davidysong
You have answered your own question.
Nipples (purchased pre-cut and threaded) are used where the assembly is required to be grouped close together. Examples: vent, drain, and instruments. These 3", 4" 6" and (sometines) 9" Nipples are made out of pipe, the same pipe you use for long distances runs.
Piping that runs long distances (i'-0" or more) are all cut in the field from pipe purchased in single random (20'-0"+/-) or double random (40'-0" +/-) lengths of pipe. This is the cheapest way to do it.
thanks for your reply pennpiper, I am still confused.
let's say 1" NPS pipe, we use pipe only for short distance (4" or 6"), if it is not in vent, drain connection. We don't use nipple in this cae. Do you know the reason?
It is the same pipe, but building a pipeline out of 12" sections and couplings (or hammer unions) would get a touch expensive and might be unacceptably prone to leakage.
davidysong - If you're using pipe for short distances like 4" or 6", then you ARE using nipples. If you're not buying them, then you're cutting & threading your own, out of pipe.
Normally, it's not cost effective to make your own threaded nipples - much cheaper to simply buy them.
Nipples in older sectional cast iron boilers such as HB Smith were short pieces of pipes threaded into the mud drums and bottom of the cast iron sections.
As I remember it, there was also the issue of the threads in nipples and/or threaded pipe couplings and tees and unions not being totally axially concentric due to manufacturing tolerance and the "rule of thumb" (using some obvious overstatement) was that if you took some short lengths of pipe nipples and some couplings, etc and threaded it all up, you would soon make a complete circle.
Now obviously that was overstatement, but I have seen some threaded runs that had several short nipple sections and were nasty looking.
