Comparison: Air Operated Double Diaphragm Pump vs. Hydraulically Actuated Diaphragm Pump

This is the second comparison in the series. The first looked at a Wanner Hydra-Cell Pro^® P400 metering pump against a Zenith C-9000 gear pump. This one compares  an air-operated double diaphragm pump next to the Wanner^® hydraulically actuated diaphragm pump.

The two designs do not share much duty. People lump them together because both use diaphragms. The notes below are so you pick the right machine, not the familiar name.

Few hydraulically actuated diaphragm pumps make real industrial flow and pressure. Most of the others are low-flow metering pumps. Call a Hydra-Cell Pro a “diaphragm pump” and a lot of plants hear AODD, because those are everywhere on transfer duty.

An AODD is almost always air-driven. Air quality matters. Dirt, compressor oil and water wear the air end and cut output. IOMs call for dry air filtered to about 5 micron — instrument air. First question: do you have enough clean, dry air?

CFM is published by model and climbs with flow and discharge pressure. An oversized AODD can use less air at the same rate.

Most AODD pumps stop under 250 PSIG. Most of the work they do is under 125 PSIG.

A Hydra-Cell Pro does not need plant air. Models cover 30 to 5,000 PSIG. Overlap with an AODD is in that 30–125 PSI band — or when the process wants more pressure than the AODD or the compressor can give.

Both designs change chamber volume with a diaphragm and use one-way check valves to set inlet-to-outlet flow.

Diaphragms

How the diaphragm is driven is the real split.

On an AODD, compressed air pushes a piston that is tied to both diaphragms. One chamber discharges while the other fills. Each chamber has a ball check into a common inlet and outlet manifold.

At mid-stroke the air swaps sides. Flow pauses. That is the pulse you see. Stroke rate is limited by the air valve, the liquid balls and a long piston travel. A 1″, 37 GPM AODD might hit 8 pulses per second; most run slower.

That long stroke fatigues the diaphragm. Higher discharge pressure speeds the rupture. When it goes, process liquid leaves through the air exhaust.

A Hydra-Cell Pro diaphragm is moved by a column of oil. Oil does not compress, so it acts like a solid piston. The patented balance keeps differential stress across the diaphragm to about 2 PSI — at 30 PSIG or at 1,000 PSIG.

Put a Post-it between your palms and press. The paper does not tear because both sides see the same load. That is the hydraulically balanced diaphragm.

All but one Hydra-Cell Pro frame uses three or five diaphragms. A three-diaphragm pump pulses on the order of 115 times per second, with a short stroke. A liquid-filled gauge sits still. An AODD gauge jumps.

Hydra-Cell Pro diaphragms fail from over-pressure, chemistry or temperature, or something stuck in the chamber (ice included). A torn diaphragm lets hydraulic oil into the process, or process liquid into the oil if inlet pressure is high. Food-contact and other special oils are available. Oil-level monitors catch the change faster.

Cost and Complexity

A Hydra-Cell Pro costs more — about five to seven times an AODD — because:

1. the pump is built heavier
2. it will run to 1,000 PSIG in this comparison band
3. you add a motor, base, coupling, guard and a pressure regulating valve

It also has more parts and wants an oil change every few months of run time.

An AODD is a simple machine. Accessories add cost and material limits: air filter/regulator, pulsation dampener, flexible connectors. Compressed air is not free. Most compressors are less efficient than a standard motor, so the AODD often takes a little more horsepower at the plant level. Sites sometimes switch because the existing compressor cannot grow and a new compressor costs more than the Hydra-Cell Pro skid.

Replacing an AODD with a Hydra-Cell Pro pays when the AODDs are disposable, or when the process needs the smoother, higher-pressure characteristics above.

Applications

AODD pumps own transfer duty: modest pressure, flow linearity not critical, and dry suction lift that beats a same-size Hydra-Cell Pro. They also fit hazardous areas without an XP motor and enclosure.

A Hydra-Cell Pro is overbuilt for low-pressure transfer. Pay for it when you need smooth flow (coating), or controllable, linear, repeatable dosing. Compared with an AODD it is the higher-pressure, smoother, more precise machine, including humid or cold rooms. It also accepts higher inlet pressure and covers a wider duty range.

The pumps a Hydra-Cell Pro usually replaces are piston and plunger pumps — similar action and price band, but packing that wants 25–150 micron filtration. A Hydra-Cell Pro is happy at 420–3175 micron, which is why it lasts on reclaimed and abrasive liquid.

Related Articles

• Comparison to piston and plunger pumps
• Comparison to external gear pumps
• Peristaltic vs AODD
• Pumping difficult liquids
• Hydra-Cell Pro design overview
• Sealless pressure regulating valves