Evaluation of Centrifugal Pump System Performance Issue

What is centrifugal pump evaluation?

Pump evaluation is a small-scale performance test of the pump in its normal operating system. For pumps with a smaller operating flow range, this test can be completed by obtaining data at a single flow rate to represent the normal operation of the pump.

In more complex systems, pumps may operate over larger flow and head ranges, and evaluations may need to include multiple separate tests covering the pump performance requirement range, or may require continuous monitoring over a period of time to capture extreme operating conditions. The evaluation typically involves measuring the pump's flow rate, head, and input power to determine its actual operating point within the system.

Why perform centrifugal pump system evaluation?

The two most common reasons for evaluating centrifugal pump systems are to improve the reliability and/or efficiency of the pump and the system.

Reliability:

The reliability of a pump depends on how close the actual operating point or range is to its best efficiency point (BEP). When the pump operates near its BEP, reliability increases. BEP, as the name suggests, is the flow and head point at which the pump achieves its highest efficiency.

Operating far away from the BEP point produces axial and radial loads generated by internal hydraulics. These forces can cause shaft deflection and bearing loads, leading to seal failure, bearing failure, internal friction, and vibration problems. Even a properly installed and maintained centrifugal pump may encounter greater reliability issues if operated far from its BEP point. Additionally, issues related to cavitation and entrained flow can be exacerbated by operating outside the design operating point.

Efficiency:

In an increasingly environmentally-friendly world, performing pump evaluations to determine whether pumps are operating as intended and efficiently has become common practice. Most facility managers believe that their pumps are operating very close to their BEP.

In reality, centrifugal pumps used in the field operate at an average efficiency of 44%, well below the average efficiency of their BEP. A quick study of the average best efficiency of each size ANSI pump sold within a year by the three largest ANSI pump suppliers in the United States showed that the average best efficiency of this subset of pumps was 65%.

These centrifugal pumps are usually smaller in size and designed primarily for reliability, not efficiency. Therefore, this efficiency sample was not intentionally drawn from the high-efficiency portion of the pump population, but still shows a 21% efficiency gap between the average best efficiency and the average operating efficiency, which is convincing enough to want to check the actual operating condition of the pump.

However, this is only the beginning of good news. With a wider view and a system-wide check, there may be partially closed valves, open bypass pipelines, excessive flow rates, and other issues. System efficiency, not pump efficiency, is the real cause of the efficiency gap. When properly installed pumps and system are running, energy costs for centrifugal pumps can be reduced by 50% and it is not uncommon for such a system.