ASME B31 Socket-Weld Question

Shooting from the hip I would assume they would be just fine, but don't take my word on it... I don't have the codes in front of me. I'm sure someone will come along who knows much more than I do to give you a real answer.

Socket (fillet) weld and butt (groove) weld are allowed by the ASME code, but you have to carry out the required calculations to satisfy yourself that the weld shall resist all imposed stresses due to various loads. But with simple comparison between the two types of welding (fillet & butt), you will find that the strength of fillet weld for applied stresses is 1/3 the strength of butt weld.

Therefore, at high pressure piping systems we have to use butt weld joints. And socket weld joints are forbidden in those piping systems subjected to vibrations.

This is an interesting issue. I work in the field of ammonia refrigeration. The normal pressures are quite modest, typically < 250 psi on the high side of the system. However, there are various scenarios of hydrostically blocked liquid, vapor-propelled liquid hammer, and condensation collapse that can produce much higher pressures.

Now here is what seems funny. Years ago, most small liquid and hot gas ammonia lines were done by oxyacetylene-welded butt-weld fittings. More recently, the trend has been toward socket-weld fittings in the smaller sizes. Apart from the knobby look, they are also weaker, but they do minimize weld slag or scaling on the inside of the pipe/fittings. Which is best may still remain undecided.

Abdel, you say that a socket weld is 1/3 the strength of a butt weld.

If the distinction can be made, between strength and reliability, do you mean that the the socket weld is 1/3 as reliable as a butt weld? By this I mean, is it the potential effects of thermal expansion forces that make the socket weld less reliable, as opposed to outright strength of the welds under equal and ambient conditions? Or is it the ability of the fluid or gas passing through the welded joint, to get between the ID and the OD of the two pipes, that make it weaker? Or something else?

I guess I just don't understand the physics behind the statement that a socket weld is so substantially and drastically weaker. It would seem to me that given the fact that in a socket weld, one part is encased within another part prior to welding, that that fact would make the socket weld inherently stronger. Where is the fault in my reasoning?

This is quite a revelation for me, since I'm no stranger to welded pipe applications. On the other hand, I rarely have dealt with true high pressure applications.

After clearing that up... could you compare it to another similar but different scenario? What would be the case when comparing the strengths of two welds of pipe that are not being used to transport anything, but are only used in a purely structural sense? Two equal diameter pipes welded together with a butt weld vs. two pipes, one fitting snugly inside the other, and welded together as a socket weld (yes I know that there are no standard pipe sizes where that is the case, but lets assume they are machined to do so). Is it merely the application of internal pressure that gives the butt weld such superiority over socket weld? Whereas, when pressure is not a factor, the reverse is true regarding structural strength?

Is my question clear?

Yep, It's clear... and I'm with you, I'm having a hard time understanding the statement... I'm eagerly waiting a response.

We use socket weld all the time in high pressure (5000PSI or less) applications, and have never once had any issues with them...

I understand the stress risers that occur in a normal fillet weld. And I understand that a butt weld between the two ends of solid rod (or any shape) is stronger than a fillet weld between the two ends of two pieces.

That's clear, for the reasons that you mentioned. What I don't yet understand has to do with the difference between a pure fillet weld, and an actual socket weld. A fillet weld doesn't necessarily have the socket aspect. Doesn't the fact that one part is inserted into the other part... and then fillet welded, compensate for the fact that the weld material is smaller? Surely there is considerable support given to the joint as a whole, when one part is fitted into another part.

Generally speaking, all proper butt welds are stronger than all proper fillet welds, yes. But not all fillet welds are also socket welds. Why does the socket not fit into the equation?

• It is clear by all codes and standards of design and fabrication that butt welded joints are the best and strictly recommended for those pressure piping systems and pressure vessels designed for high pressure, high temp., high rates of fluctuations of pressure and temp., presence of vibration, ... etc.

• By simple stress analysis, the butt welded joints are subjected to tensile stress, where we shall use the full value of allowable tensile strength S of material in equation of design. But the worst case of stress for fillet welds is its subjection to shear stress, and the value of shear stress shall not be more than 45% of the allowable tensile strength S of material as recommended by all codes and standards like ASME.

• In my designs for high pressure, high temp., presence of vibration, ... etc., I don't use fillet welds if there a chance to use butt welds.

Note. For pressure vessels and pressure piping systems, the nozzle reinforcement calculation (area of compensation) for a branch connection using a piece of pipe in lieu of standard fittings like weldolet, if we use and apply a fillet weld only, in a lot of cases its strength shall not be sufficient to withstand the applied stresses, and therefore we have to add and prepare the welded nozzle with a groove weld (butt weld) to gain its high strength to be added to the area of compensation.

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DONE remember you asked about threaded joints in high pressure system. Now, talking about sockets welds, the answer is: NO, Absolutely NO. See ASME B31.3 points K 311.2.2 and K 311.2.3. Regards, WP

WP,

Thank you for your concise and referenced answer. Do you know of a similar paragraph number regarding socket-welds in ASME B31.1, please?

OP

As mentioned in other psots, use butt-welded joints rather than socket welded. To minimize slag and buildup on the inside of the pipe, ues orbital welding with proper weld prep on the pipe ends to be joined.

Orbital welding gives clean joints that are very strong and mostly free of cracks and voids.