For the design of oil & gas valves, what is the difference in using either API 6D or ASME B16.34?

The PDF of ASME B16.34 is 206 pages and API SPEC 6D is 80 pages.  API SPEC 6D is coordinated with ISO 14313; and references ASME B16.34 for rating tables and material groups, radiographic ultrasonic testing.

API SPEC 6D valves include gates, balls, plugs and checks for pipeline, petroleum and natural gas systems.  Many generic details are illustrated, full bore openings and end dimensions defined.  Painting, marking, storage and documentation requirements are identified.

Both standards reflect some testing techniques. ASME B16.34 permits wafer bodies (not allowed? in SPEC 6D?).  ASME B16.34 includes extensive pressure temperature rating tables for material groups in a manner similar to B16.5.

I think the most important difference between them is the testing requirements, I will expound on JLS's advice,

API 6D is the primary standard for valves used in pipeline service, including gate, plug, ball and check valves. Occasionally refinery and petrochemical purchasers will reference the more stringent testing requirements of 6D although the valve may have built under API 600, 602, 608 or 609 design criteria.
    
ASME/ANSI B16.34  Steel Valves - Flanged & Buttwelding Ends
 
ASME B16.34 is the base document from which steel valve pressure/temperature ratings are derived. It also offers additional valve specification data including nondestructive examination procedures for upgrading valves to Special Class. Note: Gate valves manufactured under B16.34 wall thickness minimums may not meet the minimum wall thickness requirement of API 600 & API 602 for class 150, 300 and 600.

The most common reference for non-pipeline use are the testing requirements which differ slightly from API 598 "Valve Inspection & Test". The primary difference being 6D's zero allowable leakage on closure tests. Since most of the valves built to API 6D are resilient seated, this is no problem, however when the test standard is applied to metallic seated wedge gate, globe or check valves, compliance can be difficult.

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