ANSI B31.3 shows 1.6 mm. approx. gap before welding. I'm not sure about the standard to specify the minimum and maximum gap. This may depend on your project specification. I usually allow 3 mm. for maximum gap. This is confirmed by random RT after welding.

Pardon me, but what's a "socket weld fitting"?

"Happy the Hare at morning for she is ignorant to the Hunter's waking thoughts."

Madmango,

"Socket weld fittings" are just that....... pipe fittings that are connected to the pipe in a "socket welded design" (The pipe is slid into the fitting, then fillet welded)

These fittings are commonly installed in piping size 2 inch and smaller. ANSI B16.11 establishes dimensional and test requirements for such fittings.

regards

MJC

MJC / PAN,

Could we make such an arrangement wherein the inserted pipe touches the socket end i.e. no intentional gap is provided. In this case we can ensure that the same material is used for pipe and socket. I believe, a gap of 1.6mm is provided to take care of the differential thermal expansion of the pipe and socket. However we also need to worry about the corrosion ( crevice type) that is likely to take place due to this intentional gap of 1.6mm or so.
Your comments on this will be useful.
Thanks,
Pras

Socket weld fittings should not be used in the service of crevice corrosion. I always choose butt welding even in small bore piping in such service.

I understand that 1.6 mm. is very small gap then it should not be too difficult to follow as per ANSI B16.11.

Is there any infomation with regards to the integrity of socket welded joint, in particular the effect of large gaps (i.e 10 mm gap on a 2" NB ANSI 15.11 socket joint) on

(a) The pressure rating of the joint.
(b) The ability to withstand system stresses

Any comment on this would be ussful

Regards
colin

colinlove,
In my understanding, the excess gap depends on the depth of socket...because the pipe end slips into, and supported by the socket.

For example, 90 degree elbow size 2" with 10 mm gap has minimum depth of socket only 16 mm. The remaining insert length of pipe in socket is only 6 mm. Is this acceptable in your project requirement?

Normally I use 1/16" gap between end of pipe & end of socket or depth of socket minus 1/16". Corrosve service we use B.W. fittings and P.W.H.T.for all sizes.
Regards,
Rick699

The approx 1/16 inch gap before welding is to accomodate weld shrinkage.  If the pipe is jammed in the socket and welded, small cracks can develop at the root of the fillet weld as weld shrinkage tries to draw the pipe into the socket.  However, it should be noted that fatigue testing by Japanese indicated sockets where the pipe is jammed in before welding performed better than those with a gap, so it is probably not a big deal, although jamming the pipe in before welding would not comply with the Code.

he main points are that the B31.3 Code addresses before welding, so if you are going to radiograph after welding, provide supplemental specifications to the construction contractor requiring a gap after welding, if that is what you will be looking for.

Socket weld fittings should not be used in services subject to significant erosion, or crevice corrosion.

I personally believe that having a gap before welding larger than 1/16 doesn't significantly affect performance, as long as there is some insertion, but am not aware of any analysis or testing to confirm this.  There was analysis and testing on slip on flanges that indicate depth of insertion is not significant.

My contribution.
Concerning the strength of a socket welded joint.  At any location in a piece of pipe there are three stresses to consider: (1) the hoop stress caused by pressure in the pipe, (2) the longitudinal stress caused by pressure in the pipe and (3) bending stress caused by forces bending the pipe.  The socket welded fitting is always strong enought for the hoop stresses because in the fitting the joint has a wall thickness of both the pipe and the fitting.  So no matter how far the pipe protrudes into the joint, if the pipe has enough wall thickness for the hoop stress then the pipe and fitting definately have enough.
The fillet weld seals the joint, resists the longitudinal force trying to pull the joint apart, and the bending forces.  This is where the weakness in the joint lies.  The longitudinal stress caused by pressure in a pipe is half that of the hoop stress so the fillet weld does not require undue strength for resisting longitudinal forces caused by pressure.  Also the depth of penetration of the pipe into the socket is not important from this point of view either. If the joint is subject to high bending forces one would want a decent fillet weld and in this case, the deeper the pipe penetrates the socket the better it will be able to resist bending without relying on the fillet weld.
I have always considered the small gap in the joint required by the code to be little more than good practice particularly in joints in piping systems where the joint material is brittle and will not yield to relieve thermal stresses.  
Concerning crevice corrosion.  There will always be a crevice in a socket welded joint no matter how big the gap at the base of the joint is, and that crevice extends from the gap itself right around the pipe all the way up to the fillet weld.  Socket welded joints in a service where crevice corrosion is a problem are bad news.

"greddek" pretty well sums it all up.  There used to be some nice little crimped washers available that you inserted down the bore of the socket weld fitting which ensured that the pipe didn't bottom out.  They were designed to maintain that 1.6mm gap.  I wonder if they are still available ?

go to the following for socket weld spacers:

http://www.qtm.net/~galgage/socketweld.html

We only use SW in servicesystems i.e. water, steam, air and nitrogen.
But have problems with bad welding! Be sure that there are at least three layers of weld and absolutely no undercuts.

Laphroaig

I find using a wooden match stick ideal as it carborises during welding and is swiftly flushed away

Placy,

the problem is that it can block the flow or orifice if it didn't consumed.
We use a plastic plug that we receive with merchandise. It maintains the gap during welding and will being consumed. I tried to find similar gap-a-let wiyhout success.

See also my question and answers in AWS forum.

Regards

there are gap o lets available in the market to maintain this gap. it just like a spring washer with dif. MOC are available. Very simple and easy to use

Hi guys,
The way I have seen this done in the field by actual welders is to set this gap by simply pulling the pipe, or fitting all the way to one side, tack welding it on the opposite side , then squaring it up. This always gives you the same gap per line or fitting size. Usually a little less than 1/8" .