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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’.

Symbols used:
ρ = fluid density,
ρ_a = atmospheric pressure,
g = Acceleration due to gravity
ρ_v = vapor pressure of liquid at working
V_s = suction pipe velocity
h_f = friction head loss in suction pipe plus any other losses
H_a = pump head
σ_c = cavitation constant which depends on vane
Units should be kept consitant..

Minimum safe suction head formula and calculator:

H_min = ρ_a / ρg - ( σ_c H_a + V_s² / 2g + h_f + ρ_v / ρg )

Range of σ_c:
Safe region σ_c>0.0005N_s^1.37, where N_s = specific speed.
Dangerous region σ_c< O.OOO22N_s^1.33
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

Related:

• Centrifugal Pump Cavitation Review
• Preventing Cavitation Centrifugal Pump
• Cavitation Review Centrifugal Pump
• Centrifugal Pump - Net Positive Suction Head