“Duplex body, PREN above the specified value” can sound like a complete seawater-valve requirement. It leaves several important questions open: which grade and product form, what temperature, what crevices, what condition, and which materials are used in the remaining wetted parts?
Use PREN as a preliminary composition comparison. Approve a chloride-service valve through the actual environment, component materials, manufacturing condition and relevant corrosion evidence. Duplex and super duplex are material families, not universal seawater guarantees.
Describe the chloride environment
Chloride concentration matters alongside temperature, pH, oxygen or oxidants, deposits and flow conditions. Include chemical dosing and cleaning stages if they change the exposure. Stagnant periods can create conditions different from normal flow.
The geometry also matters. Seat interfaces, gaskets, deposits and narrow gaps can create crevice conditions that are more severe than the freely exposed surface. A laboratory pitting result or a successful open-surface coupon does not automatically qualify every crevice in the assembled valve.
Outokumpu's Forta duplex datasheet distinguishes composition-based comparison from corrosion-test information and shows that duplex grades are not immune under all chloride conditions. Its results belong to the stated materials and methods.
What PREN tells the buyer
A commonly used screening expression is PREN = %Cr + 3.3 × %Mo + 16 × %N, using mass percentages. The adopted expression and basis should be identified; modified expressions are also used in relevant contexts.
For an invented composition used only to demonstrate arithmetic, take 22% Cr, 3% Mo and 0.16% N. This expression gives 22 + 9.9 + 2.56 = 34.46. That number is not a specified ASTM grade, a test result or a permissible service temperature.
PREN does not describe the component's actual microstructure, surface condition, weld history, crevice geometry or every corrosion mechanism. A threshold can be part of a project material requirement, but it is not a universal rule that all seawater service becomes acceptable above that number.
Name the product form and condition
A cast pressure-containing part and a wrought stem need their own applicable material specifications. A familiar wrought designation should not be copied onto a casting as an automatic equivalent.
ASTM A995/A995M addresses duplex stainless castings for pressure-containing parts. Its public scope links the properties of these materials to composition and appropriate heat treatment. Use the ordered edition, grade and relevant supplementary requirements for the actual component. ASTM A995/A995M scope
| Material schedule item | What to identify |
|---|---|
| Body and bonnet | Casting/forging specification, grade, condition and heat identity |
| Stem and closure element | Actual specification and condition, rather than “same as body” |
| Seats, springs and fasteners | Exposure, selected alloy and mechanical/environmental limits |
| Welds or overlays | Qualified procedure, required condition and inspection scope |
| Seals and gaskets | Compatibility with process, cleaning chemicals and temperature |
This component schedule also exposes a common specification gap: upgrading the body while leaving a less suitable exposed internal component unchanged.
Separate chemistry verification from manufacturing quality
PMI can help identify an alloy mix-up within the capability of the selected method. It does not establish every aspect of heat treatment, phase balance or corrosion performance. A material certificate, PMI result and corrosion-test record answer related but different questions.
Where the order requires phase examination, ferrite assessment, corrosion testing or other verification, define the method, sampling, acceptance and traceability in the inspection plan. Do not invent one mandatory ferrite band for all grades and product forms. The applicable material standard and project requirements control the criteria.
A hypothetical stagnant-service review
Suppose a seawater header operates intermittently and remains wet during extended standby. One quotation lists a super-duplex body, while the seat-ring and spring materials are unspecified. The body upgrade does not close the review.
Ask the responsible materials engineer to assess the standby environment and crevices. Ask the supplier for the complete wetted-part schedule and relevant condition/inspection records. If a cleaning chemical is added during shutdown, include that stage as well. The result may be an acceptable build, a different material selection or an unresolved condition; the family name does not predetermine the answer.
Avoid substitutions by shorthand
Terms such as “316 equivalent,” “duplex trim” or “marine grade” are too broad for an approved material schedule. Record the offered specification, grade, product form and condition. Assess mechanical properties and pressure-temperature requirements together with corrosion compatibility.
If a supplier proposes another grade, retain the proposed change in a deviation register until its environmental and design requirements are resolved. A higher PREN or a more expensive alloy is not an automatic equivalent for every requirement.
Make the ZGV enquiry component-specific
Use a ZGV ball-valve or butterfly-valve reference to identify the required family. These references do not establish availability or qualification of every duplex construction.
Send the chloride-service schedule and material requirements. Request an available configuration with a complete wetted-part list, product-form identification, proposed verification and deviations. That gives procurement a reviewable manufacturer reply.
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