1、 Cause analysis:

1. The pneumatic valve and check valve of vacuum pump are jammed;

2. If the operation is improper, the vacuum pump is stopped without confirming that the pneumatic inlet valve has been closed back.

2、 The main phenomena are as follows

1. The vacuum of the exhaust device drops rapidly, and the low vacuum alarm is given;

2. The liquid level of the gas water separator of the vacuum pump drops.

3、 Treatment measures:

1. After the vacuum pump stops, if the vacuum drops, start the vacuum pump immediately.

2. Immediately ask the local personnel to open the manual valve beside the water supplement solenoid valve of the gas water separator to make water to the high water level of the steam water separator.

3. If the vacuum still drops and there is no tendency to turn back, the local personnel shall immediately close the pneumatic valve of the No.1 vacuum pump and the manual valve in front of the pneumatic valve, and start the vacuum pump No.3.

4. If the vacuum drops too fast, the load should be reduced to below 400MW manually to prevent the unit from triggering RB. If RB acts, the RB should be handled normally.

5. If the treatment is not timely and the vacuum is too low, resulting in turbine trip, it shall be treated as emergency shutdown.

4、 Preventive measures:

1. Implementation of monitoring system for major operations;

2. When switching, the correct switching method should be adopted and all possible assumptions should be made;

3. When the vacuum pump is shut down, the inlet valve can be closed first, and then the pump can be stopped after confirmation.