Sockolet vs Backwelded Threadolet 2

[Saucerito]

Hi,

Can someone provide info on advantages of welding a nipple into a sockolet vs backwelding a threaded nipple in a threadolet? I read on another forum that cracking can occur in the backweld of a threadolet due to thermal expansion of the nipple during welding. This would not occur in a sockolet because of a 1/8" gap between the olet and the nipple.

The nipple/olet would be in a high vibration environment so crack propagation would be likely if a crack existed in the weld.

Thanks

 

[bimr]

Art Montemayor answered this some time ago.

A “threadolet” is a quick and easy access to a flow stream with inherent liabilities.

By design, it is intended to be welded onto a flow pipe by means of using a fillet-lap weld. This is the weakest weld available in the boilermaker / pipefitter industry. As a welder, I always have always stayed away from this type of weak welding technique when dealing with process (or utility piping or vessels). I always insist on a “full-penetration” weld technique – which is a type of “butt-weld”. This type of weld ensures that the introduced joint is not a “weak link” in the chain – so to speak.

The technique of applying a threadolet is by simply laying it onto the main pipe and welding a fillet bead around its outer periphery and subsequently drilling a hole through the threadolet nozzle, attaching a pressure gauge or whatever to the threaded nozzle. This works for low pressure piping applications – for a while. What you have done, when you follow the above procedure, is that you have created a much weaker pipe wall at that junction and, as if that wasn’t enough, you have also introduced a stagnant, corrosive site directly under the threadolet’s “saddle”, where it rests on the outer pipe’s surface. This site is subject to static fluid and is a candidate for increased corrosion – at a point in the pipe where you have purposely weakened it! That, in my opinion is not too swift. I have always out-lawed the use of threadolets in all plants and operations that I have operated or managed.

 

[Saucerito]

Thanks bimr. Do you have a link to Art Montemayor's thread?

We have threadolets installed already (currently plugged), and want to use them with a nipple and flange for a flanged thermowell connection. Since it is hydrocarbon service, typical threaded connections are not allowed. This is why we would like to seal weld the nipple/olet connection. Is this connection going to be more prone to cracking/damage from vibration than an equivalent sockolet connection? Will the threads act as stress risers? What is the "proper" flanged nozzle connection to a small vessel?

 

[bimr]

Yes and yes. Thermowells are generally fitted to tanks as side mounted where the tip can reach the area of where the temperature is being measured. The usual process connection is flanged, 1 - 1/2" ANSI 300# flange.

 

[DekDee]

"By design, it is intended to be welded onto a flow pipe by means of using a fillet-lap weld."
"The technique of applying a threadolet is by simply laying it onto the main pipe and welding a fillet bead around its outer periphery......."

bimr,
Weldolets, sockolets, threadolets and any other O-lets are designed to be attached to the run pipe with a full penetration groove weld.
They are only attached with fillets when someone doesn't know what they are doing or can't be bothered doing it correctly.

Saucerito,
Seems strange you are allowed threaded plugs but not threaded nozzles ?

Cheers,
DD

 

[XL83NL]

Im with DekDee here, olets are integrally reinforced fittings that need full pen butt welds (Code requirements).

 

[LittleInch]

BIMR,

I also disagree with your point. By design, weldolets and similar are designed to be full penetration welds. See the start point and embedded video

http://www.bonneyforge.com/products.php?pg=branch

the fact you talk about a "saddle" seems to indicate that you are talking about a set on saddle type where the skirt extends beyond the fitting by a considerable margin. This is not a "olet" type fitting in most peoples understanding and I would then agree with your reservations.

In answer to the OP - I don't like either of those suggestions for high vibration environments.

Welding threads with a "seal weld" is not a goo solution, and socket welds can also suffer in those environments. You can find a lot of debate on these issues in this forum. If you can seal weld or socket weld, why don't you butt weld? the time taken will be worth it for that service condition.

e.g.

http://www.eng-tips.com/viewthread.cfm?qid=18266

http://www.eng-tips.com/viewthread.cfm?qid=368050

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[moltenmetal]

Seal-welded threads can be thought of as a crack waiting to happen. Doesn't mean there's never a reason to do this, but if you can eliminate it by design, you should try.

A socket weld has a higher stress intensification factor than a butt weld, but unlike the seal welded thread, it doesn't also come with a notch which has been stressed by shrinkage due to welding. If you seal weld, you will need to cover all exposed threads to reduce the locally thinned area of the nipple AND these stress risers. The trouble is, most people don't bother to do that when seal-welding, typically because they're seal-welding a joint that previously was threaded with thread sealant and then leaked. Typically, a nipple won't seat deep enough in a half coupling or thread-o-let to be covered by a normal single pass fillet weld, and people don't bother to go back for the 2nd pass.

In a high vibration environment, both are vulnerable, but the seal weld is more vulnerable in my opinion.

 

[Saucerito]

Seems strange you are allowed threaded plugs but not threaded nozzles ?

The point of this project is to get rid of threaded fittings on a two stage recip compressor's pulsation bottles . These plugs will be seal welded. I don't see a big issue with seal welding the plugs since there isn't a moment arm to cause a lot of stress on the weld/threads.

If you can seal weld or socket weld, why don't you butt weld? the time taken will be worth it for that service condition.

Seal welding would use the existing threadolet, while a butt weld would require a hole to be cut for a new nozzle.