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Self-balancing multi-stage centrifugal pump installation position and height calculation formula case

12 Apr 2021Email

Self-balancing multi-stage centrifugal pump installation position and height calculation formula case

The installation position and height of the self-balancing multi-stage centrifugal pump is a very critical issue. The floor elevation in the pump room depends on the installation height of the pump. Correctly calculate the maximum allowable installation height of the self-balancing multi-stage pump so that the pump station can Safe water supply and saving soil and construction costs are of great significance. In order to avoid the occurrence of cavitation, the calculation of the installation height of the self-balancing multi-stage pump needs to be very carefully checked.

 

Installation position of self-balancing multistage centrifugal pump

The key to the installation position technology of the self-balancing multistage pump is how to correctly determine the installation height of the pump (that is, the suction lift). For general self-balancing multi-stage centrifugal pumps, this height refers to the vertical distance from the liquid level to the center line of the pump impeller; for large-flow self-balancing multi-stage centrifugal pumps, this height should be based on the height between the highest point of the impeller population and the liquid level. Consider the distance between them. It should not be confused with the allowable suction vacuum degree. The allowable suction vacuum degree indicated in the pump product manual or nameplate E refers to the vacuum value on the section of the water inlet of the pump, and it is under 1 standard atmospheric pressure, water temperature 20'C, and rated working conditions. Measured by the following test. The degree of vacuum allowed does not consider the water flow after the suction pipe is installed. The installation height of the water pump is the value of the remaining part of the value after allowing the suction vacuum to deduct the loss of the suction pipe, and it must overcome the actual topographic suction 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 value that affects the calculated value is the resistance loss head of the suction pipe. Therefore, the shortest pipeline layout should be adopted, and the fittings such as elbows should be installed as little as possible, and water pipes with larger diameters can also be considered to reduce the flow velocity in the pipe.

It should be pointed out that when the atmospheric pressure and water temperature at the installation site of the self-balancing multi-stage pump are different from the test conditions, if the local altitude is above 300m or the water temperature being pumped exceeds 20°C, the calculated value shall be corrected. That is, it is calculated according to the atmospheric pressure at different altitudes and the saturated steam pressure when the water temperature is higher than 2°C. However, when the water temperature is below 20°C, the change in saturated steam pressure is negligible.

From the perspective of pipeline installation technology, the suction pipeline requires strict sealing, and cannot leak air or water, otherwise it will destroy the vacuum at the water inlet of the pump, reduce the water output of the pump, and even fail to pump water in severe cases. Therefore, it is necessary to do a good job in the interface of the pipeline to ensure the quality of the pipeline connection.

 

Calculation formula for installation height of self-balancing multistage centrifugal pump

The allowable geometric installation height of the self-balancing multi-stage pump is related to various conditions. The formula is as follows:

Calculation formula for installation height of self-balancing multistage centrifugal pump

[Hg]—The allowable geometric installation height of the pump, m; (calculation result is for use in design, the actual installation height must be lower than the allowable installation height)

pe—pressure of the water surface, Pa; (it is the atmospheric pressure of the water surface, the higher the altitude, the lower the atmospheric pressure)

pv—saturated steam pressure, Pa; (related to water temperature, the higher the water temperature, the higher the saturated steam pressure)

ρ—The density of the fluid, kg/m3;

g—acceleration of gravity, 9.81m/s2;

[NPSHr]—allowable cavitation allowance of the pump, m; (related to the performance of the pump, provided by the pump manufacturer)

hw—head loss in the suction pipeline, m. (Related to the design of the suction pipe, determined by the designer)

It can be seen from the above formula that the higher the altitude, the higher the water temperature, the greater the allowable cavitation margin, the greater the head loss of the inlet pipe, and the smaller the allowable geometric installation height.

 

If the allowable suction vacuum of the self-balancing multi-stage centrifugal pump is known, calculate the installation height formula of the pump. The allowable vacuum height Hs refers to the allowable vacuum degree of the pressure p1 at the inlet of the pump.

The actual allowable suction vacuum height Hs value is not a value calculated according to the formula, but a value determined experimentally by the pump manufacturer. This value is attached to the pump sample for users to check. It should be noted that the Hs value given in the pump sample is the value when clean water is used as the working medium, and the operating condition is 20℃ and the pressure is 1.013×105Pa. When the operating conditions and working medium are different, conversion is required. The operating conditions are different from the test conditions, and can be converted according to the following formula:     

Hs’=Hs+(Hs--10.33)-(Hv-0.24)(7-1)

Where: Hs’ is the allowable suction vacuum degree under the on-site condition of the pump, m.

