Pumps & Systems, November 2012
While air-operated double-diaphragm (AODD) pumps are widely accepted and used in many applications, their use on clean chemical transfer can be argued as inefficient. The word clean is emphasized in this article. AODD pumps are monsters at pumping liquids that include viscous and dirty fluids. Clean chemical is defined as having particles under 50 microns in size and less than 5 percent by volume.
Most chemicals delivered by rail, truck or tote fit into this category. Many facilities that use AODD pumps on dirty or viscous processes will also use this technology on clean processes. The availability and familiarity of AODD pumps make them an easy choice for almost any application. Unfortunately, what is sometimes overlooked is the cost associated with running an AODD pump in an application in which a sealless centrifugal pump could be used. AODD pumps have three detriments—maintenance, pulsation and air consumption.
AODD pumps require preventive maintenance on their diaphragms and other wear parts. The wear items need to be replaced before a stated number of strokes occur. To replace the wear parts, the pump must be removed from the system. This may lead to downtime if a spare pump is not on hand, although having a spare pump on hand is an added cost.
Once the pump is removed, a cost is associated with the replacement parts and the labor to rebuild the unit. Once rebuilt, the pump will require additional labor to be reinstalled. Pump rebuilding is not a one-time process. It must be rebuilt every time it approaches the defined maximum stroke limit.
AODD pumps are positive displacement pumps. Because of this, pulsation occurs when the diaphragm reciprocates. This pulsation causes pipe stress and vibration that must be monitored. Although pulsation dampeners can be installed to help alleviate the surges, this adds an expense.
AODD pumps consume air, not electricity. The cost to generate and maintain air is increasing. Plant air is not free, and air compressors are expensive. They need to be maintained and can easily be maximized by other processes in the plant. Air compressors are also inefficient. Added to this inefficiency is the fact that air lines from the compressor to the pump can leak. The AODD pump also adds inefficiency to the process. Combine these factors, and the AODD pump can be one of the most expensive pumps to operate in a plant. Compared to an electrically powered pump, an AODD pump can cost as much as 75 percent more to operate on similar services.