Minimum SW gap 3

[Joss10]

As per ASME B31.3, gap before welding approximately 1.5 mm (1/16").
I´m checking sw gap with RT, it means that any gap less than 1.5 mm is to be rejected?
Any input will be appreciated.

 

[pennpiper]

Joss10,
You said "Any input will be appreciated"
Okay here goes.
- Who is complaining about this at your Company?
- Who is responsible for setting the Gap during fit-up? Is this person qualified to be doing this? and how are they tested?
- Do you RT every Socket-Weld Joint?
- When do you perform the RT? (a)After the initial fit-up & before the Tack Weld? (b)After the Tack Weld? or (c)After the Finish Weld?
- How accurate is the "RT" readings? If they are off, then how much is too much, and is the Gap too little or too wide?
- What do you do if the RT shows an improper Gap?
- All good Pipers know the SW Gap is important but has it been blown way out of proportion at your Company?



Sometimes its possible to do all the right things and still get bad results

 

[TGS4]

You will note that the limit is specifically for prior to the welding operation. There is no limit specified for after the welding operation.

You need to have procedures in place to make that check prior to welding. Checking it afterwards with RT is of no value.

 

[Joss10]

Find here replies one by one.
Nobody is complaining.
Fitter & welder, niple adjustement is debatable because they don´t use any tool to check it.
10%
After the finish weld
Normally we see too much gap, no gap or gap less than 1,5 mm.
When no gap is showed, arrangement is rejected and is to be repaired.
In my company we have/had a lot of plant shut downs because of cracking of SW.

So, is clear for me that the main problem is when no gap is left, but my question is to fine tune rejection criteria.

 

[yousif aldossary]

good day dear
it is unfortunate that ASME didnt not give tolorance for the expansion gap .
mainly expansion gap is needed due to resuidual stresses accurs due to contraction of the weld, and to allow some thermal growth through the welded pipe .

the disadvantage in my opinion is that you will create stress concentration point between the tip of the pipe and and the inner wall of the welded fitting ( horizontally) , specially that this type of weld is used in high pressure small pipes , which can lead to cracks and even corrosion issues depending on the fluid type .

mainly the procedure that most welders do is marking the fiited pipe by pensil or scrach.

if you will allow for any tolorence it shoundt be more that +0.5mm.

regards

 

[Joss10]

TGS4,
After welding operation I think the rejection criteria is given for particular specs. based on avoid "bottoming" effect due to "no gap" on SW.
Let me tell you that in water injection piping, as soon you have sw with no gap very soon you can get cracking, that is the reason about the importance of checking sw by RT left for piping contractors/manufacturers.

 

[DVWE]

You will note that the limit is specifically for prior to the welding operation. There is no limit specified for after the welding operation.

You need to have procedures in place to make that check prior to welding. Checking it afterwards with RT is of no value.

This is absolutely correct.

You may want to google "gapolet" and see if they are something that your company can use to help establish consistent SW gaps PRIOR to welding.

 

[TGS4]

I agree that no gaps results in problems. That is why, a couple of years ago when the B31.1 Committee was trying to eliminate the SW gap, there was a technical battle (which resulted in no change to the current rules).

If you have an internal company spec that requires a minimum gap, as examined by RT, after welding - well that's certainly your prerogative. The Code is simply a minimum-requirement. Since you've experienced problems, it's definitely in your best interest to ensure some nominal gap. But since it's an internal company specification, you have the flexibility to add whatever tolerances that you want.

 

[racookpe1978]

The "gap" is required to eliminate (attempt to reduce) one source of stress in the SW joint after welding.

Logic is: The weld is applied with very hot metal that will cool (shrink) after welding.
If the pipe is bottomed out firmly in the bottom of the SW "cup" BEFORE welding, then, after the weld solidifies and begins cooling, it will shrink and try to pull the pipe further into the cup.
So, to allow for the very small length of shrinkage as the metal cools from about 1300 degrees (when it begins to become plastic from its original melted) to ambient (60 degrees), a small margin is required below the end of the pipe.
ANY margin after shrinkage that is left in the SW joint - ANY at all - is sufficient to verify that no stress is created.


If, after shrinkage (after welding) there is no verifiable gap, you cannot know how large the gap was before welding, nor can you verify how much - OR HOW LITTLE - of the original shrinkage gap was taken up before compression in the pipe metal began and stress began to rise. There might very, very little compression stress at all. That uncertainty of the residual stress is why a larger-than-needed gap (up to 1/8 to 1/16 inch) is used BEFORE welding begins, so you have a verifiable margin after welding ends.

 

[Joss10]

racookpe1978,
I understand after your explanation that you do agree to repair (cut) all sw reported by RT with no gap.
Is it right??

 

[moltenmetal]

Any gap including no gap after welding is sufficient. Someone is, as usual, trying to apply an inappropriate method of NDT to a socket weld because they're used to doing NDT on butt welds.

 

[MickMc]

Racookpe1978 is correct, I believe the minimum 1/16" was set to ensure that their would be a gap however small after welding and if after welding there is not a gap then you have not used this minimum.

 

[david339933]

I would be more concerned with the stress on the root of the weld due to expansion than contraction/shrinkage. No gap results in the relatively thin pipe having no where to expand and trying to push the pipe out of the socket....putting enormous stress on the weld.

 

[dik]

Is there any harm done by using a 2mm max gap... easier and faster fit up?

Dik

 

[weldstan]

This question has been addressed in this forum multiple times. Please do a search of this subject matter for added info.

 

[gtaw]

Here's my take on the issue, there should be a fit-up inspection to verify the welders are observing the 1/16 inch gap at the base of the socket before welding. The purpose is as stated in previous responses to allow the fillet weld to cool and contract, thus "pulling" the pipe into the socket.

Welders are human and like many humans, they take shortcuts if they see no justification for the requirements listed by the WPS or other working documents. It is only with a fit-up inspection are we assured the proper gap is used.

In some cases where the pipe diameter is large or where many welding passes are require to deposit the proper size, more gap may be necessary. If the company has experienced problems in the past, there is nothing stopping them from requiring a larger gap than 1/16 inch. 1/8 inch may be more appropriate. Likewise, the owner that is having the work performed by a contractor can impose a larger gap in the project specification. The owner has a responsibility to check the contractors work to ensure it complies with the project specification.

Any gap should be checked before welding, not after welding. The gap will be smaller after welding than before welding.


Best regards - Al