In thermal power plants, liquid ring vacuum pump is often used as condenser suction equipment. In the operation of units, the problem of low vacuum of units is often caused by the decrease of efficiency of vacuum pumps or insufficient suction capacity. This paper mainly studies how to improve the suction capacity and suction capacity of vacuum pump, and how to affect the vacuum of the unit.
Condenser vacuum is an important index to measure the thermal efficiency of thermal power plant cycle. For every 1% increase in unit vacuum, the coal consumption of generating unit power supply decreases by 2.5-3 g/kWh. At present, the eccentric liquid ring vacuum pump is widely used in domestic generating units to establish and maintain the unit vacuum, which has the advantages of simple system and low energy consumption. However, in the operation of the unit, the problem of low vacuum of the unit often occurs because of the decrease of the efficiency of the vacuum pump or the insufficient suction capacity. Vacuum pump suction capacity has a relative impact on the vacuum of the unit. With the improvement of the suction capacity of the vacuum pump, the vacuum of the unit can not always be improved. It is important to study how to improve the suction capacity of vacuum pumps and how to improve the suction capacity of vacuum pumps to influence the vacuum of units.
The working fluid of liquid ring vacuum pump in power plant is usually cooled by industrial circulating water, but the temperature of cooling water is affected by climate. Especially in summer, with the increase of circulating water temperature, the working fluid temperature of vacuum pump will also rise. The working fluid temperature of vacuum pump will also change with random load. The increase of working fluid temperature of vacuum pump will result in the decrease of pumping capacity and cavitation of vacuum pump, which will affect the safety of equipment and the economic operation of the unit.
If the saturation pressure of the working fluid is 4.241 kPa at 30 ~C and 7.35 kPa at 40 ~C, when the suction pressure of the water ring vacuum pump is less than or equal to the saturation pressure of the working fluid temperature corresponding to the vacuum of the unit, the working fluid of the vacuum pump will be vaporized. Vacuum pumps will suffer from serious shortage of pumping capacity due to the squeezing of gas generated by vaporization of pumping working medium. Therefore, to improve the working performance of vacuum pumps, we can start with changing the working fluid temperature. In our country, the ways to reduce the working fluid of vacuum pumps include changing the tube cooler to plate heat exchange or increasing cooling area capacity, changing the working fluid cooling water to central air conditioning water or installing industrial cooling device, directly adding cooling device to the working fluid, etc.
A power plant in Shandong Province adopts vacuum pump heat exchanger series compression refrigeration device to provide secondary cooling for working fluid of vacuum pump. The vacuum pump in this factory adopts 2BW4-353-OMK-Z single-stage water-ring rotary pump. Its supplementary water comes from condensate water, cooler cooling water comes from closed circulating water. When the temperature is high in summer, the temperature of closed circulating water is in the range of 40-42 C, which results in high working fluid temperature of vacuum pump and seriously affects the suction capacity of vacuum pump. In order to solve the problems existing in the vacuum system in 2012, a set of compressed refrigeration device was connected in series in the vacuum pump tube heat exchanger to provide secondary cooling for the working fluid of the vacuum pump.
This device can reduce the working fluid temperature of vacuum pump, improve the efficiency of vacuum pump and unit vacuum. Considering the continuous cooling accumulation capacity of working fluid, there is only 20t sealed water in the vacuum pump. As long as the maximum temperature drop of the original cooler and refrigeration device in a single cycle is 4-5 C, the overall temperature drop of working fluid of the vacuum pump can be met by 10 C. The normal output of the vacuum pump and no cavitation erosion of the impeller can be guaranteed. The refrigeration capacity of the compressed refrigeration unit is about 220 kW, equivalent to 60 kW of electric power. After retrofitting, the vacuum of the unit is increased by 0.5 kPa per year on average, and the coal consumption of power supply is reduced by 1.5 g/kwh.
It should be pointed out that the vacuum of the unit is not necessarily improved if the working liquid temperature of the vacuum pump is greatly reduced. The vacuum tightness of the unit and the difference between the working liquid temperature of the vacuum pump and the exhaust temperature of the condenser should also be taken into account. Generally speaking, if the vacuum tightness of the unit is over 100 Pa/min or in normal operation, the vacuum pump should also be considered. The working liquid temperature is less than 5 C lower than the exhaust steam temperature of the condenser. Reducing the working liquid temperature of the vacuum pump can further improve the vacuum of the unit, otherwise it is of little significance.