by Tom O'Donnell

The Challenge

At their simplest, metering pumps are used to inject liquids at precisely controlled, adjustable flow rates, which is a process called metering. As defined by the Hydraulic Institute's Metering Pump Section, controlled-volume metering pumps are reciprocating, positive displacement pumps typically used for the injection of chemical additives, proportional blending of multiple components or metered transfer of a single liquid.

Metering pumps pump chemical solutions and expensive additives for products manufactured in a wide variety of industries, including industrial, medical, chemical, food and dairy, pharmaceutical and biotech, environmental, fuel cell and laboratory. Metering pumps are designed to pump into low or high discharge pressures at controlled flow rates that are constant when averaged over time. Metering pumps consist of a solenoid drive or a gearbox with motor, control mechanism and pump head with valves to control the flow direction through which the liquid pumped enters the inlet connection and exits the discharge connection.

Since liquids are only slightly compressible, they can be discharged at high pressure by metering pumps. Gases, on the other hand, are much more compressible, making them incompatible with metering pump use. Therefore, problems can occur in a metering pump application when gas bubbles enter the pump head. When this happens, the pump can suffer from vapor lock, in which the pump will stop pumping the liquid that contains gas bubbles while repeatedly compressing and decompressing the bubbles.

Another challenge in the use of metering pumps can occur when the pump's outlet pressure is lower than the inlet pressure. When this situation arises, both check valves will open simultaneously and the liquid will flow through the pump head uncontrollably from inlet to outlet. A properly rated pressure-differential check valve placed downstream of the pump will arrest this undesirable flow condition.

Despite these concerns, metering pumps remain versatile, relied-upon technologies for the safe, accurate and efficient injection of a unique array of chemicals up to 20,000 cps and slurries containing up to 10 percent solids. This article will help the user define the variables to evaluate when choosing and installing the proper metering pump or complete chemical-feed system. Choosing the proper system will not only help inject liquids or slurries regardless of viscosity, but also ensure that it is done in an efficient, environmentally friendly and energy-wise manner.

The Solution

Size does matter when determining the proper metering pump for an application-specifically, the size in terms of capacity of both the pump's flow rate and discharge pressure. Simply put, metering pumps should not be oversized. In fact, a metering pump should be sized so that its maximum expected flow rate is 85 to 90 percent of the pump's capacity, which leaves additional capacity, if needed. At the other end of the spectrum, a metering pump's minimum capacity should never be less than 10 percent of the capacity; anything less will affect the pump's accuracy.

Planning a Metering Pump Installation

A metering pump installation must be planned from the day tank or liquid source up to the injection point. Remember that metering pumps will push against great pressures but they will not pull for great distances. Since it is easier to prime and more forgiving, a flooded suction is always preferred, and must be used for fluids where vapor pressure might be less than the suction lift.