Summary:
- A valve engineering specification (valve spec or piping class specification) is the document that translates your process design requirements into unambiguous procurement requirements — and a poorly written spec leads directly to wrong valves, expensive NCRs, schedule delays, and safety risks in service.
- A complete valve engineering specification covers design standard, material, pressure-temperature rating, end connections, testing, certification, inspection, marking, and documentation requirements — each section requiring specific technical choices, not generic statements.
- Learning to write a proper valve engineering specification is one of the highest-leverage skills in process and mechanical engineering — it controls procurement quality, defines the technical baseline, and protects both the designer and the end user throughout the project lifecycle.
I’ve reviewed valve datasheets from engineers at every level, from graduate trainees producing their first equipment schedules to senior engineers with twenty years of experience. The correlation between specification quality and project outcome is almost perfectly consistent: projects with well-written, detailed, technically correct valve specifications almost always procure the right valves. Projects with vague, generic, or incomplete specifications almost always generate inspection comments, non-conformance reports, and arguments with suppliers during FAT. The spec is where it all starts. Here’s how to write one properly.
What Is a Valve Engineering Specification?
A valve engineering specification is the technical document that defines all requirements for a valve or group of valves. It can take several forms depending on the project context:
A project valve specification is a general document that applies to all valves of a particular type across the project — for example, “Project Specification for Ball Valves” or “Piping Class 3A1: Carbon Steel, ASME Class 150, Ball Valves.” This document defines the standard requirements that every ball valve in this piping class must meet.
A valve datasheet (or valve technical data sheet) is a valve-specific document for an individual valve or group of identical valves. It records the specific process conditions (pressure, temperature, fluid), valve selection, and all technical requirements for that valve’s service. Valve datasheets are the primary procurement document and the reference for inspection.
A valve schedule or register lists all valves in the project, with references to the applicable specifications and datasheets. It’s a management tool as much as a technical document, used to track status through the procurement and inspection cycle.
In practice, all three documents are used together: the project valve specification defines the requirements, the datasheet applies them to the specific valve, and the schedule manages the full population.
Structure of a Valve Engineering Specification
A well-structured valve engineering specification typically contains the following sections. I’ll work through each and explain what belongs there and why.
1. Scope
Define what the specification covers and what it doesn’t. “This specification covers the design, materials, fabrication, testing, inspection, marking, and documentation requirements for carbon steel ball valves in ASME B16.34 Class 300, for installation in hydrocarbon gas service at temperatures between -29°C and +120°C.” This establishes the envelope immediately — nothing is implied.
2. Normative References
List every standard, code, and specification referenced in the document. Use the full title and edition year. “API 6D (latest edition): Specification for Pipeline and Piping Valves. ASME B16.5 (2017): Pipe Flanges and Flanged Fittings. API 607 (8th edition): Fire Test for Quarter-Turn Valves.” Specify whether the latest edition applies or whether a specific dated edition is required — for some standards (particularly API), different editions contain significantly different requirements.
3. Design Requirements
This is the core technical section. It should cover:
— Design code (ASME B16.34, API 6D, EN 13709, etc.)
— Pressure-temperature rating (design pressure and temperature, not just operating conditions)
— Valve type (ball, gate, butterfly, globe, check)
— Port configuration (full bore or reduced bore; specifying both is a common error)
— Flow direction (unidirectional or bidirectional; critical for check valves and some ball valve designs)
— End connections (flange type, face type, dimensional standard)
— Face-to-face or end-to-end dimensions (which standard governs these)
— Operating requirements (number of operating turns, handwheel size limits, actuator interface requirements)
4. Materials of Construction
Specify every pressure-containing and pressure-controlling component material explicitly. Body, bonnet, stem, ball (or gate/disc/plug), seats, seals, bolting, and body-to-bonnet gasket. Do not say “stainless steel” — say “ASTM A182 Grade F316L.” Do not say “suitable material” — name the material.
Include any special material requirements: NACE MR0175/ISO 15156 compliance for sour service (including hardness limits), low-temperature impact testing requirements for cryogenic or cold climate service, restrictions on alloy content for hydrogen service.
Seat and seal materials require particular attention. For soft-seated ball valves: specify PTFE or RPTFE or Devlon (and the temperature/chemical basis for the choice). For stem seals: specify graphite/PTFE dual packing for fire safe service. For body-to-bonnet gaskets: specify spiral wound SS/graphite for high-temperature service.
5. Testing Requirements
Specify the tests required, the test standard, the test pressure, and the acceptance criteria. A typical section for a ball valve might read:
“All valves shall be shell-tested (hydrostatic) to 1.5 × CWP for a minimum of 5 minutes. All valves shall be high-pressure seat-tested (pneumatic or hydrostatic) to 1.1 × CWP, maximum allowable leakage API 6D Rate A (zero leakage). All valves shall be low-pressure seat-tested at 0.5-1.5 bar(g) pneumatic, maximum allowable leakage API 6D Rate A. Valve operation shall be demonstrated during testing. Fire safe testing: API 607 third-party test certification required; testing shall have been conducted on a valve of identical design, same or larger size, and same or higher pressure class.”
Don’t leave test criteria open to interpretation. If the customer’s requirement is zero leakage, say “zero leakage (API 6D Rate A)” not “minimum leakage.”
6. Inspection Requirements
Define the inspection hold and review points. The standard categories are:
— Hold (H): The manufacturer cannot proceed past this point without the inspector being present. Used for witness tests and final inspection.
— Witness (W): The inspector should be notified in advance and may attend; the manufacturer can proceed after the notification period if the inspector doesn’t attend.
— Review (R): Records must be submitted for review; manufacturing can continue.
