Milroyal B Hydraulic Diaphragm Metering Pump: Selecting for API675 High-Pressure Duty

250 bar and 2616 L/h. Those are the two headline numbers on the Milroyal B brochure. Plenty of readers assume this hydraulic diaphragm metering pump does both at once. It doesn't. The two extremes come from completely different builds: 250 bar is a small plunger at a low speed ratio, while 2616 L/h is the 4-inch plunger at the highest speed ratio, and that one only manages a few bar.

Selecting a Milroyal B for high pressure comes down to reading the flow and pressure table correctly.

Where the Milroyal B Hydraulic Diaphragm Metering Pump Fits

This hydraulic diaphragm metering pump is designed and built to API675. The brochure lists petrochemical processes, hazardous chemical injection, onshore and offshore oil and gas skids, and power plant boiler, condensate, cooling water and wastewater treatment.

Compared with a mechanical diaphragm pump, a hydraulic diaphragm metering pump uses hydraulic oil to drive the diaphragm. Pressure is balanced on both sides, so it can reach far higher pressure. The Milroyal B adds Milton Roy's MARS mechanical automatic refill system and drops the process-side diaphragm support plate, so fluid passes more freely and the diaphragm is better protected. Accuracy on a traditional hydraulic head can drift slowly over time, and MARS targets exactly that: a mechanical valve refills oil automatically based on diaphragm position, no manual adjustment, and accuracy stays steadier in long-term service.

Reading the Hydraulic Diaphragm Metering Pump Table

Four variables define a pump

The Milroyal B hydraulic diaphragm metering pump has a fixed stroke length of 38.1 mm. Flow and pressure depend on four variables:

1. Plunger diameter: ten sizes from 11 mm (7/16″) to 102 mm (4″);
2. Speed ratio: six steps from 8K to 8M, giving 40–157 SPM (1450 rpm motor);
3. Motor power: 0.25–2.2 kW;
4. Operating pressure code: five steps, PB to PF.

Work through the table in this order: set the pressure, see which plunger sizes still work at that pressure, use the speed ratio to reach your flow, then check the motor size.

A few typical combinations

• 11 mm, speed ratio 8K: 5 L/h at 250 bar (0.25 kW);
• 25 mm, speed ratio 8M: 160 L/h at 24 bar (0.55 kW);
• 102 mm, speed ratio 8M: 2616 L/h at 4 bar (1.1 kW).

The bigger the plunger and the faster the ratio, the more flow and the less pressure. High pressure with low flow and low pressure with high flow are two entirely different builds of the same hydraulic diaphragm metering pump.

Pressure Codes and the Built-In Relief Valve

The operating pressure code sets the maximum setting of the built-in relief valve, so choose it by the real duty when ordering. Operating ranges and maximum relief settings by code:

• PB: 2.1–15.4 bar, relief 17.5 bar;
• PC: 15.5–31.5 bar, relief 36.1 bar;
• PD: 31.6–52.5 bar, relief 60.2 bar;
• PE: 52.6–87.5 bar, relief 100.5 bar;
• PF: 87.6–257 bar, relief 281.0 bar.

On any hydraulic diaphragm metering pump with an internal relief valve, pick a code that's too high and the relief point rises with it, so piping and valves have to be rated higher too.

Liquid End Materials and Their Limits

Standard liquid ends on this hydraulic diaphragm metering pump are 316 SS, PVC, fluoroplastic-lined, Alloy 20 and Hastelloy C22. Diaphragms and gaskets are PTFE, and valve balls are mostly SiC ceramic.

Several limits sit in the brochure footnotes and are easy to miss:

• Plastic heads are limited to 10 bar at 20 ℃, falling linearly to 4 bar at 50 ℃;
• Fluoroplastic-lined pumps are limited to 10 bar working pressure and 50 ℃ fluid temperature;
• Above 14 bar, flow drops 0.8% for every additional 7 bar;
• Adding diaphragm rupture detection cuts flow by another 5%.

So high-pressure projects are effectively limited to metal liquid ends, and flow calculations need to deduct these losses.

Other Choices to Settle

Adjustment options include a manual knob, the ACC electric stroke controller (waterproof or explosion-proof), 4–20 mA electric stroke adjustment, 0.2–1 bar pneumatic stroke adjustment, or a VFD. Choose the explosion-proof ACC for hazardous areas, and consider pneumatic adjustment on platforms without power for the controls.

Rupture detection comes as pressure gauge, pressure switch or conductivity probe double-diaphragm designs. Note that with plungers of 1″ and above, gauge and switch detection can only be used below 14 bar. For fluid temperatures of 88–121 ℃, choose the high-temperature diaphragm construction, which has no rupture detection.

Like many a hydraulic diaphragm metering pump in process service, the Milroyal B can also be multiplexed with separate flow adjustment per head, which suits skids where one drive doses several chemicals.

Data to Prepare Before Asking for a Hydraulic Diaphragm Metering Pump Quote

The model code on a high-pressure Milroyal B is long, and missing data means endless back and forth. Gather this in one go:

• Chemical name, concentration, temperature and viscosity, and whether it carries solids;
• Maximum, normal and minimum flow;
• Normal and maximum discharge pressure, and whether the inlet is flooded or on suction lift;
• Whether the area is hazardous, and the site power supply;
• Connection requirements: thread or flange, and flange rating.

Inlet conditions are the item most often forgotten. The Milroyal B oil valve has standard, medium and higher settings, each allowing a different maximum inlet pressure, so flag a pressurized inlet early. For connections, metal heads offer flanges from ANSI 150# to 1500#, and the rating should match the piping design.

Conclusion

Selecting a Milroyal B hydraulic diaphragm metering pump follows a clear sequence. Set the operating pressure code from the duty, find the plunger diameters that meet that pressure in the table, reach your flow with the speed ratio, then settle the motor, liquid end and detection options. 250 bar and 2616 L/h never appear on the same pump. Account for temperature limits on plastic heads and flow losses at high pressure, and the pump you choose will hold up.

FAQ

At 250 bar, what's the maximum flow?

Per the brochure, 250 bar only appears with the 11 mm and 14 mm plungers, with flows from a few liters up to the mid-thirties per hour depending on speed ratio and motor.

Can a PVC head on a hydraulic diaphragm metering pump work on a 20 bar line?

No. Plastic heads are limited to 10 bar at ambient temperature, and less as temperature rises.

Why is actual flow a bit below the table?

Above 14 bar, flow derates proportionally, and rupture detection takes another 5%. Build that margin in at selection.

Can this hydraulic diaphragm metering pump work offshore without reliable power?

Stroke adjustment can be pneumatic, but the drive motor still needs to suit site conditions, and explosion protection must be written into the model code up front.

What should I watch when pumping fluid around 100 ℃?

Choose the high-temperature diaphragm construction, rated for 88–121 ℃. It has no rupture detection, so you'll need another way to monitor it.

If you have a high-pressure dosing duty sheet, send it through our contact page and we'll check plunger, speed ratio and pressure code against the Milroyal B table with you.