# Pumps - Specific Suction Speed

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

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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

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 (m^{3}/h, l/s, m^{3}/min, US gpm, British gpm) at the Best Efficiency Point (BEP)

NPSH_{r}= Required Net Positive Suction Head for the pump at the 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 Required Net Positive Suction Head (NPSH_{r})_{} 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 suction speed:

**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 a pump.

*S*have the same value for geometrically similar pumps.

_{s}Empirical studies indicates that safe operating ranges from Best Efficiency Points (BEP) are more narrow at higher Suction Specific Speeds. Pump designs with *S _{sd}* (US units) less than

*11000*operates more stable and are more reliable.

### Convert between Imperial units *(gpm)* and Metric units *(m*^{3}/h, l/s)

^{3}/h, l/s)

*N*_{ss}(US gpm) = 1.63 N_{ss}(metric l/s) = 0.86 N_{ss}(metric m^{3}/h)*N*_{ss}(Metric l/s) = 0.614 N_{ss }(US gpm)*N*_{ss}(Metric l/s) = 0.67 N_{ss }(British gpm)

### Example - Specific Suction Speed

The available Net Suction Head - NPSH_{a} - 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

N_{ss}= (1750 rpm) (500^{ }gpm)^{1/2}/ (20 ft)^{3/4}

= 4138

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.

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