Threaded joints & fitting unions use in gas process plants 1

[mcisajp]

In my experience and engineering common sense the best practice in gas process plants will be to use welded joint instead of threaded one. However there is still companies fabricating / constructing using threaded joint and even using FITTING UNIONS . Recently it was known of a small fire event from a threaded joint . So, what is really the industry concern from the Process Safety perspective on the use of this practice , including the use of FITTING THREADED UNIONS and what is ASME B31.3 committed thinking about this ?

 

[DGrayPPD]

I can't exactly your answer your question from an industry perspective. I can say that I agree with your viewpoint, but I have seen many instances where the client specs all state threaded fittings in the small bore piping, even with hydrocarbon services. Some will at least say they're going to seal weld the joints, but others won't.

I don't understand the logic there either. It shouldn't take a catastrophic event, or even a small fire, for someone to step forward and make safety the first priority. But most people will tell you it all comes down to money, which I believe to an extent. But if they think having safety is expensive, try having an accident.

Just my two cents though.

"The only limits in life are those which we impose upon ourselves."

 

[LittleInch]

Well 31.3 has a specific section on it (314) and can be seen to advise against them for severe cyclic duty and for unions where there are external moments.

Screwed is I think seen as easier to field fit, which for a lot of small bore pipework it would be, and as with most things, when done well it is as good any welding.

The retrofit actions are also seen as being very useful.

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

 

[mk3223]

IMO, the use of the threaded piping can be the project decision which is based on many factors during the project execution, such as safety impact, process and piping engineering design, material availability, budget and schedule, etc., as well as the Local Laws and Regulations.

 

[mcisajp]

Thanks very much for your comments. I still would like to have some technical reasoning for using FITTING UNION . Also, what should be the leak testing pressure and fluid to determine effectively a threaded joints sealing.

 

[Dhurjati Sen]

Dear mcisajp,

Threaded piping are in vogue for instrument air, potable water and nitrogen. No piping for hydrocarbon is threaded now-a-days. You need to change your company spec, drawings etc from threaded to welded piping for all services except above.

The test pressure for threaded piping is as per B31.3 or the applicable code.

Regards.

DHURJATI SEN
Kolkata, India

 

[GD2]

Threaded fittings are almost found in any process plant, mostly in auxiliary lines. Even if the main process lines are welded (based on service fluid, service air/water (utility),lines are usually threaded), instrument connection, impulse lines are usually threaded. In many instances unions are provided for ease of removal of equipment.

It is upto the designer and the Owner to specify the fluid service category and do the correct material selection and fabrication within the rules and prohibitions under the code.

Many Owners specify their own specification built on the code. It is the experience and incidents that matures them to specify additional rules. Code rules are minimum.

B31.3 also brings the concept of Safeguarding (Appendix G).

Whether we talk of a process plant or home heating lines with natural gas, threaded joints are inevitable. A good design will be to minimize the joints, select appropriate joint type and provide safeguarding as much as possible. On the incident above, the question to ask would be: from where the heat source came? Was the joint was next to a heat source that can potentially ignite the gas. Many times, small lines that are threaded are abused by personnel working in the plant by push and pull. If the line is toxic, hazardous, flammable, the concept of safeguarding need to incorporate in the design.



GDD
Canada