Why the Standard Matters Before the Spec Sheet

Most published guides on industrial valves start with the valve type — ball, gate, globe, check — and work backward to the standard. On a real engineering project the order is the opposite. The standard is the first thing the design basis fixes, because it dictates the pressure class, the test regime, the material traceability and, ultimately, the price envelope.

For most oil & gas, petrochemical, power and marine projects built to Western specifications, three codes do almost all the heavy lifting: API 6D for pipeline valves, API 602 for small-bore forged valves, and ASME B16.34 as the foundational pressure-temperature design standard. They are not competing documents. They sit at different layers of the same selection tree, and the right answer on any given line is usually a combination, not a single standard.

API 6D — The Pipeline Valve Standard

API 6D, formally Specification for Pipeline Valves, is the code that governs the full-bore and reduced-bore ball, gate, check and plug valves used on oil and gas transmission lines. The current edition (API 6D, 25th edition) covers Class 150 through Class 2500 ratings, and it is the standard most owner companies force on a long-distance pipeline scope.

Where API 6D applies

• Crude oil, refined product and natural gas transmission mains (e.g. pipeline works built to API 5L PSL1/PSL2).
• Gathering systems, compressor stations, metering stations and terminal facilities.
• Subsea lines (where the project references the API 6DSS extension).
• Block-valve stations, scraper-trap valves and mainline check valves.

What API 6D requires that a generic industrial valve does not

• Full-bore or reduced-bore geometry to allow pipeline pigs and inspection gauges to pass.
• Self-relieving seats or external body relief to prevent thermal over-pressure in the cavity.
• Fire-safe design in line with API 6FA or API 607.
• Anti-static construction to prevent spark discharge across the ball or plug.
• Double block and bleed (DBB) capability for double-seated ball and gate valves.

Practically, an API 6D industrial valve ordered for a transmission line is bolted up to an ASME B16.5 steel flange and is shipped with full traceability from the steel melt. The associated pipe fittings and pipe flanges need to be on the same MTC chain, otherwise the inspector rejects the joint at the warehouse before it ever leaves the quay.

API 602 — The Small-Bore Forged Valve Standard

API 602, Gate, Globe and Check Valves for Sizes NPS 4 and Smaller, is the code that governs small-bore forged steel valves. It is the workhorse of the inside-battery-limit (ISBL) plant — the valves you see on instrument root lines, chemical injection quills, sample coolers and high-pressure utility branches.

Where API 602 applies

• Instrument root valves and impulse lines.
• Sampling systems and chemical injection skids.
• High-pressure utility branches (Class 800 up to Class 1500).
• Steam tracing manifolds and small-bore bypasses around control valves.

Key technical limits of API 602

• Size range: NPS 1/4 to NPS 4 (DN 8 to DN 100).
• Pressure class: Class 150 to Class 1500; Class 800 dominates the small-bore welded systems.
• End connections: threaded (NPT), socket-weld (SW) or butt-weld (BW). Class 800 is almost always welded, not flanged.
• Bonnet style: outside-screw-and-yoke (OS&Y) or compact bolting, depending on the project specification.
• Seat options: hard-faced (Stellite or equivalent) for high-temperature or erosive service; bellows seal for fugitive-emission-controlled services.

For an API 602 valve, the matching connection is normally a socket weld fitting (for NPS 2 and below) or a butt weld fitting (for NPS 2½ to 4). Threaded fittings are reserved for utility service where the system is not part of the pressure-boundary integrity case.

ASME B16.34 — The Pressure-Temperature Foundation

ASME B16.34, Valves — Flanged, Threaded and Welding End, is the underlying code that both API standards reference. It is the document that produces the pressure-temperature (P-T) rating tables everyone looks at when they want to know if a Class 300 stainless valve is still rated at 1.0 MPa at 400 °C.

What B16.34 actually provides

• Pressure-temperature ratings for Class 150 through Class 4500 across carbon, alloy, stainless and high-nickel alloys.
• Material groupings that define where each body material can be used on the temperature scale.
• Minimum wall thickness rules and the implicit link between valve rating and pipe schedule.
• End connection geometry for flanged, threaded and welding-end valves.

B16.34 is rarely used in isolation on a valve spec, but it is the document the inspector will use to decide whether your selected valve is actually rated for the design temperature. If a project references B16.34, you must check the body material on the P-T table, not just the nominal pressure class on the data sheet.

