Re: minimum pipe spool length

On Jan 6, 2007, at 9:25 AM, elie altawil wrote:

> Other than the minimum spool lengths required for upstream/downstream
> piping for measurement meters, sampling equipment, pumps,
> compressoors, etc. Most major companies policy regarding your question
> is when welding, stay away from the heat effected zone of existing
> welds, typically 2-3 inches.

My experience with field repairs and modifications has been almost universally bad, primarily because of unskilled welding and supervision. Overlapping HAZ effects is only part of the problem--when you start welding in small pieces, especially with butt welds, the weld joints are highly restrained and the residual stress is enormous. This is absolutely the case for a butt-welded short spool. I've seen such joints crack spontaneously on cooling. People think they're making things better by putting in clips and restraints to 'prevent' distortion but clamping only increases built-in stress. Think about it--you're putting very hot metal into a little V-shaped gutter. When that metal cools, it wants to shrink, and if it can't shrink forces build up to balance huge thermal strains. If you're using heat-treatable low alloy like chrome-moly, you can add possible embrittlement to the mix and if you've got stainless pipes, there's more movement and the possibility of sensitization.

Those problems are aggravated when the new welding is too close to existing welds, but it can be done if you know what you're doing, and you have experienced engineering oversight. If you think about it, a multi-pass weld comprises several separate welds a helluva lot closer than 2-3 inches. **But** the modification needs all the design and fab engineering oversight used for new construction plus careful consideration of what the additional heat is going to do to the existing construction. Weld procedures need to incorporate proper pre-heat and interpass temperature controls and complete NDT appropriate to the weld. You may also want to incorporate some form of stress relief or thermal treatment.

If you're not willing to make the effort needed for sound welding, it's best to leave it alone, because what appears to be a 'simple' modification can quickly turn into a disaster. And if you hear the words, 'Sure--no problem. When do we start?' you'll need your hat--grab it and run.

Christopher Wright P.E. |"They couldn't hit an elephant at chrisw@skypoint.com | this distance" (last words of Gen.

.......................................| John Sedgwick, Spotsylvania