Application of water ring vacuum pump performance modification in heating unit

The water to be sealed must maintain a certain degree of subcooling. If the water temperature rises, the vacuum pump seal water vaporizes. The ultimate vacuum value is the saturation pressure corresponding to the temperature.

The vacuum pump pressure at the design point should be approximately 0.7 kPa lower than the condenser back pressure to maintain normal air flow. The saturation temperature corresponding to 4.9 kPa (the rated back pressure of the company's steam turbine) is about 32.5. Considering the subcooling of 4.2 required for the normal operation of the vacuum pump, the sealing water of the vacuum pump should be lower than 28.3 to work normally. The vacuum pump seal water is self-closing water hydration. The summer closed water temperature can reach up to 35 ft. Therefore, it is difficult to ensure that the vacuum pump seal water has a degree of subcooling of 4.2 in actual operation. As the seal water temperature approaches saturation, its pumping capacity also decreases until saturated vaporization loses its ability to work.

The working pressure of the vacuum pump and the pressure of the condenser are a dynamic balancing process. Regardless of the temperature of the circulating water, the suction pressure of the vacuum pump must be lower than the pressure of the condenser to pump away the non-condensable gas in the condenser. Therefore, the vacuum pump seal water temperature must be lower than the saturation temperature corresponding to the exhaust steam pressure.

On July 17, 2009, the brother of the company's No. 3 machine experienced a vacuum drop because the water in the vacuum pump was vaporized because the water separator was not changed in time. On July 30th, the test was carried out on the 2 machine A vacuum pump. After the pump was added to the 16 cold water, the separator temperature was 16.2. After the A pump was started, the temperature quickly rose to 24.5. As the temperature increased, the vacuum dropped at 30 water temperature and the vacuum increased. The 1kPa vacuum pump current is increased by 0.85A. The working water temperature is increased by about 0.19kPa per vacuum. The summer of this test is to ensure that the vacuum pump seal water is a relatively important means of ensuring the vacuum of the unit.

3 Reconstruction method Based on the above analysis to solve the cavitation problem of the vacuum pump, there are two ways to reduce the water temperature, and the second is to reduce the condenser vacuum.

According to the actual situation of the company, the cooling water or working fluid of the vacuum pump working fluid can be modified to improve the vacuum and solve the cavitation problem of the vacuum pump.

3.1 Vacuum pump cooling water transformation In March 2009, the No. 6 machine overhaul opportunity was used to renovate the 6A machine 6A vacuum pump cooling water. The vacuum pump working fluid cooling water was originally designed as a closed water pressure of 0.4~0.6MPa closed water from open circulating water. The cooling water temperature is higher than the initial circulating water 5 left stone. Using the characteristics of continuous heating of No. 6 machine, the demineralized water is replenished into the demineralized water replenishing tank after passing through the vacuum pump cooler. This method can increase the demineralized water supply without increasing equipment input. temperature.

3.2 Vacuum pump working fluid conveying system transformation 5B vacuum pump working fluid was reformed. As shown, a water supply pipe was directly taken out from the demineralized water supply tank through the cooler to the vacuum pump. The water supply should be installed with a flow meter manual control valve, and the steam water separator is also installed. Need to install manual control valve When the vacuum pump starts, flush the pipeline. After the water quality is passed, the vacuum pump is turned on to fill the water.

Vacuum pump soda separator to the pump to seal the water back to the water once, close the second door open the ditch door to flush the pump to adjust the water volume is 7.5t / h left petrochemical test water quality pass, close the ditch door open vacuum pump steam separator to the pump Sealed water back to the secondary door, start the vacuum pump, the water flow should be increased to 12.8t / h left stone, then adjust the vacuum pump steam separator to the pump to seal the water back to the opening of the door, and maintain the vacuum pump soda separator The water level runs continuously between 150~200mm. This operation mode must ensure that the water level of the demineralized water tank is more than 4500mm to ensure the water flow is stable.

3.3 Transformation results The switching test was carried out, and the working water was switched from the original closed water to the demineralized water and changed from the original closed circulation mode to the open circulation. Since the working temperature of the vacuum pump has a great influence on the vacuum pump output, when the working fluid temperature The higher the temperature corresponding to the higher pressure of the working fluid evaporation will affect the effective working space of the vacuum pump, thereby reducing the vacuum pump output. Table 1 shows the relevant parameters before and after the switching of the cooling water of the vacuum pump working fluid cooler.

Table 1 Vacuum pump working fluid cooler before and after cooling water related parameters Item demineralized closed water load / MW unit vacuum / kPa working fluid temperature / demineralized water temperature / vacuum pump current / A test proves that the vacuum pump output can be obtained by lowering the working fluid temperature A certain degree of improvement.

Under normal circumstances, the temperature of the chemical demineralized water is 2~4 ft lower than the temperature of the closed water. The use of chemical demineralized water as the working water of the vacuum pump can reduce the working water temperature of the vacuum pump by 2~3 ft. Taking summer working conditions as an example, the closed water temperature is 32.5 left stone 1 and the salt water temperature is 29.5TX close to the circulating water temperature) left stone. If the working water of the vacuum pump is switched to demineralized water, the vacuum pump pressure can be increased by 0.6~0.8kPa. At present, the company supplies heat at 80t/h left stone plus the steam consumption of the unit during normal operation, and the demineralized water can fully meet each unit. Vacuum pump open cycle working water volume per 13m3 / h cooling water requirements.

Because the test transformation does not change the operation mode of other systems such as the desalinary water supply system and the closed cold water system. If the operation mode is changed, the unit vacuum is increased by 0.3kPa on average. According to the power generation of Unit 5 last year, the annual standard coal can be saved by 1t. According to the calculation of 630 yuan per ton of standard coal, the annual cost can be saved by about 630,000 yuan. In addition, the water temperature of the unit after the water is heated by the vacuum pump can be increased by 4, and the annual cost can be saved by about 40,000 yuan.

In short, the cooling water or working fluid modification system of the vacuum pump working fluid is relatively simple and has little investment. The flow cooling of the vacuum pump working fluid not only improves the working condition of the vacuum pump rotor, but also increases the temperature of the hydration (demineralized water). However, the premise is that the unit must be continuously supplied with heat. When the heating is stopped, the vacuum pump working fluid must be adjusted or switched during maintenance, otherwise it will have a certain impact on safe operation.

4 Other methods to solve the vacuum pump problem To solve the vacuum pump cavitation situation, increase the vacuum pump inlet pressure, and propose to add a pre-atmospheric injection device for the vacuum pump.