by Chris Dietzsch

Air operated double diaphragm (AODD) pump technology was invented and introduced to the market 55 years ago. That day in 1955 when the first AODD pump began operating remains a watershed moment in industrial pump history. AODD technology grew from humble beginnings, mainly as a way to pump wastewater in mining applications.

AODD pumps are reciprocating, positive displacement pumps. The wetted path (the area in which the pumping media is contained) consists of an inlet and discharge manifold and two liquid chambers. An air valve in the Air Distribution System (ADS) alternately directs compressed air behind the diaphragms in the air chambers. Fluid is drawn into one liquid chamber from the inlet manifold as fluid is expelled from the other liquid chamber through the discharge manifold.

AODD pumps have many strengths. They do not require electricity, are self-priming, can pump fluids with solids in suspension (rocks and debris), can run dry or be "dead headed" without damaging the pump. In addition, their performance can be tailored to specific needs by adjusting the inlet air pressure or restricting the discharge of the pump. AODD pumps were designed to operate in rugged applications. They were used in applications where most other pumps would fail and were designed to be an inexpensive, easy to maintain option for a variety of situations. They can transfer a wide range of media from wastewater to more viscous substances like slurries or even cement.

A decade of research and design innovations have led to refinements and improvements to the air valve and wetted path. The pumps that have emerged are tough and versatile enough to meet the stringent demands of the mining and heavy construction industries and the precise and sanitary requirements of the pharmaceutical and food processing industries.

The Challenge

Through the years, energy costs have increased substantially. As energy costs have increased, manufacturers have become more concerned about reducing operating costs and have focused on improving the efficiency of plant equipment.

Since their inception, one significant challenge in designing and producing an effective AODD pump has been providing the end user with a good fluid flow rate while at the same time minimizing air consumption (reducing energy costs).

AODD pumps with an Efficiency Management System (EMS) into the ADS. The EMS allows the end user to adjust the size of the air inlet ports.

All AODD pumps have an ADS in their design. Part of the ADS's job is to direct compressed air to one air chamber while exhausting air from the other air chamber. The air valve performs this task. This air valve can take many forms, but in many cases is a "spool" that slides back and forth in a cylindrical bore in the pump housing. It uncovers and blocks ports so that compressed air is supplied to the appropriate chamber and exhausted from the other. These original spools had a single diameter and air pressure was supplied to both sides of the spool. The pressure would momentarily be removed from one end of the spool to make it "shift." The shift redirected the compressed air to the other chamber.

While this single-diameter spool in early AODD pump designs was innovative and effective, its shape meant that the ADS had a higher chance of stalling if the momentary signal did not completely shift the spool. This would lead to loss of production, inefficient operation and increased pump downtime.

The Solution

A number of improvements have been made to the AODD pump's ADS to improve performance, reliability and efficiency.

One improvement was the introduction of a new "unbalanced" air valve spool. This new design did not rely on a momentary signal to shift the spool. Rather, it relied on an unbalanced signal to control the shifting of the pump. The new spool had two different diameters. The smaller of the two ends was constantly supplied with air pressure. This air pressure would force the air valve in one direction. Alternately, the larger end of the spool would be pressurized. The larger area would produce a higher force (than the small end) and shift the spool in the other direction. The forces on the spool are always unbalanced and constantly force the spool in one direction or the other.

Cross-section of an air distribution system (ADS) that incorporates an unbalanced spool to prevent the pump from dead-heating.