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### Cavitation Centrifugal Pump Minimum Safe Suction Head Formulas and Calculator

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’. 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
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

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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