Fiitting connection

By your mail I assume you want to know the length of straight pipe between say one elbow and a tee etc ?

Usually in a pipe the flow has to be laminar. However in any bends etc, the flow becomes slightly disturbed - I will not call it exactly turbulent, but on the verge of it.

The straight piece is to bring the laminar flow back - before we again disturb it (like the rest between the working hours )

As per standard practice - the thumb rule is (sorry can not trace out the literature- somewhere studied in fluid mechanics ) 0 the straight length should be minimum 5d and preferably more than 7d (where d = dia of the pipe) before you put any other fitting.

Hope it clarifies, literature may be provided by some friend in CR4 if you need it.

You wrote:

1. "Why do we need to have a spool in between Fitting to fitting connection for BW?

You don't need to put a spool (or Pup) piece between any butt weld fittings unless there is a dimensional requirement for the overall length. Any butt weld fitting can be welded directly to any other butt weld fitting. This includes ELLs, TEEs, Reducers, Flanges, etc.

2. "How much do I need to give min spool?"

If the over dimension results in a gap between two fittings and you need a "Pup" the the minimum length of the "Pup" piece should not be less than 5 times the wall thickness of the pipe used.

3. "Why ?"

With a Butt Weld there is what is called the "Heat Affected Zone" (HAZ). This means that the grain structure of the pipe material is disturbed by the heat generated by the welding process. If it is too sevier then there is the possibilty (not a surity) of structural failure at some point in the future. The HAZ is normally about one times the wall thickess from the edge of the weld. The 5 times the wall thickness rule includes a one wall thickness weld on both ends of the "Pup", a one wall thickness HAZ on both ends of the "Pup" and a one wall thickness space betwee the two HAZ areas.