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.

 

[GD2]

OP,
Look. Material selection is based on the fluid service or process condition, NOT cost saving.
Joints below NPS 2 are typically SW as you can't do butt-welding (impracticable), again NOT for cost saving.
Cast valves for smaller valves- why not? You can even use for larger valves.

GDD
Canada

 

[Xk333]

GD2,

Thanks for your reply.

I agree with you Material slection is based on fluid service and conditions which in my case is A106, SS304 and SS316. Regarding small bore for let's say 15NB SCH 80 has 3.77mm thickness which is enough for butt welding as per my knowledge.
For larger bores 50NB and above it is always casted valves. Particulary for small bores general practice is to use forged socket welded valves, I was checking the possibility of using flanged casted valves as I'm trying to avoid using socket welded fittings for SS. My main worry is the effective loss of strength and integrity of casted valves as compared to forged valves in mentioned process conditions.

 

[mk3223]

I'm trying to avoid using socket welded fittings for SS.

Don’t understand what’s the issue of the socket welded fitting for the SS piping which is typically used in the industries for years.

 

[TugboatEng]

Socket weld SS fittings are extremely sensitive to crevice corrosion. This is a problem if water is the fluid, even if treated with corrosion inhibitors. With oil there should be no issue.

A second problem is quality control. When assembling socket weld fittings the pipe must not be allowed to bottom out in the fitting. It is difficult to verify this after welding.

Fatigue is the final problem and is exacerbated by the above situation. High vibration and rapid pressure fluctuations can lead to fatigue failures.

 

[Heaviside1925]

OP,
I have typically seen socket weld for small bore for normal services (air / water / nitrogen) FOR cost savings reason. Even though you are having to buy fittings, welders can fabricate the system much more quickly. As GD2 noted, field fabrication of butt-welded fittings is difficult. That said, I agree with TugboatEng as well, which is why hazardous services are approached differently. This could even apply to the hot oil service you mentioned, where a leak could cause major life, safety and environmental issues but if your company is comfortable with this approach and the line is properly hydro'd and service tested then it could be acceptable. As far as flanged vs socket weld valves for small bore, won't you have to socket weld flanges on in the first place? Unless there is a need to remove / replace / rebuild a valve on a regular basis, why introduce flanges and another possible leak point unless they are required in the first place. Your questions are valid from just a material cost perspective but what you should be looking at, is does a 5% reduction in material cost, justify 20% increase in labor costs. I would talk to a mechanical contractor who does work on your site and ask them these questions and I anticipate you will find out from a total cost and reliability perspective why things are typically done the way they are.

 

[Xk333]

TuboatEng,

Thanks for your reply.

We hope Site contactor takes care of Socket welded bottomed out issue as a quality control. However, for Hot oil service with 350 deg celcius temperature I would refrain from using socket welds and go for butt weld even for 15NB for the reasons mentioned by you, to avoid bottoming issue at all and avoid leaks caused due to socket weld as it is a harmful service.

 

[Xk333]

Heaviside1925,

Thanks for your reply.

I agree with you Butt welds will require more end preparation time, additional welds, gasket and flanges which will add to overall cost and time required for the contractor for completion. Surely socket welds will require a lot less time for completion on site. As advised by you, I will check with my local mechanical contractor about the time cost comparison in both the cases.

 

[TugboatEng]

We use a lot of socket weld in the tugboat industry because it is the cheapest installed. We see failures on socket weld fittings on steering systems. There is more fatigue due to pressure cycles. In my experience we see failures after 10 years from install. In my experience most of the failures are due to the pipe being bottomed in the fittings. The cracks are always in the middle of the weld fillet. From what you have described a socket weld is likely acceptable. Bear in mind we don't always know everything we need to know to properly specify a system.

 

[LittleInch]

The issue with piping spec is that you don't really know when and where it will be used. I've not seen much SW stuff beyond DN 25 and most has been 15. Then you can use SW end valves.

SW is vulnerable to poor installation and fatigue from vibration and pressure pulses. Hence why adding them into a pipe spec is difficult as you don't know if that will exist where the pipe spec is used and no one reads any notes you add....

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

 

[Xk333]

Thanks TugboatEng and LittleInch for the reply.

I will be using socket welded fittings and flanges for 15NB to 40NB for CS and SS as well and for Hot oil service will use Butt welded fittings and casted flanged valves to avoid bottom out issues. Hopefully, I'll be able to convince my client.

 

[r6155]

Never use socket welding for high or low temperatures.
SS with socket?: very incorrect design

Regards

 

[Xk333]

r6155,

The temperatures I'm working with is 200 degree Celsius design. Can you please share your thoughts why can't we use sockets for SS?

 

[GD2]

OP,
"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".

Explain what fluid service the hot oil will fit in?

Hot oil can't fit in the category M Fluid Service as it is NOT toxic. Is the temperature your deciding factor?

You must read definition of category M Fluid service carefully. Paraphrase the definition and it must comply to all the definition requirements.

GDD
Canada

 

[TugboatEng]

If you're above the flash point of the oil you'll want to use fittings that are not prone to failure. If you're below the flash point you can install spray shields around the fittings to protect personnel.

 

[r6155]

(OP)
SS is a good material and the socket is the worst design.
This is not a good selection.
SS with butt weld is the nicest.

Regards

 

[georgeverghese]

In the oil/gas OpCo I used to work at, they have stopped using SW fittings some 20years ago for reasons others have stated here.

 

[Xk333]

GD2,r6155

Hot Oil service will also be normal fluid category, I mentioned it separately because it has the highest design temperature with the fluids I'm working. All other fluids have at the most around 100 to 150 degree Celsius operating temperature and 6 bar pressure.Even with such low design conditions should I avoid using socket weld fittings?

 

[Xk333]

r6155,

I'm sorry but SS socket welded fittings have been used in the industry for decades. Should it be absolutely avoided in all design conditions?

 

[Xk333]

TugboatEng,

Thanks. I will check the flash point and take preventive measures. I will also check other normal fluid service that are operating above flash point and make sure the I avoid socket welded fittings.