Cavitation phenomenon and installation height computation
一 cavitation phenomenon
Centrifugal pump cavitation erosion refers to be transported liquids in conveying temperature saturation vapour pressure of pump is equal or lower than the entrance( actually is the entrance of the blade) pressure part cavitation, cause pump vaporizing generate noise and vibration, when serious, pump flow, pressure head and efficiency significantly, apparently, cavitation phenomenon is not allowed under centrifugal pump normal operation. The key to avoid cavitation phenomenon is pump installation height must be correct, especially when conveying high temperature easy volatile liquid, need tomore attention.
二 installation height
Allowed vacuum suction height Hs refers to the pressure p1 at the entrance to the maximum allowed vacuum, while the actual allowed vacuum suction height Hs value is not calculated according to the type of value, but by the pump manufacturer measured values, this value attached to pump samples for users to check in. Pay attention to the sample given Hs value is with water as working medium, operating conditions is 20 ℃ and pressure is 1.013 x 105Pa, when operating conditions and the working medium is not the same, need to undertake conversion.
(1) Conveying water, but the operation conditions and experimental conditions are different, can depend on next type conversion
Hs1＝Hs＋(Ha－10.33) － (Hυ－0.24)
(2) Conveying other liquid when conveying liquids and villain figures conditions simultaneously.by and experimental conditions is not same, need to undertake two step conversion: the first step in accordance with the above formula pump samples discovered Hs1, the second step according to next type convert Hs1 to H ΄ s
2 NPSH Δh
To oil pump, computational installation height uses NPSH Δ h to calculate, namely: use NPSH collected by Δ h pump samples, the value is also measured by 20 ℃ water. If conveying other liquids, also need to undertake adjustment, combed through the related books.
From safe angle consideration, pump actual installation height value is less than calculation values. Again, when calculating the employed Hg is negative, the pump suction location should under storage tank of liquid level.
Example 2-3 centrifugal pump from sample check on get allow smoke vacuum height Hs = 5.7 m. Known all the inhaled pipeline resistance is 1.5mH2O, local atmospheric pressure is 9.81 x 104Pa, liquid in the suction pipe dynamic pressure head can be ignored. Try calculation:
(1) pump installation conveying 20℃ clean water
(2) Change to pump installing height conveying 80℃ water
(1) installing height conveying 20℃ clean water
known：Hs=5.7m
Hf0-1=1.5m
u12/2g≈0
Local atmospheric pressure is 9.81x104Pa, consistent with the pump manufacturer experimental conditions, so pump installation height is
Hg=5.7-0-1.5=4.2 m。
(2)conveying 80℃ water pump height
Conveying 80 ℃ water, cannot directly use pump Hs value calculation sample height of installation, need to according to the following formula to convert to Hs, namely
Hs1＝Hs＋(Ha－10.33) － (Hυ－0.24)
known Ha=9.81×104Pa≈10mH2O，By appendix check 80 ℃ water saturation vapour pressure is
Hv=47.4×103 Pa＝4.83 mH2O
Hs1＝5.7+10－10.33－4.83+0.24=0.78m
put Hs1 in formula to calculate installing height
Hg=Hs1－Hf0-1=0.78－1.5=－0.72m
Hg is negative, means pump should be installed below the pool liquid surface, at least lower than liquid surface 0.72 m.
