Metering pump types: mechanical diaphragm vs hydraulic vs solenoid vs piston — which one to choose?

Solenoid pumps use a solenoid coil to drive the diaphragm in rapid on-off pulses, making them suitable for dosing points with diluted chemicals at low flow rates. Flow rate is adjusted via stroke length combined with pulse frequency. Advantages include compact construction, no lubrication required, and immediate response to control signals — convenient for dosing systems controlled by pH/ORP sensors.

Within the Doseuro range, the SDP type S series represents this group: flow rate up to 22 l/h, pressure up to 20 bar, IP65 protection rating, solid PTFE diaphragm, available in 6 sizes. This is the option when the dosing point requires small, stable doses at moderate pressure with simple installation.

Mechanical diaphragm pumps use a motor-driven cam or eccentric mechanism to drive the diaphragm cyclically. Compared to solenoid types, these deliver higher flow rates and stable continuous operation, making them a familiar choice in drinking water and wastewater treatment. Pump head material is selected according to the fluid: SS316L, PVC, PP, PVDF, or PTFE.

The Doseuro SR type D series covers 49–1.400 l/h, pressure 3–14 bar, with ATEX versions available for hazardous areas. Common applications include dosing NaOCl/chlorine, PAC, NaOH, and polymer. When a compact plastic body resistant to acids is needed at lower flow rates, the SR type FM series (14–47 l/h, 5–10 bar) is the compact variant of the mechanical diaphragm group.

In a hydraulic diaphragm pump, the piston does not push the diaphragm directly but acts through a layer of oil. The oil distributes force evenly across the diaphragm, reducing localized stress and enabling higher working pressure. This sealed construction limits fluid leakage — critical when metering hazardous chemicals.

The SR type B series (including B and BR) covers 108–1.068 l/h, standard pressure up to 20 bar, with the high-pressure AP-B version reaching 60 bar, and ATEX options available. The BR version adds a relief valve to protect the diaphragm from overpressure. This series bridges mechanical diaphragm and piston pumps: retaining the sealed, safe advantage of the diaphragm while reaching higher pressure ranges.

Piston (plunger) pumps allow the piston to come into direct contact with the fluid inside the pump chamber. Without the diaphragm layer, this type achieves significantly higher pressure and good metering accuracy at high pressures. In return, the seal contacts the fluid directly, so material selection must match the chemical.

The SR type A series covers 156–1.458 l/h, standard pressure up to 20 bar, with the high-pressure AP-A version reaching 230 bar with SS316L pump heads — for concentrated chemicals. For oil & gas, petrochemical, and LNG processes requiring specific standards, Doseuro offers the PDP series designed to API 675 with ATEX.

The selection process comes down to four steps. First, determine the required dosing flow rate to eliminate groups that do not cover the range. Then check the pipeline pressure: below 20 bar there are many options; above this threshold, a high-pressure version (AP-A/AP-B) or piston pump must be used. Next, consider accuracy and leak-tightness requirements — hydraulic diaphragm and API 675 process pumps have an advantage here. Finally, select the pump head material (316L/PVC/PP/PVDF/PTFE) based on fluid corrosiveness. Actual metering accuracy is typically within ±1–3% depending on type; the API 675 standard requires ±1% at steady state.

All four groups can be controlled manually or via 4-20mA signal, combined with pH/ORP/flow sensors for automatic dosing according to operating conditions. The total flow rate range of the Doseuro series spans from approximately 1 l/h to 4.000 l/h, covering most industrial metering requirements.