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 to the Wanner^® hydraulically actuated diaphragm pump.

The two designs are quite different considering they both use diaphragms.

There are few manufacturers of hydraulically actuated diaphragm pumps capable of relatively high flow rates and pressures; most of those other manufacturers specialize in low flow rate metering pump applications. If you describe the Hydra-Cell pump as simply a “diaphragm pump”, many customers immediately think of air operated double diaphragm pumps because they are ubiquitous to industrial liquid transfer applications.

An AODD pump is pneumatically driven, almost always via compressed air. The quality of the compressed air is important because contaminants (particles, compressor oil, water) accelerate component wear and may directly reduce performance. AODD pump IOM manuals suggest dry air filtered to 5m referred to as “instrument quality air”. Thus, the first aspect to consider for an AODD pump application is do you have sufficient compressed air that is both relatively dry and clean?

The volume of compressed air required is noted by the manufacturer per pump model and both the desired flow rate and maximum operating pressure determine the cubic feet per minute (CFM) required. Sometimes you can increase efficiency by using an oversized pump.

The majority of AODD pumps are limited to <250 PSIG maximum discharge pressures and actually most applications are for discharge pressure <125 PSIG.

Hydra-Cell pumps do not require compressed air and can operate within a pressure range of 30 to 5,000 PSIG depending upon the pump model. Any application overlap between the two different pump designs happens within that 30-125 PSI range and sometimes customers figure-out their process would benefit from higher pressures than their AODD pumps or air compressors are capable of providing.

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

Diaphragms

This is perhaps the number one differentiation between AODDs and Hydra-Cell pumps: their diaphragm actuation method. Diaphragms used in AODD pumps are pushed forward and pulled backwards due to compressed air pushing against a piston and relieving the air behind the diaphragm. A single piston is attached to both diaphragms, one diaphragm per pump chamber. Each pump chamber has a ball check valve to control the flow path from the inlet manifold to the outlet manifold. The compressed air pushes one diaphragm forward causing the piston to pull the other diaphragm backwards. The forward stroke reduces the pump chamber volume pushing liquid into the outlet manifold and the backwards stroke increases the pump chamber volume allowing liquid to fill the pump chamber.

The mid stroke happens when the air is relieved behind one diaphragm and directed to the other, during this time there is a brief interruption in flow which causes pulsation. The pulsation is particularly noticeable because the frequency of the diaphragm stroke is limited by the air valves, liquid ball/check valves and relatively long stroke of the piston; the maximum stroke frequency for a 1” 37 GPM AODD pump might be 8 pulses per second and usually it is much less.

Their design inherently contributes to mechanical fatigue of the diaphragm, resulting in rupture; such damage is accelerated at higher discharge pressures. Diaphragm ruptures equate to fugitive emissions (leakage via the air vent).

Diaphragms used in Hydra-Cell pumps are moved by a column of oil; oil is non-compressible and thus acts just as a solid piston to move the diaphragm. The (patented) benefit of using oil as your driving piston is that you can control the mechanical stress across the diaphragm to within a maximum 2≤DPSI. That means that the diaphragm never sees more than 2 PSI of differential stress whether you are operating at 30 or 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.

While each design uses diaphragms to displace the liquid, all but one Hydra-Cell pump design has 3 to 5 diaphragms; with a 3 diaphragm design the pulsation frequency is very high compared to AODD pumps at 115 pulses per second. Hydra-Cells operate using a shorter stoke at much higher frequency’s resulting in a pulsation that is inconsequential – liquid filled pressure gauge pointers sit still whereas they jump around with AODD pumps.

It is difficult to damage a Hydra-Cell diaphragm, failure modes are over-pressurization, chemical/temperature incompatibility and foreign material lodged in the pump chamber (it could even be something like ice if the liquid is allowed to freeze within the pump head.) Damaged diaphragms tend to enable the synthetic motor oil within the hydraulic end to leak into the process fluid.

Especially if the inlet pressure is high, some process liquid may enter the hydraulic end. Special oils for food contact and other chemically sensitive applications are available to mitigate contamination concerns. There are also devices which monitor the oil level for quicker detection of a damaged diaphragm.

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 requires an oil change every few months of run time.

An AODD is a simpler design. Accessories add cost and there are material limitations: air filter/regulator, pulsation dampener, flexible connectors. Compressed air is costly to generate and a given compressor has a finite capacity. 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 is used at capacity and a new compressor is too costly, certainly more than the Hydra-Cell Pro skid.

It might be cost effective to replace an AODD pump with a Hydra-Cell pump if the AODD pumps are frequently replaced or if the process itself benefits from its aforementioned differential characteristics.

Applications

AODD pumps excel at liquid transfer applications because they typically require lower pressures and flow rate linearity isn’t much of a concern. In addition, their dry suction lift capabilities are much higher than a comparable sized Hydra-Cell pump. AODD pumps are also a good design for hazardous environments because they do not require explosion-proof motors and electrical enclosures.

Hydra-Cell pumps are over-built for low pressure transfer applications; if the application requires smooth flow (coating applications) or benefits from controllable, linear and repeatable volumetric dosing then the Hydra-Cell design would be worth the additional cost. Compared to AODDs, Hydra-Cells are typically used for higher pressures, smoother flow rates and precise flow control, even in humid or cold environments. The Hydra-Cell design accepts higher inlet pressures and has an overall broader application range compared to AODD pumps.

The most common types of pumps Hydra-Cell’s “compete” with are piston and plunger style pumps; the pumping action, capabilities and cost are comparable however the sealless Hydra-Cell design contributes to increased reliability and costs significantly less to maintain. Seals used for piston and plunger pumps require filtration within the range of 25-150µm whereas Hydra-Cell’s only require a 420-3175µm fineness of filtration, making them more suitable for reclaimed or otherwise abrasive liquid applications.

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