Tom O'Donnell is senior product specialist for Lansdale, PA-based Neptune Chemical Pump Co., an operating company with the Pump Solutions Group (PSGTM). PSG is comprised of Wilden
Remember that it is not practical to use a variable speed drive on motor-driven pumps that normally operate at less than 100 spm to 150 spm. Slowing the motor causes each stroke to take longer from start to finish and, as a practical matter, motor-driven pumps should not be operated at less than 15 spm. Electronic diaphragm pumps, which are pulsed by a solenoid, can operate at less than a single stroke per minute because the characteristic and timing of each stroke, from start to finish, is the same at all stroking speeds. The moving parts in modern diaphragm pumps offer long, reliable service at all stroking speeds. The highest stroking speeds should be avoided with viscous or abrasive chemicals.
When automatic or electric stroke positioners control a metering pump, the number of doses remains constant and the dose size is reduced, thus keeping the doses uniformly distributed in a constantly flowing line. Use of a variable speed drive changes the stroke speed, while the size of the dose injected on each stroke remains the same, but makes the doses less frequent. This, however, can produce an undesirable process result in a constantly flowing line as the discreet slugs of chemical are more widely separated than if a constant dose interval were maintained.
Finally, consider the application and quality level. Is the unit used for intermittent operation in an HVAC or light-duty application where economy is an important consideration? Is the unit for an industrial plant/waste-treatment facility/refinery/power plant where ruggedness and additional features are required? Is initial cost or life cycle cost more important?
Pumps & Systems, February 2010