Pump suction height and causes of cavitation

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To prevent cavitation in the pump, it is necessary to ensure that the liquid at the inlet of the pump impeller has an excess energy per unit weight that exceeds its vaporization pressure. Please see the following brief explanation: When the suction height of a centrifugal pump is too great and the liquid temperature is high, the pressure at the suction inlet becomes less than or equal to the saturated vapor pressure of the liquid. Under such conditions, the liquid boils and vaporizes at the pump inlet, creating a space filled with steam inside the pump casing. As the pump rotates, these bubbles move into areas of higher pressure; due to the pressure difference, they burst and re-condense. At the moment of condensation, the particles collide with each other, generating very high local pressures. If these bubbles burst and condense near the metal surface, the liquid particles act like countless small projectiles, continuously striking the metal surface and causing cracks in it; in some cases, local peeling may occur as well, resulting in a honeycomb-like pattern on the surface of the impeller. Meanwhile, certain reactive gases present in the bubbles, such as oxygen, enter the cracks in the metal surface. The heat released during bubble condensation facilitates chemical corrosion of the metal, and this phenomenon is known as cavitation. Cavitation in centrifugal pumps refers to the partial vaporization of the liquid being pumped, as the saturated vapor pressure at the pumping temperature is equal to or lower than the pressure at the pump inlet (actually at the inlet to the impellers). This leads to noise and vibration in the pump; in severe cases, it results in 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. By substituting the value of Hs1 into the formula, the installation height is obtained as Hg = Hs1 – Hf0 – 1 = 0.78 – 1.5 = -0.72 m. A negative value for Hg indicates that the pump should be installed below the water level of the tank, at least 0.72 m below it. When cavitation occurs, the pump generates noise and vibration, which causes a sharp decline in its head, flow rate, and efficiency. It also accelerates the wear of the materials and shortens the service life of the components. Therefore, it is necessary to limit the suction height of the pump to prevent excessive vaporization of the liquid and avoid the occurrence of cavitation. The suction height of a centrifugal pump refers to the height between the center of the pump’s suction inlet and the liquid surface in the reservoir. Assuming an absolute vacuum at the inlet of the impeller, zero resistance in the suction pipeline, and a standard atmospheric pressure at the liquid surface, the theoretical geometric height would be 10.33 meters. However, due to various resistance losses in the pump’s suction pipeline, the fact that a complete vacuum cannot be achieved at the impeller inlet, as well as the required net positive suction head at the pump inlet, the suction height of typical centrifugal pumps does not exceed 4–5 meters. 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 allowable suction vacuum height Hs value is not the one calculated from the formula, but rather a value determined through experiments by the pump manufacturer; this value is included in the pump manual for users’ reference. It should be noted that the Hs value given in the pump specifications applies when clean water is used 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 done using the following formula: =Hs＋(Ha-10.33)-(Hυ-0.24). 2) For transporting other liquids, when both 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 with Hs1 values obtained from pump specifications ； In the second step, Hs1 is converted to H’s using the following formula. For oil pumps, the net positive suction head Δh is used to calculate the installation height; this value is obtained from the oil pump specifications, 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. Suction lift = Standard atmospheric pressure (10.33 meters) – NPSH – Safety margin (0.5 meters). Standard atmospheric pressure can create a vacuum in the pipeline of 10.33 meters. For example: If a pump requires a net positive suction head of 4.0 meters, what is the suction lift Δh? Solution: Δh = 10.33 – 4.0 – 0.5 = 5.83 meters ； 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 inlet should be located below the liquid level in the tank. For example, it is found from the specifications of a certain centrifugal pump that the allowable suction vacuum height Hs 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) Change to the pump installation height when conveying 80°C water. Solution: When transporting water at 20°C, the pump installation height is known: Hs = 5.7 m, Hf0–1 = 1.5 μ, and 12/2g ≈ 0. The local atmospheric pressure is 9.81×10^4 Pa, which is roughly consistent with the experimental conditions under which the pump was manufactured; therefore, the pump’s installation height is Hg = 5.7 – 0 – 1.5 = 4.2 m. 3) 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 provided in the pump’s specifications to calculate the installation height; instead, Hs must be adjusted using the following formula: Hs1 = Hs + (Ha – 10.33) – (Hv – 0.24). It is known that Ha = 9.81×10^4 Pa ≈ 10 mH2O. The saturated vapor pressure of water at 80 degrees Celsius is 47.4 kPa, as stated in the appendix. Hv=47.4×103Pa =4.83mH2O hs1 =5.7+10-10.33-4.83+0.24 =0.78m