Water & Desalination

Seawater and brine are among the harshest services stainless steel faces. Chloride pitting decides the grade long before mechanics do, and a grade that serves for years inland can pit within months on a reject-brine line. We fabricate storage tanks, treatment skids, pressure vessels and process piping for desalination and water treatment facilities across Saudi Arabia, in grades selected for the chloride load of the actual stream rather than for the name on a previous project.

The Kingdom draws much of its water from the sea, so the plants on both coasts, the pumping stations that move the product inland and the treatment works in the cities all need the same thing from a fabricator: the right metallurgy for each part of the process, welded and finished so the corrosion resistance the grade promises is actually delivered.

What we fabricate for water and desalination plants

• Storage tanks: product water, chemical dosing and backwash tanks in stainless steel, with nozzle schedules, manways and internals to the drawing.
• Skid packages: dosing, chemical cleaning, pre-treatment and pumping skids, assembled and tested in the works and delivered as one lift.
• Process piping: headers, manifolds and spools in stainless and duplex grades, welded to qualified procedures with the back-purge and finish the grade needs.
• Pressure vessels: filter housings, cartridge vessels, degassers and air receivers, fabricated and tested to the project code.
• Pickling and passivation of fabricated items, so the heat tint and iron contamination left by welding do not become the first pits in service.

Choosing the grade by position in the process

The stream changes as it moves through the plant, and so should the grade. On intake and pre-treatment, where seawater is oxygenated and warm, duplex 2205 is the usual floor and super duplex 2507 is specified where temperature, stagnation or crevice geometry make pitting more likely. Reject brine concentrates the chloride further, so high-pressure brine and energy-recovery piping sit at the top of the range: super duplex, 904L or a nickel alloy such as Incoloy 825, decided by the process engineer against the design temperature.

Downstream of the membranes the picture eases. Permeate and product water piping is commonly 316L, and post-treatment tanks and dosing skids for lime, carbon dioxide and hypochlorite are specified from the chemical they hold rather than from the seawater upstream. 304L belongs on this side of the plant only where chlorides are low and the surface stays dry or freely rinsed.

The article on stainless steel for desalination plants walks through this position-by-position logic, and why 304 stains near the Saudi coast explains what happens when the floor is set too low. To compare candidate grades on paper, the PREN calculator ranks them from their composition; the grade pages give the chemistry and the resistance class with their sources.

Fabrication details that decide the service life

A duplex grade delivers its corrosion resistance only if the weld matches it. That means the right filler, controlled heat input and interpass temperature, full back-purging of pipe welds and no carbon steel tooling touching the stainless surface. Crevices are the other enemy: lap joints, threaded connections, unsealed gaps under supports and rough weld profiles all trap stagnant brine and start pits, so the shop detail avoids them wherever the drawing allows.

After welding, fabricated items are pickled and passivated, or electropolished where the specification asks for it, so the surface returns to the condition the grade was tested in. The finish inside a product-water tank is also a hygiene question, and the grinding and polishing standard is agreed on the drawing before work starts.

What a good enquiry contains

For tanks and vessels: capacity, design pressure and temperature, the fluid with its chloride content and pH, the nozzle schedule, the code and the documentation level. For piping: the line list, the piping class and the isometrics. For skids: the process and instrumentation diagram and the vendor data for the pumps, membranes or dosing sets to be mounted. Budget weights for pipe runs can be checked on the pipe weight calculator before the enquiry goes in. When the scope is defined, send the enquiry with the plant section named and the drawings attached.

Common questions

Is 316L enough for seawater intake piping?

Usually not for continuous, warm, oxygenated seawater, where 316L pits in crevices and under deposits. It is the working grade for permeate and product water once most of the chloride has been removed. For intake, pre-treatment and brine the process engineer normally starts at duplex 2205 and moves up with temperature and stagnation. The grade pages and the PREN calculator show the gap between the candidates.

Do you supply the duplex and super duplex material yourselves?

Stainless sheet, plate, pipe and fittings come from our own stock in the common grades, and duplex and higher alloys are sourced against the enquiry with their mill test certificates when the quantities call for it. Either way the certificates travel with the fabrication records, so the heat numbers on the as-built drawing trace back to the mill.

Can the skids arrive tested and ready to connect?

Yes. Dosing, cleaning and pumping skids are assembled on their base frame, piped, wired to the local panel and pressure-tested before dispatch. On site the work reduces to setting the frame, anchoring it and making the battery-limit connections. Loose-shipped instruments, if any, are listed with their tag numbers on the packing list.

What finish should a product-water tank have inside?

The drawing should say. Product water is a hygiene service, so a smooth, cleanable internal surface with ground and blended welds is the usual requirement, and pickling and passivation after fabrication removes the heat tint that would otherwise host the first corrosion. If the specification is silent, we propose a finish and record it on the drawing for approval before fabrication.