Duplex and super duplex valves for seawater service
Duplex and super duplex names do not guarantee seawater performance. Alloy grade, product form, heat treatment, welding, surface condition, temperature, chlorides and flow conditions must be controlled as one materials system.
TAZ Engineering & Export Team · · 11 min
Characterise the seawater exposure
State chloride level, temperature, oxygen, velocity, stagnation, crevices, solids, chemical dosing and cleaning. Intake water, filtered seawater, brine and hypochlorite lines can require different materials.
Assess external coastal exposure separately from the wetted process. Bolting and actuator fasteners also need a corrosion strategy.
Specify grade and manufacturing controls
Name the exact cast, forged and wrought grades for body, bonnet, trim and pressure parts. Define heat treatment, chemistry, mechanical properties and any ferrite or corrosion testing required by the project.
Mixed product forms need an approved equivalence basis. Generic descriptions such as duplex stainless are insufficient for procurement.
Control welding and repair
Use qualified welding and repair procedures with controlled heat input, interpass temperature and filler. Specify NDE, repair limits and post-repair examination.
Review casting repairs and overlay separately. Poor thermal control can damage the intended phase balance and corrosion performance.
Verify identity and final condition
Use material certificates, heat-to-serial traceability and PMI where specified. Check cleanliness, iron contamination, passivation or pickling requirements and protected packing.
Pressure testing proves containment under test conditions. It does not replace materials and corrosion controls.
Checklist
- seawater chemistry and temperature
- flow and stagnation
- exact duplex grades
- heat treatment
- welding procedure controls
- repair restrictions
- NDE and corrosion tests
- PMI scope
- heat-to-serial traceability
- cleaning and preservation