normalcy, the drop occurred due to seal steam supply was cutoff, first the seal steam supply was restored and the normalcy retained, later when found out, an RTD present in the leakoff loop was checked below a certain value and the leakoff valve of seal seam fully opened and seal steam was cutoff. Later it was checked and rectified. As it is of less importance u can also bypass this condition
2) If any valve at the vacuum pumps side is opened and is open to air, the unit gets tripped off make sure vaccum pump valves are closed.
Case study 7:
Vaccum transmitter showing the wrong value during the running condition but normal during the shutdown
solution:
A vaccum transmitter during the normal working condition of the plant seems to deviate from the normal value but when try to attend the complaint during the shutdown condition, the value seems to be normal along with the other vaccum transmitters. This happened for around six to 7 months and the condition persists. Later changed the root valve after which the transmitter is connected in the pipe. It worked for few hours, again the same problem persisted. Later changed the transmitter point to another spare location and the value seems to be fine,concluded that the problem exists in the impulse line and not in the transmitter. It is suspected to be line choke.
Case study 8:
The four temperature control valves in turbine stopped working.
Solution:
In turbine are there will be four temperature control valve namely primary water, main oil tank, control fluid, cold gas temperature control valves. All these control valves stopped operating these occurred due to failure of relay in getting the supply and there should be a second source but it did not happen and that was due to electric breaker trip. After replacing the relay and restoring the electrical supply still the valves are not operating due to the fuse blown off. Replaced the fuses and changed the TB where loose contacts are present and all valves started working.
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