Power 1

Pump Specific Speed (Ns is the US system), can be a rough indicator of the power flow relationship of a pump. Differences in pump design and construction affect behavior, therefore only the pump manufacturer's published power curve is authoritative.

At the low end of the Specific Speed range are the radial flow impellers, from about Ns = 400 to about Ns = 1800. These pumps generally always have a direct relationship of power to flow rate. Increased flow rate increases the load placed on the driver, and therefore an electric motor would consume more power as the flow rate increased.

At the high end of the Specific Speed range are the Axial Flow impellers, which are more properly propellers, with Ns values > 10000. These pumps also have a direct relationship of power to flow rate, but just the opposite of Radial Flow impellers. That is, power requirements for these pumps decreases as flow increases.

The 3 pump graphs below reveal the different relationship of power to flow rate based on the type of pump as generally indicated by Pump Specific Speed (Ns).

The Green BHP (Brake Horsepower) curve shows how much power is required to maintain the impeller rotation at a constant speed at any given flow rate, and therefore how much current (amperage) the motor will draw. BHP is indicated on the right hand scale, GPM is indicated on the bottom scale.

The Black curve line indicates Head at any given flow rate. Head is indicated in feet on the left scale, flow is indicated in GPM (gallons per minute) on the bottom scale.

Observe that BHP increases as the Flow Rate increases. This curve is prototypical (common) for pumps less then 30 hp used for irrigation , pressure boost to tall buildings, and pressure maintenance jockey pumps. Power required to operate this pump is: 2.3 BHP @ 15 gpm, 4.9 BHP @ 88 gpm, a total difference of 215%.

This type of pump can make a variable speed drive (VSD) unnecessary and a waste of money, if efficiency is the highest priority. A VSD, installed on this pump, could easily consume more power than a constant speed control with a control valve. Constant speed pressure control valves open and close to restrict flow as they maintain constant downstream pressure, and in so doing , current consumption rises and falls on this type of pump, without the complexity and expense of a VSD.

Even though the pump is held at a constant speed at all times, motor current rises and falls as flow rate increases and decreases, producing a power savings for the owner without any extra equipment and the expense or complication of a VSD. However, only analyses of the system operating conditions by an expert can determine if a VSD would be beneficial or not.

Note

Specific Speed for this pump is Ns = 844 (Radial Flow Impeller), therefore energy transfer is dominated by centrifugal force with less diffusion component.

Observe that BHP first falls slightly and then rises slightly from low flow to high flow, but BHP does not change much in relation to flow. Power required to operate this pump at a constant speed is: 17 BHP @ 150 gpm, 14.9 BHP @ 500 gpm, and 17 BHP @ 800 gpm, a total difference over the entire flow range of only 14%.

Note

Specific Speed for this pump is Ns = 3112 (Francis Vane Impeller), energy transfer is accomplished by both centrifugal force & diffusion.

Observe that BHP decreases as flow increases. Power required to operate this pump is: 65 BHP @ 2,750 gpm, and 44 BHP @ 4,750 gpm, a total difference of 148% !

This type of pump is often used for high volume low pressure applications such as flood irrigation, water features, and water transfer.

We included the manufacturer's recommended operating range or window (yellow striped area), on the graph for this pump. We did this because unlike lower specific speed pumps, pumps in this specific speed range can be especially vulnerable to operation outside the operating window. Operating this pump outside of the window can cause severe vibration, especially at flow rates below the window. In addition to severe vibration problems, when this pump is operated below the recommended flow range motor current quickly rises quickly resulting in the driver (motor or engine) being quickly overloaded and possibly damaged quickly.

Note

Specific Speed for this pump is Ns = 11226, (Axial Flow Impeller), energy transfer is heavily dominated by the diffusion component with less centrifugal component.

The 3 graphs above illustrate the various ways in which pumps consume power at different flow rates. Pump specific speed roughly indicates how the pump consumes power compared to flow rate, but only the manufacturer's published power curve is authoritative.