Centrifugal pump installation height and cavitation phenomenon

First, the installation height of the centrifugal pump Hg

Allowable suction vacuum height Hs refers to the maximum vacuum that the pressure p1 at the pump inlet can be allowed to reach

The actual allowable suction height Hs value is not a value calculated according to the formula, but an experimentally determined value by the pump manufacturer. This value is attached to the pump sample for the user to use. It should be noted that the Hs value given in the pump sample is the value of using clear water as the working medium, operating conditions at 20°C and pressure of 1.013 × 105 Pa. When the operating conditions and working media are different, conversion is required.

(1) Clean water, but different operating conditions and experimental conditions, can be converted by the following formula

Hs1=Hs+(Ha-10.33)-(HÏ…-0.24)

(2) Delivery of other liquids When the conditions of the liquid to be transported and the villains are different from the experimental conditions, two steps are required for conversion: the first step is to detect Hs1 in the pump sample according to the above formula; the second step is to use Hs1 according to the following formula. Convert to H's

NPSH Δh

For the oil pump, the NPSH Δh is used to calculate the installation height. That is, the NPSH Δh is taken from the oil pump sample, and the value is also measured with 20°C clear water. If other fluids are transported, corrections must also be made. Check the relevant books.

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

Example A centrifugal pump from the sample was found to allow suction vacuum height Hs = 5.7m. It is known that the total resistance of the suction line is 1.5mH2O, and the local atmospheric pressure is 9.81 x 104Pa. The dynamic pressure of the liquid in the suction line can be ignored. Try to calculate:

(1) Pump installation when delivering water at 20°C;

(2) The installation height of the pump when it is transferred to 80°C water.

Solution: (1) Mounting height of the pump when delivering water at 20°C

Known: Hs=5.7m

Hf0-1=1.5m

U12/2g≈0

The local atmospheric pressure is 9.81×104 Pa, which is basically consistent with the experimental conditions at the time of the pump's delivery. Therefore, the installation height of the pump is Hg=5.7-0-1.5=4.2 m.

(2) Mounting height of the pump when delivering 80°C water

When transporting water at 80°C, the installation height cannot be calculated directly from the Hs value in the pump sample. The Hs must be converted according to the following formula: Hs1=Hs+(Ha-10.33)-(Hυ-0.24)

It is known that Ha=9.81×104Pa≈10mH2O, and the saturated vapor pressure of 80°C water is 47.4kPa as found in the Appendix.

Hv=47.4×103 Pa=4.83mH2O

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

Second, the cavitation phenomenon of centrifugal pump

The cavitation phenomenon of the centrifugal pump means that the liquid to be transported is partially vaporized due to the saturated vapor pressure at the delivery temperature being equal to or lower than the pressure at the inlet of the pump (actually at the inlet of the blade), causing the pump to generate noise and vibration. The flow rate, pressure head and efficiency of the pump have dropped significantly. Obviously, cavitation is not allowed in the normal operation of the centrifugal pump.

The key to avoiding cavitation is the correct installation height of the pump, especially when transporting volatile liquids with high temperatures.

Substituting the Hs1 value into the equation to obtain the mounting height

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

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