Hydraulic Diaphragm Metering Pump Specifications: How to Read Model Codes for High Pressure

The two numbers on the cover of a brochure are the easiest part of hydraulic diaphragm metering pump specifications to misread. Milroyal B lists 2616 L/h maximum flow per head and 250 bar maximum discharge pressure, but no single pump does both. The big flow comes from the big plunger. The high pressure comes from the small one. This guide is for engineers and buyers sizing pumps for high-pressure injection, and it shows how to break down a model code, read the flow and pressure table, and find the limits hidden in footnotes. For the operating basics, see how a hydraulic diaphragm pump works.

Start With Series Range

Hydraulic diaphragm metering pump specifications vary widely from series to series, so screen by size first:

Milton Roy Milroyal B: up to 2616 L/h per head, up to 250 bar, 10:1 turndown, ±1% steady-state accuracy, built to API675.
Milton Roy Milroyal D: 0.11 to 345 L/h, up to 500 bar, 25.4 mm stroke, fluid up to 110°C, up to 13 heads.
SEKO NEXA: up to 9000 L/h and 200 bar, hydraulic double diaphragm, built to API675.
SEKO NYVA: up to 660 L/h and 124 bar.

Low flow at very high pressure points toward Milroyal D. Large flow at medium to high pressure points toward Milroyal B or NEXA. This only rules out the wrong series. Within one series, capacity can differ by a factor of dozens between plungers, so the real choice happens in the model code and the table.

Breaking Down the Model Code

Take Milroyal B. The code starts with MBH, followed by plunger diameter and material, speed ratio, operating pressure, motor mounting, capacity control, connection, lube oil valve, base, diaphragm detection and number of valve balls. Three segments matter most when you read hydraulic diaphragm metering pump specifications.

Plunger Diameter and Material

Plunger size is the first thing to pin down in hydraulic diaphragm metering pump specifications. Plungers run from 7/16" to 4". Code 071 means a 7/16" plunger with a 316 liquid end. Code 162 means a 1" plunger with PVC. A bigger plunger means more flow and less pressure capability.

Speed Ratio

8K is 36:1, giving 40 spm at 1450 rpm. 8M is 9.25:1, giving 157 spm. Same plunger, faster strokes, more flow. In hydraulic diaphragm metering pump specifications, stroke rate also affects pressure, as the table shows.

Operating Pressure Code

Five codes, PB to PF. PB covers 2.1 to 15.4 bar and PF covers 87.6 to 257 bar, each tied to a maximum built-in relief valve setting. Pick the code that matches your actual operating pressure, not the highest one available.

Reading the Flow and Pressure Table

The Milroyal B table uses a 38.1 mm stroke. Each row lists plunger diameter, speed ratio, maximum flow at 14 bar, then flow and pressure with 0.25 kW, 0.37 kW and 0.55 kW motors.

Take the 11 mm (7/16") plunger with 8J ratio. At 14 bar it gives 9 L/h. With 0.25 kW it reaches 123 bar at 8 L/h. With 0.37 kW it reaches 244 bar at 7 L/h. With 0.55 kW it reaches 250 bar at 7 L/h.

Two lessons. Higher pressure means less flow. And the same flow and pressure may be out of reach if the motor is too small. Look at the same plunger with 8M and 0.25 kW: only 33 bar. Faster strokes lower the pressure a given motor can hold.

A common mistake is to find a flow that looks close and stop there, without checking pressure, or reading pressure from the wrong motor column. On site, the pump then cannot reach design pressure and the relief valve keeps lifting. So read in this order: pressure decides the usable plungers, flow decides the speed ratio, and motor power must cover your pressure.

Footnotes in Hydraulic Diaphragm Metering Pump Specifications

The small print under the table often matters more than the table, and it is where hydraulic diaphragm metering pump specifications most often get misread:

Material limits. Milroyal B fluoroplastic-lined pumps are limited to 10 bar and 50°C fluid temperature.
Flow correction. Above 14 bar, flow drops 0.8% for every additional 7 bar. Diaphragm rupture detection takes another 5%.
Leak detection range. Detection options C5, SN and SE are limited to below 14 bar when the plunger is 1" or larger, and are not available on PVC liquid ends.
High-temperature diaphragm. Code HT covers 88°C up to 121°C, with no rupture detection.

Miss one of these and you end up short on flow or with an alarm you cannot fit. For the full Milroyal B code walk-through, see Milroyal B for API675 duty.

Conclusion

Read hydraulic diaphragm metering pump specifications in three segments, plunger, speed ratio and pressure code, then use the flow and pressure table to match pressure, flow and motor power, and finally check every footnote on materials, flow correction and detection limits. Always confirm against the latest brochure for the series. To check a model against your duty, contact us, or browse hydraulic diaphragm metering pumps and the SEKO brand page.

FAQ

Can hydraulic diaphragm metering pump specifications give maximum flow and maximum pressure together?

No. Maximum flow comes from the largest plunger and maximum pressure from the smallest. Read across a single row.

Why does the same plunger reach different pressures with different motors?

Higher pressure needs more force, and a smaller motor cannot supply it.

Is it safer to choose a higher pressure code?

Not really. The code sets the relief valve, so a code that is too high delays protection when the system overpressures.

Why does rupture detection reduce flow in hydraulic diaphragm metering pump specifications?

The Milroyal B brochure states a 5% flow reduction with detection fitted. Deduct it when sizing.

Can a fluoroplastic-lined pump go on a high-pressure line?

Milroyal B lined pumps are limited to 10 bar and 50°C. For high pressure, choose a metal liquid end.