Different Thicknesses and Grinding the Inside Diameter Equal Them

• So why use thick pipe when thinner pipe will do? The strength of the pipe cannot exceed that of the thinner pipe!
• So why use thin pipe when thicker pipe is needed? The strength of the pipe cannot exceed that of the thinner pipe!

Someone here has screwed up and is bodging something, which is why such practices are not covered in API.

I MEANT when welded flange as GOST with thickness 13mm with pipe as GOST thick 17mm in case those just available

also i think the answer regarding this issue in ASME b31.3

Here I'm ASKING REGARDING API Code found or not ?

There isn't a section about "bodging".

That is a universal problem with defective standards that fail to contemplate common occurrences.

If this was water pipe (Reticulation mains) and you've reached a point where a lower pressure rated line can be used, then a welded joint would not be appropriate. (Eventually you get high enough up the hill that you don't need the heavier grade OR the heavier grade isused for external loading like underneath a shallow road crossing where the extra strength is called for.)

Our local convention would be to use a fitting to join the different pipe grades, probably a stop valve.

This enable isolation of the sections for maintenance, inspection and such and also provides decisive indication of where the pipe grade changes.

"Bodged". Quite.

See LANL Procedure according to API 1104 & Asme B31 8
http://www.lanl.gov/orgs/eng/engstandards/esm/welding/vol2/WFP%202-01-Att-3-R1.pdf
Fig 15
WP

A common standard is 1:3 taper; hence D (as in the picture, but not in the given formula) = 12 mm.

No API code on your suggested procedure.

Welded pipe couplings are required to be of the same ANSI class which ensures the wall thickness and inside diameters are the same matched surfaces and of the same composition.

This requirement is to ensure all fittings and pipe used in an application meet applicable standards and minimize cavitation which can be detrimental to the pipe and fitting inside surfaces.

Machining the inside diameter surface of the fittings to match the pipe will eliminate or minimize the chance of cavitation occurring but the increased cost due to lost time to the fabrication schedule and the added labor is high.

You do not say what type of fittings or welding procedure(s) will be used to mate the surfaces which can also drastically affect the outcome and safety of modifying the fittings.

Depending on the process application pressure, temperature, fluid type, and conveying velocity there may be significantly complicated annealing and/or tempering procedures required to ensure the fittings and pipe maintain their critical properties.

These procedures will be different for the fittings than for the pipe which may cause serious safety and/or reliability issues with the installation.

It is a much better safety and cost control option to purchase the correct fittings that match the pipe being installed.