Hs a standard state (or given by the sample) allowable suction vacuum, m;

Ha--atmospheric pressure under the on-site condition of the pump, m;

10. 33-Atmospheric pressure under standard conditions, m;

Hv——The vaporization pressure of the liquid at the current temperature, m;

0. 24-The vaporization pressure of water under standard conditions, m.

Calculation formula for installation height of self-balancing multistage centrifugal pump

Where: Hg—— pump installation height, m;

Hy——liquid surface pressure, m;

Ha——standard atmospheric pressure, m;

νs——the average speed of pumping population, m/s;

hw-Hydraulic loss of suction pipe, m.

When the calculated value Hg is negative, it means that the suction port of the pump should be below the liquid level of the storage tank, that is, backflow.

The installation height of the pump can also be calculated from the cavitation margin. If the required NPSH of the pump is known, then the installation height of the pump needs to be calculated according to the following formula.

Hg = (Hy-Hv) - △ h-hw

Where: Hv——liquid saturated vapor pressure, m;

△h——Necessary cavitation margin, m.

When the calculated Hg is a negative value, it means that the suction port of the pump should be below the liquid level of the storage tank, that is, backflow.

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

Hs1=Hs+(Ha-10.33)-(Hυ-0.24)

(2) Transporting other liquids When the conditions of the liquid to be transported and the villain are different from the experimental conditions, a two-step conversion is required: the above formula will be the Hs1 found in the pump sample; the second step will convert Hs1 to the following formula H?s

 

Calculation of cavitation margin Δh for self-balancing multi-stage centrifugal pump

For self-balancing multi-stage centrifugal oil pumps, the cavitation allowance Δh is used to calculate the installation height, that is, the vacuum degree of the pump allowed to suck liquid, that is, the allowable installation height of the pump, in meters. Use the NPSH Δh to check from the oil pump sample, and its value is also measured with 20℃ clean water. If other liquids are to be transported, calibration is also required, and the relevant books should be checked in detail.

Suction range = standard atmospheric pressure (10.33 meters)-cavitation margin-safety amount (0.5 meters)

The vacuum height of the standard atmospheric pressure energy pressure pipeline is 10.33 meters.

For example: a certain pump must have a NPSH of 4.0 meters, what is the suction stroke Δh?

Solution: Δh=10.33-4.0-0.5=5.83 meters

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

 

Self-balancing multi-stage centrifugal pump installation height calculation formula case

For example, a self-balancing centrifugal pump found from the sample that the allowable vacuum height Hs=5.7m. It is known that the total resistance of the suction pipeline is 1.5mH2O, the local atmospheric pressure is 9.81×104Pa, and the dynamic pressure head of the liquid in the suction pipeline is negligible. Try to calculate:

(1) Pump installation when delivering 20℃ clean water;

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

Solution: (1) The 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 in line with the experimental conditions when the pump leaves the factory, so the installation height of the pump is Hg=5.7-0-1.5=4.2m.

(2) The installation height of the pump when delivering 80℃ water

When delivering 80°C water, the Hs value in the pump sample cannot be used directly to calculate the installation height. The Hs time line must be converted by the following formula, namely

Hs1=Hs+(Ha-10.33)-(Hυ-0.24)

It is known that Ha=9.81×104Pa≈10mH2O, and the saturated vapor pressure of water at 80℃ is found to be 47.4kPa from the appendix.

Hv=47.4×103Pa=4.83mH2O

Hs1=5.7+10-10.33-4.83+0.24=0.78m

Substitute the value of Hs1 into the formula to obtain the installation height

Hg=Hs1-Hf0-1=0.78-1.5=-0.72m

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

 

Example: The NPSH of a brand of MD450-95X8 multistage centrifugal pump is [NPSHr]=3.29m. If you want to work at an altitude of 500m, the highest water temperature in summer in this area is 40℃. If the head loss of the suction pipe is 1m, Then the local operating geometric installation height [Hg] of the pump is calculated as follows:

Suppose: the surface pressure of water absorption is the local atmospheric pressure, and the atmospheric pressure head at an altitude of 500m is 9.7m from the table; when the water temperature is 40℃, the saturated vapor pressure head of the water is 0.752m;

Calculated: [Hg]=9.7-0.752-3.29-1=4.658m

Previous post: Solutions to the high temperature of the motor bearing of the pump motor in addition to salt water in the power plant

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