Types of Metering Pumps: Solenoid, Mechanical Diaphragm, Hydraulic Diaphragm and Plunger

The four main types of metering pumps are solenoid diaphragm, mechanical diaphragm, hydraulic diaphragm and plunger pumps. When buyers ask us which one they need, the answer mostly comes down to two numbers: how much flow and how much pressure. This guide sets out the range each type covers and where it fits, using figures from manufacturer catalogs, so you can narrow the choice before asking for a quote.

How the types of metering pumps are defined

The split comes from the drive and the liquid end. A solenoid pump uses an electromagnet to push the diaphragm. A mechanical diaphragm pump uses a motor and eccentric to push the diaphragm directly. In a hydraulic diaphragm pump, a plunger pushes oil and the oil moves the diaphragm. In a plunger pump, the plunger itself touches the chemical. You can split dosing pumps further by control method or head material, and our metering pump types overview covers those. Here we stay with the four families.

Solenoid diaphragm pumps

Solenoid pumps are compact, easy to wire and the lowest entry cost of all types of metering pumps. They suit low flows. In the NEWDOSE DFD series, the DFD-02-16 gives 2 L/h at 16 bar while the DFD-50-02 gives 50 L/h at 2 bar. The pattern is clear: as flow goes up, pressure capability comes down.

Typical duties: small water treatment dosing, cooling towers, pool disinfection, lab systems and compact skids. Limits are single-unit capacity and a pulsing output that becomes noticeable on long discharge lines.

Mechanical diaphragm pumps

A motor drives a connecting rod that pushes the diaphragm, and capacity rises a long way above solenoid pumps. The NEWDOSE NDWS-8/1.6 is a small 8 L/h, 16 bar size. The NDWL series uses a 120 mm diaphragm at 4.5 to 7 bar, and the large ND2000 sizes use 155 to 160 mm diaphragms at 3 bar. The Milton Roy G series catalog lists up to 1800 L/h at 12 bar, 10:1 turndown and ±1% accuracy.

The same rule applies: bigger diaphragm, more flow, less pressure. Among the types of metering pumps, this is the workhorse for PAC, polymer and hypochlorite in municipal water and wastewater plants. The trade-offs against solenoid pumps are covered in solenoid vs mechanical diaphragm pumps.

Hydraulic diaphragm pumps

Here the diaphragm is balanced by oil on one side and process fluid on the other, so it never takes the full mechanical load. That lets it run at higher pressure. The NEWDOSE HND series uses a 90 mm diaphragm with a 4 mm stroke and is rated at 30 bar. SEKO's PS2E hydraulic diaphragm range covers 40 to 1000 L/h, with a maximum of 20 bar for stainless steel heads and 10 bar for PVC heads.

Typical duties: boiler chemicals, reactor feed in chemical plants, and petrochemical injection where pressure is high and leaks are not acceptable. The design is more complex and the hydraulic oil needs checking. A detailed comparison is in hydraulic vs mechanical diaphragm pumps.

Plunger pumps

Plunger pumps reach the highest pressure of all types of metering pumps. The Milton Roy PRIMEROYAL catalog lists up to 4410 L/h and up to 1034 bar, with ±1% steady-state accuracy between 10% and 100% of rated flow. Its NX design with a 10 mm plunger is rated at 1000 bar.

Because the plunger contacts the fluid and relies on packing or seals, be careful with toxic, volatile or expensive chemicals. A hydraulic diaphragm pump is often the safer choice there. Typical duties are oilfield chemical injection, high-pressure methanol injection and high-pressure chemical feed. See the plunger metering pump range.

Picking among the types of metering pumps quickly

Work through this order and you will rarely pick the wrong family:

1. Low flow, moderate pressure: solenoid diaphragm pump.
2. Medium to high flow, low to medium pressure: mechanical diaphragm pump.
3. Higher pressure with zero leakage: hydraulic diaphragm pump.
4. Very high pressure where some seal weepage is tolerable: plunger pump.

The exact model then depends on chemical compatibility, viscosity and temperature, which also set the head material.

Mixing the types of metering pumps on one site

Most plants use more than one family. A mid-sized wastewater plant might run mechanical diaphragm pumps for PAC and polymer, solenoid or small mechanical pumps for hypochlorite, and solenoid pumps for antiscalant on a reuse RO unit. Hydraulic diaphragm and plunger pumps are less common in municipal plants and show up more in power, chemical and oilfield work.

So pick the type per dosing point, then try to standardise brand and series to keep spare parts simple.

Common buying mistakes

Sizing on maximum flow only. A metering pump is most accurate in the middle of its range. Running it near the bottom for years hurts accuracy, so choose a size where normal flow sits mid-range.

Ignoring the flow-pressure link. Within one series, larger sizes often have much lower pressure ratings. This holds across all types of metering pumps.

Thinking like a centrifugal pump buyer. The working principles differ completely, as explained in metering pump vs centrifugal pump.

Conclusion

Each of the types of metering pumps has its job: solenoid for small flows, mechanical diaphragm for medium to large flow at low to medium pressure, hydraulic diaphragm for high pressure without leaks, and plunger for very high pressure. Settle the type from flow and pressure first, then choose materials for the chemical. Send us the chemical, flow, pressure and application through our contact page and we will check catalog models against your duty.

FAQ

Which of the types of metering pumps is cheapest?

Solenoid pumps usually have the lowest entry cost, but for larger flows several solenoid pumps can cost more than one motor-driven pump.

Can a mechanical diaphragm pump reach 30 bar?

Based on the catalogs, common mechanical diaphragm sizes stay within 16 bar and large sizes only reach a few bar. For 30 bar, look at hydraulic diaphragm pumps.

Why do plunger pumps leak?

The plunger touches the fluid and is sealed by packing or seals. As these wear, weepage appears, so plan regular inspection and replacement.

Are peristaltic pumps one of the types of metering pumps?

They are used for dosing but sit outside these four families. They squeeze a hose and suit low flows and fluids with particles.

How often should hydraulic oil be changed?

Follow the manufacturer's manual, and check promptly if the oil level drops or the oil changes colour.