Created on:2021-07-27 09:58

Turbine hydraulic failure_ EH oil pressure low fault

Turbine hydraulic failure_ EH oil pressure low fault

Ni35-13.42-535/535-ki51 steam turbine is used in unit 1 of a power plant, and its speed control system adopts pure digital electro-hydraulic regulation (DEH). Both speed control system and distributed control system (DCS) are XDPS-400.

(1) Fault condition of unit 1

One day, the operator switched the feed water pump from pump a to pump B. However, due to the misoperation, the flow of feed water pump was low, resulting in protection action. The feed water pump tripped in combination, resulting in the boiler tripping (MFT) and steam turbine tripping due to low boiler water level. After the turbine trip, the operator adjusted the parameters in time for impulse starting. However, due to the low EH oil pressure, the impulse starting was unsuccessful for many times. When the impulse starting speed reached a fixed value, the turbine would trip. See the table below for the situation.

 

The highest speed of turbine tripping due to low EH oil pressure during impulse starting

Times

时 刻

转速/r·min-1

系统EH油压/MPa

过程时间/s

第1次

第2次

第3次

第4次

第5次

第6次

第7次

第8次

16:45:05

19:35:21

19:42:30

20:08:18

20:38:09

21:20:50

22:20:54

23:12:30

3000

434

575

91

114

99

87

2056

从14.5降至9.5

从14.5降至9.5

从14.5降至9.5

从14.5降至9.5

从14.5降至9.5

从14.5降至9.5

从14.5降至9.5

保持14.5

4

2

2

2

2

2

2

2

 

(2) The process of finding the cause of failure

Turbine trip is caused by EH oil pressure drop. The reasons for EH oil pressure drop are as follows: 1) system leakage, such as (servo valve, unloading valve, one-way valve, directional valve), pipeline, hydraulic cylinder or accumulator leakage, etc; ② Hydraulic pump failure; ③ Improper operation mode.

Therefore, we can start from the above points to find out the cause of EH oil pressure drop step by step in the process of unit impulse starting.

First, check the field system for leakage. During the second impulse start of the unit, EH oil pressure was low and dropped rapidly, which led to the unit trip. In the field inspection of the control valve, main steam valve hydraulic cylinder, pipeline and other places, no leakage was found. However, when the OPC and AST solenoid valve test blocks were inspected, it was found that there was a lot of oil leakage on the test blocks. After careful inspection, it was found that the bolt gasket of one of the ast solenoid valve body fell off, and EH oil continuously leaked from the valve body. After the gasket was replaced, the external leakage was handled, and the unit switched on again. When the speed reached 91r / min, the system oil pressure dropped from 14.5mpa to 9.5Mpa, and the unit tripped. Therefore, the leakage on site is not the cause of EH oil pressure drop.

Then, check for EH problems. Start up two pumps at the same time. When the unit is switched on again, the system oil pressure decreases with the increase of speed, and the current of hydraulic pump reaches 45A, the turbine is switched off. This shows that the pump failure is not the cause of EH oil pressure drop.

Finally, the DEH equipment manufacturer was contacted, and the other party thought that it was possible that the air bag of the high-pressure accumulator was leaking and the air pressure was insufficient. The reason is that the system oil pressure begins to drop after each regulating valve is opened. When the high and medium pressure main steam valves are fully opened, the regulating valves also start to act. At this time, the system oil quantity is not enough, and the system pressure needs to be supplemented by high-energy accumulator. If the high energy accumulator leaks or the by-pass valve of the accumulator is loose, the oil from the by-pass valve to the oil return pipe will also cause the system oil pressure to drop in the process of impulse starting. Therefore, using a special tool to measure the nitrogen pressure of each accumulator, the measurement results are about 9.8MPa, which is normal. Therefore, the by-pass valves of all accumulators were re tightened, and the unit switched on again. However, when the speed increased to 99R / min, the unit tripped again due to low system oil pressure.

After verifying the possible causes and failing to try various methods, the on-site equipment was carefully checked again, especially the servo valve and hydraulic cylinder of each regulating valve. Later, it was found that the rotation handle of the unloading valve of the high-pressure regulating valve on the left and right sides was abnormal and about 5mm was ejected, so the rotation handle of the unloading valve was locked again. After careful inspection of other control valves, the unit switched on and started. This time, the speed rise was normal, and the system oil pressure did not drop again. However, due to the long shutdown time (up to 9h) of the unit and the large temperature difference between the upper and lower cylinders, the unit was forced to stop due to large vibration. After eliminating the temperature difference between the upper and lower cylinders, the unit operates normally.

(3) Cause analysis of low EH oil pressure fault

After the simulation operation of the steam turbine, the full stroke test of each high and medium pressure regulating valve was carried out separately, and no low oil pressure fault occurred again. By checking the historical curve, it is found that when the steam turbine turns to about 3000r / min, because the speed of the steam turbine is difficult to stabilize at 3000r / min and fluctuates within 2950-3050r / min, the regulating valves act frequently. The control valve moves from 20% to 50% in a short time, and then drops to 20% in a short time. In this way, it moves back and forth frequently and the body vibrates greatly, which causes the rotating handle of the needle valve of the unloading valve to spring open and unloads the unloading valve of the high pressure control valve. When the valve cannot be opened to the command position, the command makes the opening of the servo valve increase continuously to the maximum. In this way, the high-pressure oil of the system goes through the servo valve to the unloading valve, then to the pressurized oil return pipe, and then returns to the oil tank, causing the oil pressure to drop. When the system oil pressure drops to 9.5Mpa, the machine trips.

(4) Summary

The following lessons can be learned from this low oil pressure accident.

① During the first impulse, the operator did not find the problem of low oil pressure in time. When the problem was found, the time to deal with the fault was delayed. This is the performance of the operator's inexperience.

② The process of finding out the cause of low oil pressure fault shows that we should proceed from reality and analyze it carefully. First, we should eliminate several possible causes of the fault, and then check and verify one by one to find out the real cause of the fault.

③ The scope of inspection and verification should be expanded, sometimes beyond the possible causes; In addition, we should strengthen the inspection and understanding of EH oil system, so that the time of troubleshooting can be shortened.

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