Toleraces on Multiple Welded Pipe Fittings 1

[bigfoot22]

Forum
B16.9 tells me the dimensional tolerances on pipe fittings
any spec. or anything out there that can tell me pipe tolerances when I weld 2 or more pipe fittings together?
for example what is the length tolerance when I weld a sch. 10 weld neck flange to a sch. 10 reducer?

thank you

 

[DekDee]

bigfoot22,
There can be no tolerance because you can have totally different root gaps in two different joints which will give you two different lengths when the same fittings have been welded together.

 

That's a workmanship question.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[racookpe1978]

On a real-world basis, you should NEVER "design" pipe fittings so closely that the manufacturing "tolerance" of assembly of successive "parts" causes a gap or an interference or a access problem (a valve too close to a piece of steel to operate or to remove the actuator.

"Pipe" welding requires a butt weld gap at each weld, threaded and socket weld piping requires gaps for assembly tightening and for the SW fitup/expansion. Threaded pipes usually have connectors between threaded joints so the pipes rotate and close.

Clearly, after assembly of every spoolpiece, there can be neither gaps nor overlaps: the whole pipe must fit near-perfect from start to finish without jamming nor . The pipe must be able to move (expand) with heat, load, weight and live loads, and with the occaisional water hammer. Supports attach at the middle of pipes but have to be designed (specified) to fit the function and the support steel they attach to. Support locations do need to be worked out so differences in the as-built conditions can be accommodated.

So, to the specific question: Don't design the pipe components so end-to-end "tolerances" matter, your pipe fitters and pipe welders HAVE TO BE ABLE to cut the pipe stubs for weld gaps and assembly.

 

[racookpe1978]

In this case, the final distance of your assembly must be within the dimension (length) YOU decide. (The distance you need between the resulting two ends of the mating assemblies, minus 2x the weld gap - usually 1/8 inch each end.

Now, having said that, the two ends of the reducer can be trimmed slightly, but no so far that the straight lengths are eliminated: How else can they "fit" the OD of the pipe they are connecting to? A Sch 10 weld neck flange also has a short tangent on one end - that end can also be trimmed slightly - but not so much as to eliminate the straight. That's for the weld and HAZ around the weld not going too far into the flange.

 

[DVWE]

bigfoot22,

You will reference PFI (Pipe Fabricating Institute) ES-3 - Fabricating Tolerances.

Here is a snippet of 2.4, which will apply for fitting to fitting. You should get a copy, because Figure 1 in the spec is important.

 

[bigfoot22]

DVWE
thank you for the information, this gives me a guideline to go by on the few piping assemblies that we weld together
will look into purchasing the spec.

thanx everyone for your great comments

 

[DekDee]

bigfoot22,
I didn't explain my comment very well.
You can have tolerances in total length (as far as I am aware the PFI is only a guideline unless stated in contract documents) but the end result is what you need to worry about.
It really comes back to a competent Piping Engineer and/or Welding Engineer.
If you have 6 x fittings in a pipe spool you will have 6 x weld joints.
If welded GTAW the root gap will be approx 4 mm.
If welded SMAW the root gap will be approx 2 mm.
Big difference in completed length if you are looking at 6 x weld joints.

Generally a Piping Designer will nominate the length between A & B and it is then up to the contractor to calculate cut lengths based on weld process.
Hope that helps,
Cheers,
Shane