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Pumps & Systems, July 2007

When is a metering pump not just an ordinary metering pump? When it exceeds API 675 performance standards with "pulse-free" linear flow that is being accomplished through the latest technological improvements.

While many institutions and associations have developed performance criteria for metering pumps, the American Petroleum Institute (API) Standard 675 for controlled-volume, positive displacement pumps is perhaps the most widely cited. It specifies accuracy (performance at a set point) of ±1 percent, linearity (speed/flow rate relationship) of ±3 percent, and repeatability (return to set conditions) of ±3 percent.

The reciprocating metering pump is commonly used in applications requiring precise chemical injection. A good example is polymer feed systems at water and wastewater treatment plants, where polymers need to be delivered without damage in accurate, repeatable quantities.

Although metering pumps are noted for accuracy, linearity and repeatability, there is the potential for leakage, lost motion, and pumping inaccuracies during stroke adjustments. In addition, the intermittent, pulsating flow of metering pumps places a strain on the system.

By incorporating electronic flow control and a multiple-diaphragm design, new metering pump technology has been developed to overcome these operational drawbacks and give new definition to what constitutes a metering pump. For example, in the instance of polymer dosing, certain metering pump technology now incorporates variable frequency drive (VFD) electronic flow adjustment to create very low sheer and protect the integrity of the polymers during pumping.

Layout showing all of the equipment used in a complete metering solution for a pumping system.

Metering Applications Uncovered for Sealless Pumps

In the early 1970s, multiple-diaphragm pump manufacturers introduced sealless pumps that were designed for agricultural applications and the burgeoning car wash market. Field-proven reliability and durability soon made some of these the pumps of choice in these industries. As potential new users learned the advantages of these seal-less pumps, sales for them expanded into different markets.

This led some manufacturers to compile market data about the use of their products, which revealed that certain seal-less pumps were operating successfully in applications that demanded accurate dosing. In fact, those seal-less pumps would often replace standard diaphragm metering pumps.

When tested under the same operating conditions at the same flow and pressure, certain multiple-diaphragm pumps provide smoother, almost pulse-less performance compared to a traditional, single-diaphragm metering pumps.