Metering Pump Drive Systems

Controlled volume pumps have never been greatly impacted by changes in system conditions, and the newest technologies in metering pumps automatically adjust to keep the output accurate to the set point, even when discharge pressure changes. At the same time, recent innovations have made it easier for the operator to set metering pumps to the desired discharge rate with minimum or no need to calculate between capacity change due to stroke length setting versus capacity.

Many controlled volume pumps today incorporate both stroke length and stroke frequency/speed in order to provide the user with up to 100 to 1 capacity variation. Today's electronics enable wider speed variation and extreme accuracy, eliminate the need to vary the stroke length and avoid the hydraulic difficulties inherent to running pumps at less than full stroke length.

The first controlled volume metering pumps incorporated an electric motor with a belt and pulley drive. The drive types commonly used today include standard AC motors. Solenoid drive systems are probably the most popular today due to competitive pricing. AC and DC variable speed motors are used in conjunction with stroke length control in some types. Pneumatic and hydraulic power sources are found in specialty applications like natural gas and agriculture. Advanced electric motors, taken from the robotic and computer industry (i.e., stepper motors), have recently eliminated the need for stroke length control, simplifying setup and operation.

In the original metering pump, capacity was changed by adjusting the stroke length of the piston, which was located on the outside of the pump. Figure 3 is essentially the same concept, but represents a technology introduced about 30 years after the first metering pump. In this concept, the stroke length adjustment function was moved to an internal, oil bathed gear system. This drive type is readily available today.

Figure 3

The choice of drive system is important to the application as it relates to durability, accuracy, automation levels, maintenance availability and cost. From the simplest timed triggering of a solenoid to electric motor driven drives as depicted above, all incorporate some capacity change. While speed was initially a secondary control method introduced to extend the turndown ratios from the 2 to 1 or 3 to 1 limits and up to 10 to 1, speed was immediately seen as a preferred capacity control in some applications. Speed brought problems for relative accuracy.

The Value of a Metering Pump

Within the applications best served by dosing or metering pumps, economics should be considered to select the best value for a particular application. Unlike most other pumps, cost of ownership or life cycle cost of dosing pumps is usually not heavily influenced by power consumption and repair costs. A more likely case can often be made for savings in the cost of chemicals metered due to high level accuracy or the quality of a final product.

Users choosing metering pumps are often attracted to the simple convenience of a pump with a compact package and a built-in capacity adjustment. Factors such as steady state accuracy (see Figure 2), repeatability and linearity represent even greater value in the majority of applications where dosing pumps are found today. Today's controlled volume pump accuracy can dispense the desired amount of liquid during extended periods of time, even as the process requires the pump's discharge rate to be changed. The capacity ranges as much as 1,000 to 1 in some styles of these pumps.