Normally for well head and API 3000/ 5000 We use API 6A for Choke Valve.however I want to know the design standard for ASME CL 600 Choke valve used for Oil services.
Choke valves must be always to API 6A design whether to API 3000 or API 20000 or ASME 150 or ASME 2500 class,as they are considered part of the wellhead equipment.
Choke valve can be with API flanges or ASME flanges, in your case with ASME 600 flanges, but the Choke design should be always to API 6A.
I am not sure if I can say that a Choke valve to ASME design is just an Angle Control valve supplied as a choke valve ! Generally, Choke valves with ASME flanges are not permitted to have API monogram on their body. Somebody can correct me if I am wrong.
By Cl 600 choke valve, I can only assume you mean a standard control valve of which there are many different types. "Choke valve" has become recognized as a particular type of high pressure control valve associated with well head assemblies, but is just a particular type of control valve.
pages 175 onward, doesn't' list a specific code for a control valve, but simply it normally conforms to something like ASME B 16.34 for dimensions and pressure rating and has flanges to ASME B 16.5 and the internals are up to the vendor.
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....and all this based on that we are only talking about US standards, as implied by the question! (Just to remind you that there is other standards in other parts of the world, although US standards are dominating for the oil industry ! Sorry, coldn't resist the comment!)
Choke valves can be to API 6A design or ISO 10423 design standards, but there is no ASME standard choke valves even if the choke is required with ASME flanges.
Also, for choke valves , user should specify material classification to API 6A - CC, DD,EE,FF or HH and testing to PSL -1/2/3/4