Valves for Desalination & High-Purity Water

SWRO plants require careful material selection due to seawater's high chloride content (28,000–38,000 ppm Cl⁻ in typical seawater). For seawater-wetted components: Duplex stainless UNS S31803 (PREN ~35) is the standard minimum for body, disc, and trim; Super Duplex UNS S32750 (PREN ~42) is preferred for warm seawater (above 30°C) and brine concentrate. SS 316 (PREN ~25) is insufficient for long-term seawater service and will develop pitting corrosion. On the permeate side (pure water): SS 316L is suitable, or PVDF/CPVC thermoplastics for aggressive pure water service.

SWRO (Seawater Reverse Osmosis) systems typically operate at 55–80 bar feed pressure (depending on seawater salinity and temperature). This corresponds to ASME Class 300 (approximately 51 bar at ambient temperature in SS 316) to Class 600 (approximately 103 bar). Most SWRO designers specify Class 600 rated valves (in SS 316 or Duplex) for membrane train isolation to provide an adequate safety margin above the 80 bar maximum operating pressure. High-pressure booster pump stages may require Class 900 in some ultra-high recovery designs.

Desalination pre-treatment uses several chemical dosing streams: antiscalant (scale inhibitor), sodium hypochlorite (biocide/disinfection), coagulant (ferric chloride or alum), and sodium bisulphite (chlorine scavenger before RO membranes). For dosing of sodium hypochlorite and sodium bisulphite: PVDF-bodied ball or diaphragm valves — resistant to oxidising hypochlorite. For ferric chloride (acidic, pH 2–3): PTFE-lined or PVDF valves. For antiscalant (generally neutral): SS 316L ball valves are suitable. Dosing pump discharge check valves should be SS 316L or PVDF depending on the chemical.

An energy recovery device (ERD) in SWRO captures the high pressure of the brine reject stream (which exits the RO membranes at near-feed pressure) and uses it to pre-pressurize the RO feed, significantly reducing the energy consumption of the high-pressure pump. Common ERD types include pressure exchangers (PX, isobaric) and turbochargers. Valves associated with ERDs include high-pressure block valves (Class 600, Duplex) on ERD inlet and outlet connections, bypass ball valves for ERD maintenance, and anti-slam check valves to protect the high-pressure pump. All must be Duplex or Super Duplex for brine service.