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Technical requirements for boiler deaeration feed water pump
The function of the boiler deaerator feed water pump is to transport the feed water with a certain temperature in the deaerator water tank to the boiler, remove oxygen and other non-condensable gases in the boiler feed water to ensure the quality of the feed water. If oxygen is dissolved in the water, the metal in contact with the water will be corroded. At the same time, if gas accumulates in the heat exchanger, it will increase the heat transfer resistance and reduce the heat transfer effect of the equipment.
According to the characteristics of the boiler operation, the deaerator feedwater pump must be continuously operated to ensure the boiler feed water, thereby ensuring the requirements of safe production of the boiler. Feed water pumps generally use multi-stage centrifugal water pumps. Filling cylinders are installed at both ends of the inlet section and outlet section of the water pump. In order to cool the filler, some water pumps are also provided with a cooling water chamber outside the filler barrel, with cooling water flowing inside, and cooling water is used to cool the filler. Therefore, there are certain technical requirements for the boiler deaeration feed water pump:
1. High outlet pressure
The outlet pressure of the medium-pressure power plant boiler deaeration feedwater pump (inlet water temperature is 104°C) is 55 kg/cm^2, and the outlet pressure of the high-pressure power plant feedwater pump (inlet water temperature is 150°C) is above 150kg/cm^2. In order to obtain a higher outlet pressure, two structural measures are often used:
1. Design the feedwater pump as a multi-stage pump, such as the domestically made electric feedwater pump with 2950 revolutions per minute, DG270-150 is 10 stages, and DG375-185 is 12 stages. In this way, the pressure of the entire pump is the sum of the pressures at all levels, the more the number of stages, the higher the pressure.
2. The feedwater pump is designed as a high-speed pump. After the number of revolutions is increased, although the number of impeller stages does not increase, the outlet pressure increases rapidly with the square of the number of revolutions. The more the speed increases, the faster the pressure increases.
Of the above two measures, the latter measure is better. When the second measure is adopted, the pump has fewer stages, the pump shaft is shorter and the deflection is smaller, so a smaller sealing gap can be used, which improves a certain efficiency. In addition, when the number of stages is small, the volume of the pump is greatly reduced and the weight is greatly reduced. Therefore, the feed water pump is becoming more and more high-speed. The revolutions of foreign large-capacity feedwater pumps have reached more than 7000 revolutions per minute. These feedwater pumps are generally driven by a steam turbine. Because the speed of the steam turbine is easy to adjust, the pressure and flow rate under different loads can be obtained by changing the speed, so it has good economic efficiency.
The boiler deaeration feed water pump is operated when the inlet water temperature is high and the outlet pressure is high. The outlet pressure of the feedwater pump mainly depends on the working pressure of the boiler drum. In addition, the water outlet of the feedwater pump must overcome the resistance of the water supply pipeline and valve; the resistance of the heaters at all levels; the resistance of the feedwater adjustment valve; the resistance of the economizer; the boiler The resistance formed by the pipe distance and geometric height between the water inlet and the outlet of the water pump; the pressure in the boiler steam drum; the pressure required by the once-through boiler to promote water circulation. According to experience, the minimum outlet pressure of the general feedwater pump is 1.25 times the maximum working pressure of the boiler.
2. Set under the deaerator
The boiler feed water delivered by the deaerator feedwater pump is saturated water under a certain pressure, and the temperature is high, and it is easy to vaporize at the inlet of the feedwater pump, which will cause cavitation and cause the water outflow to be interrupted. In order to prevent the feed water from vaporizing, the suction height of the pump should be a negative value, so that the static pressure at the inlet of the feed water pump is higher than the corresponding saturation pressure of the inlet water temperature. Therefore, the feedwater pump is usually installed at a certain height below the feedwater tank of the deaerator to increase the static pressure at the inlet of the feedwater pump, avoid vaporization, and ensure the normal water supply of the pump. Generally, the feedwater pumps are installed at 6-7m below the deaerator.
Because the feed water pump delivers saturated water under a certain pressure, the saturated water will be vaporized if the pressure on the saturated water drops slightly, causing the water supply to the water pump to be interrupted. For this reason, the feed water pump must be set under the deaerator, and the backfill height is used to increase the pressure at the pump inlet. In addition, it should also be considered that the inlet water will rise very much when the pump is under low flow conditions (because at low flow, the water stays in the pump for a longer time and is stirred by the impeller at high speed, causing friction and heat. This part will be heated after the temperature rises. The water from the balance plate returns to the pump inlet along the balance pipe, so it will cause the inlet water temperature to rise, the smaller the flow, the higher the inlet water temperature). In order to prevent vaporization even after the water temperature rises, the pump inlet must have a larger pressure head, which means that there must be a larger backflow height. The higher the inlet water, the greater the backfill height should be. However, due to the arrangement, it is impossible to arrange the deaerator water tank very high, so the temperature rise at the pump inlet must be limited, generally not exceeding 8C
Then consider the hydraulic loss of the pipeline, the cavitation allowance required by the pump and the fluctuation of the deaerator pressure during operation. Therefore, the backfill height of the general atmospheric deaerator should not be less than 6 meters, and the 6 absolute atmospheric pressure deaerators should not be less than 14 M. For modern high-speed feed water pumps, the allowable cavitation margin is also very large (up to 30 meters or more). In order to make the pump inlet have the necessary suction head value, the installation height of the deaerator must be increased, but This is economically and technically difficult. Therefore, large-capacity units are equipped with booster pumps in front of their feedwater pumps. The booster pump has a low number of revolutions and allows a small cavitation margin, so the installation height of the deaerator can be reduced, and the pump is not easy to vaporize. The feedwater that has been boosted by the booster pump then enters the inlet of the feedwater pump.
Features: The front end (conveying end) of the curved pipe pressure balance compensator has two upper bellows, and the rear end (yu end) is composed of a balanced bellows. The working bellows absorbs the lateral displacement generated from the elbow end. The balanced corrugation absorbs the axial displacement of the straight pipe section. When the elbow needs to produce lateral displacement, the spherical nut on the tie rod rotates around the spherical washer, and the tie rod bears the internal thrust. Therefore, the compensator is often used at the mountain pass of pumps, tanks, compressors, and steam turbines. , In order to reduce the force on the above equipment. Commonly used for displacement at the mountain pass of pumps, tanks, compressors, and steam turbines: axial and lateral pressure and thrust are balanced, working temperature ≤420℃, fatigue life 1000 times
The balanced compensator can compensate the axial and lateral displacement without generating internal pressure thrust on the piping system. The bellows pressure balance compensator is installed on the bend of the pipeline or the space pipeline connected with the equipment. It can compensate axial displacement and lateral displacement without generating internal pressure thrust on the piping system or other equipment. It is often used in pumps, compressors, gas turbines and other load-sensitive pipeline systems, and can absorb vibration and reduce noise to ensure the safe operation of equipment.
3. Flat characteristic curve
In order to adapt to the needs of boiler load changes, it is required that after adjusting and changing the feed water volume, the outlet pressure of the feed water pump should be small to make the operation stable. Therefore, the characteristic curve of the boiler deaeration feed water pump must be relatively flat.