[ Desalination ]
coloured green , are made from sheets of high-alloy steel .
Steels in reverse osmosis desalination The operating conditions to which steels are subjected to during desalination by reverse osmosis are : - fluid : seawater , brine , permeate ; - the presence of oxygen : some ppm depending on the solubility of oxygen in seawater ;
- the presence of chemical substances : the seawater inlet may be exposed to the occasional presence of hypochlorite in solution ( expected max . 2 ppm ), acid / base dosing for pH correction and for surface cleaning ; - operating temperature : from 20-35 º C ; - working pressure : up to 70 bar .
Subsequently , the selection of materials is as follows : - aerated seawater or aerated brine : EN
1.4410 with PREN > 40 ; - permeate , drinking water : EN 1.4404 .
The indicated selection concerns : - equipment such as pumps and valves that are made from castings ; - high-pressure seawater and brine pipes .
Reverse osmosis systems are considerably more ‘ compact ’ than thermal systems , whether they are Multi-Stage ( MSF ) or Multiple Effect ( MED ). Figure 4 shows the overall view of a multi-stage plant . Figure 5 shows the vertical pipes for feeding seawater to the membranes : for a reverse osmosis system , they work
Figure 5 . Vertical pipes for conveying seawater to the reverse osmosis membranes .
at a pressure of about 60-65 bar and are constructed from EN 1.4410 . Pipes operating at lower pressures at 5-10 bar are usually constructed from plastic materials such as fibreglass . Figure 6 shows the high-pressure pump used to feed seawater to the reverse osmosis membranes constructed from casting Gr 5A duplex ( ASTM A995 ).
Quantities of stainless steel in desalination plants The quantities of high-alloy stainless steel used in desalination plants depend on the type of system ( thermal / reverse osmosis ) and plant capacity .
Experience shows that a mediumsized thermal desalination plant , which produces about 1500 m 3 / h of distillate , requires 900-1000 tons of stainless steel . This evaluation includes the steel required to construct the desalinator , connecting pipes , equipment such as pumps and valves for the desalinator , and the main auxiliary systems . For reverse osmosis systems , the use of high-alloy stainless steel is limited to the pumps , valves and tubes working at higher pressures . These plants require far less stainless steel than thermal desalination plants ; for example , a plant producing 1500m 3 / h of permeate requires about 30-40 tons of the aforementioned materials . Like all industrial installations , even desalination plants , whether thermal or reverse osmosis , are subject to a scale factor . Therefore , an increase in the plant ’ s production capacity does not result in a linear increase in the quantities of stainless steel required .
Acknowledgement : Thanks to the company Fisia Italimpianti SpA for the material provided
Figure 6 . High-pressure pump in duplex stainless steel .
About Centro Inox
This article was provided by Centro Inox ( through its magazine Inossidabile ), a nonprofit association involved in the study and development of stainless steel applications in Italy . For information visit www . centroinox . it /
30 Stainless Steel World November 2022 www . stainless-steel-world . net