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Cavitation Centrifugal Pump Minimum Safe Suction Head Formulas and Calculator

Pump Engineering and Design

If the suction pressure of a pump falls to a very low value, the fluid may boil at a low pressure region (e.g. at the vane inlet). A formula is given for the minimum suction head, which depends on the fluid density and vapor pressure at the operating temperature and also on the ‘specific speed’.

Cavitation Centrifugal Pump and Minimum safe suction head

Symbols used:
ρ = fluid density,
ρa = atmospheric pressure,
g = Acceleration due to gravity
ρv = vapor pressure of liquid at working
Vs = suction pipe velocity
hf = friction head loss in suction pipe plus any other losses
Ha = pump head
σc = cavitation constant which depends on vane
Units should be kept consitant..

Minimum safe suction head formula and calculator:

Hmin = ρa / ρg - ( σc Ha + Vs2 / 2g + hf + ρv / ρg )

Range of σc:
Safe region σc>0.0005Ns1.37, where Ns = specific speed.
Dangerous region σc< O.OOO22Ns1.33
A ‘doubtful zone’ exists between the two values.

Alternative cavitation formula

Net Positive Suction Head Available

NPSH-A = (((Pe – Pv)/ρ) x 10.2) + Hz – Hf + (V² / 2g)

where:

Pe =  Absolute pressure in pumped vessel (bar)
Pv =  Vapour pressure of fluid (bar)
ρe =  Density of fluid (kg/dm³)
Hz =  Minimum fluid level above pump (m)
         (negative term if below pump)
Hf  =  Friction losses in suction side pipework (m)
Ve =  Fluid velocity in pump flange (m/s)
g    =  Acceleration due to gravity (9.81m/s²)

Reference:

Mechanical Engineering Data Book, Butterworth Heinemann, Carvill, James 2003

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