Types of Metering Pumps: Drive, Liquid End and Control Explained

The types of metering pumps look confusing in a catalog, but almost every model can be placed on three axes. By drive, a pump is solenoid, motor or pneumatic. By liquid end, it is mechanical diaphragm, hydraulic diaphragm or packed plunger. By control, it is manual, externally controlled (pulse or 4-20 mA) or digital with communication. A real pump is one pick from each axis, for example "motor drive, mechanical diaphragm, 4-20 mA input." This guide is written for engineers and buyers specifying their first chemical feed package. It explains what each class is for and links to deeper articles on how each one works.

What kind of pump is a metering pump?

Every metering pump is a reciprocating positive displacement pump. A diaphragm or plunger moves back and forth, and each stroke draws in and pushes out a nearly fixed volume. Stroke means one full travel of that part from one end to the other. Flow equals displacement per stroke times stroke rate, so you change output by adjusting stroke length, stroke speed or both. Within the rated range, discharge pressure has little effect on flow.

A centrifugal pump cannot do this. Its flow slides along the curve whenever system resistance changes, which is why it struggles to hold a low, steady chemical dose. So whichever of the types of metering pumps you look at, the core is the same reciprocating principle.

Types of metering pumps by drive

The drive sets how the pump is powered and roughly fixes its flow and pressure class.

Solenoid drive

A solenoid metering pump has no motor. An electromagnet pulls the diaphragm forward when energized and a spring returns it. These pumps are compact and cover low flows: cooling tower inhibitor, RO antiscalant, pool chlorination. NEWDOSE DFD, DP, DM and DC are all solenoid diaphragm pumps. The stroke cycle is covered in how a solenoid metering pump works.

Motor drive

An electric motor turns a worm gear or eccentric that converts rotation into reciprocation. Flow and pressure go well beyond solenoid units, so municipal wastewater, power plants and chemical processing mostly use this class. Standard induction motors are most common; some designs use variable speed or stepper motors for finer turndown, such as the ECO series EFD stepper motor pump.

Pneumatic drive

Compressed air drives the stroke. You see this where there is no power or where hazardous area rules are strict, and it is far less common than the other two. One warning: most "air operated diaphragm pumps" are transfer pumps and do not belong among the types of metering pumps at all. Their output is nowhere near as repeatable, so do not swap one for the other.

Types of metering pumps by liquid end

The liquid end is everything wetted by the chemical: pump head, diaphragm or plunger, and the suction and discharge check valves. Of the three ways to sort types of metering pumps, this one decides how much pressure the pump can build and whether it can leak.

Mechanical diaphragm pumps link the diaphragm directly to the drive rod. There is no dynamic seal, so the chemical stays inside. They suit low to medium pressure duty with acids, caustic, coagulants and sodium hypochlorite. NEWDOSE NDJ, NDWS, NDWL, ND1000 and ND2000 belong here, and the HND high pressure version is rated up to 3.0 MPa in the catalog.

Hydraulic diaphragm pumps push the diaphragm with a chamber of hydraulic oil. Pressure is balanced on both sides of the diaphragm, so it sees low stress and can work against much higher pressure with toxic or costly fluids. Milton Roy Milroyal B, built to API 675, is one example. For the trade-offs, see mechanical vs hydraulic diaphragm metering pumps.

Packed plunger pumps put the plunger in direct contact with the fluid and seal it with packing. They reach the highest pressures of the three. The cost is a small, normal weep through the packing, so plan for leak collection with hazardous chemicals. Details are in how a plunger metering pump works.

Types of metering pumps by control

Control is the third way to sort types of metering pumps, and it decides how the pump ties into your plant.

1. Manual: an operator sets stroke length or stroke rate with a knob. Fine where dose rarely changes, like the NEWDOSE DFD.
2. External control: the pump follows a signal. Pulse input usually comes from a water meter for flow-paced dosing; 4-20 mA usually comes from a PLC or a pH or chlorine analyzer. NEWDOSE DP takes pulse and DM takes current. For help choosing, read 4-20 mA vs pulse control for metering pumps.
3. Digital and networked: a display and menus for batch or timer modes, and on some models a fieldbus. The LMI EXCEL XR, for instance, offers Profibus DP and Modbus RTU modes.

How to choose between types of metering pumps

Many buyers start by picking a drive. That order tends to cause trouble. We suggest this sequence instead:

1. Pressure and fluid first, which settles the liquid end. High back pressure or toxic fluid points to hydraulic diaphragm or plunger; ordinary water treatment chemicals are fine with mechanical or solenoid diaphragm.
2. Flow and power next, which settles the drive. Low flow with normal mains power, a solenoid pump is simplest. Large flow or high pressure, go motor driven.
3. Control signal last. Ask whether the PLC sends pulse or 4-20 mA before you order, not after the pump arrives.

Two field mistakes come up again and again when buyers compare types of metering pumps. A drawing says "diaphragm pump" and purchasing buys an air operated transfer pump that cannot be calibrated. Or a solenoid pump is picked on flow alone, then installed on a long, high pressure line where it struggles and under-delivers.

Conclusion

The types of metering pumps fall along three axes, drive, liquid end and control, and every pump is a combination of one choice from each. All types of metering pumps are reciprocating positive displacement pumps. Choose the liquid end from pressure and fluid, the drive from flow and power, and the control from your signal. You can browse the NEWDOSE metering pump range or filter every class in our metering pump product category. Send flow, pressure, chemical and signal type and contact our engineers to match a model.

FAQ

Is a dosing pump the same as a metering pump?

Mostly. "Dosing pump" describes the job; "metering pump" describes the accuracy. In water treatment the two usually mean the same machine, but a centrifugal or progressive cavity pump used for dosing is not a metering pump.

Can a solenoid pump match the flow of a motor driven pump?

Usually not. Solenoid force is limited, so once flow or back pressure climbs, move to a motor driven mechanical diaphragm pump.

Which types of metering pumps leak less, diaphragm or plunger?

The diaphragm pump. The diaphragm fully separates fluid from drive with no dynamic seal. A plunger pump weeps slightly through its packing by design and needs periodic adjustment or repacking.

Can a 4-20 mA pump also accept pulse signals?

It depends on the model. Some accept both and switch in the menu; others take only one. Put the signal type in your specification.

Is a "digital pump" one of the types of metering pumps?

It belongs to the control axis, not a separate class. Inside, a digital pump is still solenoid or motor driven, with a diaphragm or plunger liquid end.