Sizing And Selecting The Proper Metering Pump
Chemical Engineering World|August 2018

In process application, large amounts of chemicals are used during daily operations and the volumes of chemicals are large and very precise, with exact amounts delivered according to strict injection schedules. Thus the pressures - from very high to very low – for injecting the chemicals should be governed precisely. Metering pumps have—whether mechanically, hydraulically or electronically actuated—raised to the fore as a technology of first choice for the injection of chemicals in processing and agricultural applications, but within the realm of metering pumps there are many different styles and modes of operation. The article explains the different metering-pump technologies and illustrates how the user should select the proper size and style depending on the injection application for which the pump will be tasked.

A typical process application can consume large amounts of chemicals during daily operations. While the volumes may be large, though, they must also be very precise, with exact amounts delivered according to strict injection schedules. The pressures—from very high to very low—that the chemicals are injected at must also be governed precisely. This demand for precise flow rates at predetermined schedules and pressures requires the use of a very specific pumping technology, one that can ensure successful, reliable adherence to the injection schedule and fluid volumes.

Metering pumps have—whether mechanically, hydraulically or electronically actuated—risen to the fore as a technology of first choice for the injection of chemicals in processing and agricultural applications. Today’s metering-pump technology can be easily set to deliver exact volumes according to a set schedule, and at varying pressures.

But within the realm of metering pumps there are many different styles and modes of operation.

Know the Variables

When determining which metering pump is the best option for a chemical injection application there are a number of variables that must be taken into consideration, including:

• Flow Rate: Metering pumps should never be oversized, meaning that determining the exact flow rate that is required for the application is of paramount importance. With that in mind, a metering pump should be sized so that the maximum expected flow rate is 85 to 90 per cent of the pump’s capacity, which will leave room for additional capacity, if needed.

• Materials of Construction: Metering pumps are available in a variety of materials, most commonly 316 stainless steel, C-20 stainless steel, PVC and Kynar PVDF. When selecting a metering pump’s materials of construction, the corrosion, erosion and solvent action of the chemical must be taken into consideration. For example, solvent-based chemicals may dissolve plastic-headed pumps, while acids and caustics may require stainless-steel models. The effects of erosion must also be considered when the chemical takes the form of abrasive slurry.

• Chemical Makeup: Chemicals come in many formulations, from extremely thin to highly viscous, while they can also be classified as a slurry or off-gas when transferred. Standard metering pumps are typically able to handle clear liquids with viscosities ranging from water-like to 1,500 cPs. Chemicals with viscosities that approach 5,000 cPs or have light suspensions will require special liquid ends. Those with viscosities up to 20,000 cPs or that contain up to 10 per cent solids will require special diaphragms, while ones that automatically vent accumulated gas will need their own variety of liquid ends.

• Driver: Drivers, which can be powered by electricity, water, gas, air or the sun, must be selected according to the utilities that are available. Driver selection must also take into account any environmental hazards that may be found in the operating area, with the operator realising that pumps used in remote locations may not be able to be inspected as often as those in controlled environments.

• Environment: Determine if the pump will be operating indoors or outdoors. If used outdoors, the pump must be sheltered from direct sunlight. Any pumps that will be used in freezing temperatures can only pump fluids that will not freeze at that temperature.

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Sizing And Selecting The Proper Metering Pump

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