Since the sealless, high pressure pump for RO is a positive displacement system, the flow rate remains constant even as pressure increases. This reduces the cost-per-gallon of water, the critical measuring stick when gauging efficiency. To stretch efficiency further, many customers are now incorporating energy recovery devices as well.

Since the flow of the pumps is proportional to the shaft speed (and independent from the pressure), it is easier to control permeate flow more accurately while matching it to the RO system curve for greater efficiency. Just a small change in flow and pressure can result in a large change in efficiency and operating costs.

RO pumps with a seallesss design can handle particulates of up to 500 microns. There is no need for fine filtration and its associated costs, whereas axial piston pumps with dynamic seals may require filtration in RO applications to protect the seals and packing.

Reliability and Longevity

Approximately 80 percent of typical pump maintenance, according to Wanner, involves the replacement of leaking seals and packing. A sealless design is an advantage in maintenance and downtime. In addition, these high pressure RO pumps feature a hydraulically-balanced diaphragm, so they are capable of handling virtually all particulates. According to Wanner, the pumps can handle solids of up to 500 microns, while other pump types may require the expense of a filter to prevent particulates larger than 10 microns from damaging the pump.

"We always tell our customers our Hydra-Cell positive displacement pumps handle the really miserable fluids and abrasives that destroy other pumps," he said. "Our design enables charged and dirty liquids to be processed without the need for fine filtration to protect the pump. This reduces maintenance and allows the pump to operate continuously at high pressure."

Wanner notes that most of these RO pumps can operate at any pressure from less than 40 psi up to at least 1,000 psi, with some models capable of operating at up to 2,500 psi. By comparison, rotodynamic pumps need to be started with 750 psi of pressure to avoid damaging the pump.

When compared to piston and plunger pumps, sealless RO pumps offer total separation of the pumped liquid from the lubricating oil, extending the life of the oil by a factor of four and reducing breakdowns.

"Axial piston pumps internally driven by water need five microns of absolute fine filtration," Wanner added. "The filters typically cost around $500, and the elements, which should be changed every 15 days, cost about $180 each," he said. "Those maintenance costs add up quickly, and failure to maintain the filter means very costly breakdowns."

Maintenance cost is not the only concern. When one pump goes down for maintenance, it reduces the overall treatment capacity. This means that additional pumps are required in large treatment facilities for redundancy. In smaller applications, it means two pumps are required when one maintenance-free pump would have sufficed.

Meeting the same performance criteria, but operating more efficiently with smaller motors, RO positive displacement pumps use significantly less energy than multistage centrifugal pumps. This saves the user hundreds, if not thousands per year in RO applications while allowing for a smaller, space-saving system footprint.

Application Flexibility

Although we tend to associate RO with water desalinization plants, it is also being used for innumerable applications in industries worldwide:

• Treatment of wastewater streams from food plants and industrial plants
• Treatment of waste solvent streams from chemical and pharmaceutical plants
• Treatment of recycled water for reuse
• Treatment of water used in boiler feed applications to prolong system life and efficiency
• Treatment of final rinse water in carwashes and in cleaning processes to aid drying and improve final result
• Disaster management water purification

Since pump seals deteriorate in the presence of corrosive chemicals, a sealless design is often considered a necessity in everything from normal to severe-duty applications.

The Tides of Need and Demand to Rise

As the need for fresh water continues to rise and more laws regulating the treatment of wastewater are enacted, the demand for economical, energy efficient water treatment will increase. Part of this trend, driven by the rising cost of fresh water is toward localized treatment plants and units. Given the significant advantages of reverse osmosis filtration, it is no surprise that Wanner predicts this market will grow by 68 percent during the next five years.

References

1.      A Global Outlook for Water Resources to the Year 2025; ISSN0920-4741 (Print) 1573-1650 (Online)

Tags: June 2009 Issue , Pumps

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