The Supporting Standards the Procurement Team Will See

Beyond the three primary codes, four supporting standards show up on most valve datasheets. They are not optional — they are referenced by API 6D and API 602 and they are required at the inspection stage.

A 4-Step Selection Logic for the Valve Specification

Rather than memorising every clause in every code, run the valve selection through four questions. Each question eliminates a class of valves and narrows the standard combination you need to cite.

1. Where is the valve installed? If it is on a long-distance pipeline, mainline block-valve station or scraper trap, the answer is API 6D. If it is inside the plant battery limits, ｄｒｏｐ to API 602 (small-bore) or API 600 (large-bore cast gate). If the project does not enforce API, ASME B16.34 plus API 598 is the fallback for general industrial service.
2. What size is the line? NPS 4 and below defaults to API 602. NPS 6 and above on a pipeline defaults to API 6D. NPS 6 and above inside the plant usually defaults to API 600 (gate) or API 608 (ball).
3. What is the end connection? Small-bore welded systems (NPS 2 and below) almost always end in socket-weld or threaded connections. Pipeline valves and large refinery valves go to RF or RTJ flanges per ASME B16.5, matched to pipe flanges on the line.
4. What is the service fluid? Sour service (NACE MR0175), cryogenic (special body extension and trim), high-temperature steam (A217 grades), and seawater (Cu-Ni bodies) each impose an extra material overlay. The selected standard is still API 6D, API 602 or B16.34, but the body material gets overlaid with the appropriate code.

Common Selection Mistakes (and the Right Way to Specify)

A few mistakes show up so often in RFQs that they are worth calling out directly. Each one of them is fixable in the office, before the valve has been manufactured.

Mistake 1: Specifying API 6D on a plant-side valve

API 6D is a pipeline code. It requires full-bore geometry, self-relieving seats and DBB capability — features that are unnecessary and expensive on a process-side block valve. For a refinery or petrochemical plant, the right combination is API 600 (gate), API 608 (ball) or API 602 (small-bore), with ASME B16.34 setting the rating.

Mistake 2: Forgetting the B16.34 P-T check

A Class 300 stainless valve at ambient temperature has a different working pressure than the same valve at 300 °C. The B16.34 P-T table tells you exactly how much margin you have left at design temperature. If the specifier skips this check, the inspector will catch it at the FAT, and the project will absorb a costly rework.

Mistake 3: Using a cast steel body on a small-bore Class 800 line

Class 800 small-bore service should be a forged body (API 602), not a cast body. Forged bodies have a finer grain structure and are better suited to the high pressure and frequent thermal cycling typical of small-bore branches. Cast bodies are reserved for NPS 2½ and above, usually under API 600.

Mistake 4: Mismatched bolting on the flange joint

The valve, the flange and the gasket, stud bolt and nut set are one system. The stud-bolt grade must match the flange class, the gasket must match the flange face, and the torque procedure must be issued before hydrotest. Skipping this is the single most common cause of a leak on a freshly commissioned valve station.

How the Standard Choice Ripples into the Rest of the Bundle

A valve is rarely the only line item on the PO. The same engineering basis that picks API 6D, API 602 or B16.34 will also pick the pipe standard, the flange standard, the gasket type and the stud-bolt grade. Treating these as one bundle — rather than seven independent items — is what a project-centric supplier is set up to deliver.

For a buyer, the practical benefit of getting this right is straightforward. One supplier, one MTC trail, one delivery slot, one phone number if anything needs re-issuing. That is the same model EZ STEEL INDUSTRIAL applies across its carbon steel, stainless steel and copper-nickel alloy lines out of Changsha, including the matching pipe fittings, pipe flanges, heat efficiency tubes and gasket / stud-bolt kits.

A Practical 8-Point Checklist Before the RFQ Goes Out

1. Confirm the standard combination (API 6D / API 602 / B16.34) for every valve line.
2. State the design pressure and design temperature — both are required for the P-T check.
3. List the fluid, concentration and chloride / H₂S content — not just "process water" or "crude".
4. Fix the end connection (RF, RTJ, BW, SW, NPT) per line.
5. Lock the flange class, face type and facing finish.
6. Specify the body material and trim material against ASTM or EN grades.
7. Reference the inspection and test standard (API 598 is the usual default).
8. State that delivery is bundled with pipe, fittings, flanges, gaskets and stud bolts on a single MTC.