Diaphragm pumps are similar to piston and plunger pumps, except that the reciprocating motion of the pump causes a diaphragm to flex back and forth, which in turn causes the liquid to flow into and out of the liquid end of the pump. As with all reciprocating pumps, diaphragm pumps require check valves at the inlet and outlet ports . The diaphragm is usually made of an elastomeric material to allow it to flex. The diaphragm can be mechanically attached to the reciprocating member or it can be separated and actuated by a reservoir of hydraulic fluid, often with a contour plate to control the travel limits of the diaphragm
One very common application for diaphragm pumps of the type described above is for metering applications. Metering pumps, or dosing pumps as they are called in Europe, have relatively low flow rates, usually measured in gallons or liters per hour rather than per minute. These pump types are highly accurate in measuring flow (usually having an accuracy of better than ±1%), and the diaphragm makes the pump leak-free and compatible with a variety of liquids. above Figure shows a hydraulically actuated diaphragm-metering pump with stroke adjustment capability to vary the flow rate.
Another style of diaphragm pump is the solenoid metering pump, used in light-duty metering applications. This style of diaphragm pump uses an electrical signal to magnetically move the plunger/diaphragm assembly.
Much larger versions of hydraulically actuated diaphragm pumps are used in process services, where their high pressure capability and sealless pumping make them an interesting alternative for special services. These pumps, with metal diaphragms and remote heads, can pump liquids to 900°F.
Another type of diaphragm pump is the air-operated, double-diaphragm pump . In this pump design, compressed air enters the air chamber behind one of the diaphragms , flexing the diaphragm and thus forcing the air or liquid on the other side of the diaphragm out the discharge check valve. Simultaneously, the second diaphragm in below Figure is pulled inward by a rod connecting the two diaphragms, creating a suction stroke with that diaphragm, with liquid coming in through the inlet check valve on that side of the pump. Then a shuttle valve causes the air distribution to shift, sending air to the other chambers and reversing the stroke of the two diaphragms.
Any type of diaphragm pump has the distinct advantage, compared with piston or plunger pumps, of being sealless, that is, not requiring any packing assembly or mechanical seal. Air-operated diaphragm pumps offer the additional advantage of being able to accommodate large solids, abrasives, and corrosives. They are self-priming and can run dry. Their versatility makes them a good choice for pumping wastewater, acids, and foods. The shortcomings of air-operated diaphragm pumps are that they require air to operate (this may actually be a benefit if the pump is in an area where compressed air is available but electricity is not), they have limitations on flow and pressure, they produce fairly large pressure pulsations, and they are quite energy inefficient. Some designs also have problems with the air valves stalling or freezing up, and some air valves require periodic lubrication.
Another type of diaphragm pump, is known as a wobble plate pump. This pump type has the reciprocating action of several pistons or diaphragms caused by a rotating plate mounted eccentrically on the shaft. Advantages of this pump type include quite high pressure capability, sealless pumping, self-priming, and the capability of running dry or with a blocked inlet line. Disadvantages include relatively low flows and many moving parts.
