Water. desalination + reuse

TECHNOLOGY November-December 2014 | Desalination & Water Reuse | 43 | valves are suitable for installation between flanges, downstream dismantling or dead- end service. GaLvaNiC COrrOsiON As seawater is highly conductive, galvanic corrosion is one of the highest risks faced by metals used in contact with it. Traditionally, butterfly valve discs are manufactured of corrosion-resistant bronze aluminium or duplex stainless steel. Since prices for these materials have significantly risen during the past few years so solid material discs have become very expensive. A look at the photograph at the start of the article which shows an employee standing next to a DN 3700 butterfly valve at the French KSB factory in La Roche- Chalais makes clear how much stainless steel and bronze can be saved by using ethylene chlorotrifluoroethylene (ECTFE)-coated nodular cast iron discs. Hard-rubber coated discs are a traditional alternative which has been used in the water industry for more than 25 years. But it is an expensive and time-consuming process. The hard rubber coating is applied to favourably priced carbon steel or nodular cast iron using vulcanisation in an autoclave where coatings of 5 mm in thickness are produced. PLasTiC COaTiNG iNsTEad Of ExPENsivE mETaL ECTFE, better known under its brand name of Halar, has become established as a cheaper alternative to vulcanised metals. It is a fluoropolymer which is applied to the hot work piece in the form of powder. The powder is blown onto the work piece and melts on its surface. To ensure perfect adhesion of Halar powder, the disc's surface must not only be very smooth and clean, but must also be subjected to a special treatment. The objective is to ensure an even coating of 600-800 microns in thickness across the disc's surface. After the fluoropolymer application, each disc must undergo a high-voltage sensor test to check that there are no defects in the coating. A valve disc protected by such a coating is highly resistant to chemical attack. In the chemical processing industry, this material is already well known and has proven its worth. The result is a butterfly valve that is able to withstand the corrosive nature of seawater and treated water. ECTFE- coated discs are used as a standard by European desalination plant contractors and operators. CONTraCT mOdELs PrOmOTE NEw TECHNOLOGiEs In the past, Middle East operators often preferred valve discs made from bronze aluminium or duplex stainless steels. This preference is quite understandable given that the salt content of seawater in the Persian Gulf is about 65 mg/l which is about twice as much as is encountered the North Sea near Helgoland. As prices for metals such as copper, nickel and chrome have considerably increased in recent years, users are more prepared to employ Halar-coated discs. Globally, operating contractors from France and Spain with an interest in innovation have contributed to the propagation of this new technology. In addition, the move to build-own operate-transfer contract models – which place the financial risk for the application of new technologies with the contractor instead of the operator – have created a greater confidence in this technology among operators. As a result there is an increasing number of seawater desalination projects that use coated-valve discs. l Going with the flow: centred-disc butterfly valves are established in seawater desalination. Figure 3. Sealing principle at the flange faces and seat/disc interface on a butterfly valve.

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