D'oh! Guess I should have read the OP's post more carefully!
Yes, I'm aware of the difference between slip on and lap joint flanges! Sorry for confusing matters!
Here too, we've seen slip-on flanges used as body flanges on small pressure vessels fab'd from pipe, to 1500# class. But in piping, where moments resulting from thermal expansion etc. have to be contended with, most people prefer to use weld neck flanges above 300# class.
I don't know if there's anything official limiting their use in piping below certain classes or certain temperatures- it seems to be rule of thumb. The only code reference I've seen is toward their use with a single fillet weld, which we never do anyway.
They're handy in piping because they allow a fair bit of angular adjustment during fit-up without a lot of fitting labour. They also conserve expensive metal somewhat relative to weld necks, though not as much as the lap joints I was babbling on about before NozzleTwister corrected me.
NozzleTwister's caveat about using them with spiral-wound gaskets is well taken: if you use a standard spiral wound gasket between these flanges, the inner diameter of the gasket is smaller than that of the flange faces, leaving an exposed lip which can catch in the fluid flow, break the tack weld and cause the gasket to "spool" into the bore of the pipe. Very bad situation, and not solved merely by specifying an inner ring. Special dimension spiral-wound gaskets are required for use with these flanges. The question is: even if you know it, will your customer's pipefitters be smart enough to know this in future?
One thing I've seen which is interesting: some fitters drill a small (1/16" or so) hole on an angle from the back face of the flange to about the midpoint of the air gap between the two fillet welds and the pipe wall. It allows them to complete the second weld without contending with hot air attempting to leave this space. It also acts as a telltale or weep hole to detect any leakage past the first fillet weld during the hydrotest, or subsequently during operation. You can imagine that this gap would be a pretty nasty place for crevice corrosion if it were left to fill with a corrosive process solution.
