Air-operated double-diaphragm (AODD) pumps have earned their place across a wide range of industrial applications largely because of their simple design. But simplicity in principle doesn't mean simplicity in practice. Air consumption, dry-run capability, deadhead tolerance and the ability to withstand continuous operation – these are the details that separate a pump that merely works from one that performs reliably over the long haul.
Understanding how an AODD pump handles those real-world operating demands can make all the difference when selecting the right pump for an application. With that in mind, we've answered some of the most common questions about Wilden® Pro-Flo® SHIFT Series AODD Pumps, covering air efficiency, continuous operation, dry-run tolerance, deadheading and cavitation.
Why does air efficiency matter so much on an AODD pump?
Air efficiency matters because compressed air is the pump's entire energy input. Pro-Flo SHIFT Series AODD pumps run solely on compressed air, meaning every gallon pumped comes at a cost at the compressor. The more efficiently the pump uses air, the lower your operating costs.
That air is expensive. According to ENERGY STAR and the U.S. Department of Energy, compressed air is one of the most expensive energy sources in a plant. The overall efficiency of a typical compressed air system can be as low as 10-15%, and roughly 10% of the electricity consumed at a typical industrial facility is used to generate it.
This is what the Pro-Flo SHIFT Air Distribution System (ADS) is engineered to address. The air control spool allows air to flow freely into the air chamber for the majority of each pump stroke, then significantly restricts the flow of air into the chamber when activated by the inner piston near the end of each stroke. Only the air needed to sustain the pumping process is introduced.
The result is lower air consumption per cycle and the elimination of air chamber overfilling, with no reduction in flow rate. The system is mechanically actuated with no electronic components, and eliminating overfilling helps maximize mean time between repairs (MTBR).
What air savings should I expect from a Pro-Flo SHIFT Series AODD pump?
Pro-Flo SHIFT Series AODD pumps achieve up to 60% savings in air consumption compared to other AODD pump technologies, along with up to 34% greater flow rates and more product yield per SCFM. This significantly reduces operating costs.
Because savings vary by installation, the Wilden Pro-Flo SHIFT Savings Calculator uses your specific operating details to give you a precise figure rather than a rough estimate.
Cost of entry matters too. Because the Pro-Flo SHIFT ADS is mechanically actuated rather than electronically controlled, it costs roughly 50% less than an electronically actuated air distribution system, so the efficiency gain does not carry a control-system premium.
Do I need a pressure relief valve on an AODD pump?
Most AODD pump installations do not need an external pressure relief valve. If the discharge pressure meets or exceeds the air supply pressure, the pump will automatically stop. No bypass or pressure relief valve is needed, and there is no risk of damage to the pump. This is known as a deadhead condition. To restart the pump, simply reduce the fluid discharge pressure or increase the air inlet pressure.
This sets it apart from other positive displacement pumps, which can sustain damage when deadheaded. It also means discharge throttling is a legitimate control method. Partially closing a valve in the discharge line controls the discharge rate, since the added friction loss reduces flow.
There is one case where a relief valve earns its place. If pressure-sensitive equipment sits downstream and cannot handle the pressure the pump reaches at deadhead, a relief valve protects that equipment. In that situation, the valve is protecting the downstream system rather than the pump.
Why do some AODD pumps stall, and how does the Pro-Flo SHIFT Series prevent it?
AODD pumps can stall when the ADS reaches a position where it cannot shift the pump's air valve and continue cycling. This is often referred to as a dead-center condition. It can occur at startup, at low air pressures, or when friction, wear, contamination or icing interferes with the air valve or diaphragm movement.
Modern AOD pump designs incorporate features specifically intended to prevent stalling and improve reliable restarting. For instance, Pro-Flo SHIFT Series AODD Pumps use an unbalanced spool in the air valve to prevent stalling.
The small end of the air valve spool is continuously pressurized while the large end is alternately pressurized and then exhausted to move the spool. The spool directs pressurized air to one chamber while exhausting the other. When the shaft reaches the end of its stroke, the inner piston actuates the pilot spool, which controls air to the large end of the air valve spool, and repositioning that spool routes air to the opposite chamber.
If a pump is running slowly or not running at all, check the air supply before suspecting the valve:
- Air inlet pressure should be at least 0.4 bar (5 psig) above startup pressure.
- The differential between air inlet and liquid discharge pressures should be no less than 0.7 bar (10 psig).
- The air inlet filter should be clear of debris.
- The air line must be large enough to supply the air volume required for the desired pumping rate.
- A 5-micron air filter should be installed upstream of the pump.
Figures vary by model, so check the specifications for your pump.
What happens if a Pro-Flo SHIFT Series AODD pump runs dry?
Pro-Flo SHIFT Series AODD pumps can run dry without significant damage. That tolerance makes them a good fit for processes with variable or interrupted flow, and it is a key factor in their self-priming capability. These pumps are self-priming by design and can operate from a completely dry start. However, a pump may take several cycles to prime completely depending on the application.
It's important to note that dry running will still cause wear on the diaphragms. While the occasional dry-run event is unlikely to cause immediate damage, excessive dry running can eventually lead to diaphragm failure at the accelerated cycle rates it produces.
The real concern is duration rather than the occasional event. A pump that empties a tank and continues cycling unnoticed can accumulate considerable wear before anyone intervenes.
Can a Pro-Flo SHIFT Series AODD pump run 24/7?
Yes. Pro-Flo SHIFT Series AODD pumps can operate continuously, 24/7, making them suitable for demanding process and manufacturing environments.
Continuous duty is also where the efficiency argument is strongest. Pro-Flo SHIFT Series AODD pumps have the highest air efficiency on the market, resulting in lower energy costs than competing pumps, particularly in long-duration applications.
What causes cavitation in an AODD pump?
Cavitation in an AODD pump is a suction-side problem. When a pump runs but delivers little or no product, check for cavitation, slow the pump speed so the material has time to flow into the liquid chambers, and verify that the vacuum required to lift the liquid is not greater than the vapor pressure of the material being pumped.
Cavitation can disrupt flow and may cause some pitting on metallic inlet valve seats. Pro-Flo SHIFT Series AODD pumps are less susceptible to cavitation damage than other pumps, and specifying elastomeric seats rather than metallic ones helps mitigate its effects.
Most of the remedy lies in the installation. The suction pipe should be at least as large as the diameter of the pump's suction inlet, and the suction hose should be a non-collapsible, reinforced type, since these pumps pull a high vacuum. The best hook-up is the shortest and straightest the site allows, avoiding unnecessary elbows, bends, and fittings. All fittings and connections must be airtight or suction capability is reduced or lost. Where the pump is running faster than the fluid can fill the chamber, an air regulator reduces pressure and a needle valve reduces volume.
Have more questions about improving the performance and service life of your Pro-Flo SHIFT Series AODD Pumps?
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