- A poorly written valve specification is a false economy — it produces an ambiguous contract that results in either expensive clarification during procurement or, worse, valves that meet the letter of the specification but fail to meet the engineering intent, discovered only during installation or commissioning.
- The valve datasheet is the primary vehicle for communicating technical requirements to valve manufacturers, and its format — what information is provided by the purchaser, what is to be confirmed by the manufacturer, and what is left undefined — directly determines the quality of the resulting equipment.
- Reference standards (API, ASME, ISO) in valve specifications must be cited with specific edition numbers, and the engineer must understand what those standards actually require before citing them — many specifications cite standards that the writer has never read, resulting in requirements that conflict with the stated engineering intent.
Why Valve Specifications So Often Fail
I have reviewed hundreds of valve purchase specifications over a career in valve engineering, and the failure modes are remarkably consistent. The most common: specifications that are copied from a previous project with minimal modification, carrying forward the assumptions and constraints of a different service condition. Specifications that cite standards without understanding what those standards require, creating requirements that are either unachievable, redundant, or conflicting. Specifications that leave critical parameters undefined (“heavy-duty construction” without defining what that means) and then complain when the supplied valves are not what was intended. And specifications that specify the solution (a specific valve type) rather than the requirement (the isolation and flow control performance needed), preventing the use of better technical solutions.
Good valve specification writing starts with a clear understanding of the purpose: the specification is the technical communication between the purchaser’s engineering team and the valve manufacturer’s engineering team. Its job is to convey exactly what the valve must do, under what conditions, to what standards, with what documentation, in an unambiguous manner that leaves no room for misinterpretation but also doesn’t unnecessarily restrict the manufacturer from using their design expertise to provide the best solution.
The Valve Datasheet: Architecture and Information Flow
The valve datasheet — typically a table-format document with line items for each technical parameter — is the primary technical document in the valve procurement package. Its structure must clearly distinguish between information that the purchaser provides (process conditions, performance requirements, and mandatory standards), information that the manufacturer is expected to confirm (design details, material selections within permitted alternatives, and calculated parameters), and information that the manufacturer completes (their specific design data, component selections, and test results).
The purchaser-provided section must contain: fluid description (name, phase, composition for multicomponent streams); process conditions (operating pressure and temperature, design pressure and temperature, minimum and maximum credible temperatures); flow conditions (normal, minimum, and maximum flow rates, and whether these are simultaneous or alternative cases); inlet and outlet pipe sizes and schedules; end connection type and standard; leakage classification required (ANSI/FCI 70-2 class); actuator type (if actuated) and fail-safe position; control signal type (if controlled); and the applicable standards for design, materials, testing, and documentation.
The manufacturer-confirmation section should contain: proposed body and trim materials (confirming compatibility with the specified service); calculated Cv at normal and maximum flow; selected trim Cv and characteristic; noise prediction at design conditions; weight; face-to-face dimension; actuator model and torque output; and compliance statements for each specified standard.
Common errors in datasheet structure: providing design pressure only (without operating pressure, the manufacturer cannot determine whether live-loaded packing is required or whether cavitation will occur at normal operating conditions); providing flow in mass or molar units only (without density information, the manufacturer cannot calculate volumetric flow or Cv); and providing temperature as a single value (without the operating range, the manufacturer cannot assess packing temperature, seat material temperature limits, or body-bonnet joint design requirements for thermal cycling).
Material Specification Approach: Specifying What You Need, Not How to Get There
Material specification in valve datasheets frequently falls into one of two traps: either over-specifying (requiring specific alloy grades with specific heat treatment and mechanical property requirements that go beyond what the service demands) or under-specifying (saying “stainless steel body” without specifying the grade, form, or condition).
The correct approach specifies the required material properties and the environment to which the material must be resistant, and allows the manufacturer to select specific alloys and heat treatments to meet those requirements. For example: “body material shall be suitable for continuous service in 98% sulphuric acid at 60°C, shall have a maximum corrosion rate of 0.1 mm/year in that service (verified by corrosion data or test), and shall comply with ASME B16.34 material requirements for the specified pressure class” is a better specification than “body: Hastelloy C-276.” The former communicates the requirement; the latter prescribes a solution that may or may not be optimal and leaves the manufacturer no flexibility to propose a better alternative.
When a specific alloy is required — because the design is based on proven experience with that alloy in the service, because spare parts interchangeability requires consistency with existing plant, or because there is a specific material limitation in the process (contamination concerns, for example) — specify it explicitly with the grade designation, product form, and applicable material standard. “316 stainless steel to ASTM A182 Grade F316L” is a complete specification. “Stainless steel” is not.
