Pumps and Systems, June 2009

Water is an essential substance to sustain life and one of the most valuable resources a nation can possess.

Most of what is available, however, is of little use in its natural state.

Water seems readily obtainable since it covers more than 70 percent of the Earth's surface, but clean, fresh drinking water comprises barely a fraction of all available water. In fact, 97.5 percent of the world's water is saline. Of the remaining 2.5 percent, 70 percent is locked up in the polar ice caps, and 30 percent resides in underground aquifers.

Demand for fresh water increases steadily, and researchers including Surendra N. Kulshreshtha of the University of Saskatchewan predict that half the world's population could face water vulnerability by 2025 [1]. It is no surprise that the United Nations devotes much attention to the issue of safe, fresh water, particularly in places that have been underserved in the past. The UN released a new report in March 2009, Water in a Changing World, which is the third in series.

Equally concerning is ensuring that harmful toxins and contaminants are removed from water that manufacturing processes release in the environment. Wastewater from factory production often has levels of chemicals, pharmaceutical byproducts and other pollutants that can pose a significant risk to human beings and other life.

Water and other liquids can be filtered and purified in many ways. One of the most effective methods is simple and straightforward: reverse osmosis (RO) filtration.

Depending on the pump model and configuration selected, sealless, high pressure pumps for reverse osmosis offer flow rates from 3 to 37 gpm with maximum discharge pressures ranging form 1,000 to 2,500 psi.

Design Principle

In RO filtration, liquid is forced through a semi-permeable membrane at high pressure, producing purified liquid. The contaminants are concentrated in a portion of the liquid that does not pass through the membrane and are discharged in a reject stream. Efficient RO systems employ positive displacement diaphragm pumps with a sealless design and high pressure capability to purify large volumes of water or liquid at a time. The sealless principle is appealing because of its design simplicity, low maintenance and operating efficiency.

Bill Wanner, CEO of Wanner Engineering, Inc. (Minneapolis, Minn.), explained how this design simplicity makes these pumps easy to operate and inexpensive to maintain.

A sealless design "removes the most maintenance intensive element compared to many other type pumps," he said. "This not only permits the pump to operate in severe environments such as chemical treatment, but allows it to run dry indefinitely without damage, thereby avoiding a situation where operator error causes an equipment breakdown."

Wanner added that the relatively small footprint of these RO membrane pumps is a tremendous advantage in mobile water treatment systems, such as those used by emergency management teams, offshore oil platforms and even in submarines, where space is at a premium. Due to its unique design, the sealless, high pressure pump boasts a far more compact package than can be achieved with other pump technologies, such as multistage centrifugal pumps.

Energy Consumption

Energy usage is a major factor in the operational cost of water purification systems, so companies and organizations that rely on water treatment are extremely conscious of efficiency. It is not only important to keep costs low, but electrical power is also extremely coveted in certain environments. Oil platforms, desert desalinization plants and mobile treatment facilities are prime examples of how remote locations rely on efficient energy usage.

Manufacturers are likewise under increasing pressure to find efficiencies in energy usage, not only from their accountants, but from the general public. Going "green" is not just sensible for the bottom line, but is smart PR, as well.

"We engineered [our pump] to operate so that a customer can use a smaller, lower wattage motor to acquire the same treatment capacity as other, less efficient types of pumps," said Wanner.

Wanner points to a number of case studies demonstrating energy savings. In one such example, a single 12 kW sealless PD pump replaced a pair of multistage centrifugal pumps totaling more than 18 kW, but with the same pumping capacity. The total energy savings was more than 50,000 kWh/year. Proportionate savings were shown in smaller applications, as well.