Pumps - Specific Suction Speed

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[h=Specific Suction Speed may be used to determine what general pump design to use for maximum efficiency]2[/h]
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_r^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_r = pump NPSH required at best efficiency point (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.

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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.

[h=Convert between Imperial units (gpm) and Metric units (m³/h, l/s)]3[/h]

• N_ss (US gpm) = 1.63 N_ss (metric l/s) = 0.86 N_ss (metric m³/h)
• N_ss (Metric l/s) = 0.614 N_ss (US gpm)
• N_ss (Metric l/s) = 0.67 N_ss (British gpm)

[h=Example - Specific Suction Speed]3[/h] 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) (500gpm)^1/2 / (20 ft)^3/4
= 4138

Well below the limit 8500 to avoid cavitation.
[h=Double Suction Type Pumps]3[/h] For a double suction pump the flow at the inlet is divided by two. Using a double suction pump is one way of meeting system NPSH and obtaining a higher head.