Milton Roy G Series Selection: How to Choose a GM or GB Mechanical Diaphragm Pump

Say an older system runs a Milton Roy GB pump, and the dosing point pressure has crept up to around 8 bar. Can you just move up a size? Open the brochure and you'll find everything from the GB0700 upward tops out at 4 bar, so a bigger pump wouldn't even get chemical into the line. That's the most common trap in Milton Roy G series selection: the more flow a model delivers, the less discharge pressure it allows.

The Milton Roy G series is a mechanical diaphragm metering pump range for municipal, industrial water and pool dosing, split into GM and GB branches. Below we go through the data sheet, materials and controls in the order we check them ourselves.

What Separates GM From GB

Both Milton Roy G series branches share the same construction: a mechanically driven diaphragm with a variable eccentric to set stroke, adjustable stopped or running. The difference is mainly the flow band:

• GM: GM0002 to GM0500, 2.25–500 L/h per head;
• GB: GB0080 to GB1800, 82–1800 L/h per head.

The two overlap in the middle. If you need around 400 L/h, both the GM0400 and GB0450 cover it, and pressure and motor decide. GB motors run a size larger than GM across the board, so measure up first if installation space is tight.

Many people start Milton Roy G series selection by checking flow. It's easier to do that step after pressure.

Reading the Milton Roy G Series Data Sheet by Pressure Class

The flow and pressure table in the brochure is the page to study. We regrouped it by pressure class, with maximum flow first and maximum discharge pressure second:

• GM0002–GM0025: 2.25–25 L/h, 12 bar;
• GM0050: 50 L/h, 10 bar;
• GM0090–GM0240: 85–235 L/h, 7 bar;
• GM0330–GM0500: 315–500 L/h, 5 bar;
• GB0080–GB0450: 82–416 L/h, 10 bar;
• GB0500–GB0600: 464–583 L/h, 7 bar;
• GB0700–GB1200: 666–1200 L/h, 4 bar;
• GB1500–GB1800: 1500–1800 L/h, 3 bar, or 4 bar with the 1.1 kW motor.

Shared specs across the Milton Roy G series: 10:1 turndown, ±1% steady-state accuracy, suction lift up to 3 m water column, maximum inlet pressure 2 bar, and fluid temperature up to 60 ℃.

With the table in hand, strike out models that can't meet the actual injection pressure, then pick flow from what's left. Leave 20–30% headroom on the maximum dose so the pump normally runs mid-range, and the 10:1 turndown is plenty. Oversize it and the pump spends its life at short stroke, which makes dosing less steady. Stroke rates marked with an asterisk in the table (180 and 206 SPM) can't take a 60 Hz motor, which matters for export projects.

Choose the Milton Roy G Series Liquid End by Chemical, Not by Habit

Milton Roy G series liquid ends come in PVC, PVDF, 316 and 316L, and the GM adds black PP. The diaphragm is always PTFE, while seats, balls and seals change with the liquid end.

Beyond material codes there are special heads: V for high viscosity, K for slurry, M for mixtures (GM only), and F, a PVC liquid end dedicated to sodium hypochlorite. For hypochlorite on a Milton Roy G series pump, choose F rather than making do with a standard PVC head.

One more detail people miss: even with a PVC liquid end, the seat and ball materials on small GM models and large GB models aren't identical. For unusual chemicals, send the name, concentration and temperature to the factory and let them check each item.

Controls and Motors for the Milton Roy G Series

Three automatic options

Manual stroke adjustment is standard. For automatic dosing there are three routes:

1. An electric stroke controller (E-STROKE) that sets stroke length from 4–20 mA;
2. A motor controller that switches a three-phase motor on and off, GM only;
3. A VFD controller that varies stroke speed, which needs a VFD-rated motor for 5–50 Hz operation.

For flow-proportional dosing, electric stroke or VFD both work. For simple timed start/stop, the motor controller is enough. Decide on control at the selection stage. The model code has a dedicated adjustment position, and adding an electric stroke controller later means rechecking the model and parts.

Motors and explosion protection

Standard motors are IP55 with class F insulation. Explosion-proof (ExdIIBT4) and 60 Hz motors are available. Near chemical areas or fuel depots, put explosion protection into the model code when ordering. Changing it later is painful.

Order Accessories Up Front

The Milton Roy G series brochure is explicit: of the strainer, calibration column, pulsation dampener, safety valve and back-pressure valve, the safety valve is mandatory. If a mechanical diaphragm pump's discharge gets valved shut, pressure keeps climbing. Running without a safety valve is gambling.

GM0002 to GM0050 pumps with PVC or PVDF heads ship with an injection valve, foot valve, weight and 6 m of tubing (except high-viscosity heads), so small projects can skip buying some parts.

Where diaphragm rupture is a concern, choose a double-diaphragm head with a pressure gauge or pressure switch for rupture detection. To feed stroke counts into a PLC, add a stroke counter sensor.

Conclusion

For a Milton Roy G series pump, the core rule is pressure first, flow second. Screen the GM and GB ranges by injection pressure, size flow from the maximum dose plus headroom, choose the liquid end by chemical, set the adjustment method by your control needs, and quote the safety valve, back-pressure valve and other accessories together. Check which models sit in each pressure class before you commit.

FAQ

GM or GB where the Milton Roy G series flows overlap?

Look at pressure first. Around 400 L/h, the GM0400 tops out at 5 bar and the GB0450 at 10 bar. If injection pressure is on the high side, take the GB. If not, the GM's smaller motor does the job.

Can a Milton Roy G series pump handle chemicals above 60 ℃?

The brochure rates fluid temperature at up to 60 ℃. Above that, the G series isn't recommended. Switch series or ask the factory for a specific assessment.

Will it prime if mounted above the tank?

The G series is rated for up to 3 m water column suction lift, but that's an ideal figure. High viscosity and long lines reduce it, so flooded suction is best. Fit a foot valve and strainer at the tank bottom so sediment can't lodge in the check valves.

What's the difference between VFD and electric stroke control?

A VFD changes stroke speed and electric stroke control changes stroke length. Both accept 4–20 mA. Which suits you depends on site power and the control panel.

Can I skip the safety valve to save money?

No. The brochure lists it as a mandatory accessory. If someone closes a discharge valve by mistake, it protects the pump head and piping.

To check GM and GB models against your duty, put together the chemical, flow and injection pressure and send them through our contact page. We'll go through the selection with you.