Hardness limitations — particularly for sour (H2S) service — are mandatory specifications that must be stated explicitly, not left to the manufacturer’s default. NACE MR0175/ISO 15156 maximum hardness requirements (e.g., 22 HRC maximum for carbon and low-alloy steel body and trim components in H2S service) are not default requirements in any design standard — they must be explicitly invoked in the specification. Specifying “in accordance with NACE MR0175” is the correct approach, but the engineer writing the specification must understand what this actually requires for each component in the valve assembly to verify compliance when the manufacturer’s material certifications are received.
Reference Standards: What to Cite, How to Cite It, and When Not to Cite It
Reference standards in valve specifications must be cited precisely and the engineer writing the specification must understand what those standards actually require. Citing a standard you haven’t read is one of the most common and consequential errors in specification writing — you may be requiring something that is impossible, redundant, or that conflicts with another requirement in the same specification.
The minimum information for a standards citation is: standard number, title (for clarity), and edition or year. “ASME B16.34-2020” is correct. “ASME B16.34” is incomplete — the edition matters because requirements change between editions, and a manufacturer who supplies to the 2009 edition when you intended the 2020 edition may technically comply with your specification while not meeting your intent.
Common specification standard errors: citing API 598 and ISO 5208 simultaneously without specifying which takes precedence (they have different test pressures and leakage allowances, so both cannot apply simultaneously); citing ASME B16.34 for a valve outside its scope (B16.34 does not cover all valve types or sizes — check the scope carefully); citing API 6D for valves that are actually process plant valves rather than pipeline valves (the longer face-to-face dimensions of API 6D create interchangeability problems with existing plant B16.10 dimensions); and citing the ASME N-stamp requirement for a valve that is in non-nuclear service (which adds enormous cost and qualification requirements that are unnecessary).
Actuator Specification Requirements
Actuator specification is frequently separated from the valve specification and handled by the instrumentation and control discipline — which is appropriate from a project discipline perspective but creates a risk that the actuator specification and valve specification are not coordinated. The valve engineer and the instrument engineer must agree on: the valve torque requirement (which the valve engineer calculates, not assumes), the actuator mounting standard (NAMUR for rotary, standard coupling dimensions for linear), the supply pressure available (which determines the actuator sizing for pneumatic designs), the control signal type and range, the position feedback requirements, the fail-safe direction and spring specification, and the ingress protection requirement for the actuator and positioner.
Actuator datasheets should specify: output torque at minimum supply pressure (for pneumatic) or minimum battery voltage (for battery-backed electric), confirming that the actuator can perform its safety function under worst-case conditions; stroke time from fully-closed to fully-open at normal and minimum supply; spring return torque at mid-stroke position (the minimum available torque during fail-safe operation); and the actuator’s duty cycle classification.
One of the most commonly under-specified actuator requirements is the in-situ torque test — a test that verifies the installed, actuated valve assembly can be stroked under the actual process pressure and fluid conditions, not just under bench-test conditions with no fluid load. In-situ torque testing is the ultimate verification that the actuator sizing is adequate, and it should be a commissioning requirement for all safety-critical actuated valves and for any control valve where the calculated torque margin is less than 30%.
Documentation Requirements: The Paper Trail That Protects You
The documentation requirements in a valve specification define what paper trail will exist for the supplied valve for the lifetime of the plant. Getting this right protects the purchaser in three ways: it provides the evidence needed to demonstrate regulatory compliance during inspections; it provides the reference data needed for condition monitoring and remaining life assessment; and it provides the evidence needed to pursue warranty claims or liability claims if a valve fails prematurely.
Minimum documentation requirements for process plant valves should include: material test certificates (MTCs) for all pressure-containing components, traceable to specific material heat numbers; dimensional inspection records for critical dimensions; test certificates for all specified factory tests (shell, seat, backseat, functional); and the manufacturer’s declaration of conformance confirming compliance with all specification requirements. For safety-critical valves, additional requirements include PMI records, NDE records, and the manufacturer’s quality control records showing hold point sign-offs during fabrication.
Electronic document management for valve documentation — delivery of documentation in defined formats (PDF for certificates, XML or structured data for valve parameters) to defined naming conventions — is increasingly specified on modern projects. The investment in structured documentation during procurement pays dividends throughout the plant’s operating life when maintenance engineers need to retrieve valve data quickly. A plant’s valve documentation stored in a searchable electronic format accessible from a mobile device is immeasurably more useful than a filing cabinet full of paper certificates that nobody can find when they’re needed.