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First, the centrifugal pump cavitation phenomenon
Centrifugal pump cavitation phenomenon refers to the liquid being transported as a result of the saturated vapor pressure at the delivery temperature is equal to or lower than the pressure at the pump inlet (actually at the inlet of the blade) and partially vaporized, causing the pump to produce noise and vibration, serious, Pump flow, pressure head and the significant decline in efficiency, obviously, cavitation phenomenon is the normal operation of centrifugal pumps are not allowed to happen. The key to avoid cavitation phenomenon is to install the pump to the correct height, especially when transporting volatile liquids with higher temperatures.
Second, the installation of centrifugal pump height Hg
Allowable suction height Hs refers to the maximum degree of vacuum that the pressure p1 at the pump inlet allows
The actual allowable suction height, Hs, is not a value calculated according to the equation, but a value determined experimentally by the pump manufacturer, which is attached to the pump sample for user investigation. It should be noted that the value of Hs given in the pump sample is the value of water used as working medium, operating conditions of 20 ℃ and pressure of 1.013 × 105Pa, when the operating conditions and working medium are different, need to be converted.
(1) Convey water, but the operating conditions and experimental conditions are different, according to the following conversion
Hs1 = Hs + (Ha-10.33) - (HÏ…-0.24)
(2) Transfer other liquids When the conditions of liquid and villain are different from the experimental conditions, two-step conversion is required: the first step will be Hs1 detected in the pump sample according to the above formula; the second step will be Hs1 Converted into Hs
2 NPSH Δh
For the oil pump, calculate the installation height using the NPSH Δh to calculate, that is
Naphthenic residual amount Δh by the pump in the sample, its value is also measured with 20 ℃ water. If you transfer other liquids, also need to be corrected, detailed investigation of the books.
From a safety point of view, the actual installed pump height should be less than the calculated value. Also, when the calculated Hg is negative, it indicates that the suction port of the pump should be below the level of the sump.
Example 2-3 A centrifugal pump from the sample was allowed to suction on the vacuum height Hs = 5.7m. It is known that the total resistance of the suction line is 1.5 mH2O and the local atmospheric pressure is 9.81 × 10 4 Pa. The dynamic head of liquid in the suction line is negligible. Try to calculate
(1) pump installation at 20 ℃ water delivery;
(2) Change the pump installation height to 80 ° C.
Solution: (1) delivery of 20 ℃ water pump installation height
Known: Hs = 5.7m
Hf0-1 = 1.5m
u12 / 2g≈0
The local atmospheric pressure is 9.81 × 104Pa, basically in line with the experimental conditions of the pump factory, so the installation height of the pump is
Hg = 5.7-0-1.5 = 4.2m.
(2) The installation height of pump when conveying 80 ℃ water
When conveying 80 ℃ water, you can not directly use the Hs value in the pump sample to calculate the installation height.
Hs1 = Hs + (Ha-10.33) - (HÏ…-0.24)
Ha = 9.81 × 104Pa ~ 10mH2O is known, the saturation vapor pressure of water at 80 ℃ is 47.4kPa.
Hv = 47.4 × 103 Pa = 4.83 mH2O
Hs1 = 5.7 + 10-10.33-4.83 + 0.24 = 0.78m
Substitute the Hs1 value for 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 tank level, at least 0.72 m below the liquid level.