1、 Key installation techniques for centrifugal pumps

The key to the installation technology of pipeline centrifugal pumps lies in determining the installation height (i.e. suction head) of the water pump. This height refers to the vertical distance from the water surface of the water source to the centerline of the water pump impeller. It cannot be confused with the allowable vacuum suction height. The allowable vacuum suction height indicated on the water pump product manual or nameplate refers to the vacuum value on the water inlet section of the water pump, and is determined through testing at 1 standard atmospheric pressure and a water temperature of 20 degrees Celsius. It did not take into account the water flow situation after the matching of the suction pipeline. The installation height of the water pump should be the remaining value after deducting the head loss of the suction pipeline from the allowable vacuum suction height, which needs to overcome the actual terrain suction height. The installation height of the water pump cannot exceed the calculated value, otherwise the water pump will not be able to draw water. In addition, the magnitude of the calculation value is affected by the resistance loss head of the suction pipeline. Therefore, it is advisable to use the shortest pipeline layout and minimize the installation of elbows and other accessories. It is also possible to consider adding larger diameter water pipes to reduce the flow velocity inside the pipeline.

It should be pointed out that when the elevation and water temperature of the installation site of the pipeline centrifugal pump are different from the test conditions, if the local altitude is above 300 meters or the water temperature being pumped exceeds 20 degrees Celsius, the calculated values need to be corrected. The atmospheric pressure at different altitudes and the saturated vapor pressure at water temperatures above 20 degrees Celsius. However, when the water temperature is below 20 degrees Celsius, the saturated vapor pressure can be ignored.

In terms of pipeline installation technology, the suction pipeline requires strict sealing and must not leak air or water. Otherwise, it will damage the vacuum degree at the water inlet of the pump, reduce the water output of the pump, and in severe cases, it may even be unable to pump water. Therefore, it is necessary to carefully carry out the interface work of the pipeline to ensure the construction quality of the pipeline connection.

2、 Calculation of installation height Hg for centrifugal pumps

The allowable suction vacuum height Hs refers to the maximum vacuum degree that can be achieved by the pressure p1 at the pump inlet.

The actual allowable suction vacuum height Hs value is not calculated based on the formula, but is determined experimentally by the pump manufacturer. This value is attached to the pump sample for user reference. It should be noted that the Hs value given in the pump sample is the value obtained with clean water as the working medium, operating conditions of 20 ℃, and pressure of 1.013 × 105Pa. When the operating conditions and working medium are different, conversion is required.

(1) Transport clean water, but the operating conditions are different from the experimental conditions, which can be converted according to the following equation

Hs1=Hs+(Ha-10.33) - (H ù -0.24)

(2) When transporting other liquids and the conditions of the transported liquid and villain are different from the experimental conditions, two steps of conversion are required: the first step is to convert the Hs1 detected from the pump sample according to the above formula; The second step is to convert Hs1 to H according to the following equation? s

2. NPSH Δ h

For oil pumps, the installation height is calculated using the cavitation margin Δ h, which is the vacuum degree that the pump is allowed to suction liquid, that is, the allowable installation height of the pump, in meters. Take the NPSH Δ h from the oil pump sample and measure its value with 20 ℃ clean water. If transporting other liquids, calibration is also necessary and relevant books should be thoroughly consulted.

Suction lift=standard atmospheric pressure (10.33 meters) - NPSH - safety margin (0.5 meters)

The standard atmospheric pressure can pressure the vacuum height of the pipeline to 10.33 meters.

For example, if a certain pump requires a NPSH of 4.0 meters, calculate the suction head Δ h?

Solution: Δ h=10.33-4.0-0.5=5.83 meters

From a safety perspective, the actual installation height of the pump should be less than the calculated value. When the calculated Hg is negative, it indicates that the suction position of the pump should be below the liquid level of the storage tank.

Example 2-3: A certain centrifugal pump found that the allowable vacuum suction height Hs=5.7m from the sample. The total resistance of the suction pipeline is known to be 1.5mH2O, and the local atmospheric pressure is 9.81 × 104Pa. The dynamic pressure head of the liquid in the suction pipeline can be ignored. Trial calculation:

(1) Installation of pumps for conveying 20 ℃ clean water;

(2) Change to the installation height of the pump when transporting water at 80 ℃.

Solution: (1) The installation height of the pump when conveying 20 ℃ clean water

Known: Hs=5.7m

Hf0-1=1.5m

U12/2g ≈ 0

The local atmospheric pressure is 9.81 × 104Pa, which is basically consistent with the experimental conditions when the pump was manufactured, so the installation height of the pump is Hg=5.7-0-1.5=4.2 m.

(2) The installation height of the pump for transporting 80 ℃ water is 80 ℃

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