Defining Impeller Specific Speed

Specific speed is defined as the speed in revolutions per minute at which a geometrically similar impeller would operate if it were of such a size as to deliver one gallon per minute against one foot head.

Although that definition of specific speed may be a bit confusing, it is best to think of it as simply an index number that can predict certain pump characteristics. Pump designers use it to compare the efficiency of designs across a large range of pump models and types. Pump users reference it to know what efficiency can be expected from a particular pump design. The efficiency of pumps with the same specific speed can be compared, providing a starting point for the user or designer during the selection process. ( a benchmark for pump selection). There are four basic categories of impellers; Radial flow, Francis vane, Axial flow and Mixed flow.  Each category tends to have overlapping ranges of Impeller Specific Speeds.

Before we get into the range of each impeller type lets look at how Specific Speed is calculated. As stated before it is a non-dimensional design index , although the formula is the same between the metric system and the imperial system. The metric specific speed is different than the imperial specific speed and therefore care must be taken not to mix metric and imperial specific speeds. Imperial specific speed is normally represented as ”Ns” where the metric equivalent is sometimes represented as  “Nsm”. In either case care must be also taken to use the correct units. ( see below)

The general formula is:       

The “imperial units”  to use are as follows:

• N = Pump speed in RPM
• Q = Capacity in gpm at the best efficiency point
• H = Total head  in feet per stage at the best efficiency point

The “Metric units”  to use are as follows:

• N = Pump speed in RPM
• Q = Capacity in meters cubed per second at the best efficiency point
• H = Total head per stage in meters at the best efficiency point

The graph below shows how larger pumps with the same specific speed generally have higher efficiency ratings .I am showing this primarily to illustrate how the imperial specific speed shown on the lower horizontal axis differs from the values for the metric specific speeds in m3 & seconds shown on the upper horizontal axis.

You will find that the majority of graphs and charts currently published reference specific speed are based on the imperial system. This may be as a result of the metric system to date not being standardized on the units used to calculate the specific speed . Should you need to at any time convert from imperial to metric or visa versa shown below are the various conversion factors.

• To meters cubed/hour (m3/h) and meters: multiply Imperial (Ns) by 0.861.
• To liters/sec (l/s) and meters: multiply Imperial Ns by 0.437.
• To meters cubed/sec (m3/sec) and meters: multiply Imperial Ns by 0.017

Regarding the range of each type of impeller it is best defined by the illustration below.( note Ns values are in imperial units) From this illustration it is easy to see that there is no defined line between the different impeller designs but more so a blending from one category into another. At the lower end of the scale we have Radial flow impellers that develop head principally through centrifugal force and are generally low flow, high head designs. At the other end of the scale we have Axial flow impellers that develop head principally through axial forces and are generally high flow, low head designs.  An axial flow or propeller pump, with a specific speed of 10,000 or greater, generates it’s head exclusively through axial forces. In between we have Francis vane and Mixed flow impellers filling in the middle area  providing more of a compromise between the high head or high flow offered by radial or axial designs.

A simple and rough way of defining an impeller type is by comparing  outlet diameter to inlet .  As the specific speed increases, the ratio of the impeller outlet diameter, D2, to the inlet or eye diameter, D1, decreases.

Radial vane impellers generally have ratios of over 4, while this ratio becomes 1.0 for a true axial flow impeller.

Before I leave you today I should add, specific speed values of less than 500 are associated with positive displacement pumps.

I hope I have provided some definition of impeller specific speed.  If you are interested in how it can be used as a tool, stay tuned it will be covered next time.

Cheers

RJ