By Gene Vogel
EASA Pump & Vibration Specialist
Many EASA service centers have installed facilities for operational testing of pumps. Equipping a pump test facility for variable speed operation can provide greater flexibility and expand opportunities to evaluate pump performance. For close coupled pumps (integral pump-motor units), a variable frequency drive is all that’s needed. If an independent motor is required for testing, other variable speed options such as clutches and DC motors will also work.
A test facility may not be adequate to test a pump at full speed, full load operation. But for most centrifugal pumps, an accurate evaluation of pump performance can be achieved by running the test at a lower rotating speed. Most centrifugal pumps are designed for operation with various impeller diameters. Pump curves often illustrate this with head-flow performance curves for the various impeller diameters. (See Figure 1). It is common practice for impeller diameter to be trimmed to achieve a desired operating point close to the best efficiency point (BEP) of the impeller. Impeller trimming may be done by the manufacturer on a new pump or by a pump user or service center to recommission a pump. Pump performance software available at many pump suppliers’ websites typically have features to generate pump curves for a range of impeller diameters. Similarly, pumps may be specified for operation at different speeds, and corresponding pump curves will be available.
Both reducing impeller diameter and reducing speed directly and proportionately affect the vane tip velocity of the impeller and the corresponding pump performance. As the Affinity Laws (Figure 2) illustrate, a reduction of either rotating speed or impeller diameter affects flow, head and power in equal proportions.
Note that power is proportional to the cube of either impeller diameter or pump speed. So, a reduction of rotating speed from 1800 RPM to 1500 RPM will reduce load by 42% (from 100% to 58%). So even if a pump test facility does not have capacity for full load testing, testing at a reduced speed, with reference to a suitable pump curve, can verify proper pump operation. In regards to having a suitable pump curve, the curve can be for reduced rotating speed or for reduced diameter, since both have virtually the same effect.
It is not uncommon for a service center to be asked to trim an impeller to match a customer’s flow-head requirements. Since there is always fear that too much could be trimmed (running the lathe backwards won’t fix that), a 15% margin is often observed. The result is often an impeller that is “almost” right. But by using variable speed at the test facility, the proper proportion for trimming can be accurately gauged. Just operate the pump at the speed that produces the desired flow and head, determine the proportion of that speed to the full speed pump curve, and trim the impeller by the same proportion.
There are no downsides to equipping a pump test facility for variable speed except the cost of the variable speed equipment. For many service centers, the benefits are worth the investment.
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