Desalination Pipe Fittings: Grade Selection for SWRO, MSF, and MED Plants

The Three Desalination Process Types

SWRO (Seawater Reverse Osmosis): High-pressure membrane process, typically 55–75 bar operating pressure, seawater feed at ambient to ~35°C. Brine reject concentration up to ~2× seawater salinity. Main material driver: resistance to pitting and crevice corrosion in aerated, often chlorinated, seawater and brine at pressure.

MSF (Multi-Stage Flash): Thermal distillation, seawater heated to 90–120°C then flashed through successive stages. Main material driver: elevated temperature brine corrosion and erosion-corrosion at stage transfer nozzles. The brine is de-aerated, which reduces its corrosivity compared with aerated hot seawater, but de-aerator upsets let oxygen back in.

MED (Multi-Effect Distillation): Similar to MSF but lower peak temperatures (60–70°C), which reduces scaling and corrosion. Widely used in the Gulf, Mediterranean, and North Africa, often coupled to power or process plants.

Grade Selection by Service Zone

Titanium and lined or non-metallic piping are also used by some plants in dosing and brine service; they sit outside the buttweld metallic fitting scope covered here. For a closer comparison of the two main seawater grades, see super duplex 2507 pipe fittings for seawater and the 254 SMO grade page; for thermal plants, copper-nickel fittings for seawater.

Why 316L Is Not Sufficient for Seawater Service

316L (PREN ~24) suffers pitting and, above all, crevice corrosion in natural seawater at moderate temperatures, and chlorination makes this worse. Gulf seawater reaches 35°C in summer, well beyond any safe margin for 316L. Early desalination plants that used 316L for seawater and brine piping saw repeated pitting and crevice failures, which is why super duplex and 6Mo austenitic grades became the normal choice for raw seawater and brine. Crevices at flange gaskets, under deposits and at weld roots are where attack starts — see crevice corrosion in stainless steel pipe fittings. Even super duplex and 6Mo have temperature limits in chlorinated seawater; industry practice such as NORSOK M-001 sets conservative limits, and these should be checked for each hot or chlorinated line.

Cathodic Protection Interaction

Many offshore desalination structures and SWRO intake structures use cathodic protection (CP). Sacrificial-anode CP in seawater commonly polarises steel to potentials around −1000 mV Ag/AgCl or more negative, and at these potentials duplex and super duplex stainless steels can absorb hydrogen and suffer hydrogen-induced stress cracking (HISC) at highly stressed locations. DNV-RP-F112 gives design guidance for duplex under CP. Electrical isolation of duplex components from the CP-protected structure, or design to limit local stress, is the usual solution. Galvanic coupling between alloys is the other side of the same problem — see galvanic corrosion at dissimilar metal joints.

Welding and Fabrication

• Super duplex 2507: heat input and interpass temperature control to keep the ferrite/austenite balance and avoid intermetallic phases; matching or over-alloyed filler; root shielding gas to protect the bore.
• 6Mo austenitic: welded with an over-alloyed nickel-base filler, because matching filler segregates and loses pitting resistance in the weld.
• Post-weld cleaning: remove heat tint inside and out by pickling; heat tint on the bore is a common initiation site for pitting in seawater.

Inspection and Testing to Specify

• Ferrite content measurement on duplex and super duplex fittings.
• ASTM A923 testing for detrimental intermetallic phases in duplex grades.
• ASTM G48 pitting corrosion test at a specified temperature, where the project specification requires it.
• Impact testing where the design temperature or project specification calls for it.
• PMI on every fitting — 2205, 2507 and 6Mo look identical.

RFQ Checklist for Desalination Fittings

• Grade by UNS and EN number (e.g. S32750 / 1.4410, S31254 / 1.4547), not by trade name only.
• Dimensional standard: ASME B16.9 or EN 10253-4 (Type A or Type B), schedule or wall thickness.
• Supplementary tests: ferrite, ASTM A923, ASTM G48, impact test temperature.
• Surface condition: pickled and passivated, bore free of heat tint.
• Certification: EN 10204 3.1 or 3.2 with the named inspection body.

Arshya Supply for Desalination Projects

Arshya supplies Duplex 2205 and Super Duplex 2507 buttweld pipe fittings to EN 10253-4 Type A and Type B and ASME B16.9, 6Mo (254 SMO) fittings, and Inconel 625 fittings to ASME B16.9 dimensions (EN 10253 does not cover nickel alloys; the fitting specification is ASTM B366). EN 10204 3.1 certificates are standard; 3.2 certificates with third-party inspection are available on request. Fittings are manufactured at our group manufacturing facility, which holds ISO, PED (TÜV), AD 2000, IBR and ASME approvals.

Frequently Asked Questions