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Cavitation in Centrifugal Pumps and Installation Height I. Cavitation in Centrifugal Pumps Cavitation in centrifugal pumps occurs when the liquid being pumped vaporizes to some extent, as the saturated vapor pressure of that liquid at the pumping temperature is equal to or lower than the pressure at the pump inlet (actually at the inlet to the impellers). This phenomenon causes noise and vibration in the pump; in severe cases, it leads to a significant decrease in the pump’s flow rate, head, and efficiency. Clearly, cavitation is not something that should occur during the normal operation of a centrifugal pump. The key to avoiding cavitation is to ensure the correct installation height of the pump, especially when transporting volatile liquids at high temperatures. II. Installation height Hg of the centrifugal pump The allowable suction vacuum height Hs refers to the maximum degree of vacuum that can be achieved at the pump inlet pressure p1. The actual value of the allowable suction vacuum height Hs is not calculated using formulas, but is determined through experiments by the pump manufacturer; this value is provided in the pump’s documentation for users’ reference. It should be noted that the Hs value given for the pump samples is based on clean water as the working medium, under operating conditions of 20°C and a pressure of 1.013×105 Pa; conversions are required when the operating conditions or the working medium differ. (1) For transporting clean water, but when the operating conditions differ from those in the experiment, conversion can be carried out using the following formula: Hs1 = Hs + (Ha – 10.33) – (Hυ – 0.24). (2) When transporting other liquids, and when both the properties of the liquid being transported and the operating conditions differ from those in the experiment, two steps of conversion are required: the first step involves using the formula above to determine Hs1 based on the values provided for the pump ； In the second step, Hs1 is converted to H΄s using the following formula. 2. Cavitation head Δh: For oil pumps, the cavitation head Δh is used to calculate the installation height. The value of Δh is obtained from the specifications for the oil pump, and it is also determined using clean water at 20°C. If other liquids are to be transported, corrections are also required; consult relevant books for details. From a safety perspective, the actual installation height of the pump should be less than the calculated value. Furthermore, when the calculated Hg is negative, it indicates that the pump’s suction port should be located below the liquid level in the tank. Example 2-3: For a certain centrifugal pump, the allowable suction vacuum height Hs as determined from the specifications is 5.7 m. It is known that the total resistance of the suction pipeline is 1.5 mH2O, the local atmospheric pressure is 9.81×104 Pa, and the dynamic head of the liquid in the suction pipeline can be ignored. Try to calculate: (1) Pump installation for transporting water at 20°C ； (2) Changed to the pump installation height for transporting 80°C water. Solution: (1) Installation height of the pump when transporting water at 20°C. Given: Hs = 5.7 m, Hf0-1 = 1.5 m, u12/2g ≈ 0. The local atmospheric pressure is 9.81×10^4 Pa, which is roughly consistent with the conditions under which the pump was tested at the factory; therefore, the installation height of the pump is Hg = 5.7 – 0 – 1.5 = 4.2 m. (2) Installation height of the pump when transporting water at 80°C When transporting water at 80°C, it is not possible to use the Hs value given in the pump specifications to calculate the installation height; instead, Hs must be adjusted using the following formula: Hs1 = Hs + (Ha – 10.33) – (Hυ – 0.24). Given that Ha = 9.81×10^4 Pa ≈ 10 mH2O, the saturated vapor pressure of water at 80°C is 47.4 kPa, as stated in the appendix. Hv=47.4×103 Pa＝4.83 mH2O Hs1＝5.7+10－10.33－4.83+0.24=0.78m The installation height is determined by substituting the value of Hs1 into the formula: Hg=Hs1－Hf0-1=0.78－1.5=－0.72m A negative value for Hg indicates that the pump should be installed below the water level in the tank, at least 0.72m below it.