Help with standards that govern welding valves to gas pipelines

[aggieengineerrmb]

Does anyone know what industry standards are commonly used to govern the design and procedures used to determine the wall thickness of butt weld adapters used on ball valves for the gas pipeline market? Also, does anyone know what standards govern when it is necessary to use a transition piece when welding two sections of pipe together with different wall thickness?


Background To The Questions:

On large diameter ball valves used in the construction of gas pipelines, we are seeing different valve manufacturers offer valves of the same size, type, and pressure class but with with large differences in the ID of the butt weld adpater flanges that are used to connect these valves into the pipeline.

Some have told us that ANSI B31.3 and/or B16.34 standards are typically used for formulas, allowable wall thickness or allowable material stress to determine required wall thickness of a valve's buttweld end. These are conservative standards that tend to lead to very thick wall sections that are in some cases 2-2.5 times thicker the the wall thinkness of the actual pipeline (when high stregnth X60 pipe is used). Others don't provide that much detail about what drives their standards, but provide much thinner adapter sections and state that there valves meet all design standard.

Who is right? It all comes down to cost, as some of our engineers feel that there is no way to weld a valve with say a 1" wall thickness to a gas pipeline that has a .375" wall without the use of a transition piece. Those components add lots of extra expense that I want to avoid if possible.

 

[CWIC]

I can only try to answer part of your post as follows:
a.) API Specification 6D - Specification for Pipeline Valves (Gate, Plug, Ball, and Check Vlaves)
b.) API Standard 1104 - Welding of Pipelines and Related Facilities

I'm not sure if any one standard covers specific component thickness(es) of materials, this would be material/service/pressure-specific information. The 6D specification provides standard dimensional properties of certain elements of valves such as bore sizes, flange properties, etc., and operating pressures (class specific).

Can the theoretical valve (1"T) be machined to accomodate the thinner wall pipe (0.375"T) eliminating a pup piece reducer?

 

[stanweld]

Refer to ASME B31.8 Fig. 15 for acceptable designs for unequal wall thickness. Please note that gas pipeline design is based on Specified Minimum Yield Strength (SMYS)- not on Specified Minimum Tensile Strength as for B31.3 and the minimum wall thicknes of your low strength valves may be less than that required for B31.3 designs.