For more than 50 years, air-operated, double-diaphragm (AODD) pumps have played a role in many companies’ modern global manufacturing and fluid-handling operations.
The original pump concept has changed little over time and works because of its simple operation and construction.
With a major focus on operating improvements and cost reductions from businesses, AODD pump manufacturers have introduced new air distribution systems (ADS) to increase pump efficiency and meet these demands. The new ADS technologies range from electronic controls to mechanical innovations. However, the resulting savings in compressed air usage and increase in pump efficiency vary for each system.
This article presents the results of head-to-head comparisons of AODD pumps with different ADSs and the final data acquired during those tests. The results help illustrate the impact that different ADS technologies have on air consumption, energy use and overall flow rate.
In AODD pump operations, energy use is directly related to the rate at which the pumps use air to drive each stroke during the pumping cycle. With increasingly high energy costs, facility operators understand that the initial purchase and installation of a new pumping system is only a small part of the total cost of ownership (TCO) and operation. The routine costs of energy and maintenance remain the primary cost drivers in the TCO.
Manufacturers have introduced new air distribution systems to improve AODD pump efficiency. (Article images and graphics courtesy of Wilden.)
Much like cars, pumps can be rated according to efficiency. The goal is to use the least amount of air to pump the greatest amount of product. The ultimate objective is to reduce the rate of air consumption in relation to the product flow rate and minimize the amount of air that is not creating product yield (therefore, wasting energy) during the pumping cycle.
If attained, this combination can positively impact an operation’s bottom line. The profitability is determined by the volume of air being consumed and the energy required to compress it.
In addition to wasting energy, air overfilling unnecessarily stretches the diaphragm. This stretching produces excessive diaphragm wear, resulting in more frequent replacement and downtime and increased operating costs.
Although the technology is more than 50 years old, AODD pumps can still be made more efficient by recent ADS evolution. These developments better control the air flow and prevent wasteful air overfilling, which reduces air consumption and operating and maintenance costs.
Two primary methods control air flow and prevent overfilling by cutting off the air supply to the air chamber before the end of the pump cycle: electronically controlled and mechanically actuated ADSs.
The electronic control method has two shortcomings. First, the electronics raise an additional set of maintenance and operational concerns, including the prohibition of their use in explosive environments, such as mines, and an inability to be submerged.
Second, electronic ADS technology requires a “learning period” of 30 to 40 seconds during which it monitors the operation of the pump before it can estimate when to cut off the air supply prior to the end of the stroke. This can result in erratic and sometimes wasteful flow rates during the learning period.
During the time period from the end of each stroke to the completed shift of the valve, the air is allowed to “overfill” the air chamber without any corresponding displacement of fluid. This volume of compressed air is entirely wasted.
The mechanically actuated method is a simple and extremely effective way to control the air with each stroke and prevent wasteful overfilling.
The air is constantly mechanically controlled to maximize efficiency, minimize complexity and maintenance considerations, and allow for operation in explosive and submerged pumping environments.
The Controlled Comparisons
To create real-world conditions to measure the effectiveness of different ADS technologies industry-wide, a controlled, objective head-to-head test was arranged comparing several AODD pumps that were fitted with traditional, electronically controlled and mechanically actuated ADSs. Common conditions of service that are seen in the field were replicated in the laboratory.
This simulated application required each pump to produce a flow of 320 liters per minute (lpm)—85 gallons per minute (gpm)—against a head pressure of 2.1 bar (30 psi).
To achieve a fair comparison between these competing pumps, each pump was run at the inlet air pressure needed to meet the common condition of service. Air-consumption data was then recorded (see Table 1 for the results for four pumps).
Table 1. In a controlled head-to-head laboratory test comparing eight AODD pumps operating under common parameters seen in the field, this sample of four illustrates Pump D’s standout results and the wide range of energy and strokes required to achieve similar fluid output.
The data gathered in the objective AODD testing show that Pump D—which used the ADS that restricts air flow into the filling air chamber to minimize wasted compressed air—had better performance in all three points of measurement.
It had the lowest air consumption in standard cubic feet per minute (SCFM), the lowest strokes per minute and the highest volume of fluid pumped per SCFM.
Pump D averages an overall 30 percent reduction in air consumption compared with Pumps A, B and C.