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Pumps & Systems, January 2008

Q.  Most centrifugal pumps have a horsepower characteristic curve which rises as rate of flow increases to some level and then levels off or turns downward. However, other pumps have more level power curves or even some that are higher at low rate of low. Why is this, and can it be a problem?

A. The shape of the pump characteristic curves depends or the specific speed value of the pump (NS). See ANSI/HI 1.1-1.2 Centrifugal Pumps for Nomenclature and Definitions, Section 1.1.4 Impeller Designs.

Impellers with low NS values of 1000 (U.S. customary units) are narrow compared to the outside diameter, whereas high NS impellers, such as 10,000, are wide compared to their outside diameter. High NS impellers are often referred to as propellers (or axial pump designs) and are used in low head applications.

As the NS increases, the power at low rate of flow increases as well as total head (see Figure 2.2 and Figure 2.3). This curve shows that pumps with NS of 5000 have a relatively flat horsepower curve. Above NS of 5000, the power and total head rise steeply with decreasing rates of flow, especially near zero flow or shut off.

Figure 2.2. Pump Characteristic Curves

The greatest concern with high NS design pumps is that they should not be started with a closed discharge valve since the allowable pressure of the system or the driver starting capability may be exceeded.

Figure 2.3

Q. With the increasing emphasis on energy conservation, turbines are being installed on sources of hydraulic power that are normally wasted. Is it necessary to install small turbines in such cases or can centrifugal pumps running backwards be used?

A. Centrifugal pumps of all sizes, types and specific speeds may be operated in reverse rotation as hydraulic turbines.

While running in the turbine mode, the performance characteristics of a PAT (pump as turbine) differ significantly from pump operation (see Figure 1.51). The applied head is usually constant, so the other parameters are shown as they vary with speed. The discharge nozzle of the pump becomes the inlet of the turbine, the suction nozzle of the pump becomes the outlet of the turbine, and the impeller of the pump, rotating in reverse direction, becomes the runner of the turbine.  The impeller orientation to the casing is the same for both pump and turbine.

Reverse running pumps are an excellent alternative to conventional turbine designs. A common application is hydraulic power recovery turbines (HPRT). The potential for power recovery from high-pressure liquid streams exists any time a liquid flows from a higher pressure to a lower pressure in such a manner that throttling occurs. Reverse running pumps are used instead of throttling valves to recover the power in the high-pressure liquid.

Figure 1.51