What API 600 adds on top of ASME B16.34
ASME B16.34 sets the pressure-temperature ratings and the minimum wall a body must have to hold them. API 600 is a purchase specification layered on top: it calls up heavier minimum wall sections than B16.34 alone, sets minimum stem diameters so the stem is not the weak link when someone puts a cheater bar on the handwheel, requires a backseat, mandates renewable seat rings rather than integral seats machined into the body, and defines the numbered trim combinations so that "13Cr trim" means the same thing from every supplier. That last point is the practical reason API 600 exists — it makes gate valves from different manufacturers genuinely comparable on a bid sheet.
Choosing between API 600, 603, 602 and 6D
Four standards cover overlapping ground and picking the wrong one is a common and expensive error. API 600 is cast steel, bolted bonnet, from NPS 2 upward — the refinery and process plant default. API 603 covers corrosion-resistant valves in a lighter wall for Class 150 and 300 only; it is the right call for stainless service where API 600 wall thickness is simply paying for metal you do not need. API 602 is the compact forged standard for small bore, generally NPS 2 and below, and is where an A105 forged body belongs. API 6D is a pipeline standard with different testing and traceability requirements — specifying it for an in-plant isolation valve adds cost without adding anything you will use. Match the standard to the service before you match it to the size.
Trim selection and third-party inspection
API 600 defines trim by number, and the choice is driven by the service rather than the body material. A 13Cr trim is the general-purpose baseline. Hard-faced seating surfaces — typically a cobalt-based overlay such as Stellite — are the normal answer for steam, for lines carrying particulate, and for any duty where the valve will be cycled frequently under differential pressure. Austenitic trim such as F316 goes in where the process attacks 13Cr. Note that the body material and trim are specified independently: a WCB or A105 body with stainless hard-faced trim is an entirely ordinary combination.
If the valves are going into a project with a witnessed inspection regime, say so on the enquiry. Hold points for shell and seat testing to API 598, material certification to EN 10204 3.1, and third-party witness by an agency such as SGS or BV all have to be planned into the production schedule — they are difficult and expensive to add retrospectively.
What to include on the enquiry
- Size and class, and confirmation that API 600 is the right standard for the service
- Trim number, and hardfacing where the duty is steam, erosive, or frequently cycled
- Body material, specified independently of the trim
- Bonnet gasket type and face-to-face dimensions to ASME B16.10
- Testing scope to API 598, plus any witnessed hold points and third-party inspection
- NACE MR0175 / ISO 15156 for sour service, and ISO 15848 if a fugitive-emissions class applies
Material behaviour and standard references on this page are general engineering guidance for grade selection. Confirm the final specification against your line list and the applicable design code for the project.
