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  • Available NPSH and Required NPSH
  • (1)Net Positive Suction Head (NPSH)
    Liquid vaporization within a pump intake is called “cavitation”. Cavitation reduces a pump performance and will damage the pump. To prevent cavitation, the pressure reduction in the pump intake must be compared to the vapor pressure entering the pump to determine whether the liquid will vaporize.
    The amount of excess pressure above the vaporization pressure is called net positive suction head (NPSH).
    The Net Positive Suction Head NPSH can be expressed as the difference between the suction head and the liquids vapor head and expressed like
    NPSH = hs – hv = ps / γ + vs2 / 2 g - pv / γ
    Where
    hs - suction head close to the impeller
    hv - vapor head
    ps - static pressure in the fluid close to the impeller
    γ - specific weight of the fluid
    vs - velocity of fluid
    g - acceleration of gravity
    pv - vapor pressure
    (2)Available Net Positive Suction Head (NPSHa)
    NPSHa refers to the NPSH determined by pump system (based on liquids at rated flow rate and at normal pumping temperature), also is known as efficient NPSH. The value of NPSHa is determined by parameters of pipeline system and flow rate in pipeline and has nothing with pump structure.
    (3)Net Positive Sction Head Required (NPSHr)
    The NPSHr, called as the Net Positive Suction Head as required by the pump in order to prevent cavitation for safe and reliable operation of the pump.
    NPSHr (which is calculated by liquid column in meter) is determined by pump manufacturer according to test (The test is usually carried out with 20℃ water at rated flow rate).
    (4)Pump installation height
    Pump installation height also is known as liquid suction height of pump, which refers to the height difference from pump reference surface to inhaled liquid surface.
    (5)Cavitation curve
    Both NPSHa and NPSHr change with the changes of flow rate. Generally speaking, the value of NPSHr increases with the increase of flow rate and NPSHa decreases with the increase of flow rate, which are shown as follows. Pump performance curve provided by pump manufacturer generally includes NPSHr-Q curve.
    (6)NPSHa safety margin S for centrifugal pump
    In order to avoid cavitation, it is required that NPSHa be greater than or equal to NPSHr. That means that a safety margin “S” for centrifugal pump is required to meet the requirements: NPSHa-NPSHr≥S.
    For common centrifugal pumps, S=0.6~1.0m. According to the regulation in SH/T 3139 and SH/T 3140, the NPSHr for horizontal pumps at rated point should be lower than NPSHa by 0.6m.
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