Centrifugal pump cavitation phenomenon and installation height

1. Cavitation Erosion of Centrifugal Pumps The cavitation phenomenon of a 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 pumping. Noise and vibration are generated. In severe cases, the flow rate, pressure head, and efficiency of the pump are significantly reduced. 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.

Second, the installation height of centrifugal pump Hg

The allowable suction height Hs is the maximum vacuum to which the pressure p1 at the inlet of the pump can be allowed, while the actual allowable suction height Hs is not the value calculated according to the formula, but the 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 the operating conditions and experimental conditions are different, can be converted according to 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. Converted to H ́s

2 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 2-3 A centrifugal pump was checked from the sample 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 is known: Hs=5.7m

Hf0-1=1.5m

U12/2g≈0

The local atmospheric pressure is 9.81×104 Pa, which is basically in line with the experimental conditions of the pump. Therefore, the installation height of the pump is Hg=5.7-0-1.5=4.2 m.

(2) When pumping 80°C water, the installation height of the pump can not be calculated directly from the Hs value in the pump sample when delivering 80°C water. The Hs conversion should be performed as follows: 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=4.83 mH2O

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

Substituting the Hs1 value into the equation to obtain the installation 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.