Low-pressure valves for desalination: time for a new look at the butterfly?
KSB Pumps, Germany
The centred-disc butterfly valve is a proven technology and, according to KSB's Amri Water Valve Division Competence Centre in La Roche Chalais, France, it is in widespread demand from all types of desalination plant. It is a simple yet effective design principle whereby the disc rotates around an axis at right angles to the flow. When open, the fluid passes around both sides of the disc.
"Low pressure valves are rarely considered as a critical item," says Pascal Viaud, "but if not correctly specified at the outset of a project problems can occur over the long term. Wrong selection of valves can be the cause of up to 40% all operational problems, so the customer must consider the real importance of valves at the specification stage. Even though valves may only constitute 1% of the plant investment costs, it only takes a single valve to create problems and seriously affect plant production."
Butterfly valves are installed in the low-pressure areas of a desalination plant, usually in the early stages of production upstream of the membranes and then again at the end on the permeate lines. By keeping the butterfly valves open each valve creates a pressure loss and with the centred disc the differential pressure is much lower than an offset disc valves. Where a high number of valves, say 1,000 units, are installed in a desalination plant the accumulation of pressure saved on each valve will contribute to lower energy savings.
The full article first appeared in the November/December 2008 issue of D&WR magazine
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