Plunger Metering Pump Specifications: Plunger Diameter, Stroke and Speed

When you open a data sheet, plunger metering pump specifications boil down to three numbers: plunger diameter, stroke length and strokes per minute. Multiply them and you get theoretical flow. The plunger diameter also caps the pressure, because the same drive force spread over a bigger plunger area produces less pressure. Everything else on the sheet, motor power, connections, materials, packing, gets checked after flow and pressure are settled. This guide is for engineers, buyers and maintenance leads who need to compare quotes or pick a model from a catalog.

The three core plunger metering pump specifications

A plunger pump pushes the liquid directly with a reciprocating plunger, with suction and discharge check valves doing the switching. There is no diaphragm in between, so the logic of the numbers is very direct.

Plunger diameter

Usually given in mm, sometimes in inches on US catalogs. A thicker plunger moves more liquid per stroke but delivers lower maximum pressure on the same frame. That is why a single series runs from small-flow, high-pressure models to large-flow, low-pressure models. In the Nanfang JW plunger series, for example, the range goes from about 75 L/h at 0.4 MPa up to 20 MPa at the smallest flows.

Stroke length

Stroke length is the distance the plunger travels from back to front. On the Milton Roy Milroyal D, the plunger stroke is 25.4 mm. That is the fixed maximum of the frame; in daily operation you adjust the effective stroke between 0 and 100%. Do not confuse the maximum stroke on the sheet with your working stroke.

Stroke rate

Strokes per minute (spm) come from motor speed and gear ratio. A higher rate gives more flow, but the check valves open and close more often, which is bad news for viscous chemicals. For thick liquids, choose a slower stroke rate option.

How to calculate flow from plunger metering pump specifications

Displacement per stroke = π/4 × diameter² × stroke length. You can verify this against real plunger metering pump specifications. The Milroyal D U and N plunger heads list plunger diameters of Φ3.2 to 22.2 mm with a 25.4 mm stroke. The formula gives roughly 0.20 to 9.83 cm³ per stroke, which matches the published stroke volume.

Theoretical flow = displacement × spm × 60. Real flow is a little lower because of volumetric efficiency, and efficiency drops as discharge pressure rises. So treat the rated flow as a nominal value at rated conditions, not a promise at every pressure. Near the pressure limit, verify output with a calibration column.

Reading model codes in plunger metering pump specifications

Many Chinese manufacturers write flow and pressure straight into the model number. Nanfang's JW series uses series + flow/pressure: JW75/0.4 means 75 L/h at 0.4 MPa, and JW1/20.0 means roughly 1 L/h at 20 MPa. The same plunger metering pump specifications also list the boundaries you must respect:

1. Maximum discharge pressure 20 MPa and maximum flow 75 L/h, never on the same model.
2. Suction lift up to 1.5 m water column.
3. Liquid temperature -10 to 100 °C, ambient up to 40 °C.
4. Clean liquids without solids; viscous liquids are fine, but the catalog advises against hazardous chemicals.

Western brands use long configuration codes instead. The Milroyal D code includes positions for frame, plunger diameter, liquid end type, plunger material, packing option and single or double ball valves. Decode it position by position from the catalog table rather than guessing.

The 50 Hz vs 60 Hz trap

Most plunger metering pump specifications are published at 50 Hz. Milton Roy states that flows at 60 Hz are 1.2 times the 50 Hz figures, and Nanfang notes a 20% flow increase with an optional 60 Hz motor. If you are exporting a skid, or running a VFD above 50 Hz, include this. When you slow the pump with a VFD, flow drops in proportion, and motor cooling at low speed needs attention.

How to size using plunger metering pump specifications

Pick pressure first, then flow, then materials and packing.

• Pressure: take the maximum line pressure plus friction and injection point pressure, then add margin. The pump rating must be above it, and the relief valve set point below the pump and piping rating.
• Flow: size around 1.3 times the maximum dose so normal operation sits at about 30 to 80% stroke, where adjustment and repeatability are comfortable.
• Materials: JW liquid ends come in 304SS or 316SS, plungers in stainless or stainless with ceramic, with graphite fiber packing. Aggressive or crystallizing chemicals need a separate check on materials and flushing.

One point buyers often miss: a plunger pump seals with packing, and slight weepage at the packing is normal by design. If the chemical is toxic, volatile or must not leak at all, stop comparing plunger metering pump specifications and look at a hydraulic diaphragm pump instead.

Conclusion

Reading plunger metering pump specifications is simple once you know the rule: diameter × stroke × speed gives flow, and diameter limits pressure. Find the models that hold your pressure, choose within that range by flow and stroke margin, then confirm frequency, materials, packing and suction lift. For the mechanics, read how a plunger metering pump works. For turndown, see plunger pump stroke adjustment with frequency control. For other pump types, our guide to metering pump specifications and material codes helps. Browse models in the plunger metering pump range and on the Milton Roy brand page.

FAQ

Is the rated flow at full stroke?

Yes. Rated flow in plunger metering pump specifications normally means 100% stroke, rated speed and 50 Hz power. At 50% stroke, flow roughly halves, and accuracy at very low stroke settings is weaker.

Why do pumps with the same plunger diameter have different pressure ratings?

Different frames. A frame with a stronger drive pushes the same plunger to a higher pressure, so always read the frame and the plunger together.

Can I change the stroke rate myself?

With a VFD, yes, but order an inverter-duty motor. Nanfang specifies a drive with vector control and heavy-duty capability. A standard motor on a VFD tends to overheat at low speed.

Is 1.5 m of suction lift enough?

The suction lift in plunger metering pump specifications is usually enough for thin liquids at normal temperature. Subtract losses from the suction line and foot valve. For viscous or volatile chemicals, mount the pump below the tank level for flooded suction.

What should I send a supplier for a quote?

Chemical name and concentration, temperature, maximum and normal dose, discharge pressure, suction conditions, power frequency and any hazardous area rating. With that, we can match the right model quickly, so contact our engineers.