What makes a good metering pump?

To understand what makes a good metering pump, you need to first understand what an application’s requirements are. A metering pump (also called a dosing pump) adds small but precise (pre-determined) volumes of liquid from one fluid stream to another or from one vessel to another within a specified timeframe.

A metering pump must be able to meter and transfer highly accurate, measured amounts of the fluid across a range of discharge pressures. A metering pump can be used to transfer any type of viscous liquid, as well as acids, bases, and solvents.

When is a metering pump necessary?

Metering pumps are often used for the following reasons:

• To maintain a constant flow rate regardless of pressure or fluid viscosity
• To provide variable flow rates to maintain a set parameter (for example, pH)
• To inject a discrete dose (typically required when metering batches such as when dispensing chemicals for disinfection, filtration, or coagulation). Dosage rates can change according to process conditions

Metering is required by many industries, including the pharmaceutical industry, municipal water and wastewater treatment, the food & beverage industry as well as industries that process chemicals, petrochemicals or oil and gas. The liquids transferred could be anything from perfumes to the pigments used to produce toiletries or food, or the acid/alkaline solutions for chemical processes and water treatment.

Metering with positive displacement pumps

Any type of positive displacement pump can be used for metering, because PD pumps have a linear flow rate with RPM/SPM. A positive displacement pump allows a small change in flow due to pressure fluctuations, while being capable of high pressure. It can also handle low flows with high accuracy.

Centrifugal pumps can also be used for metering; however they are not as common in metering applications for the following reasons:

Metering with centrifugal pumps

A centrifugal pump can be useful to deliver a controlled flow, particularly in applications where low viscosity liquids must be delivered at high flow rates and low pressures. While often the most cost-effective solution, centrifugal pumps are more complex as control is non-linear. They also have a limited range, with 90% of flow occurring over ~25% speed range.

Other issues with centrifugal pumps are that they cannot handle low flows or maintain the minimum flow required for seal or bearing cooling.

Now that we’ve discarded centrifugal pumps in our choice of the best metering pump, let’s look at the types of positive displacement pumps available.

Types of positive displacement pumps:

There are many different types of positive displacement pumps, including:

• Diaphragm,
• Peristaltic,
• Gear (magnetic-drive),
• Rotary vane (magnetic drive),
• Progressive cavity,
• Piston/Packed plunger,
• Lobe,
• Flexible impeller…and many more.

To find out which PD pump makes the best metering pump, you’ll need to read the next article in our series, which looks at one of four positive displacement pumps used for metering (hint: the best metering pumps eliminate dynamic seals).

Are you left scratching your head when trying to decide on the best metering pump for your system? Give us a call on 1-800-367-4180 (toll-free). As your metering pumps supplier, we’re here to help you choose, install, maintain, and monitor a variety of equipment. And to answer questions about things you’ve previously tried gone wrong.