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Seawater intake pumps for reverse osmosis (RO)
desalination plant

Q - Is there a relative advantage or disadvantage between a horizontally- or
vertically-mounted seaware intake pump for a large reverse osmosis (RO) plant
with a capacity of 25,000 to 45,000 gallons per minute? Is there any special
requirement for the pumps in desalination plants?

A - Large-scale reverse osmosis desalination plants generally require dozens of
pumps of various configurations depending on which stage of the process the
pumps are used.
They include seawater intake pumps (horizontally- or vertically-mounted) to supply
seawater to the filtration system, low pressure filter pumps to feed the cartridge
filters, high pressure pumps to feed water to the reverse osmosis (RO)
membranes, booster or transfer pumps, pumps for filter backwash, flushing,
chemical cleaning, etc.

Most commercially available single stage and multistage pumps can be used in
RO plants with minimal design changes. Notable among these design changes
is the upgrade to corrosion and abrasion resistance materials such as the use of
duplex stainless steel, or those materials with high Pitting Resistance Equivalent
Number (PREN).

The selection of the type of seawater intake pumps to use may require special
hydraulic considerations due to limited net positive suction head available
(NPSHA) in typical desalination plants. For this reason, vertically submerged
pumps are preferred for intake application whereas horizontal pumps are the
norm for other applications in the desalination plants.

At the intake site, the net positive suction head available (NPSHA) is provided
solely by atmospheric pressure, unless the pump impeller is well submerged
below sea level. Horizontal pumps will be severely restricted in their flow rates, or
operating speed, if they are to operate with acceptable suction specific speed to
prevent cavitation..

Vertically submerged pumps provide the flexibility of increasing the site NPSHA by
locating the impeller below the water level, as needed, to increase the NPSHA
higher than those required by the pumps for the given flow rate and operating
speed. The NPSHA provided by atmospheric pressure will increase by an amount
corresponding to the submergence of the impeller.

The ability to increase the site NPSHA by the amount of submergence provides
the flexibility to select the intake pumps with the optimum capacity and speed that
will yield the highest operating efficiency. And with the option to use single stage,
two-stage, or multistage design the pumps can be easily selected on the basis of
optimum specific speed and their corresponding life cycle cost.

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