The key to the installation of the multistage pump is how to correctly determine the pump installation height (ie suction stroke). For a general centrifugal pump, this height refers to the vertical distance from the liquid level to the center line of the pump impeller; for large-flow centrifugal pumps, this height should be considered according to the distance between the highest point of the impeller population and the liquid level.
It can not be confused with the allowable vacuum. The pump specification or the nameplate E indicates the allowable vacuum to refer to the vacuum value on the water inlet section of the pump, and it is at 1 standard atmospheric pressure, the water temperature is 20'C, and the rated condition. It was measured by the test. Allowing the vacuum to be absorbed does not take into account the flow conditions after the suction pipe is fitted.
The installation height of the pump is the value of the remaining part that allows the suction vacuum to be deducted from the loss of the suction pipe. It has to overcome the actual topographic water absorption height. The installation height of the pump should not exceed the calculated value, otherwise the pump will not be able to pump water. In addition, the magnitude of the influence calculation value is the resistance loss head of the suction pipe.
Therefore, it is advisable to use the shortest pipe arrangement and to install accessories such as elbows as little as possible. It is also conceivable to equip the pipe with a larger diameter to reduce the flow rate inside the pipe. It should be noted that when the atmospheric pressure and water temperature at the installation location of the multistage pump are different from the test conditions, such as the local altitude above 300 m or the water temperature of the pumped water exceeds 20 ° C, the calculated value should be corrected. That is, the atmospheric pressure at different altitudes is higher than 2. The saturated vapor pressure at °C water temperature is calculated. However, when the water temperature is below 20 ° C, the change in saturated vapor pressure is negligible.
From the pipeline installation technology, the suction pipe requires strict sealing, and it cannot leak or leak water. Otherwise, the vacuum at the water inlet of the pump will be damaged, and the water output of the pump will be reduced. In severe cases, you can't even get water. Therefore, it is necessary to carefully do the interface work of the pipeline to ensure the construction quality of the pipeline connection. The installation height of the pump can be calculated from the seven vacuum degrees allowed. If it is known that the pump is allowed to absorb vacuum, the calculated installation height of the pump is calculated according to equation (7-2). Allow to absorb the vacuum H. Refers to the pressure p at the pump population; the maximum vacuum that can be tolerated. The actual allowable vacuum height H is allowed. The value is not a calculated value. It is the value measured experimentally by the pump manufacturer. This value is attached to the pump sample for user review.
However, it should be noted that the H value given in the pump sample is the use of clean water as the working medium. C and the atmospheric pressure are 1.013 × lOsPa, when the operating conditions and working medium are different, need to be converted. The operating conditions are different from the test conditions and can be converted according to formula (7-1): Hs'=Hs+(Hs--10.33)-(Hv-0.24)(7-1) where: Hs' allows for a pump in the field state Aspirate vacuum, m. Hs is allowed to absorb vacuum in a standard state (or given by the sample), m; Ha--atmospheric pressure in the field state of the pump, m; 10.33 - atmospheric pressure in the standard state, m; Hv - liquid at that time Vaporization pressure at temperature, m; vapor pressure of water at 0.24-standard state, m. Where: Hg - pump installation height, m; Hy - liquid level pressure, m; Ha - standard atmospheric pressure, m; mouth s - average speed of pumping population, m / s; hw - suction pipe hydraulic Loss, m.
When the calculated value H: is negative, it indicates that the suction port position of the pump should be below the liquid level of the tank, that is, it is inverted. The installation height of the pump can also be calculated from the NPSH margin. If the necessary NPSH of the pump is known, then the height difference of the pump is calculated according to equation (7-3): Hg=(Hy-Hv)-△h-hw(7-3} where: Hv - liquid saturated vapor pressure. m; △ h - necessary NPSH, m. When the calculated Hg is negative, it means that the suction port of the pump should be below the liquid level of the tank, that is, backfill.
