Different allowable stress basis, and different Committees doing the Code development.

Pressure vessels have lots of other stress and forces and also need to have a higher FoS due to the stored energy.

It could easily be that "most" PV's are relatively large with lots of openings, dished ends etc, whereas pipe under pressure is rarely > 48" and in most plants is more likely in 1" to 24".

The two codes don't work well at their extremes.

"This a common issue with chemical plants when they design a small vessel using the Piping code " Is it?? There have been many debates here on whether you can just make something out of pipe to say B31.3 or other code as opposed to ASME VIII and the usual answer is no. PV definition is a legal and safety issue so I don't recognise your phrase as being correct IME.

It annoys me when pig traps on pipelines get designed as a Pressure vessel, because that really is an extension of the pipeline and is universally recognised as such in the pipeline design codes, but for a process plant??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Those are some good points. I didn't think of the extra nozzles and openings that vessels have unlike pipes. One could argue that pipe contains store energy so for those that design very small vessels with very few openings, it almost seems it can get away with being built as a "pipe".

I know that's not what the code says though.

I agree with you

@LittleInch

1) The piping and the PV can have the same storage energy.
2) The different allowable stresses between PV and piping are not the cause, they are a consequence.
3) PVs have more complex detail fabrications, so higher risks and more inspections and NDEs are required.
4) Most PVs have longitudinally welded shells, some with two of them.
5) Most PVs are made in the shop. Lifting, transport and erection are other risks.

The above mentioned (and others) are the cause of greater thickness than the piping.

Regards

Failure modes are different. Sizing is different. Lids are different. Even in the B31 codes, like power vs process piping, allowable stresses and equations are different. One such reason for that is that chemica plant may be located in different areas than power plants, one committee member once told me. Hence risks are different, resulting in other allowables.
And so forth

Huub
- You never get what you expect, you only get what you inspect.

To OP's point, how are the ASME VIII exemptions determined?

why is a pressure vessel with a diameter less than 6" exempt from the code?

I wish these codes contain more background information on have reached their requirements.

Buy one of the ASME guide books or the companion guides. They might shed some light on this

Huub
- You never get what you expect, you only get what you inspect.

I checked and sadly not much information there.

Which sources did you check? Maybe I can help

Huub
- You never get what you expect, you only get what you inspect.

I checked the ASME companion guide, read various papers, books regarding pressure vessels. I haven't found the answer yet.

How are the Section VIII Exemptions determined? By discussion and negotiation among the members of the Subgroup on General Requirements (SG-GR) of Section VIII. Basically, to best understand the logic and reasoning behind the rules, you have to attend the meetings where the Scope is discussed. There are a number of competing interests, and the engineering community (by way of the members of the Code Committees) makes their engineering judgements on what's in the best interest of the public.