Water. desalination + reuse

TECHNOLOGY Is the axial piston pump the next big efficiency gain? _________ John P MacHarg, Ocean Pacific Technologies, and Bradley Sessions, AQC Ltd, USA ___ Editor's Note: Centrifugal pumps are currently the dominant solution for high-pressure pumping used in large-scale seawater reverse-osmosis systems. A preliminary review of a modular positive-displacement waterlubricated axial-piston pump system reveals this new concept is not only a cost-effective alternative to traditional centrifugal pump systems, but that it is simpler to operate and brings other advantages during the SWRO facility operating period. This is a shortened and updated version of a paper presented at the International Desalination Association's 2013 World Congress in Tianjin, China. WATER-LUBRICATED axial-piston (AP) pump technology has been growing in popularity and size in the seawater reverse-osmosis (SWRO) industry for more than ten years. A "tipping point" exists for this technology to replace the less efficient centrifugal (CF) high-pressure pumps that have dominated in large-scale SWRO systems for more than 30 years. AP pumps can be efficiently and economically linked together in parallel to provide high-pressure feed to systems of any size, including large municipal-scale plants. This kind of modular application in pumping systems was pioneered by Energy Recovery Inc (ERI), whose modular energy-recovery Pressure Exchanger (PX) arrays are now the standard for SWRO. Both operate on the same revolutionary technology platform of water-lubricated bearings. When ceramic material is used, these bearings can provide maintenance-free operation for tens of thousands of hours. Furthermore, both devices employ a positive-displacement AP design that results in very high efficiencies and virtually pulsation-free flow. The progression in size of the AP pump has also been similar to the PX. The initial commercial PX units in 1995 produced only 9 m3/h and were therefore only considered suitable for smaller scale systems. But as the size of the individual PX units grew, and their reliability in modular arrays was proven, the technology became applicable to the largest systems in the world. Similarly the modern water-lubricated AP pump has grown in size from approximately 10.2 m3/h, at its original introduction into the SWRO market in 2000, to capacities up to 43 m3/h available today. The pump has also proven its compatibility with modular-array applications. The AP pump array requires up to 25% less power 1.1mm 5.5mm Figure 1. Six-pump array provides up to 257 m3/h at 90% efficiency. than the current SWRO industry's standard centrifugal (turbine) pump. Power is the single greatest operating cost in the SWRO process. The efficiency advantage of the AP pump system shown in Figure 2 represents the largest single gain in desalination efficiency in 30 years. Looking at the X-axis, one can see that as the train size increases AP pumps are added onto a pumping array. For example, a 4,000 m3/d train would require four AP pumps running in parallel to produce 172 m3/h at a specific energy of 2.3 kWh/m3 (approximately 90% pump efficiency) compared with the centrifugal pump's 2.7 kWh/m3 (approximately 76% pump efficiency). The 15% savings adds up to approximately US$ 58,000/year @ US$ 0.10/kWh. In Figure 2, one can see how the CF pump energy-consumption improves as the plant size increase. This is because CF pump efficiency improves as flow-rate increases. This fact diminishes the AP efficiency advantage as the CF pump size increases. | 28 | Desalination & Water Reuse | November-December 2013

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