Centrifugal pump installation height and cavitation phenomenon

First, the centrifugal pump installation height Hg allows the vacuum suction height Hs refers to the pressure p1 at the pump inlet allows the maximum vacuum and the actual allowable 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 above formula will be detected by the pump sample Hs1; The second step by the following formula will be converted into H's Hg NPSH Δh For the pump, calculate the installation height with steam Etch margin Δh to calculate, that is, the use of Naphtanean margin Δh extracted from the pump sample, the 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 of 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 = 4.2 m. (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 = 47.4 × 103 Pa = 4.83 mH2O Hs1 = 5.7 +10 -10.33-4.83 +0.24 = 0.78m Second, the phenomenon of centrifugal pump cavitation Centrifugal pump cavitation phenomenon refers to the liquid being transported due to the saturation vapor pressure at the delivery temperature Is equal to or lower than the pressure at the inlet of the pump (actually at the inlet of the vane) to partially vaporize, causing the pump to generate noise and vibration. In severe cases, the flow rate, pressure head and efficiency of the pump are significantly reduced. Obviously, Centrifugal pump normal operation is 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. Hs1 value into the formula 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 tank level, at least 0.72m lower than the liquid level.