Doseuro Metering Pump Principles: Plunger vs Hydraulic Diaphragm for High Pressure

Once line pressure climbs into the tens of bar, the question we get most is plunger or hydraulic diaphragm. Doseuro builds both, and the model list is long enough to make anyone's eyes glaze over. So let's start with how each Doseuro metering pump design works. Once the mechanics are clear, selection is mostly a matter of matching the fluid and the pressure.

How a Doseuro Metering Pump Works: Two Designs

In a plunger pump, the plunger reciprocates directly in the pumping chamber, sealed against the body by packing. It's simple, it reaches very high pressure, and it's easy to service. The trade-off is that packing always weeps a little. The Doseuro metering pump literature is blunt about it: plunger pumps suit fluids that are non-abrasive and not dangerous, where a drip-proof system isn't essential. Flushing or leakage recovery can be added if needed.

In a hydraulic diaphragm Doseuro metering pump, the plunger still reciprocates, but it pushes on a chamber of hydraulic oil, and the oil moves the diaphragm. The chemical only touches the diaphragm and the head, completely sealed from the outside. That makes it the right fit for toxic, corrosive fluids where leakage isn't acceptable. The literature also says it handles fluids carrying small amounts of suspended solids.

The SD, SDI and MR I Doseuro metering pump models use a sandwich double diaphragm with rupture detection between the two layers. If one layer fails, the system flags it before chemical reaches the oil. A built-in safety valve in the hydraulic circuit also protects the diaphragm if the discharge is blocked.

Spring Return vs Positive Return

The second thing to understand about any Doseuro metering pump is the return mechanism, because it decides which family a model belongs to.

The spring return (SR) family covers the type A plunger pump and the B&BR and SD hydraulic diaphragm pumps. The motor is mounted vertically, a worm and wheel gearbox reduces speed, and a spring pulls the plunger back. It's a compact layout.

The positive return family covers the A-I plunger pump and the SDI and MR I hydraulic diaphragm pumps. The literature states API 675 and ATEX compliance, with an oil-bath lubricated internal worm gear, and pump heads can be combined into multi-head units. The US catalog gives accuracy of ±1% within 10% to 100% of rated capacity. Oil and gas or petrochemical projects that call for API 675 usually start their Doseuro metering pump search here.

In either family, stroke length on a Doseuro metering pump can be adjusted with the pump running or stopped, manually or with a 4–20 mA, fieldbus or pneumatic actuator.

Doseuro Metering Pump Specs: Plunger Models

In the positive return A-I series, the standard A I 175 delivers about 385 L/h at 50 bar, while the A I 250 and A I 350 reach about 1265 L/h and 3569 L/h at 40 bar.

For more pressure, look at the AP high-pressure versions. The AP-A I 175 gives about 44 L/h and the AP-A I 350 about 477 L/h, with maximum pressure listed between 230 and 250 bar and higher pressures available on request. The spring return type A also comes in an AP version rated up to 250 bar.

Standard wetted materials are 316L, PVC and PVDF, and type A adds PP and PTFE. At high pressure it's mostly stainless steel; plastic heads only make sense at lower pressure.

Flushing and leakage recovery are options, not standard. If the fluid is harmless but you'd rather not have drips on the floor, add it when ordering. Retrofitting later is far more hassle.

Doseuro Metering Pump Specs: Hydraulic Diaphragm Models

Each series has its own role:

• B&BR: single diaphragm. B125N gives about 108 L/h and B250N about 1068 L/h, up to 20 bar. The BR version adds a safety valve in the hydraulic circuit.
• SD: sandwich double diaphragm. SD 175N gives about 418 L/h at 14 bar; the high-pressure AP SD 175N about 247 L/h at 40 bar.
• SDI: positive return. SD I 350 gives about 4028 L/h at 14 bar, and AP SD I 250 about 711 L/h at 80 bar.
• MR I: high-pressure sandwich diaphragm. All three sizes are rated 200 bar, up to about 2390 L/h, in 316L as standard. The literature notes it suits very low inlet pressure.

So the hydraulic diaphragm side of the Doseuro metering pump range covers everything up to 200 bar quite thoroughly. Above that, plunger pumps take over.

Choosing a Doseuro Metering Pump for High Pressure

This is the order we work through:

1. Toxic, flammable, or no leakage allowed: rule out plunger pumps and look at SD, SDI or MR I;
2. Clean, harmless fluid above 200 bar: AP plunger pumps;
3. Small amounts of suspended solids: hydraulic diaphragm, since packing doesn't like abrasion;
4. API 675 or explosion-proof requirements: pick from the positive return family.

Two oil and gas uses listed by Doseuro show how this works. Methanol injection at the wellhead is flammable and high pressure, so rule one knocks out plunger pumps and you choose between MR I and AP SDI by pressure. High-pressure water reinjection is harmless but demanding on pressure, and here an AP plunger pump makes more sense: simpler, with easier spares. Same pressure class, different fluid, opposite answer.

A few site traps. Fit a relief valve on the discharge, because a closed valve lets the pump build pressure far beyond its rating. Add a back pressure valve when injection pressure is low, or the flow won't be accurate. Check plunger packing regularly and adjust or replace it when leakage grows, before the drip turns into a stream.

Conclusion

Every Doseuro metering pump follows one of two paths. A plunger pushes the fluid directly, giving high pressure and a simple build at the cost of slight leakage. Oil pushes a diaphragm, giving zero leakage and tolerance for toxic fluids with small solids, up to 200 bar without strain. Dangerous fluid means hydraulic diaphragm. Clean fluid at extreme pressure means plunger. API 675 projects belong in the positive return family.

FAQ

How often does the hydraulic oil need changing?

The literature gives no fixed interval; it depends on hours and temperature. Check oil level and color on your maintenance schedule, and change it if it turns dark or milky.

Is a little leakage from a plunger pump normal?

Slight weeping from packing is normal, which is why plunger pumps are recommended for harmless fluids. A clear increase means the packing is worn and needs adjusting.

What happens if a diaphragm ruptures?

With a sandwich double diaphragm, the detector gives a signal when one layer fails, and the other layer generally keeps the fluid isolated, so you can plan a shutdown. The single-diaphragm B type has no such backup; a rupture lets chemical into the oil.

Can plunger and diaphragm heads share one multi-head unit?

The positive return family can combine heads of different sizes and stroke settings. Whether plunger and diaphragm heads can be mixed should be confirmed with the factory for your project.

For around 30 bar, is there a simpler option?

Consider a mechanical diaphragm pump. The NEWDOSE HND high-pressure mechanical diaphragm pump reaches about 3 MPa with no hydraulic oil to maintain, and suits some medium to high pressure duties. We can compare options with you.

Send us your fluid, flow and line pressure through our contact page and we'll pick the right Doseuro metering pump design with you.