[Bill Dornfeld]

Root Gaps in 2D CAD Programs
Years ago I was on an AutoCAD 2D piping design job, on the world’s largest free-standing hospital in California. I was helping draft new changes using AutoCAD as it was being built. I noticed that every drawing had a gap between the fittings and pipe. Every fitting, flange, and pipe. I had to place the root gap on everything as the Union Foreman in charge of the entire operation walked up to me. He was adamant about it. I had to obey his command. I had to match the rest of the project… I had to add a root gap at every weld!

I had been taught that the fabrication shop accounts for the 1/8″ root gap, or first weld-pass that initially joins the components together. I had always joined the AutoCAD fittings together… never with a gap between them. I worked in a fab shop making “pipe spools” from plans and/or Isos. I know that the root gap is accounted for in the fab shop.But, since this was a first for me… I started calling around to all my fellow Pipers to get their opinion. “Naw,” they said, “the shop accounts for that root gap!” Another Piper said, ” in fitting make up… yes we do account for the gaps and subtract it in the straight run of pipe adjacent to the fitting make up spool.” Regardless, the hospital was built as scheduled, on time. I’m still edgy about that incident.

Incidently, in 3D CAD design, the root gap is “accounted for.” We do not leave a root gap. we couldn’t make an Iso even if we did leave a root gap. What’s your comment on root gaps?

[James Pennock]

In my opinion that Foreman had his head stuck where the sun doesn’t shine.

The Design room (manual or CAD) should NOT get involved with Root Gaps (graphically or dimensionally).
The Pipe Fab Shop or the Field Welders will take care of it.

More date:
http://www.eng-tips.com/viewthread.cfm?qid=336582

[Mike Vezina]

JOP is absolutely correct, that the foreman and the project engineering team, had no idea what they were doing.
In the real world, we do not know the actual length of any manufactured component, nor the actual cut length of pipe, due to the industry standard of manufacturing tolerances.
Until all components are assembled for final installation, the actual length is not known, hence the assembly crew, field or shop, must adjust the final cut length for piping.
Also if the assembly involves no piping, but a number of manufactured components, then there is no way to know the final overall length, until after final assembly on site.
Hence the old rule that at least one pipe spool must be included in any long assembly of manufactured components so the field has the ability to trim the assembly length to suit the overall required length.
Also the larger the pipe diameter, the larger the allowable manufacturing dimensional tolerances.

[Mark Layport]

I agree with everyone else, especially JOP’s on the sunshine comment! Designers should document what the end dimension should be, or Mike Vezina comment about a “long” leg spool. It’s up to the fabricator to apply and root gaps THEY feel should be used for THEIR efforts to make all the spools required. In fact every fabricator I’ve had dealings with, ALL generate their own fabrication spools drawings documenting each spool and what THEY want for a weld gaps, after all they are responsible for their work. Remember thought they are working from your drawings, so you need to be correct and present that info in a clear and uncluttered manner!

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