Specific Suction Speed can be used to determine what general pump design to use for maximum efficiency
Specific Suction Speed can be used to determine what pump geometry - radial, mixed flow or axial - to use for maximum efficiency and to prevent cavitation. Suction Specific Speed is commonly used to estimate pumps safe operating ranges.
Suction Specific Speed is dimensionless and can be expressed as
Nss = ω q1/2 / NPSHr3/4 (1)
Nss = Suction Specific Speed
ω = pump shaft rotational speed (rpm)
q = flow rate capacity (m3/h, l/s, m3/min, US gpm, British gpm) at the Best Efficiency Point (BEP)
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 suction speed:
Note! When comparing pumps and their documentation - be aware of the units used.
Suction Specific Speed - Ss - is useful in determining the required operating conditions on the suction side of a pump. Ss have the same value for geometrically similar pumps.
Empirical studies indicates that safe operating ranges from Best Efficiency Points (BEP) are more narrow at higher Suction Specific Speeds. Pump designs with Ssd (US units) less than 11000 operates more stable and are more reliable.
Convert between Imperial units (gpm) and Metric units (m3/h, l/s)
- Nss (US gpm) = 1.63 Nss (metric l/s) = 0.86 Nss (metric m3/h)
- Nss (Metric l/s) = 0.614 Nss (US gpm)
- Nss (Metric l/s) = 0.67 Nss (British gpm)
Example - Specific Suction Speed
The available Net Suction Head - NPSHa - of a pump is determined to be 20 ft. With rotational speed 1750 rpm and flow rate 500 US gpm the Specific Suction Speed can be calculated as
Nss = (1750 rpm) (500 gpm)1/2 / (20 ft)3/4
Well below the limit 8500 to avoid cavitation.
Double Suction Type Pumps
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.
- en: suction specific speed pumps
- es: bombas de velocidad específica de succión
- de: Saugzahl Pumpen