Application of Water Ring Vacuum Pump Performance Modification in Heating Unit

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The sealed water must maintain a certain degree of subcooling. For example, when the water temperature rises, the vacuum pump sealed water vaporizes and its limit vacuum value is the saturation pressure corresponding to the temperature.

When working at the design point, the pressure of the vacuum pump should be about 0.7kPa lower than the back pressure of the condenser to maintain the normal flow of air. The saturation temperature corresponding to 4.9kPa (rated back pressure of the company's steam turbine) is about 32.5. Considering the subcooling degree of the 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 sealing water is taken from the closed water makeup water. The temperature of the closed water in summer can reach up to 35 zhangs, so it is difficult to ensure a 4.2 degree of subcooling of the vacuum pump sealing water in actual operation. As the sealing water temperature is close to saturation, its pumping capacity will continue to decline until it is saturated and vaporized and loses its working capacity.

The working pressure of the vacuum pump and the pressure of the condenser are a dynamic balance 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 in order to pump away the non-condensable gas in the condenser. Therefore, the sealing water temperature of the vacuum pump must be lower than the saturation temperature corresponding to the exhaust steam pressure.

On July 17, 2009, the reason why the vacuum dropped in the company's No.3 machine was that the sealed water in the vacuum pump vaporized due to the failure to change the water from the steam-water separator in time. On July 30, a test was carried out on the vacuum pump of machine 2 a. after 16 cold water was added to pump a, the separator temperature was 16.2. after pump a was started, the temperature soon rose to 24.5. as the temperature increased, the vacuum increased by about 1kPa at 30 water temperature. the vacuum pump current increased by about 0.85a. The vacuum increases by about 0.19kPa for every 1 drop in working water temperature. According to this test, the lower temperature of the sealing water of the vacuum pump is an important means to ensure the vacuum of the unit.

Based on the above analysis, there are two ways to solve the cavitation problem of vacuum pump, which are to reduce the water temperature and the vacuum of condenser.

According to the actual situation of the company, the cooling water or working fluid of the vacuum pump working fluid is 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 cooling water of 6A vacuum pump of Unit 6 was transformed by using the opportunity of overhaul of Unit 6. The working fluid cooling water of the vacuum pump was originally designed to have a closed water pressure of 0.4 ~ 0.6MPa. The cooling water temperature of the open circulating water was higher than that of the initial circulating water. The continuous heating of Unit 6 was used to add the demineralized water to the demineralized water supply tank after passing through the vacuum pump cooler, this method does not need to increase equipment investment and can increase the temperature of demineralized water replenishment.

3.2 Vacuum Pump Working Fluid Delivery System Transformation 5B Vacuum Pump Working Fluid Transformation As shown in the above, a water replenishment pipe is directly led out from the demineralized water replenishment tank to the vacuum pump through the cooler. A flowmeter manual control valve shall be installed for water replenishment. A manual control valve shall also be installed at the steam-water separator to flush the pipeline when the vacuum pump is started. After the water quality is tested to be qualified, the vacuum pump shall be opened to fill with water at the same time.

Vacuum pump steam-water separator to the pump seal water backwater primary door, close the secondary door, open the trench door to flush the pump, adjust the water volume to 7.5 t/h, then close the trench door, open the vacuum pump steam-water separator to the pump seal water backwater secondary door, start the vacuum pump, when the water flow should be increased to 12.8 t/h, then adjust the opening of the steam-water separator of the vacuum pump to the primary backwater valve of the pump seal water, and keep the water level of the steam-water separator of the vacuum pump in continuous operation between 150mm and 200mm. This operation mode shall ensure that the water level of the demineralized water makeup tank is above 4500mm to ensure stable water flow.

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

Table 1 Relevant parameters before and after the cooling water switch of vacuum pump working fluid cooler: demineralized water closed water load/MW unit vacuum/kPa working fluid temperature/demineralized water temperature/vacuum pump current/A test proves that the output of vacuum pump can be improved to a certain extent by reducing the working fluid temperature.

Under normal circumstances, the chemical desalination water temperature is 2~4 zhangs lower than the closed water temperature. The use of chemical desalination water as the working water of the vacuum pump can reduce the working water temperature of the vacuum pump by 2~3 zhangs. Take the summer working condition as an example. The closed water temperature is 32.5. The demineralized water temperature is 29.5TX close to the circulating water temperature). If the working water of the vacuum pump is switched to demineralized water, the pressure of the vacuum pump can be increased by 0.6~0.8kPa. At present, the heat supply of the company is 80 t/h left stone 1 and the steam and water loss during normal operation of the unit, and the water replenishment amount of demineralized water can fully meet the requirements of 13m 3/h cooling water amount per vacuum pump open cycle working water amount.

Because the test modification does not change the operation mode of other systems such as demineralized water supply system and closed cooling water system. If the vacuum of the unit is increased by 0.3kPa on average after the operation mode is changed, calculated according to the power generation capacity of unit 5 last year, 1t of standard coal can be saved annually. According to the calculation of 630 yuan per ton of standard coal, the annual cost saving is about 630000 yuan. In addition, the water temperature of the unit's supplementary water can be increased by 4 years after being heated by the vacuum pump, which can save about 40000 yuan in cost.

In short, the transformation system of the cooling water or working fluid of the vacuum pump working fluid is relatively simple and less 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 water (demineralized water). But the premise is that the unit must be continuous heating. When the heating is stopped and overhauled, the working fluid of the vacuum pump must be adjusted or switched, otherwise it will have a certain impact on the safe operation.

4 Other methods to solve the problem of vacuum pump In order to solve the cavitation of vacuum pump, it is proposed to increase the inlet pressure of vacuum pump.

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