A typical valve inspection plan might hold on final assembly, witness pressure tests, and review material certificates. For critical safety valves, you may hold on all tests and require the inspector to witness everything.
7. Marking and Identification Requirements
Specify what information must be marked on the valve body, the marking method (cast, stamped, stencilled), and the applicable marking standard (API 6D, MSS SP-25). Minimum marking typically includes: manufacturer’s name or trademark, body material grade, pressure class, flow direction arrow, size, heat number or lot number, and any special service markings (sour service, fire safe).
8. Documentation Requirements
List every document required in the manufacturer’s data record (MDR) and specify whether originals or copies are acceptable. Typical requirements include: material test certificates (to EN 10204 Type 3.1 or 3.2 for pressure-containing parts), dimensional inspection reports, pressure test reports, NDE reports, fire safe test certificate, NACE compliance certificate (if applicable), and operating and maintenance (O&M) manuals.
The EN 10204 Type designation for material certificates is important: Type 2.2 is a works certificate validated by the manufacturer; Type 3.1 is a specific test certificate validated by the manufacturer’s authorised inspector; Type 3.2 requires validation by both the manufacturer’s and purchaser’s (or independent) inspector. For safety-critical and pressure-containing components, Type 3.1 minimum is the standard requirement.
Common Specification Writing Errors and How to Avoid Them
Using “shall” and “should” inconsistently — in engineering specifications, “shall” is mandatory; “should” is recommended. Every requirement that must be met for the valve to be acceptable must use “shall.” Using “should” for mandatory requirements invites suppliers to treat them as optional. Go through your specification and ensure every true requirement uses “shall.”
Referencing outdated standards — major standards are revised periodically. A specification that references “ASME B16.5-1988” will get you valves designed to an obsolete standard. If the latest edition is required (which it usually is), say “ASME B16.5 (latest edition).” If you need a specific edition, use that edition consistently and explain why in the scope section.
Specifying “as per manufacturer’s standard” for critical requirements — “pressure testing as per manufacturer’s standard” gives the manufacturer complete freedom to define their own test requirements. If you have specific test requirements (leakage class, test medium, test duration), specify them explicitly. “As per manufacturer’s standard” is only acceptable for non-critical aspects like packaging and shipping.
Leaving face-to-face dimensions unspecified — if you have a valve that needs to fit into an existing installation with a fixed face-to-face distance, specify the required face-to-face dimension and the dimensional standard that governs it. If you just say “ASME B16.34 Class 300 ball valve,” suppliers will provide their standard face-to-face which may not match your installation. ASME B16.10 is the governing standard for face-to-face dimensions for steel valves.
Not specifying hard-facing requirements — for ball valves in erosive, abrasive, or corrosive service, hard-facing or HVOF coatings on the ball and seat are often required to achieve design life. If this isn’t in the specification, the supplier will provide standard seat material, which will fail early. Specify hard-facing explicitly: “ball shall be hard-faced with HVOF tungsten carbide, minimum Rockwell C 70, minimum 200 microns DFT.”
Datasheet Best Practices
A well-structured valve datasheet should be self-contained — a competent supplier should be able to manufacture and inspect the correct valve using only the datasheet and the referenced specifications. Every field should have a definitive entry; “TBD” (to be determined) fields are a procurement risk.
Key datasheet fields that are consistently poorly completed:
Design pressure and temperature — engineers often list operating conditions but omit design conditions. The valve must be rated for the design condition (maximum allowable working pressure at design temperature), not just normal operating conditions. The difference can push a valve from Class 150 to Class 300.
Fluid composition — “natural gas” is not a fluid composition. The manufacturer needs to know the specific gravity, H2S content, CO2 content, and any other components that affect material selection, leakage classification, and fire safe requirements. Even if the detail isn’t critical, completing the fluid composition field shows that the designer has considered the service conditions properly.
Actuator requirements — if the valve will be actuated, every actuator-related parameter must be on the datasheet: actuator type, signal type, fail position, power supply, area classification, and actuator mounting pad standard (ISO 5211). Missing actuator information is the single most common cause of schedule delays in valve procurement.
Painting and coating requirements — specify the external painting system, minimum DFT, and colour. If the valve is for offshore service, specify the corrosion category (ISO 12944 C5-M) and the system number from the project’s painting specification.
Aligning Specifications with HAZOP and SIL Requirements
For safety-critical valves — Emergency Shutdown Valves (ESDVs), safety relief valves, high-integrity pressure protection system (HIPPS) valves — the valve specification must incorporate requirements derived from the HAZOP and Safety Instrumented System (SIS) safety requirements specification. This typically includes:
— Proof test interval requirements (how often the valve must be tested to maintain the required SIL)
— Diagnostic coverage requirements (does the valve need partial stroke testing capability to provide online diagnostic coverage?)
— Redundancy requirements (is a single valve sufficient or is a 1oo2 or 2oo3 arrangement required?)
— Demand mode and proof test requirements from the SIL assessment
The valve specification for SIS service should cross-reference the SIS safety requirements specification (SRS) and confirm that the specified valve’s reliability data (PFDavg and Lambda-D values) has been validated against the SIL requirement. Valve manufacturers increasingly provide IEC 61508 failure mode data to support SIL verification — specify that this data must be provided as part of the documentation package.
Writing a valve engineering specification well is a disciplined, methodical exercise. It requires knowledge of standards, understanding of service conditions, and the willingness to be specific rather than general. Generic specifications generate generic (and often wrong) valves. Precise specifications generate precisely what the project needs. Invest the time at the start — it pays back many times over through the procurement, installation, and operating life of the facility.