Specific Suction Speed for Pumps

Specific Suction Speed may be used to determine what general pump design to use for maximum efficiency

Suction Specific Speed may be used to determine what pump geometry - radial, mixed flow or axial - to use for maximum efficiency and prevent cavitation. Suction Specific Speed is commonly used as a basis for estimating the safe operating range of a pump.

Suction Specific Speed is dimensionless and are expressed as

N_ss = ω q^1/2 / NPSH_a^3/4 (1)

where

N_ss = Suction Specific Speed

ω = pump shaft rotational speed (rpm)

q = flow rate capacity (m³/h, l/s, m³/min, US gpm, British gpm) at Best Efficiency Point BEP

NPSH_a = available Net Positive Suction Head (m, ft)

Specific Suction Speed can be compared with Specific Speed but instead of using the total head of the pump, the available Net Positive Suction Head N.P.S.H._a is used.

As a rule of thumb the Specific Suction Speed should be below 8500 (calculated with US gpm) to avoid cavitation.

Online Specific Suction Speed Pump Calculator

The calculator below can used to calculate pump specific speed:

ω - pump shaft rotational speed (rpm)

q -  flow rate (m³/h, l/s, m³/min, US gpm, British gpm)

NPSH_r - required Net Positive Suction Head (m, ft)

Note! When comparing pumps and their documentation be aware of the units used.

Suction Specific Speed - S_s - is useful in determining the required operating conditions on the suction side of the pump. For geometrically similar pumps the S_s have the same value.

Empirical studies indicates that higher Suction Specific Speed is, the narrower is the safe operating range from its Best Efficiency Point (BEP). Pump models with S_sd (US units) less than 11000 have more stable operation and are more reliable.

Convert between US units (gpm) and Metric units (m³/h)

• N_ss (US) = 1.63 N_ss (metric)
• N_ss (Metric) = 0.614 N_ss (US)
• N_ss (Metric) = 0.67 N_ss (British)

Example - Specific Suction Speed

The available Net Suction Head - NPSH_a - of a pump is determined to be 20 ft. With a rotational speed of 1750 rpm and a flow rate of 500 US gpm the Specific Suction Speed can be calculated as

N_ss = 1750 (rpm) 500 (US gpm)^1/2 / 20 (ft)^3/4

    = 4138

Well below the limit 8500 to avoid cavitation.

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