Criteria for the inclusion of a pup piece in a piping system:
- "Need" - The distance from the butt weld end of one object to the butt weld end of another adjoining object exceeds the normal weld gap. Example: a 10" NPS line is in a pipe rack and reaches the point where it needs to leave the pipe rack to go to a piece of equipment. The center to end of a 10" long radius Elbow is 15" (10"D x 1.5). Two elbows welded together would be 30" (15" + 15"). The distance between the perpendicular levels of travel is 36". This leaves a distance (or Gap) of 6" to be filled with a short length of pipe. This short length of pipe is called a "Pup".
You will get a lot of advice about the minimum length of a "Pup". I believe a pup piece can be as short as four times the pipe wall thickness when the circumstances require it. Others will tell you there are minimum lengths rules such as "One Pipe Diameter" or 4" minimum. The bottom line is simply, every situation needs to be evaluated on it's own merits.
To give some room to make a cut out and still reweld it back into the pipeline is usually why they are ordered. They are good to have on valves as welding pipeline steel to a pup is less likely to damage the valve internals from excess heat.
I'm with BI on this, but a bit more explanation. I think what you mean is pre welded pups. They are very useful to have on pipeline fittings mainly so that the construction contractor can use his normal weld procedure as you have hopefully managed to supply the vendor with some spare line pipe. This avoids having to do a separate weld procedure for each fitting, costing money and time. On valves it allows the main weld to the BW end is done before any valve internals are fitted and allows end caps to be welded on for pressure tests and then cut off without affecting the valve body itself.
Other fittings are less sensitive to testing issues, but not the welding one so if you can weld line pipe to a fitting made from perhaps lower strength material in a shop where they already have suitable weld procedures then it's worth it.
Otherwise Penn piper has it nailed for piping pp's. Thin ones just don't look good, but if you need one then insert it. You should always have some"green" length if going fitting to fitting, just try and make it 1D if you can, but if you can't go for the longest you can.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
I have required pup pieces to be placed on large bore fittings in the shop where they could be backwelded to assure full pemetration, and subsequently facillitate fit up in the field (pipe to pipe). The roundness and thickness of fittings relative to the adjoining pipe can be really troublesome to fit up.
