How to Write a Valve Specification That Suppliers Cannot Misread

“Gate valve, DN200, Class 300, WCB.” Five words that will be quoted by four suppliers at four different prices, producing four different valves, none of which may be what the project needs. That is exactly what a valve specification is meant to prevent.

A valve specification is the only document that determines what gets built. When it is incomplete, the supplier fills the gaps with the cheapest interpretation that satisfies the written text — not the interpretation the buyer had in mind. That is not dishonesty; it is the only rational commercial behaviour when the requirement is ambiguous.

This guide lists the data a valve line item needs, shows a properly completed valve specification example, and identifies the omissions that cause most disputes.

The 14 data points every valve specification line item needs

Add a fifteenth field to the valve specification if the service is demanding: documentation and inspection level — whether an EN 10204 3.1 works certificate is sufficient or an independent third-party inspection to 3.2 is required.

A properly completed line item

Here is what an unambiguous line item looks like, using a sour service example:

Tag: GV-1204 Service: Sour produced water, intermittent isolation duty Medium: Produced water with H₂S, chloride content approx. [value] ppm Design temperature: 0 °C to 120 °C Size / class: DN200 (NPS 8), Class 300, rating per ASME B16.34 Ends: Flanged, ASME B16.5, raised face, RF Face-to-face: ASME B16.10, Class 300, DN200 Body / bonnet: ASTM A216 WCB Trim: 13Cr disc and seat with Stellite 6 overlay on both seating surfaces Seat: Metal-to-metal, lapped Operation: Handwheel, rising stem, OS&Y Design standard: API 600, current edition Testing: Shell and seat test per API 598; seat leakage acceptance ISO 5208 Rate D or better Special requirements: Materials and hardness per NACE MR0175 / ISO 15156; body and bonnet stress relieved; marking to include heat numbers Documentation: EN 10204 3.1 material certificates, hydrostatic test report, dimensional inspection report Quantity: 6

Compare that with “DN200 Class 300 WCB gate valve, x6” and the number of decisions the supplier is silently making becomes clear — trim, seat type, face-to-face, torque, testing criteria and documentation, six judgement calls with six cost implications.

The five omissions that cause the most disputes

1. No trim specification.Body material gets all the attention and trim gets none, yet the trim is what determines whether the valve survives. A WCB body with a 13Cr trim is a completely different valve from a WCB body with CF8M trim and a Stellite overlay, and the price difference can reach 30%. The material selection logic is covered in our body and trim selection guide.
2. No face-to-face standard.Two valves with identical size and class can differ by 100 mm or more depending on which series is specified. In a retrofit, that is a rework order. Always cite the standard and the series — for instance ASME B16.10, or EN 558 Series 14 where the project is European.
3. No leakage acceptance criteria.“Zero leakage” is not a specification. It is a number defined by a standard, and it differs between soft-seated and metal-seated designs. Say “ISO 5208 Rate A” or “Rate D or better”, as described in our guide to API 598 and ISO 5208 testing.
4. No actuator torque basis.Specifying “electric actuator” without stating the differential pressure at which the fail-safe position is required invites a marginally sized actuator. State the differential pressure, the required operating time, the supply voltage and the enclosure class.
5. No documentation level.If you need third-party witnessed testing, the manufacturer needs notice — commonly two to four weeks depending on the agency and the location. Discovering this after the valves are painted means either a delay or an inspection that never happened.

Format: keep it in a table, keep it consistent

A valve specification in table form is easier to review and quote when every valve uses the same field order, even the valves where a field does not apply — enter “N/A” rather than leaving it blank, because a blank field is ambiguous between “not required” and “forgotten”.

Three practical habits that prevent most errors:

• State units explicitly, and stay in one system per order. Mixed DN and inch sizing in the same line item invites a conversion mistake.
• Reference the standard, not the habit.“Our usual valve” travels badly into a written valve specification, especially across languages.
• Include the revision where you can.Standards change; the paragraph you relied on in 2015 may have been rewritten.

Frequently asked questions

How much detail is enough? Enough that two competent manufacturers, working only from the document, would build functionally identical valves. If they would not, the document is still incomplete.

Can I just reference the project specification instead? Yes, if it exists and is attached. Referencing an external document that is not attached is worse than writing nothing, because both parties assume the other has read it.

Should I specify the manufacturer’s brand? Only where interchangeability is genuinely required, for instance where spare parts must match an existing installed base. Otherwise, specify performance and let the market compete.

Is a datasheet enough, or do I need a purchase specification? For single valves, a complete datasheet is enough and a formal valve specification is not needed. For projects, a specification covering inspection, documentation, packaging and warranty prevents the arguments that a datasheet cannot address.

What if a supplier offers a deviation? Accept it only in writing, with the technical consequence stated. A deviation accepted verbally to protect a schedule is the origin of most commissioning surprises.

Send us your line list in any format. We will return a priced quotation with every unspecified field flagged rather than quietly chosen — and we will tell you where the specification is likely to cause a problem on site.