Water. desalination + reuse

TECHNOLOGY November-December 2014 | Desalination & Water Reuse | 41 | _________ Christoph P. Pauly, KSB Aktiengesellschaft, Frankenthal ___ Editor's note: lower cost materials and coatings used in addition to corrosion-resistant valve designs – both urged on by preferences in plant construction contracts – have reduced the capital and operational costs of valves used in membrane and thermal seawater desalination. The design of the valve too has a significant role to play in keeping a rein not only on corrosion but also cost of operation. SEAWATER DESALINATION operating costs are the focus of much attention for the industry's advocates and critics alike with energy costs at the centre of the debate. But operators and developers of systems and components have other cost concerns and corrosion is among them. Desalination systems handle chemically aggressive fluids, and the cost of corrosion protection required for long-term and trouble-free operation is significant. Shut-off butterfly valves have an intrinsically greater resistance to corrosion than other commonly used valve designs because they have less contact with the seawater being transported. In gate and globe valves, seawater wets a large proportion of the entire flow path. These valves therefore must be made from costly seawater-resistant material or at least provided with a plastic coating or an elastomer lining where possible from a manufacturing point of view. TeChniCal benefiTs Shut-off butterfly valves (figure 1) are more cost- effective than other valve designs because only their liner and disc are in contact with the medium being handled. Thanks to their particularly suitable design and the specific operating conditions encountered in desalination, centred-disc butterfly valves have become established in the global seawater desalination industry. So butterfly valves ranging from 40 mm to 4000 mm in diameter have become an essential part of most desalination plants. They are employed in the low-pressure areas of desalination plants using reverse osmosis or in multi- stage flash distillation plants. The valves are typically installed in the pre-treatment stages upstream of the membranes and at the end of the permeate lines. The liner is the most important component as it ensures perfect shut-off as well as reliable sealing at the flanged line connections and the shaft passages. It can either be inserted into the body without being glued to it or be vulcanised into the body or onto an additional support ring fitted inside the body. The advantage of the "inserted" mounting option is that the Cutting the costs of corrosion in seawater desalination with shut- off butterfly valves Material gains: the cost savings in valve manufacture from replacing stainless steel with plastic coated iron are clearly big.

• Cover