A committed and effective long-term solution to South Africa’s water challenges must be found as Day Zero for Cape Town approaches. It is impossible to manage the weather, and the region’s drought trends are expected to get worse. It’s time to cease counting on rainfall and dam levels as the only reliable sources of clean water.
Why desalination
Given that South Africa has a coastline that stretches for more than 2,500 kilometers, it should view the oceans as a plentiful source of water.
Desalination is a tried-and-true method that has been used all over the world to turn seawater into potable water. Desalination is the process of reducing the salinity of water—the amount of dissolved salts and minerals. There are several methods for doing this, but reverse osmosis is the only one that meets the criteria for handling huge volumes, environmental effect, and cost.
Number of desalination plants in South Africa
Around ten desalination facilities may be found in South Africa, spread out along the coastline from Lambert’s Bay in the west to Richards Bay in the east. Each produces a relatively tiny amount of goods that are only used by nearby homes.
The stainless steel involved
The majority of materials used to create the parts of a desalination plant are austenitic stainless steel in the 300 series (316L and 317L). In situations with a marine atmosphere, type 316L is useful.
Although the lack of oxygen in hot seawater lessens the tendency of stainless to pit, the majority of contemporary MSF facilities do not utilize grade 316L. The evaporators’ preferred material in the past has been 316L. However, the majority of desalination system suppliers now use duplex substitutes for this application.
Corrosion is effectively resisted by duplex stainless steel, particularly by stress corrosion cracking (SCC). SCC refers to the unanticipated, rapid breakdown of typically ductile metals that have been subjected to a tensile stress in a particular corrosive environment, particularly at high temperatures. Due to their mechanical characteristics, duplex stainless steel grades are more economical and have two times the strength of austenitic steel grades. Additionally, they make great engineering materials. Both the Melittah facility in Libya and the Skikda plant in Algeria have already used MSF evaporator shells made of duplex grade 2205. (constructed in 2004 and 2005).
Cost-effective duplex stainless steel called lean duplex contains less nickel and molybdenum than 2205. Corrosion resistance is extremely similar to grade 316L’s.
Compared to austenitic 300 series grades, lean duplex’s strong strength allows for gauge reductions of up to 50%. The actual savings, however, are only about 35% to 40% due to design code limits.
There is a risk of corrosion if the incorrect stainless grade is utilized in the high pressure pipe, according to experience from 27 seawater reverse osmosis (SWRO) units that are now in operation. Corrosion resistance is insufficient in neither grade 316L nor 317L.
