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Abstract: The low oil pressure often occurs when the oil pump of the original transmission and induced draft fan system of 700 MW unit in Zhuhai Power Station is switched to the standby oil pump. The standby pump can not play a reliable backup role. To this end, the phenomenon was analyzed for the following reasons: the blade of the oil pump is worn and the output of the pump is decreased; the pump switching period is long, the standby pump starts to dry, and the sound is loud. To this end, developed a transformation measures: the position of the exhaust valve to move closer to the direction of the pump body; pump outlet automatically drain the air tube from the tank surface to the bottom of the liquid level, to ensure that the reserve pump import and export pipeline full of oil; The entrance of the two pumps added two contact doors. After the transformation, the pump started without any abnormal sound, the outlet pressure is stable.  Key words: 700 MW unit; Axial blower; Axial draft fan; Oil system; Oil pump; Oil Zhuhai Power Plant Phase I project is a 2 × 700 MW coal-fired unit. The boiler is a sub-critical pressure produced by Mitsubishi Heavy Industries , A middle reheat, quadrilateral tangent, balanced ventilation, solid waste slag control cycle drum boiler. Each boiler is equipped with two MLHIR140-type 50% capacity adjustable-blade axial flow blower, two ML -HI-R175 50% capacity adjustable-blade axial-flow fan. 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 At 17:08 on June 11, 2003, the pressure loss of the busbar caused by the action of the busbar switch relay during the switch-over of 400 V PC2A12 section (boiler section) of Unit 2 in the power plant led to the operation of the 2B draft fan operating oil pump Power loss, standby pump start but the establishment of hydraulic pressure is too long, the system lubricating oil pressure is "low" state, B-side fan full trip. 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. Analysis of the causes Unit 1 and unit 2 of Zhuhai Power Station have been operating for 4 years since they were put into operation in 2000 respectively. The phenomenon of low oil pressure switched by the multi-fan pump in the first two years was not obvious, but the switch to low oil pressure Often appear. The reason, in the following two aspects:  a) French-made vane pump blades after running for a period of time blade wear, virtually make the pump's inner circle gap is too large, pump output decreased; b) as every 15 d (Or two weeks) to switch the oil pump, the exhaust pipe outlet pump connected to the upper part of the tank, through the atmosphere (the exhaust pipe has a spring valve, when the oil pressure spring valve will automatically shut down), under the action of atmospheric pressure, In ten days, almost all of the oil accumulated on the outlet of the backup pump drains out to the tank (flowing back to the tank). Therefore, the pump will make a loud noise when the standby pump is switched on and sometimes the starting is unsuccessful, and sometimes the hydraulic pressure is established slowly When a few days when the pump switch, the hydraulic pressure to establish faster. 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 normal hydraulic pressure, we can see standby pump starting time is not reliable, standby pump in this state can not be a reliable backup, the system must be transformed . 3, Reconstruction measures After 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 closer to the pump body to increase the pump start emptying speed. 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, so as to ensure that the spare pump inlet and outlet lines are full of oil.  c) In the two pumps at the entrance to the addition of two contact doors (diameter smaller), the contact door is often open the door, two pumps in normal (one run, one spare), the two contact doors are open, and further Ensure that the spare pump body and elbow parts are filled with oil for a long time, which is an important reason for the oil pump to start and establish the fast hydraulic pressure. 4, the transformation effect After the transformation, the test pressure time in 5s, the pump started smoothly no abnormal sound, 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.