Socket Welds vs. Butt Welds

I will try to briefly answer your 2 questions (posts) in one shot here.

The main reasons why threads/socket-welds/butt-welds are allowed/disallowed/preferred are due to pressure (strength), temperatures, corrosion and ease of installation/disassembly.

Threaded joints are ease to install/reassemble and are preferred where this must happen on a regular basis - such as attaching hose connections. However, the main problem with threads is that you must remove pipe material and reduce the thickness of the pipe so that its pressure/temperature rating is reduced. Also, you can run into trouble with temperature fluctuations as the threads may loosen - a big problem if you join two dissimilar pipe materials due to coefficients of thermal expansion. Lastly, you can have problems with erosion/corrosion at threaded joints.

Socket-welds are also easy to install and are preferred for small bore pipine - however, their limitations include:

1. There must be approx. 1/16" gap at the bottom of the pipe to socket to allow for thermal expansion. It is possible that this gap is missed and can lead to problems of over stress/cracking the fillet weld of the fitting.
2. SWs are very troublesome in corrosive fluid service due to crevice corrosion in the fittings.
3. SWs are susceptible to erosion as well due to the discontinuity of the smooth pipe internals.
4. Fatigue cracking can be a concern due to temperature fluctuations and stress concentrations on the fillet welds.

Butt-welds are the "best" in terms of strength, fatigue, corrosion, erosion, and temperatures - however, they are more difficult to fit-up and weld properly.

From strength point of view, the fillet weld (FW) resist 1/3 the strength of butt weld (BW), and we use the BW when we need a welded joint with a strength not to be lower than the strength of the base metal itself.

For all piping systems subjected to higher pressure, higher temperatures, and/or expansion or vibration, all codes and standards don't recommend using of threaded or fillet welded joints.

a) Better strength

Nooooooooooooooooooo,

Please refer to my post #4, it is a rule of thumb in all piping handbooks in addition to the recommendations of codes and standards that BW strength is better than FW. And in serious operating conditions, all codes and standards prohibit using the fillet and threaded connections and recommend using of BW instead.

In all my piping designs which accompanied with fine calculations for any critical piping operations, we have to use BW connections.

Ask any pipe welder had he rather weld a 1/2" OD butt weld or 1/2" OD socket weld I am pretty sure he would say socket weld every time. The socket welds make pipe installation on smaller diameters easier not to mention faster to install and easier to make the pipe square and so on IMO. Most socket weld fittings come in 1500, 3000 or 6000 lb. classes so they can be used for many different pressure ratings. While it is true that on services that are very corrosive or erosive a butt weld may be better because of turbulence socket welds should be used on everything else that is below 2" in diameter just because of the ease of installation if nothing else. I have used socket welds in just about every service including steam pipe, water, air, process water and in up to 1500 psi systems. Socket weld fittings usually cost more than but weld fittings but the extra cost is made up very quick in terms of labor when installing them. The pipe is square cut and does not need to be beveled or prepared other than cleaning the outside, the pipe can be a little out of square or even cut a little short with socket welds, with most alloys purging is not needed with socket weld fittings. Unless you have ever tried to fit and/or weld small butt weld fittings it is hard to understand how hard it is to keep them square while welding. I have seen welders waste allot of time and in some instances damage the weld by re-welding and/or cooling welds in order to get a butt weld fitting to "pull" back into square. With all this being said when someone asks me what kind of fittings should I use on a particular system I nearly always answer the same...Follow the proper specification for that service! This is easy for me to say because we have a specification for every service we use and it leaves nothing to guess when it comes to all aspects of the piping, fittings, valves, gaskets etc. selected for a particular service. if your company does not have a spec book for piping services they use then I would recommend that you invest into setting one up.

pipewelder