How to Choose a Plunger Metering Pump for High Pressure, Low Flow

When someone asks how to choose a plunger metering pump, they've usually hit a high-pressure, low-flow duty: a few liters to a few tens of liters per hour, at pressures in the teens or tens of bar, sometimes more. A plunger metering pump is simple and builds pressure easily. That's its home ground. It also has one trait you can't design away, and you need to decide up front whether you can live with it.

Strengths, and the One Trait You Can't Avoid

The plunger reciprocates directly in the pumping chamber, one suction and one discharge per stroke, sealed against the body by packing. With no diaphragm layer, the build is simple, pressure can go very high, and servicing is easy.

The trait you can't avoid: packing always weeps a little. Doseuro from Italy puts it plainly in its catalog. A plunger metering pump suits fluids that are non-abrasive, where a drip-proof system isn't essential and high pressure is needed. The literature also mentions flushing or leakage recovery, but that's an option. If you don't specify it, you don't get it.

So the first step isn't the model list. Ask three questions: Is the chemical toxic? Is any drip acceptable? Does it carry particles? If any answer rules out leakage or abrasion, look at a hydraulic diaphragm pump instead.

Plunger Diameter Sets Pressure, Stroke Rate Sets Flow

With the same driving force, a thinner plunger reaches higher pressure and moves less liquid. The SEKO PS1 plunger metering pump catalog shows this clearly. Maximum pressure with stainless steel heads:

6–30 mm plunger: 20 bar
38 mm: 17 bar
48 mm: 10 bar
54 mm: 8 bar
64 mm: 6 bar

On flow, the same plunger at 116 strokes per minute instead of 58 delivers exactly twice as much: a 6 mm plunger gives 1.5 L/h and 3 L/h, a 64 mm plunger 152 L/h and 304 L/h.

That's the logic behind "plunger pumps for high pressure, low flow." The higher the pressure, the thinner the plunger, and the smaller the flow per pump.

Estimate the Flow Yourself First

Theoretical displacement of a plunger metering pump is plunger area × stroke length × strokes per minute.

For example: a 6 mm plunger, 15 mm stroke, 58 strokes a minute. Area is about 28.3 mm², times 15 mm gives about 424 mm³, or 0.424 ml per stroke. That's about 24.6 ml a minute and about 1.48 L an hour, close to the 1.5 L/h in the catalog.

The bigger the plunger, the wider the gap between theory and catalog. Run the same math for 64 mm at 116 strokes and you get about 336 L/h in theory, against 304 L/h in the catalog, roughly a tenth lower. So use your own estimate to narrow the range, and rely on the catalog and site calibration for the final figure.

Reading a Plunger Metering Pump Model Code

Every maker writes codes differently, but the content is similar. Take PS1D064C31: P means plunger pump, S1 is the drive type, D stands for stroke length, 064 is a 64 mm plunger, C means 116 strokes a minute (A is 58), and 31 is a PVC head (21 is SS316L), followed by motor and option codes.

When you choose a plunger metering pump from its code, focus on three parts: plunger diameter, stroke rate and head material. Between them they set pressure and flow.

Packing, Plunger and Head Material

Head material changes the pressure ceiling directly. In the same PS1 series, most PVC head sizes top out at 10 bar, and the 64 mm only at 4 bar. Maximum fluid temperature is 40 ℃ for PVC and 90 ℃ for SS316L. The PVC version uses a ceramic plunger, the stainless version an SS316L plunger, with FPM or EPDM seals.

Higher still, you need purpose-built high-pressure models. The Milton Roy Milroyal D series offers U and N type plunger heads, with the series reaching up to 500 bar. Packing options include double-stage packing or single-stage packing with a lantern ring, and plungers are available in 316 stainless with a chromium oxide coating. The right packing and plunger surface cut both leakage and wear considerably.

Choosing a Plunger Metering Pump: A Worked Example

Say the chemical is non-toxic and non-abrasive, the peak dose is about 20 L/h, and the injection point is at 15 bar. With a 20% margin, the target is around 24 L/h.

In the PS1 table, a 17 mm plunger at 116 strokes gives 22 L/h. Pressure is fine, but flow is only about 10% over demand, which is tight. A 25 mm plunger at 58 strokes gives 25 L/h at 20 bar with a stainless head, leaving room on both flow and pressure at a lower stroke rate. Written out, that's PS1D025A21 plus motor and option codes. PVC heads drop out here: 10 bar can't hold a 15 bar line.

In practice it comes down to three steps. Use pressure to rule out plunger sizes and head materials that fall short, pick by flow and margin from what's left, and when two options both work, take the slower stroke rate.

Don't Skip These at Installation

Discharge relief valve: if the outlet is blocked, a plunger metering pump builds pressure far past its rating. Fit the relief valve between the pump and the nearest shutoff valve.
Back pressure valve: fit one when injection pressure is low, to stop flow from swinging.
Pulsation dampener: on long, high-pressure lines it noticeably reduces pipe vibration.
Packing checks: watch leakage regularly and adjust or replace packing when it increases, before the drip becomes a stream.

Bottom Line

To choose a plunger metering pump, start with the fluid. Only if it's non-toxic, non-abrasive and slight leakage is acceptable does a plunger metering pump qualify; toxic or no-leak duties go to hydraulic diaphragm pumps. Once it qualifies, size the plunger diameter by pressure, the stroke rate by flow, then match head, plunger and packing materials to the fluid and temperature. Fit the relief valve, back pressure valve and dampener with the pump, check packing regularly, and a plunger metering pump will run a long time on high-pressure, low-flow work.

FAQ

Is a little leakage from a plunger pump normal?

Slight weeping at the packing is normal, which is exactly why it's only recommended for harmless fluids. A clear increase means the packing is worn and needs adjusting or replacing.

Is there a leak-free option under 30 bar?

Yes. The NEWDOSE HND high-pressure mechanical diaphragm pump reaches about 30 bar, and its diaphragm design has no packing leakage. For ordinary chemicals it's worth comparing.

Can PVC heads be used at high pressure?

We don't recommend it. In the catalog, PVC heads generally have a lower pressure limit than stainless at the same size, and temperature only goes to 40 ℃. High pressure almost always means stainless.

Is a plunger pump accurate enough for tight dosing?

The plunger metering pump itself meters steadily; what matters is packing condition and back pressure. Calibrate under real conditions after installation, and again once the packing wears.

Can one plunger pump serve several injection points?

We don't recommend it. Even small pressure differences send the chemical toward the lowest-pressure point. For multi-point high-pressure injection, use a multiplex pump, for example the Milroyal D can be built with up to 13 heads, one per point.

Send your fluid, flow, injection pressure and temperature through our contact page, and we'll help you decide between a plunger metering pump and a diaphragm pump.