I already know the answer to this but, it worth throwing it out there for everyone. In Alberta I see tie-ins shown at fittings all the time on piping drawings. I actually learned that way if I'm truthful. I know now years later through field experience the rule of thumb is actually to be a min 1" or 4 times Wall thickness away from a weld. Other issues like equipment/work space access, safety etc also play important roles as well. Can you guys give us some good reasons why it is better to cut a pipe before your fitting weld a few inches away on the pipe rather than at the fitting weld itself? Alot of people don't understand why this is done.
Thanks guys

I think your answer may lie in this topic...

http://pipingdesigners.com/phpBB3/viewtopic.php?f=1&t=47

Thanks WNRF but that doesn't give an answer to (Why do we leave a section of pipe to tie-into, near a fitting rather than cutting a fitting at the weld, grinding off the weld..... and then tying in right at the end of a fitting). We know what the absolute min is....1" or 4 times the W.T., we know what a good pup standard is and thats to use the diameter of your pipe between fittings. This we know from old posts but.....my question is WHY shouldn't we tie-in directly at the end of a fitting, allowing a small length of pipe???? Forget the other factors like safety or working access....thats common sense. Think only welding and installation.

To be honest I never really gave it much thought, strangely never an issue for me.
Ill start by saying that I have not worked in the oil and gas sector, just pharmaceuticals and heavy industry. With regards to HAZ it must be industry or process specific because its never been an issue for any line I have worked on. When you have slip on flange there are 2 welds in close proximity, any fitting on seam welded pipe has over lapping welds on 45 deg elbows the welds can also be quite close, so I never really understood that one. I always worked on the principal as long as the welds dont touch your ok. Comes from a BS standard. So I dont think thats the reason. The only thing I can think of is that for the larger sizes on site to get a square cut you need to have some straight pipe either side of the cut to be able mount the jig for gas cutting the pipe.

If you already know the answer then I'm not sure why you are asking the question. So, with the caveat that it depends on what type of fitting you are talking about and I assume you are talking normal Shedule pipework here then it is down to the schedule of the pipe not being of uniform thickness on for example, a tee or WN flange. So, if you cut off the prepared beveled end of a tee to put in your tie-in then it is impossible to prepare an new bevel to match the ID of the inserted pipe or fitting exactly, thus having a 'lip' inside the pipe.

Of course if you're talking Biobore tube for example then this wouldn't apply.

Well, Did I get it right? Do I get a gold star if I do?

"IF" you cut on a weld seam and re-weld you get an over-lapping heat affected zone, so in high pressure system you'd have to do a PWHT and probably in the field too, so it would be expensive ...adding to the total cost and delaying operation! BUT if you add a 4" pup or so on the fitting you can now cut in the new section and weld it up without generating a over-lapping heat affected zone, and not have to go thru the expensive PWHT procedure.

Thanks 11echo....as well the end preparation is more expensive at a fitting weld. You need to cut the weld seam, grind off the existing weld and then prepare it for the tie-in as you say.

11echo nailed it. The pipe basically has to be stress relieved due to the affected heat zone due to welding. This is safer to have that minmum pup length between fitings as described in 11echo's post. Also by cutting and grinding a fitting you could go out of round and out of accepted weld tolerance. A square cut can be made on a pipe and adjusted if need be. If done on a fitting you are restricted and the cut could compriomise the weld. Sometimes we try to minimise dead legs by welding close to a branch with a valve for a tie-in but you have created a small dead leg to the valve anyway so that small length does not make a difference.