Socket welded fittings vs Butt welded fittings

[Xk333]

I'm preparing a Pipe Material Specification which mostly consists of CS (A 106 Gr B) and SS304 SS316 material. For Small bores from 15NB to 40NB particulary for CS material I'm using socket welded fittings and for SS instead of Socket welded fittings to save cost can I use Butt welded pipes and fittings as we avoid forged fittings in this case for SS304 and SS316.

Another doubt is it a good practice to use casted flanged valves for small bores instead forged valves for cost saving proposes?

All the piping is generally 150# with Category D and Category Normal fluid services, except Hot Oil with 350 degree Celcius and 6 bar pressure

Thanks in advance for your responses.

 

[Xk333]

How about CS and socket welded fittings?
We are using CS for caustic lye, alcohol and phthalates with design conditions below 200 degree Celsius and 6 bar.
Would it cause problems if we use CS socket welded fittings as well?

 

[GBTorpenhow]

Socket welds have inherent crevices and can be subject to crevice corrosion. As CS would usually not be chosen for corrosive fluids in the first place CS socket welds are very common in noncorrosive fluid services.

The crevice also serves as a stress riser and essentially a preinstalled crack so fatigue life suffers. This becomes important at extreme temperature swings and high cyclic rates.

You will have to make your own judgement on whether crevice corrosion and/or fatigue life is important to your service. In many services SS socket welds are fine.

 

[1503-44]

Many oil gas Companies will not allow less than 2" pipe, or socket welds for anything other than instrument connections.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[davefitz]

TugBoatEng is correct, but there have been developed automated machines ( orbital welders) that can be used for small diameter SS tubing , originally developed for aircraft but applied to Heat Exchanger tubing as well. There could be clearance issues to allow the machine's components to fit around the individual tube.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[Krausen]

Any competent site contractor will tell you it's much easier to socket weld 1/2" - 1-1/2" size pipe with quality compared to buttwelding pipe in this size range. From a purist engineer perspective, you can't beat buttwelds. But the reality is ensuring a min 1/16" socket gap prior to socket welding is standard practice for any competent site contractor & much easier & faster compared to buttwelding small bore piping (especially in the 1/2" - 3/4" size range)

 

[weldstan]

I have seen far more failures in socket welds due to poor workmanship (no gap to too large a gap). Controling workmanship in the field can prove diificult. And as Daveitz has noted above, automatic orbital tube/pipe welding equipment have been used in petrochem and power projects for decades on small diameter stainless steel pipe/tubes.

 

[r6155]

@ Krausen
You confuse standard practice with CODE.

ASME B31..3 2022 CHAPTER IX
K306.1.2 Specific Fittings
(a) Socket welding fittings are not permitted.

Regards

 

[r6155]

I do not know automatic orbital welding for connecting pipe branches.

Regards

 

[GD2]

Orbital welding has a special place and is used primarily in "High Purity Fluid systems' like Food processing and Pharmaceuticals. No one uses it in ASME B31.3 Normal Fluid Service.
Going back to OP's original question of cast valves as opposed to forged valves cost - forged valves (limited by size) has increasingly become as cheap as cast valves, and even cheaper.

GDD
Canada

 

[davefitz]

GD2- actually there are a few niche applications for orbital welded SS tubes in the HRSG ( heat recovery steam generators)industry . The canadian company IST makes a once thru steam generator using small diameter incolnel tubes that are attached to stubs on the inlet and outlet headers using an orbital welder, and also Alstom ( GE) had a series of once thru HRSG's that used HP and LP steam generators that recovered heat from the compressor air discharge using incolnel and SS tubes that were attached using an orbital welder. In general, though, it is rarely used in most industrial applications.

In the case of fabricating a high temperature header for a power boiler ( eg, using advanced ferritic material such as P91) there are difficulties meeting the high pre-heat weld requirements while protectng the manual welders from being overheated beyond OSHA limits. An ideal use of an orbital welder would be in such an application, as it would also allow the innovative use of reinforced nozzles attached to the header, which then allows the use of a thinner walled header so that thermal fatigue limits could be extended for faster ramp rates and faster startups, without overheating manual welders during fabrication. However, it now seems that western industries are no longer in an innovative mode and are apparently de-industrializing.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[r6155]

@ davefitz

Thank you !!

Regards

 

[Krausen]

@ r6155
You are the confused one. Chapter IX of B31.3 applies to High Pressure piping (>2500# design pressures) only. In this case we are talking standard Category D & Normal fluid services with a design pressure of only ~90 psig (6 bar). Socket weld fittings are both acceptable by code & very common in this case.

 

[r6155]

@ Krausen
ASME B31.3 2022
FK3OO GENERAL STATEMENTS
The rules in Chapter IX provide an alternative to those in
Chapters I through VI. They include special considerations
for thick-walled piping components such as the theory of
failure and the approaches to fatigue and thermal stresses,
generally resulting in lower design factors with respect to
burst strength than the base Code. Chapter IX rules may
allow thinner piping components than the base Code,
resulting in lower piping weight, balanced against
increased requirements for material testing, fatigue
analysis, fabrication, and examination. Although High
Pressure Fluid Service is often considered to be service
exceeding that allowed by the ASME 816.5, Class 2500
pressure-temperature rating for a particular material
group, there are no pressure limitations for the application of these rules.
The decision by an owner to specify
High Pressure Fluid Service is based on the economic,
technical, and other issues pertaining to the piping.

 

[XL83NL]

Another Code-quote by r6155 without indicating the relevance of this argument.
The randomness in your posts is becoming more and more confusing.

Huub
- You never get what you expect, you only get what you inspect.

 

[r6155]

Sorry if you don't understand, I can't do anything for you.
Regards