01 System equipment
There are two 1,000 MW ultra-supercritical coal-fired units (No. 3 and No. 4) installed in Power Plant B of a power generation limited liability company, each of which is equipped with two feedwater turbines to drive the feedwater pumps, and no electric feedwater pumps.
Each feedwater pump turbine is equipped with an independent lubricating oil system, including two sets of small plate heat exchangers, one in operation and one in standby, using closed cooling water as the cooling water source. The heat exchanger plates are 0.5 mm thick and each plate is equipped with a sealing gasket. Plates are equipped with a frame, the angle of rotation between adjacent plates is 180 °, the plate bundle to form a series of parallel channels, heat exchanger plate on both sides of the oil, water and two kinds of media into the appropriate channel, rely on the heat exchanger plate to transfer the temperature difference to achieve the role of cooling oil temperature.
02 Accident
Closed cooling water pump regular rotation, start the No. 2 closed cooling water pump, running normally, 11:07 stop No. 1 closed cooling water pump, No. 1, No. 2 feed pump turbine tripped one after another, 85 ms apart, MEH tripped the first out of the "lubricating oil pressure is low" (fixed value of 0.07 MPa). Check the history curve found that the feed pump turbine lubricant pressure low, stator cooling water flow low alarm, issued at the same time with the closed cooling water pump switching. 11:07:08 stop No. 1 closed cooling water pump; 11:07:11 closed cooling water pipe pressure from 0.78 MPa suddenly fell to 0.56 MPa (minimum), water pressure fluctuation range of 0.22 MPa; 11: 07:11 two feed pumps tripped at a interval of 85 ms, MEH tripped the first out of the "lube oil pressure" (fixed value of 0.07 MPa). 07:11 2 feed pump turbine lubricant pressure low Ⅱ switch both reached the action value (0.07 MPa), feed pump turbine tripped; 11:07:11 stator cooling water flow low protection reached the action value, due to the existence of the protection of the 30 s delay, the protection did not trigger the unit tripping logic (stator cooling water cooler for plate heat exchanger).
03 Cause analysis
From Figure 1, it can be seen that the heat in the lubricating oil is released to the closed cooling water system through the plate heat exchanger. Plate heat exchanger core element heat transfer plate is made of stainless steel, thickness of about 0.5 mm. usually with a mould will be pressed into a variety of slots or corrugated plate (horizontal straight corrugated, herringbone corrugated, diagonal corrugated, etc.), not only to increase the rigidity of the plate, but also to prevent the corrugated plate deformation of the pressure, enhance the degree of turbulence of the fluid and the area of heat transfer, and at the same time to withstand the difference in pressure on both sides. Therefore, the plate plate structure form is to ensure that the plate heat exchanger efficient heat transfer fittings. Due to the plate heat exchanger plate is thin, when the heat exchanger on both sides of the pressure difference changes, will affect the shape of the plate, the heat exchanger on both sides of the volume will be followed by a slight change in this feature is easy to be ignored.
In the process of rapid fluctuation of closed cooling water pressure, the plate cooler heat exchanger plate sheet is affected by the pressure difference between the two sides of the micro-deformation occurs, the water side of the space is rapidly contracted, the oil side of the space expansion, the lubricating oil in the system expands and fills up the increased space on the oil side of the heat exchanger, resulting in instantaneous fluctuations of the lubricant oil pressure of the feed pump turbine to less than 0.07 MPa, which reaches the value of the action of lubricant oil pressure low Ⅱ switch, and the 2 feed pump turbines are successively The two feed pump turbines tripped one after another.
After the unit tripped, in order to verify the relationship between the pressure fluctuation of the closed cooling water system and the lubricant system pressure of the feed pump turbine, the closed cooling water pump switching test was re-conducted, and the lubricant pressure gauges of the two feed pump turbines were monitored in situ. Start No. 2 closed cooling water pump, start normal, stop No. 1 closed cooling water pump, feed pump turbine lubricating oil pressure instantaneous rapid decline in closed cooling water pressure in 3 s from 0.81 MPa to 0.65 MPa, No. 2 feed pump turbine lubricating oil pressure is low Ⅰ switch action, and the stator cooling water system flow rate has a significant impact.
04 Treatment measures
a. In the protection logic of feed pump turbine lubricating oil pressure low Ⅰ start standby lubricating oil pump, additional delay 3 s conditions, oil pressure instantaneous fluctuation <3 s, standby oil pump is too late to start, in order to ensure that the turbine does not burn tile accident, usually lubricating oil pressure low Ⅱ value in the accident lubricating oil pump at the same time to trigger the feed pump turbine tripping protection. In order to avoid false tripping caused by lubricant pressure fluctuation of feed pump turbine, the 3 s delay condition of standby pump can be cancelled, but it is required that the oil pressure is low Ⅰ switch action is safe and reliable, otherwise it is easy to cause the standby oil pump to start frequently. Lubricating oil pressure low Ⅱ trigger feed pump turbine trip protection, can increase 0.5 ~ 1 s delay, but increased the risk of shaft tile burn.
b. Due to the plate heat exchanger micro-deformation caused by low lubricant pressure, only instant fluctuations, the system oil pressure can be restored in a short time, therefore, in the lubricating oil system oil supply pipeline (especially in the pressure tube near the point of taking oil) to install an accumulator, in the lubricating oil pressure for a short period of time when the lubricating oil pressure is lowered, the accumulator stored in the release of lubricating oil, to compensate for the system pressure, can effectively alleviate the impact of instant fluctuations of lubricating oil pressure on the system.
05 Technical Summary
Currently plate heat exchanger is widely used in the new installation unit, in the actual application of the heat exchanger plate micro-deformation characteristics are often ignored, and this characteristic of the system's safe and stable operation of the existence of a greater risk. Therefore, in the use of plate heat exchanger system, especially the more important oil system, in the design should pay special attention to the problem, it is recommended to add a sufficient capacity of the accumulator.
