Aug 18, 2003

Selecting a Pump for Seawater Reverse Osmosis

Tech Update

Reverse osmosis (RO) has become a significant process used
in the purification of water. Selecting the correct pump is a significant step
in a well-designed and properly operating system. Although the pump is a
relatively small part of the overall cost of an RO system, expenses resulting
from downtime and inefficiency quickly can diminish the productivity of the
system and your return on investment. Due to the corrosive atmosphere and high
demands on these sea water RO systems, special attention is required in the
pump selection process. There are several key factors that will help to ensure
optimum pump and system performance.

Pump Compatibility

Pump compatibility is a key element in the selection
process. Due to the fact that seawater is a corrosive liquid, it may contain a
variety of microorganisms and the water temperature may vary, the materials of
construction of your pump are critical. Some locations and water conditions may
tolerate a liquid-end construction of nickel aluminum bronze. Other locations
may demand more corrosion-resistant materials such as 316SS and Duplex
Stainless Steel. Each installation must be reviewed on its own merits, but
having these pump options assures the best fit to the installation.

Pump Efficiency

Pump efficiency is a key element in keeping the costs of
providing fresh water at an affordable price. Generally, positive displacement
(PD) plunger pumps operate in the 80-90 percent efficiency range, while
centrifugal pumps are between 30-60 percent efficient. With the lower
efficiency pumps, the addition of an energy recovery system is required. When
these energy recovery systems are coupled with the higher efficiency PD pumps,
you can achieve maximum output and return on your investment.

Pump Speed

Lower revolutions per minute (rpm) operation is highly
recommended when working with corrosive liquids such as seawater. Higher rpm
operation will further aggravate the corrosion-erosion wear process initiated
by seawater. Lower rpm also helps to reduce the risk of cavitation.

Ease of Installation

Easy and compact installation is a consideration. Many RO
systems are on board cruise ships, naval vessels, freighters, off shore
platforms or at hotels and resorts. Space and weight are two primary
considerations. Convenient, easy connection of inlet and discharge plumbing is
preferred. Installation is easier when system accessories conveniently attach
to the pump head or immediately in the inlet and discharge lines. This
minimizes delays in valve response time and facilitates periodic visual checks
of the system performance. The pump should be adaptable with booster pump feed
as most often the seawater is lifted from a well or through the ship hull valve
or sea chest.

Accessories for Your System

System accessories are nearly as important as the pump
itself. Without proper pressure relief installed in the system, the pump is
subject to deadhead and destruction should the system fail or become blocked.
Pulsation is damaging to the RO membranes. A pulsation dampener will minimize
pulsation and prolong the life of the entire system. A suction stabilizer or
captive acceleration tube will minimize the inlet pulsations and reduce the
risk of cavitation. A pressure gauge is essential in the system as a monitor of
system performance. As soon as pressure drops or begins to fluctuate, a
maintenance inspection should take place. Prolonged operation with worn
components can result in more serious and costly repairs.

Service and Support

Pump serviceability is a significant factor in selecting an
RO pump. Delivering fresh water requires a continuous-duty system and downtime
is not tolerated. Periodic maintenance should be minimal, easy to perform and
require few tools. Easily removed pump valves and seals, and replaceable heads
are preferred. Good technical support and readily available parts are essential.
Selecting a manufacturer that can technically support your pump once it's in
operation is not only a comfort but a necessity. Knowing your pump can be
serviced within 24 hours anywhere around the world is a benefit for you and
your RO system.

Proven Dependable

Dependability is an absolute necessity. RO is an application
where a supplier's reputation for dependability weighs heavily in the selection
process. With lives and livelihoods depending on fresh water delivered by your
system, you do not want to be experimenting with an unproven pump. You will
want to select a pump manufacturer who has experience with this application and
has a history of dependable

About the author

Darla Jean Thompson is the director of marketing and advertising at CAT PUMPS. CAT PUMPS is a manufacturer of high pressure reciprocating plunger pumps and pumping systems up to 320 gpm, 7,000 psi. For more information, call 763-780-5440; [email protected];