Tips: Centrifugal pump cavitation phenomena and installation height

Cavitation of Centrifugal Pumps Cavitation of centrifugal pumps means that the liquid being transported is partly vaporized due to the pressure of the saturated vapor pressure at or below the pump inlet (actually at the inlet of the vanes) at the delivery temperature, causing the pump Produce noise and vibration, serious, the 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 allows the vacuum suction height Hs refers to the pressure p1 at the pump inlet allows the maximum degree of vacuum and the actual allow suction vacuum height Hs value is not calculated according to the value, but by the pump Manufacturer experimentally measured value, this value is attached to the pump sample for user investigation. It should be noted that the Hs value given in the pump sample is the value of the water used as the working medium, the operating conditions of 20 ° C and the pressure of 1.013 × 105Pa when the operating conditions and working medium are not the same, need to be converted. (1) Conveying clear water, but the operating conditions and experimental conditions are different and can be converted into Hs1 = Hs + (Ha-10.33) - (Hυ-0.24) according to the following formula. The conditions are different, the need for two-step conversion: The first step according to the formula will be detected by the pump sample Hs1; The second step according to the following formula Hs2 Hs2 NPSH Δh For the pump, calculate the installation height When using Naphthic allowance Δh to calculate, that is, use Naphthenic residue Δh from the pump 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 20% water delivery; (2) to the delivery of 80 ℃ water pump installation height. Solution: (1) The installation height of the pump when delivering 20 ℃ fresh water is known: Hs = 5.7m Hf0-1 = 1.5mu12 / 2g≈0 The local atmospheric pressure is 9.81 × 104Pa, which basically agrees with the experimental conditions of the pump factory Pump installation height Hg = 5.7-0-1.5 = 4.2m. (2) Installation height of pump when conveying water at 80 ℃ When installing water at 80 ℃, the installation height can not be directly calculated by using the Hs value in the pump sample. Hs1 = Hs + (Ha-10.33) - (Hυ-0.24) Known Ha = 9.81 × 104Pa≈10mH2O, from the appendix to find the saturated vapor pressure of water at 80 ℃ 47.4kPa. Hv = 47.4x103Pa = 4.83mH2O Hs1 = 5.7 + 10-10.33-4.83 + 0.24 = 0.78m The Hs1 value is substituted into the equation to find the installation height Hg = Hs1-Hf0-1 = 0.78-1.5 = -0.72m Hg is negative Value, indicating that the pump should be installed below the tank level, at least 0.72 m below the liquid level.