what should be the expansion gap for socket weld joints??

Date 07-14-2011 14:42

1.6mm gap? You gotta be kidding?
There is no requirement before or after welding. And especially with after welding you get what you get. You cannot control shrinkage that way.
The usual procedure is to either use gapolets or some such thing, or tilt the pipe on fit, tack one side, and square the pipe up and tack the other.
Viola! Gap.
Its not that technical.

By joe pirie (***)

Date 07-14-2011 15:02

When i worked at San Onofre Nuke they made a special tool that scribed a line around
the pipe. During Fitip inspection if the fitting wasn't gapped correctly according to the scribe
mark it was rejected. There is no gap requirement before welding? That depends on what the wps
says. Not that technical all depends what your welding on!!

By js55

Date 07-14-2011 16:00

Joe,
Even Section III says approximately 1/16" (Fig. NB-4427-1(d)).

By 46.00

Date 07-14-2011 15:41

According to B31.1, there is a requirement for a 2.0mm (1/16") gap before welding. There is NO requirement for an 'after' weld gap! However, it is my experience that most contract documents will stipulate a percentage of socket welds are radiologicaly examined for this gap after welding.

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By js55

Date 07-14-2011 15:49

I stand corrected on the before gap. Thanks. Memory fails.
But I do notice Fig 127.4.4(c) says Approximately 1/16".

By js55

Date 07-14-2011 16:02

I also noticed the OP said "approximately". So I stand corrected there too. Thats what happens when you hit and run.

By 46.00

Date 07-14-2011 16:07

Yes, it does! But the title of the fig also says 'Minimum Welding Dimensions'!

I can see the reason for a gap 'before' welding, but I have given up arguing about radiography of completed socket welds for 'gap'.

By 99205

Date 07-14-2011 16:29 Edited 07-14-2011 16:32

May not be appropriate for all situations but they do say time.
http://www.galgage.com/measuring_gap-a-let.html
http://caltechindia.com/GAPALET-Socket-Weld-Contraction-Rings.htm

By js55

Date 07-14-2011 16:44

The problem with Gapalets is they've been known to come apart and end up in equipment.

By 99205

Date 07-14-2011 20:07

Interesting, I've never heard of these having failures.

By joe pirie (***)

Date 07-14-2011 23:07

someone please correct me if im wrong. I thought the purpose of the 1/16 gap was to
allow for expansion of the pipe so that the pipe wouldn't hammer against the fitting.
using one of these gapolets seems to defeat the purpose, you really don't have a gap
if the pipe is jammed against this steel ring it would defeat the purpose of the gap cause
essentually you wouldnt have a gap doesn't make sense to me

By 99205

Date 07-15-2011 00:32 Edited 07-15-2011 00:35

Take a closer look at the links. Basically the rings are designed to crush, preventing weld compression from cracking the fitting.

By js55

Date 07-15-2011 12:09

I've never known it to happen with those we've used. Used many of them over the years. They are a handy little gizmo. Though you wouldn't always know of course. I've just read of such.

By mbrush

Date 07-15-2011 12:53

http://www.aquasolwelding.com/products/solugap.html

These could be helpful - they do the same thing but dissolve in water after the system has been filled.

By Shane Feder (****)

Date 07-15-2011 14:19

Jeff,
Surprised you weren't aware of required gap as both B31.1 & B31.3 state it, required gap before welding but nothing after welding.
Have had some very heated exchanges with misinformed bosses who want radiography of socket joints after welding and then want them rejected for gaps less than 1.6 mm (1/16") after welding ????,
Regards,
Shane

By js55

Date 07-15-2011 15:32

Shane,
Good catch. I am embarrassed. :red:

I suppose I forgot because gapping is a matter of standard operating procedure for us.
We just do it. We've had projects with gap RT and no issues.
No excuse.

By qcrobert (***)

Date 07-20-2011 18:16

I have verified that the gap remains after welding from RT and also from VT when the pipe fitting is small enough to check (by finger nail and/or inspection mirror).

Common practice I have seen is to place pipe in fitting (example: 90 deg elbow) butted up, draw a line around the pipe, then withdraw pipe a min. of 1/16", place first tack, straighten pipe and place second tack, then while "hinged", rock pipe to 90 deg of face of fitting and place remaining two tacks.

I have personally seen where 50# steam lines of a common make-up warm air unit in the sub floor of paper machines have failed because there was no room for expansion. In 1992, one make-up unit in particular at a local Weyerhaeuser paper mill had 23ea (2" dia) socket welds fail at the toe of the weld because of this. These welds were made during a shutdown just a week before. Will try to round up the pix for your perusal.

By Joey

Date 07-21-2011 02:06

I refer to OP, B31.1 and ASME Sec III suggest a 1/16-in gap. The reason behind this gap is to avoid unwanted shear stress in the fillet and possible cracking during operation, especially those high-temperature service when pipe inside the socket will expand to a greater degree than the socket itself. The global practice (GP) of an American company I worked states “A minimum of five percent of all socket welds shall be radiographed to inspect for: the presence of a gap after welding and for weld defects such as lack of penetration”.

~Joey~

By joe pirie (***)

Date 07-21-2011 18:37

ok that was the point i mentioned in earlier post if you use a gapolet it is essentialy
defeating the purpose of the gap or does it smash into little pieces and contaminate your system??