How to Prime a Milton Roy Pump: First Startup and After a Chemical Change

If you want to know how to prime a Milton Roy pump, the routine is short: get liquid into the suction line, take the pressure off the discharge, run at 100 percent stroke and let the pump push the air out. When the discharge runs solid liquid with no bubbles, reconnect the line and return the stroke to its working setting. What changes from one model to another is where the vent point is and how you relieve discharge pressure. This guide is for operators and commissioning technicians working on G Series GM/GB mechanical diaphragm pumps, LMI solenoid pumps and the process side of mRoy and Milroyal hydraulic diaphragm pumps.

Identify the pump family before you prime

How to prime a Milton Roy pump depends on the design. The G Series is a motor-driven mechanical diaphragm pump; its datasheet quotes a suction lift of up to 3 m of water, a maximum inlet pressure of 2 bar and an oil-bath lubricated drive. LMI pumps are the solenoid range under the Milton Roy group, with small strokes that air-lock easily. mRoy and Milroyal are hydraulic diaphragm pumps. The mRoy datasheet mentions a venting system on the liquid end for first startup and restart after service, and the Milroyal B lists a built-in relief and vent valve with automatic venting on the hydraulic side.

Hydraulic pumps have two kinds of air. One sits in the hydraulic oil chamber and is handled by the pump's own venting or refill system. The other sits on the process side, and that is the air this guide on how to prime a Milton Roy pump deals with. For oil fill and refill on a specific hydraulic model, follow that model's manual rather than opening valves on the pump body.

Pre-start checks before you prime a Milton Roy pump

• On the G Series, check the drive oil level; on hydraulic pumps, check the hydraulic oil as well. Levels follow the manual.
• Fit the foot valve and strainer, and tighten every suction joint. A tiny suction leak means the pump never fully primes.
• Open the suction and discharge isolation valves and note where any back pressure valve sits.
• If the layout allows a flooded suction, keep the tank level above the pump head so liquid fills the head by gravity.

Suction lift decides how hard it is to prime a Milton Roy pump. A G Series pump can lift 3 m, but a dry suction line with dry check balls at that height can take a long time to catch.

How to prime a Milton Roy pump step by step

1. Disconnect the discharge line at the pump, or open the discharge vent or bypass, so the pump discharges to atmosphere into a container.
2. Set stroke length to 100 percent, and on pumps with adjustable speed, set speed to maximum.
3. Start the pump and watch the discharge: first air, then foamy liquid, then solid liquid.
4. When the flow has run bubble-free for about half a minute, stop, reconnect the line or close the vent.
5. Return stroke to the working value, restart and check that the discharge gauge is steady.

Why is relieving discharge pressure so central to how to prime a Milton Roy pump? If the head is full of air, the diaphragm only compresses that air. It never builds enough pressure to open the discharge check valve, so no liquid is drawn in. Against atmosphere, the air can leave. For setting stroke afterwards, see our article on Milton Roy pump flow adjustment.

On LMI models, how to prime a Milton Roy pump is simpler: pumps with a FASTPRIME liquid end have a bleed knob, so you do not need to break the discharge line. The procedure is covered in how to bleed an LMI FASTPRIME liquid end.

How to prime a Milton Roy pump after a chemical change

Two situations look similar but are not. If you simply swapped to a fresh drum of the same chemical and the suction line pulled air, repeat the steps above. If you are switching to a different chemical, flush first.

When switching between an acid and sodium hypochlorite, flush the head and lines thoroughly with clean water. Mixing the two releases chlorine gas, and the pump head is the worst place for that to happen. Wear goggles and chemical gloves, point the open discharge into a container, never toward people, and confirm that the new chemical is compatible with the head material, since PVC, PVDF and 316 SS heads tolerate different chemicals.

After flushing, move the suction line into the new tank and follow the same steps for how to prime a Milton Roy pump. A thicker or thinner chemical will change how long the prime takes.

When Milton Roy pump priming will not finish

If the pump keeps discharging only air, the pump itself is rarely the problem. Check these first:

• an air leak on the suction side at a fitting, foot valve or strainer seal;
• a stuck check ball or a valve reassembled upside down, common after maintenance;
• back pressure still on the discharge from a back pressure valve or the injection point;
• an off-gassing chemical such as hypochlorite that keeps producing gas in the head.

Working through these is faster than repeated stop-start attempts, and it is the part of how to prime a Milton Roy pump that quick guides skip. Our Milton Roy G Series troubleshooting guide gives a fuller sequence, and the general method is in how to prime a diaphragm metering pump.

Conclusion

How to prime a Milton Roy pump comes down to one sequence: flood the suction, relieve the discharge, run full stroke, wait for solid liquid, then reset. G Series and LMI pumps prime reliably this way. On mRoy and Milroyal hydraulic pumps, deal with the process side only and leave the hydraulic side to the manual. Always flush between incompatible chemicals. You can view the range on our Milton Roy brand page. If a pump still will not prime, send us the model, chemical, suction lift and installation layout and contact our engineers.

FAQ

When learning how to prime a Milton Roy pump, do I really need 100 percent stroke?

Yes, it helps. The bigger the volume per stroke, the easier the air is pushed out. Set the working stroke again once primed.

With a flooded suction, do I still need to know how to prime a Milton Roy pump?

Yes, but it is much quicker. Gravity pushes most of the air out of the head and liquid appears within moments of starting.

Do I vent the hydraulic oil chamber on a Milton Roy hydraulic pump myself?

Usually not. The Milroyal B has a built-in relief and vent valve and the mRoy liquid end has a venting system. Follow the manual for the exact model when learning how to prime a Milton Roy pump of this type.

Can the pump run dry for a long time while priming?

Avoid it. Check balls and the diaphragm wear faster without liquid. If nothing comes after a few minutes, stop and check the suction side.

How do I avoid repeated priming in the future?

Fit a low-level alarm so the suction never uncovers, use a flooded suction where possible, and choose a degassing head for off-gassing chemicals like hypochlorite.