Hydraulic Diaphragm Pump Parts: Inside a Milton Roy Refill and Relief System

If you are taking over a Milton Roy plant or reviewing a spare parts quote, it helps to know the hydraulic diaphragm pump parts by location before you open anything. A hydraulic diaphragm metering pump has three separate fluid spaces: lube oil in the gearbox, hydraulic oil behind the diaphragm, and process fluid in front of it. The plunger never touches the chemical. It pushes oil, and the oil flexes a PTFE diaphragm. Two valves keep that oil space healthy: the refill system and the relief valve. This article uses Milroyal B, mRoy A/B and Primeroyal literature as the reference.

How to Read the Drawing: Three Zones of Hydraulic Diaphragm Pump Parts

Split any Milton Roy cross-section of hydraulic diaphragm pump parts into three zones and it stops looking complicated.

1. Drive end. Motor, worm gear set and the stroke adjustment mechanism. mRoy uses a worm gear running in an oil bath. Milroyal B combines forced lubrication with oil-bath lubrication.
2. Hydraulic chamber. Plunger, hydraulic oil, the MARS refill valve and the relief and vent valve.
3. Liquid end. Diaphragm, suction and discharge check valves and the head cover. This is what most people call the pump head.

The Primeroyal PH outline drawing labels the oil ports clearly. R1 is the lube oil fill, V1 the lube oil drain and L1 the lube oil sight glass. R2, V2 and L2 are the same three points for hydraulic oil. Two oils, two sets of ports. Filling the wrong one is a classic commissioning mistake.

Drive End: Where Stroke Length Comes From

The motor drives the worm gear, which drives the plunger back and forth. Primeroyal uses a variable eccentric: shifting the eccentric changes stroke length, shown as a percentage on the adjuster. Flow is set by stroke length and stroke rate together.

Drive end trouble usually shows up as noise, heat or a falling gearbox oil level. That is a different problem from a pump that runs smoothly but under-delivers, so separate the two before you order hydraulic diaphragm pump parts.

Hydraulic Chamber: The Core Hydraulic Diaphragm Pump Parts

MARS Automatic Refill

Of all the hydraulic diaphragm pump parts, this one does the most quiet work. A small amount of hydraulic oil slips past the plunger on every stroke. Without makeup, the chamber slowly runs short, the diaphragm stops reaching full travel and flow drops. Older designs use an adjustable refill valve that someone has to set at startup. Milton Roy uses MARS, a mechanical automatic refill system.

The Milroyal B literature breaks the cycle into three steps. With the diaphragm and plunger forward, the refill valve is pushed forward and the poppet valve stays closed, so no oil enters from the makeup line. On the return stroke the refill valve moves back with them. If the oil volume is correct, the poppet stays shut. If oil is short, the refill valve keeps moving back, pressure falls, the poppet opens and oil flows into the chamber.

In plain terms, it only adds oil when the diaphragm has fully retracted and the chamber is actually short. Milton Roy points out that startup is simpler because there is no fill valve to adjust, and the high-performance head removes the traditional disc-shaped diaphragm support plate on the process side.

Relief and Vent Valve

The Milroyal B head drawing labels a combined vent and relief valve. Air trapped in the oil gets compressed on each stroke and eats into displacement, so the vent clears it. If a discharge valve gets closed by mistake or the line blocks, oil pressure climbs to the set point and the relief valve dumps oil back to the reservoir.

The relief setting is tied to the operating pressure code. On Milroyal B, code PB covers 2.1 to 15.4 bar with a maximum relief setting of 17.5 bar. Code PD covers 31.6 to 52.5 bar with 60.2 bar. Pick the code for your real operating pressure and leave the relief valve alone afterward. This applies to most hydraulic diaphragm pump parts in the oil circuit: they are matched as a set.

Liquid End: Diaphragm and Check Valves

The wetted hydraulic diaphragm pump parts on Milroyal B come in 316 SS, PVC, fluoroplastic-lined, Alloy 20 and Hastelloy C22. Diaphragm and gaskets are PTFE and valve balls are mostly SiC ceramic. For more safety, a double diaphragm with rupture detection adds a pressure gauge or switch between the two layers. Our hydraulic double diaphragm pump guide covers how that alarm works.

One easily missed note: Milroyal B flow drops 5% with rupture detection fitted, and another 0.8% for every 7 bar above 14 bar. Allow for that when sizing.

Inspection Order Based on Hydraulic Diaphragm Pump Parts

Work from the outside in. Low flow? Check the L2 hydraulic oil sight glass for level and color first, then the suction and discharge check valves, and only then suspect the diaphragm. Milky oil, or oil tinted with process color, usually means a failed diaphragm. Before opening the liquid end, relieve discharge pressure, drain and flush the head and wear protection for the chemical. Use the oil grade and level given in the manual for that model.

For the general operating cycle, see how a hydraulic diaphragm pump works. For high-pressure sizing, see Milroyal B for API675 duty. More models are under hydraulic diaphragm metering pumps and the Milton Roy brand page.

Conclusion

Remember the hydraulic diaphragm pump parts as three chambers and two valves: lube oil, hydraulic oil, process fluid, plus MARS refill for oil volume and the vent and relief valve for air and overpressure. Read the drawing by zone, then identify the ports. If you need to service or replace a Milton Roy unit, send us the nameplate model, operating pressure and fluid through our contact page.

FAQ

Do the MARS refill hydraulic diaphragm pump parts need adjustment?

No. Milton Roy states that MARS makes startup simpler because there is no fill valve to adjust. That is the main difference from older hydraulic pumps.

Is hydraulic oil the same as gearbox lube oil?

No. They are filled and checked separately. On Primeroyal the ports are marked R2 and R1. Follow the model manual for oil grade.

Can I raise the relief setting to get more pressure?

It is not recommended. The setting matches the operating pressure code, and raising it can push other hydraulic diaphragm pump parts past their rating.

What is the poppet valve on the drawing?

It is the MARS valve that opens the makeup oil path when the chamber is short and pressure drops, letting hydraulic oil in.

Why vent a new pump before startup?

Air in the oil chamber or head compresses, so strokes are not fully delivered and flow reads low. The mRoy liquid end has a vent system for exactly this, at first startup and after service on hydraulic diaphragm pump parts.