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Duplex Usage in Desalination Plants

Introduction:

Desalination Plants are known to be very severe environments due to the high chloride, high temperature conditionsi. In the past, austenitic stainless steels in the ASTM 300 series tended to be the material of choice in desalination plants. In cases in which even higher corrosion resistance has been needed, the typical material solution has been higher alloyed Austenitic grades that go beyond the 300 series, such as austenitic 6Mo grades (UNS NO 8367)ii. More recently, Duplex 2205 (UNS S32205/S31803) and other duplex steels have emerged as the more optimal material in many situations.

Duplex 2205 and other duplex steels have gained in popularity in the desalination industry for a number of reasons. First, duplex steels have good corrosion resistance and in many cases can be offered at a cost savings over the comparable austenitic steel. For example, Duplex 2205 has similar pitting and crevice corrosion properties to 904L (UNS NO8904), but is available at a lower cost due to the fact that its chemical composition includes lower percentages of expensive metalsii. In addition, duplex steels have high strength. Specifically, Duplex 2205′s strength is nearly double that of austenitic steels. Thus, less weight of material is typically needed which can lead to further cost savings.

Duplex stainless steels have been successfully implemented in many applications within desalination plants, including evaporators, SWRO and BWRO high-pressure piping, and numerous components in connection with product water processingii. Here are a few specific examples.

Solid Duplex and DualDuplex™ Evaporators:

The breakthrough for duplex in the desalination industry occurred in 2003 when Duplex 2205 was selected to be installed in the Melittah MSF plant in Libya, built by REGGIANE of Italy. Duplex 2205 made it possible to build the evaporators with thinner plates, requiring less material and less welding compared to traditional evaporator materials, such as Austenitic 316L (UNS S31603). Additional benefits include easier handling and less overall environmental impacti. Both the Melittah plant in Libya and the Skikda plant in Algeria (constructed in 2004 and 2005) have successfully demonstrated the ability of Duplex 2205 to meet the needs of these environmentsiv.

Following the successful adoption of the 2205 evaporators, Outokumpu proposed the DualDuplex™ Evaporator concept. DualDuplex Evaporators use two different types of duplex stainless steel in the structures of evaporators in order to maximum cost and material savings. Duplex 2205 is used for parts exposed to the most hostile conditions and lean duplexes, LDX 2101® (UNS 32101) and Duplex 2304 (UNS 32304), are used for parts exposed to less hostile conditions[iii]. The high strength of lean duplex makes it possible to reduce the gauge for these components by up to 50% compared with austenitic grades, although restrictions in design codes limit the real savings to around 35 to 40%iv. DualDuplex™ evaporators have successfully been implemented in multiple desalination plants, including the Taweelah B extension in Abu Dhabi, the Jebel Ali L plant in Dubai and the Ras Abu Fontas desalination plant in Qatar. iv

SWRO High Pressure Piping:

Experience from numerous Seawater Reverse Osmosis (SWRO) plants already in service has shown that there is a high risk of corrosion if the wrong stainless steel is used in the high-pressure piping needed for this environment. The 316L (UNS S31600) and 317L (UNS 31603) grades do not possess sufficient corrosion resistance properties to stand up to this environmentiv. Consequently, highly alloyed austenitic steels, such as 254 SMO (UNS S31254), had been deemed more or less mandatory for yearsv. In recent times, however, super duplex grades such as SAF 2507 (UNS S32750) have made strong headway into this area. SAF 2507 is a desirable solution due to the fact that is has almost the same resistance to pitting and crevice corrosion as 254 SMO, it has twice the strength, and the cost is far lowerv. The Singspring desalination plant in Singapore, which provides about 10% of Singapore’s fresh water needs, is a great example of SAF 2507s successful implementation. SAF 2507 is used for the high-pressure piping that feeds the incoming seawater through the initial seawater reverse osmosis (SWRO) pass for the plantiv. Singspring has been in operation since 2005 and the SAF 2507 has continued to perform welliv.

References:

  1. “Practical Guidelines for the Fabrication of Duplex Steels.” www.imoa.info. International Molybdenum Association. Web.
  2. Olsson, Jan, and Malin Snis. “Duplex – A New Generation of Stainless Steels for Desalination Plants.” www.outokumpu.com. 0utokumpo Stainless, 15 Nov. 2006. Web.
  3. Milestones in Duplex Application. Outokumpu, 2009. Web.
  4. “Desalination in Stainless Steel.” www.worldstainless.org. International Stainless Steel Forum, 2010. Web.
  5. Alvarez-Armas, Iris. “Duplex Stainless Steels: Brief History and Some Recent Alloys.” Bentham Science Publishers LTd. (2008): 51-57. 12 Nov. 2007. Web.

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Duplex Usage in Desalination Plants