Summary of principle and application scope of various pumps

Key installation technology of pipeline centrifugal pump: selection of pump installation height, i.e. suction head

1、 Key installation technology of centrifugal pump

The key to the installation technology of pipeline centrifugal pump is to determine the installation height (i.e. suction head) of the pump. This height refers to the vertical distance from the water source surface to the centerline of the pump impeller, which cannot be confused with the allowable vacuum suction height. The allowable vacuum suction height marked on the pump product manual or nameplate refers to the vacuum value on the pump inlet section, and is measured by the test under 1 standard atmospheric pressure and 20 ℃ water temperature. It does not consider the water flow condition after the supporting of the suction pipe. The installation height of the water pump should be the value that is left after the allowable suction vacuum height deducts the loss head of the suction pipe. It should overcome the actual terrain suction height. The installation height of the water pump shall not exceed the calculated value, otherwise, the water pump will not be able to pump water. In addition, the calculation value is affected by the resistance loss head of the suction pipe. Therefore, the shortest pipeline layout should be adopted, and elbows and other accessories should be installed as little as possible. It can also be considered to properly equip larger caliber water pipes to reduce the flow rate in the pipe.

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 elevation is more than 300 meters or the water temperature being pumped is more than 20 degrees Celsius, the calculated value should be corrected. That is, the atmospheric pressure at different altitudes and the saturated steam pressure when the water temperature is higher than 20 ℃. However, when the water temperature is below 20 ℃, the saturated steam pressure can be ignored.

In terms of pipe installation technology, the suction pipe shall be strictly sealed, and shall not leak air or water, otherwise the vacuum degree at the water pump inlet will be damaged, the water output of the pump will be reduced, and even the water can not be pumped in serious cases. Therefore, it is necessary to do a good job in the pipeline interface work to ensure the construction quality of pipeline connection.

2、 Calculation of installation height Hg of centrifugal pump

The allowable suction vacuum height Hs refers to the maximum allowable vacuum degree at the pump inlet pressure p1. The actual allowable suction vacuum height Hs is not the value calculated according to the formula, but the value measured 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 uses clean water as the working medium, the operating condition is 20 ℃ and the pressure is 1.013 × The value at 105Pa shall be converted when the operating conditions and working medium are different.

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

Hs1＝Hs＋(Ha－10.33) － (H υ－ 0.24)

(2) When conveying other liquids, when the conditions of the liquid to be conveyed and the villain are different from the experimental conditions, two steps of conversion are required: the first step is to convert the Hs1 found in the pump sample according to the above formula;

Step 2: convert Hs1 into H according to the following formula ΄ s

2 NPSH Δ h

For oil pump, NPSH is used to calculate the installation height Δ H, that is, the vacuum degree that the pump is allowed to suck liquid, that is, the allowable installation height of the pump, in meters. NPSH Δ H Check from the oil pump sample, and measure the value with clean water at 20 ℃. If other liquids are transported, they should also be calibrated and relevant books should be carefully checked.

Suction head=standard atmospheric pressure (10.33m) - NPSH - safety amount (0.5m), and vacuum height of standard atmospheric pressure pressure pipeline is 10.33m.

For example, the required NPSH of a pump is 4.0m, and the suction head is calculated Δ h？

Solution: Δ H=10.33-4.0-0.5=5.83m

From the perspective of safety, 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 port of the pump should be below the liquid level of the storage tank.

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

(1) Installation of pump when delivering 20 ℃ clean water;

(2) It is changed to the installation height of the pump when delivering 80 ℃ water.

Solution: (1) Installation height of the pump when delivering 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 test conditions when the pump leaves the factory, so the installation height of the pump is Hg=5.7-0-1.5=4.2 m.

(2) Installation height of pump when delivering 80 ℃ water

When transporting 80 ℃ water, the installation height cannot be directly calculated by using the Hs value in the pump sample. It is necessary to convert Hs according to the following formula, that is

Hs1＝Hs＋(Ha－10.33) － (H υ－ 0.24)

Known Ha=9.81 × 104Pa ≈ 10mH2O. According to the appendix, the saturated vapor pressure of 80 ℃ water is 47.4kPa.

Hv=47.4 × 103 Pa＝4.83 mH2O

Hs1＝5.7+10－10.33－4.83+0.24=0.78m

Substitute Hs1 value into the formula to obtain the installation height

Hg=Hs1－Hf0-1=0.78－1.5=－0.72m

Hg is a negative value, indicating that the pump should be installed below the liquid level of the pool, at least 0.72m lower than the liquid level.