Double Diaphragm Metering Pump: How the Intermediate Chamber Detects a Rupture

A double diaphragm metering pump works like any hydraulic diaphragm pump: a plunger pushes hydraulic oil, and the oil flexes the diaphragm that moves the chemical. The difference is that two diaphragms replace one, with a sealed intermediate chamber between them. In normal service that chamber is cut off from both the oil and the process fluid. If either diaphragm fails, fluid enters the chamber, the pressure or conductivity inside it changes, and a detector raises a rupture alarm. This guide is for process engineers, instrument techs and maintenance leads who need to know how that alarm actually works and how to specify it.

One naming note first. Air-operated double diaphragm (AODD) transfer pumps use two diaphragms side by side to move fluid. A double diaphragm metering pump stacks two diaphragms in one head for containment. Same words, very different machines.

How a double diaphragm metering pump moves fluid

Read a hydraulic diaphragm head from the inside out: plunger and oil chamber, the oil-side diaphragm, the intermediate chamber, the process-side diaphragm, then the process chamber with its suction and discharge check valves. On the discharge stroke, oil pressure pushes the first diaphragm, which pushes the second through the chamber, and the chemical is expelled. On the return stroke both diaphragms pull back together and the pump fills.

For the two diaphragms to move as one, the chamber cannot hold compressible air. Oil refill, relief and air bleed work exactly as in a single diaphragm head; see how a hydraulic diaphragm metering pump works for that part.

There is a small capacity cost. The Milton Roy Milroyal B brochure notes that adding diaphragm rupture detection reduces flow by 5%, so size the double diaphragm metering pump on the derated figure.

Two ways to build a double diaphragm metering pump head

Sandwich (contact) type. The two diaphragms sit against each other with only a thin detection passage between them. In the Milroyal D order code, pressure detection goes with this construction. Doseuro calls its SD model a sandwich double diaphragm and lists toxic, corrosive, no-leak duties that require a rupture detector.

Separated type. The diaphragms are spaced apart and the gap is filled with an intermediate liquid. Milroyal D pairs this with electrolyte-based detection, and the mRoy options include an intermediate-liquid double diaphragm rupture detector with or without a probe. Which liquid is used is set by the manufacturer.

Pressure-type rupture detection on a double diaphragm metering pump

In normal running the chamber is sealed and close to zero pressure. Once one diaphragm tears, pressure from the oil side or the process side leaks in and climbs with each stroke.

You can read it locally on a gauge during rounds, or fit a pressure switch that changes state at a set point and signals the control panel or DCS. Milton Roy offers gauge-only, gauge plus NEMA 4 switch and gauge plus NEMA 7 explosion-proof switch versions.

Pressure detection has limits. The Milroyal B literature states that pressure-type detection is not available on PVC liquid ends, and with plunger diameters of 1 inch or more it is limited to service below 14 bar. Other brands may carry similar notes, so check every line of the data sheet.

Conductivity-type detection in a double diaphragm metering pump

The separated design watches the intermediate liquid instead. Process fluid leaking in changes its conductivity or resistivity, and a probe trips the alarm. Milroyal D lists resistivity and charge-density detection, plus intrinsically safe alarm versions for hazardous areas.

This only works if the leaking fluid changes the reading. Acids, bases and salt solutions usually do. A fluid that is almost non-conductive may not, and a pressure-type double diaphragm metering pump is the safer choice there.

Choosing the alarm for a double diaphragm metering pump

1. Conductivity of the process fluid. Conductive fluids suit either method; near-insulating fluids point to pressure detection.
2. Liquid end material and pressure. PVC heads, large plungers and high pressure all need a check against the pressure-type limits.
3. Hazardous area. Specify explosion-proof switches or intrinsically safe probes where required.
4. Remote signal. A local gauge nobody reads on the night shift is not an alarm. On critical duty, wire a switch into the control system.

After the alarm: can a double diaphragm metering pump keep running?

The SEKO Nexa brochure puts the benefit plainly: if one diaphragm fails, the protection system warns immediately while the second diaphragm keeps the pump running, so there is no forced shutdown and maintenance can be planned.

Keep running does not mean run indefinitely. You are down to one barrier, so schedule a diaphragm change soon. Stop the pump, close the suction and discharge valves, relieve pressure and follow the chemical's safety data sheet before opening the head. For what the failure looks like in the field, see hydraulic diaphragm rupture warning signs.

Two field mistakes defeat the whole design: not restoring the intermediate chamber as the manual specifies after a diaphragm change, and jumpering out an alarm that keeps tripping. Either one turns a double diaphragm metering pump back into a single diaphragm pump.

Conclusion

The real value of a double diaphragm metering pump is the intermediate chamber, not the extra diaphragm. It stays isolated in normal service, and when a diaphragm fails the change in pressure or conductivity trips the alarm. Sandwich heads pair with pressure detection, separated heads with conductivity-type detection. Choose by fluid conductivity, liquid end material, pressure, area classification and the need for a remote signal. For toxic or no-leak duties, start with our hydraulic diaphragm metering pump selection guide, then browse hydraulic diaphragm pumps and the Milton Roy range. Send fluid, pressure, flow and area classification and contact our engineers to confirm the right detection option.

FAQ

Is a double diaphragm metering pump less accurate?

It reduces capacity slightly. Milton Roy states a 5% flow reduction on Milroyal B when rupture detection is added, so size with margin and take accuracy from the data sheet.

Can the alarm tell which diaphragm failed?

Usually not. It only shows that something entered the chamber. Inspect both diaphragms and replace them as a pair.

Is pressure detection available on PVC heads?

On the Milton Roy Milroyal B, no; the literature excludes PVC liquid ends. For other brands, check the data sheet rather than assuming.

Do mechanical diaphragm pumps offer a double diaphragm?

Yes. The Milton Roy G series GM/GB offers a double diaphragm head with gauge and pressure switch detection.

Should the rupture alarm be tested?

Yes. After a diaphragm change or at the annual overhaul, confirm the alarm circuit works as the manual describes, and that the signal really reaches the control system.