Desalination Pipe Fittings: Grade Selection for SWRO, MSF, and MED Plants
Desalination plants handle seawater, brine concentrate, and chemical dosing in high-temperature, high-pressure cycles — some of the most corrosive environments in industrial process engineering. Specifying the wrong grade means repeated replacements within the plant's 25-year design life.
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 ~2× seawater salinity. Main material driver: high PRE for concentrated chlorides and pitting resistance at pressure.
MSF (Multi-Stage Flash): Thermal distillation, seawater heated to 90–120°C then flashed through successive stages. Main material driver: elevated temperature seawater corrosion (hot seawater is far more aggressive than ambient); scaling/erosion-corrosion at stage transfer nozzles.
MED (Multi-Effect Distillation): Similar to MSF but lower peak temperatures (60–70°C). Standard in the Gulf, Mediterranean, and North Africa.
Grade Selection by Service Zone
| Zone | Process | Recommended Grade | Reason |
|---|---|---|---|
| SWRO high-pressure feed | SWRO | Super Duplex 2507 | PRE ≥40, 55–75 bar, ambient seawater |
| SWRO brine concentrate | SWRO | Super Duplex 2507 | 2× seawater chlorides, high pressure |
| MSF stage transfer / flash nozzles | MSF | Super Duplex 2507 / Inconel 625 | 90–120°C seawater + erosion-corrosion |
| MED effects piping (60–70°C) | MED | Duplex 2205 or Super Duplex 2507 | Hot seawater — 2205 marginal at >60°C |
| Chemical dosing (anti-scale, chlorine) | All | Hastelloy C-276 or Super Duplex | Oxidising chemicals + chloride |
| Product water (permeate) | SWRO | 316L | Low chloride, non-aggressive |
| Seawater intake screening | All | Duplex 2205 | Ambient seawater at <35°C |
Why 316L Is Not Sufficient for Seawater Service
316L (PRE ~24) will pit in natural seawater within months at temperatures above approximately 20°C. Gulf seawater reaches 35°C in summer — well above 316L's critical pitting temperature in seawater (~10–15°C). Numerous desalination plants in the 1980s and 1990s specified 316L for seawater piping and experienced catastrophic pitting failures within 1–3 years. The industry standard since the late 1990s is Duplex 2205 as minimum for ambient seawater and Super Duplex 2507 for concentrated brine and temperatures above 35°C.
Cathodic Protection Interaction
Many offshore desalination structures and SWRO intake structures use cathodic protection (CP). Under cathodic protection, duplex stainless steel can be susceptible to hydrogen embrittlement if over-protected (potential below -1000 mV Ag/AgCl). Most offshore CP systems target -850 mV Ag/AgCl for structural steel — this can result in local over-protection at duplex stainless attachments. The design of CP systems on mixed-material structures must account for this; electrical isolation of duplex components from the CP-protected structure is often the preferred solution.
Arshya Supply for Desalination Projects
Arshya supplies Duplex 2205, Super Duplex 2507, and Inconel 625 buttweld pipe fittings to EN 10253-2 Type B for desalination projects in Saudi Arabia, UAE, Oman, Qatar, and North Africa. EN 10204 3.1 certificates standard; TPI by Lloyd's Register, TÜV, or BV on request. PED 2014/68/EU DoC available for European EPC contractors.