Analysis of Low Oil Pressure Switching in Large Axial Fan Pump

Key words: 700 MW unit; axial blower; axial draft fan; oil system; oil pump; oil pressure

The first phase of Zhuhai Power Plant is a 2 × 700 MW coal-fired unit. The boiler is a sub-critical pressure produced by Japan's Mitsubishi Heavy Industries Co., Ltd., with an intermediate reheat, quadratic tangential, balanced ventilation, and solid slag discharge control cycle drum boiler. Each boiler is equipped with two MLHIR140-type 50% capacity adjustable-blade axial fans and two ML-HI-R175 50% capacity adjustable-blade axial-flow fans. Each fan oil system is equipped with two vane pumps mounted in parallel above the tank, one for operation and one for backup, providing control oil and lubricants to the system. In normal operation, the hydraulic pressure at the outlet of the pump is 6.86 MPa; when the system control hydraulic pressure is lower than 4.9 MPa, the hydraulic pressure "low" alarm signal is generated and connected with the standby oil pump; when the system control hydraulic pressure is lower than 2.45 MPa, Low "alarm signal, delay 3 s fan tripping. The system lubricating oil pressure is normal 0.125 MPa; when the system lubricating oil pressure is lower than 0.098 MPa, the standby oil pump is self-starting; when the lubricating oil pressure is lower than 0.049 MPa, it sends a "low" alarm signal and the delay time of 20 s fan trips. When the main pump is out of power, the standby pump electrical interlock automatically start. Therefore, the normal operation of the pump is an important guarantee for the safe operation of the fan.

1, accident example

2003 at 17:08 on June 11, because Unit 2 plant power 400 V the PC when the segment 2A12 (boiler section) switch causing operation of mother of the switch of the relay bus bar pressure loss, resulting in 2B ID fan pump power loss, The backup pump is started but the hydraulic pressure is set too long, the system lubricating oil pressure is "low" and the B-side fan is fully tripped. In addition, due to A air preheater zero speed power taken from the plant power 400V PC2A12 segment, air preheater zero speed relay power failure contact closure start, A side fan tripping, resulting in boiler MFT action, the unit solution. After this accident, the air preheater zero speed power supply is changed by the unit uninterruptible power supply UPS.

2, the reason analysis

Units 1 and 2 of Zhuhai Power Station have been operating for 4 years since they were put into operation in 2000 respectively. In the first two years, there was no obvious change in the low oil pressure of the multi-fan pump, but the low-pressure switch of the fan frequently occurred. The reason, in the following two aspects:?

a) The leaves of the French-made vane pump run through for a period of time and the vanes wear out, causing the inner circle clearance of the pump to be large and the pump output dropping.

b) Since the pump is switched every 15 days (or two weeks), the exhaust pipe of the pump outlet is connected to the upper part of the fuel tank through the atmosphere (the exhaust pipe has a spring valve and the spring valve automatically closes when oil pressure is applied) Under atmospheric pressure, the oil accumulated on the outlet of the backup pump will drain off (drain back to the tank) for about ten days, so the pump will make a loud noise when starting the standby pump, and the start-up may not be successful , And sometimes the hydraulic pressure builds up slowly, when a few days when the pump is switched over, the hydraulic pressure build up fast.

In order to keep track of the actual starting time for the fan oil system pump to switch the backup pump, we track the time it takes for the 2A and 2B blower pumps to switch the standby pump to establish hydraulic pressure during September and October 2003. Press the time record in Table 1. ?

From Table 1 we can see we shorten the pump switching cycle, but the pump is still unable to establish the normal hydraulic pressure, we can see standby pump start-up time is not reliable, standby pump in this state can not be a reliable backup, the system must be transformed.

3, transformation measures

After a full demonstration, we use the crew during the minor repairs in 2004, the two units of 8 sets of fan oil system transformation. Before the transformation of the fan oil system shown in Figure 1, the modified fan oil system shown in Figure 2.

Transformation measures are as follows:?

a) The position of the outlet exhaust valve moves closer to the pump body to increase the speed at which the pump starts to empty.

b) The pump automatically exhausts the air pipe from the liquid level in the tank to the bottom of the liquid level so that the air pump can not enter the air line when the pump is out of service, ensuring that the standby pump inlet and outlet lines are full of oil. ?

c) Two liaison gates (smaller diameter) are added to the entrance of the two oil pumps. The liaison door is normally open. When both pumps are normal (one operation and one standby), both liaison doors are opened to further ensure Spare parts of the pump body and the elbow are filled with oil for a long time, which is an important reason that the oil pump starts to establish a fast oil pressure.

4, transformation effect

After the transformation, the test start-up time is within 5 s, the pump starts smoothly and has no abnormal noise, and the outlet pressure is stable. Equipment put into operation more than six months, 8 typhoon oil system switching test oil pump pressure are established normal, no abnormalities appear. ?

The transformation of the system changes less, less investment , obvious results, to ensure that the standby pump spare capacity, greatly improving the safety and stability of fan operation.