cutting a b16.9 90deg elbow at site 3

[ghkskn]

What do you think about cutting the standard fabricated CS elbows at site to suit pipe routing for low pressure piping systems with design temperature max 60degC.

Piping designer has drafted the system to use "on site cut elbows" but 3rd party inspector says "NO !".

Thanks in advance for the replies.

Gokhan

 

[TD2K]

What service, what size?

How exactly do you plan to have the field fabricate the elbow, are you thinking of a mitred elbow made out of pipe?

Personally, I can't see this being cost effective in terms of the labor involved over buying a fabricated elbow. What is the reason for wanting to go this route?

 

[JStephen]

If you're thinking of replacing forged steel ells with mitered ells, that's a design issue as well as workmanship.

In terms of cutting mitered ells in the shop, or in the field, I'd suggest you look at how it's to be done in each place. If they're using the same equipment either place, not much reason to choose one over the other.

 

[NozzleTwister]

gkes,

I assume that you're talking about trimming the ends of 1.5D elbow fittings because your offsets are something other than 90 deg. or 45 deg.

This is done all of the time and does not compromise the piping system. Ideally these spools are fabricated in a pipe fab shop where the controls and tools are better, but there is no reason this cannot be done in the field if done right.

I would ask your inspector for justification for prohibiting the use of trimmed elbows. Possibly there is a local code addressing this, he needs more than his personal preference to say no.

Good luck,

NozzleTwister
Houston, Texas

 

[TD2K]

I have a feeling now that I've re-read the OP that NozzleTwister's is correct what they meant by this question.

 

[ghkskn]

Thanks again for all the replies..

Pipe sizes are ranging from 4" up to 24". We are building a new tank farm and layout of tanks, piping routing and tank nozzle heights mandate to trim the elbows other than 45 and 90deg. And I do not want to use miter elbows especially on pump suction lines.

The main concern our inspector has is that when standard elbow is trimmed the trimmed end will not be a circle. it will be slightly elliptical and that may create problems with welding and NDT. But the service is just not high pressure, high temperature, cycling, vibrating etc.

Gokhan

 

[NozzleTwister]

gkes,

Depending on how you are trimming the elbow, you inspector may be correct.

The line that you cut the elbow needs to be normal to the tangent of the elbow centerline at the cut, otherwise you will get an elliptical end. You may need two trimmed elbows to get the offset and angle that you require.

A Piping Designer will be able to calculate the angles and provide the layout so all of the elbows are cut square.

Good luck,

NozzleTwister
Houston, Texas

 

[JStephen]

Concerns about roundness of the cut cross section would be just as valid in the shop as in the field- not sure that I see the issue here.

I don't know what kind of roundness tolerance those ells hold- guess you could measure one and see. I know the thickness can vary some away from the ends.

 

[ghkskn]

Thanks NozzleTwister and JStephen,

We made a trial and cut a 4" and a 10" 90deg elbow at 45deg. The cuts were normal to the tangent line but still we got a slight elliptical shape (2~3mm at diameter). I will check what the diameter tolerance given in B16.9 Otherwise, as NozzleTwister points, we will have to have two trimmed elbows for a non standard degree turn.

Thanks again for the replies.

Gokhan

 

[MTPipeliner]

This may be too late, but I'll tell you anyway.

You can buy fittings that are made to be cut. They are known as segmentable and have a stricter tolerance for out or roundness and wall thickness variations.

On small diameter piping <6" it doesn't seem to be much of a problem, but with larger piping the out of roundness can be quite difficult to deal with when fitting up the pieces.

Pat

 

[JohnBreen]

They should be able to make a "nice square" cut in the fab shop by jigging the elbow and cold cutting it with a band saw or a disk cutter type saw. Of course the end would still have to be prepped for welding.

One thing not mentioned so far is the loss of flexibility. Of course if an elbow has a flange on one end it will lose the flexibility of about 30 degrees from the flange (the flange will cause the elbow to "keep its roundness" under bending and so the additional flexibility of an elbow will be compromised). We have a Note (note 5) and a Figure (Table D300, chart B) that addresses flanged bends in Appendix "D". For example, if you put flanges on both weld lines of a 90 degree, B16.9 elbow you will take away about 2/3 of the "elbow flexibility". Consider a 45 degree elbow with flanges on both weld lines - there is really no "elbow flexibility" left in that (although the typical computer program will still use the Code flexibility factor). So, if you have a 45 degree elbow with no flanges, the matching straight runs of pipe will ovalize (for some distance) under bending but not enough to provide the same flexibility as a 90 degree elbow. A "trimmed" elbow will also "give away" some of the "elbow flexibility" too so what should we tell the computer program? Food for thought.

Regards, John.