Cavitation and installation height of centrifugal pump
First, the cavitation phenomenon of centrifugal pump
The cavitation phenomenon of the centrifugal pump means that the liquid to be delivered is partially vaporized due to the pressure of the saturated vapor 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 noise and vibration of the pump. The flow rate, head pressure 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 that the pump should be installed at the correct height, especially when transporting volatile liquids with high temperatures.
Second, the installation height of the centrifugal pump Hg
The allowable suction vacuum height Hs refers to the maximum vacuum that the pressure p1 at the pump inlet can allow, and the actual allowable vacuum height Hs value is not a value calculated according to the formula, but is a value experimentally determined by the pump manufacturer. Values ​​are attached to the pump sample for user review. It should be noted that the Hs value given in the pump sample is the value when the water is used as the working medium, the operating conditions are 20 ° C and the pressure is 1.013 × 105 Pa. When the operating conditions and working medium are different, the conversion is required.
(1) Conveying clean water, but the operating conditions are different from the experimental conditions, and can be converted according to the following formula
Hs1=Hs+(Ha-10.33) - (HÏ…-0.24)
(2) Conveying 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 first step is based on the Hs1 detected in the pump sample; the second step is as follows: Hs1 Converted to H?s
2 NPSH Δh
For the oil pump, the calculation of the installation height is calculated by using the NPSH Δh, that is, the NPSH is taken from the oil pump sample, and the value is also measured with 20 °C water. If other liquids are transported, corrections are also required. 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 indicates that the suction port position of the pump should be below the liquid level of the sump.
Example 2-3 A centrifugal pump was found to allow the vacuum height Hs = 5.7 m from the sample. It is known that the total resistance of the suction line is 1.5 mH2O, the local atmospheric pressure is 9.81 x 104 Pa, and the dynamic head of the liquid in the suction line is negligible. Trial calculation:
(1) Installation of the pump when conveying 20°C clean water;
(2) The installation height of the pump when transporting water at 80 °C.
Solution: (1) Installation height of pump when conveying 20°C 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 leaves the factory, so the installation height of the pump is
Hg = 5.7-0 - 1.5 = 4.2 m.
(2) Mounting height of the pump when transporting 80°C water
When transporting water at 80 °C, the installation height cannot be calculated directly from the Hs value in the pump sample.
Hs1=Hs+(Ha-10.33) - (HÏ…-0.24)
It is known that Ha = 9.81 × 104 Pa ≈ 10 mH 2 O, and the saturated vapor pressure of water at 80 ° C was found to be 47.4 kPa by 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 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 pool level, at least 0.72m below the liquid level.
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