[ Desalination ]
- the presence of oxygen : concentration from a few ppb to a few ppm depending on the sections of the plant ;
- the presence of chemicals : in the seawater inlet section , hypochlorite in solution is present ( max . 1 ppm expected ); possible occasional dosage of acids used for cleaning ;
- operating temperature : from 20-35 ° C at the inlet , up to 110-112 ° C within the plant ;
- working pressure : from absolute vacuum to 6 bar .
Figure 3 . Cross-section of a multi-stage thermal evaporator .
In de-salted osmosis desalination plants ( Figure 2 ), seawater is previously filtered and purified . Powerful high-pressure pumps ( about 60-65 bar ) bring the water to the semipermeable membranes . Some water passes through the membranes to be collected as permeate , while the brine is concentrated and discharged . Reverse osmosis desalination plants require approximately 2.5-4m 3 of seawater for every m 3 of permeate produced .
Material selection In thermal and reverse osmosis desalination plants , particular attention must be given to selecting the construction materials as these remain in contact with seawater or saline solutions throughout the plant ’ s entire life . The two techniques described require different considerations , leading to the use of different types of stainless steel and , above all , different quantities . The main factors that must be taken into consideration when selecting the materials to be used are : - the fluid in contact with the system ; - the presence of oxygen or chemicals ; - operating temperature ; - operating pressure ; - the part of the plant or equipment under consideration and its required structural and corrosion resistance .
Thermal desalination In thermal desalination , the steels are selected based on the working conditions : - fluid : seawater , brine , distillate , non-condensable gases ( CO 2 , O 2 , chlorinated mixtures , bromine ) in the presence of water vapour :
These factors and experience have led to the selection of the following stainless steels in the various plant sections : - aerated seawater : EN 1.4547 ( UNS S31254 ) or EN 1.4410 ( type 2507 ) with a PREN value > 40 ;
- deaerated seawater or deaerated brine : UNS S31803 or EN 1.4162 (( UNS S32101 );
- distilled , drinking water : EN 1.4404 ( type 316L );
- non-condensable gases : EN 1.4404 , preferably in the versions with high molybdenum content ( EN 1.4432 / EN1.4435 ), or EN 1.4410 .
Equipment such as pumps and valves are made from castings . The desalination units are largely constructed using sheets with thicknesses from 4-8 mm for non-structural details and from 8-15 mm for structural parts , partitions , the roof and floor of the evaporator . Sheets are also used to make larger diameter pipes . Figure 3 shows a cross-section of a Multi-Stage Flash desalinator . The dividing walls , floor , roof and the lateral walls of the evaporator , which are
Figure 4 . Aerial view of a Multi-Stage Flash desalination plant .
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