Metering and Dosing Pumps for Precise Chemical Injection
Chemical metering pumps sized so the dose lands in the middle of the range, with wetted materials matched to your chemistry.
Do I need a metering pump or a regular transfer pump?
If you have to deliver a precise, repeatable amount of a chemical, you need a metering pump, not a transfer pump throttled by a valve. A metering pump displaces a fixed volume per stroke, so the delivered dose is controlled and consistent even as the discharge pressure varies. If you only need to move fluid from one tank to another without a controlled dose, a transfer pump is simpler and cheaper. The deciding question is whether accuracy of the delivered amount matters.
What a metering pump does
A metering pump, also called a dosing pump or chemical metering pump, injects a precise and repeatable volume of liquid into a process stream. Most use a diaphragm driven so that each stroke displaces a fixed volume, and the delivered rate is set by stroke length, stroke frequency, or an electronic control. Because the volume per stroke is fixed, the pump holds its dose even as the injection point pressure changes, which is what separates true metering from a throttled flow.
These pumps show up wherever a chemical has to be added in a controlled amount: water treatment, pH adjustment, coagulant and polymer feed, disinfection, scale and corrosion inhibition, and process chemical injection.
Accuracy, turndown, and control
Two properties drive metering performance. Accuracy is how close the delivered volume is to the setpoint, and repeatability is how consistent it is stroke to stroke. Turndown is the ratio between the highest and lowest controllable output; a wide turndown lets one pump cover a broad dose range without swapping heads. When the dose has to track a changing plant flow, the pump is paired with a flow signal so the injection scales in proportion. Selecting for the normal dose to fall mid-range preserves room to trim in both directions.
Chemical compatibility comes first
The chemistry sets the materials before anything else. The diaphragm, check valves, seals, and pump head all touch the fluid, so acids, caustics, oxidizers, and solvents each demand a compatible combination of plastics and elastomers. Off-gassing chemicals such as sodium hypochlorite need a degassing head or a valve arrangement that clears vapor, or the pump gas locks and under-doses. Nail the wetted materials and the pump lasts; get them wrong and it leaks the exact chemical you are metering.
Sizing checklist
- Dose rate converted to a volume per hour or per day.
- Discharge pressure at the injection point, including any back pressure.
- Turndown needed if the dose has to follow a variable flow.
- Chemical identity and concentration, to set the wetted materials.
- Suction conditions and whether the fluid off-gasses or is viscous.
Metering pump lines available through Brabazon
For metering and dosing duty we supply the Grundfos DDA line of digital diaphragm dosing pumps, chosen for accurate, repeatable chemical injection with wide turndown and control options.
Industries we serve
Service and failure considerations
Metering pumps are precision devices, so their failure modes are subtle: a worn check valve or a hairline diaphragm crack quietly throws the dose off before it stops the pump entirely. Off-gassing fluids cause gas locking and lost prime, and abrasives wear the valve seats. Routine diaphragm and valve service keeps the delivered dose honest. When a rebuild is needed, our factory-trained technicians service dosing pumps through pump repair.
Metering and dosing pump questions, answered first
What is a metering pump and how is it different from a dosing pump?
A metering pump delivers a precise, repeatable volume of liquid per stroke or per unit time, and dosing pump is simply the common name for a metering pump used to inject chemicals. The terms are used interchangeably in the field. Both work by displacing a fixed volume with a diaphragm, so the delivered amount is controlled and consistent rather than just a flow you throttle with a valve.
How do I size a chemical metering pump?
Start with the required dose rate in the units your process uses, then convert it to a volume per hour or per day, and add the discharge pressure the injection point sees. You want the normal dose to land in the middle of the pump capacity range, not at the top or bottom, so you have room to trim up and down. Turndown, the ratio of maximum to minimum controllable output, matters when the dose has to track a changing flow.
What determines the wetted materials on a dosing pump?
The chemical being injected determines the wetted materials, because the diaphragm, valves, seals, and pump head all contact the fluid. Aggressive chemicals such as acids, caustics, and oxidizers each call for specific compatible plastics and elastomers. Selecting the wrong material leads to swelling, cracking, or attack of the wetted parts and a leak of the very chemical you are trying to control.
Why is my metering pump losing prime or under-dosing?
Loss of prime and under-dosing are most often caused by gas locking on off-gassing chemicals, worn or fouled check valves, a cracked diaphragm, or air leaks on the suction side. Viscous or off-gassing fluids need degassing heads or the right valve configuration. Because these pumps deliver a controlled dose, even a small valve leak throws the delivered amount off, so the valves and diaphragm are the first things to check.
Size a dosing pump to your chemistry and dose
Send us your chemical, dose rate, injection pressure, and required turndown and an engineer will size a metering pump and quote it. We answer 24/7.
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Tell us the chemical, dose rate, injection pressure, and turndown you need and a pump specialist from your nearest branch will size and quote the pump. Or call 800.825.3222, answered 24/7.
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