For any centrifugal pump user, knowing the answers to the following six questions is key to reliable and efficient pump operation.
1. What is efficiency?
When we speak of the efficiency of any machine, we are simply referring to how well it can convert one form of energy to another. If one unit of energy is supplied to a machine and its output, in the same units of measure, is one-half unit, its efficiency is 50 percent.
The overall efficiency of a centrifugal pump is simply the ratio of the water (output) power to the shaft (input) power and is illustrated by the equation below:
Ef = PW / PS
Pw= the water power
Ps= the shaft power
2. What is specific speed, and what is its effect on the pump curve?
Specific speed was first applied to centrifugal pumps in the latter 1800s and was a modified version of one developed for water turbines. Many pump designers see specific speed as the most important contributor to centrifugal pump design. It allows the use of existing design and test data to design similar higher and lower flow pumps because the specific speed of a pump is independent of its size.
Figure 1. Pump profile comparisons (courtesy of www.pumpfundamentals.com)
As Terry Henshaw stated in “Centrifugal Pump Specific Speed” (Pumps & Systems, September 2011), the definition of specific speed can be confusing. It is best to think of it as an index number that can predict certain pump characteristics. Viewed this way, specific speed can be useful when selecting a pump for a particular application and predicting premature failure due to off best efficiency point (BEP) operation.
3. What are individual efficiencies that affect operation?
- Hydraulic efficiency. The shape and spacing of the impeller vanes have an effect on overall pump efficiency. Although the ideal impeller would have an infinite number of vanes, the real world limits us to five to seven for clear water pumps and even fewer for pumps that handle larger solids.
- Volumetric efficiency. Whether the volumetric efficiency of a centrifugal pump is a function of the volute or the impeller is debatable (it is probably both), but I will include its effect here. Volumetric efficiency represents the power lost due to flow leakage through the wearing rings, the vane front clearances of semi-open impellers and the balancing holes in the rear shroud.
- Mechanical efficiency. The final piece of the pump efficiency puzzle is that of mechanical losses, although some of these losses are not always included in published efficiency curves. In the case of a frame-mounted pump, these losses are caused by the shaft bearings and the mechanical seal or packing. For close-coupled pumps, bearing losses are figured into the motor efficiency. Again the rule of thumb follows that of volumetric efficiency, and losses increase as flow and/or specific speed decrease.
- Combined efficiency. When looking at the overall efficiency of a pump in operation, the efficiency of the driver must be included, and in many instances, that driver will be an electric motor. When the Energy Independence and Security Act of 2007 went into effect in December 2010, it raised the bar on motor efficiency. Today, all new motors must meet premium efficiency standards. Obviously, a higher efficiency motor will increase the overall efficiency of a pumping system, but by how much? How do we calculate the combined efficiency of pumps and motors?
4. How do you preserve efficiency?
An important part of the volute is the tongue, or cutwater. Its purpose is to maintain flow into the throat while minimizing recirculation back into the case. The optimum clearance between the tongue and the impeller periphery is the smallest distance that does not give rise to pressure pulsations during vane tip passing. A well-designed pump will have a full-size impeller that meets these clearance criteria. When an impeller is trimmed, this distance increases and allows more fluid to recirculate back into the case. As recirculation increases, hydraulic efficiency decreases.
5. How does curve shape affect efficiency?
A typical performance curve is relatively flat at low values of specific speed (Ns) and becomes steeper as Ns increases. Pump efficiency is lowest at low values of Ns (500 and below) and increases as Ns increases. It reaches its maximum in the mid-to-high 2,000 range and begins to decrease above 3,000. However, the decrease above 3,000 is much smaller than it is below 1,000.
Steeper curves usually offer a greater range of control when operated under variable speed control against some fixed elevation or pressure head. These pumps can be problematic when running in parallel or starting against varying system head conditions. … Flatter curves work fine in across-the-line applications as long as the static or pressure head remains relatively constant. They also work well in closed-loop (and most open-loop) circulation applications when operated under variable speed control.
6. When is efficiency important?
The power required by a pump is directly proportional to both the flow and the head that it produces. As flow and/or head increase(s) so does the power required. Conversely, power is inversely proportional to hydraulic efficiency. For the same flow and head, an increase in efficiency reduces the power requirement.
As the cost per kilowatt hour increases, so will the savings due to increased pump efficiency.