welded vs flanged 1

[Ahmad Ishfaq]

Hi Everyone

What is criteria for choosing welded joint vs flanged for pipe to pipe in hydrocarbon services.

flanged joints are not recommended. do we have any criteria of joining pips

 

[1503-44]

Flanges: Ease of Maintenance of connected equipment, but may need maintenance themselves. Difficult to disconnect atteached equipment and devices when using welded connections. Electric and cathodic isolation. Joining dissimilar pipe or wall thicknesses that can't be welded. No heat or sparks generated when working in hot zones. Usually cheaper than welding. Good Joining heat sensitive devices. Harder to insulate. Easier planning for field work. Its like USB for pipes.

Welded: High Joint integrity. No leaks. Good transmission of forces and stress. Poor for maintenance of connected equipment. Provides electric and thermal contenuity. Hot zone safety and work permits needed. Close fit-up tolerance. Field joint preparation. Coating over completed weld. Requires relatively highly skilled work force and more inspection and testing. Can be expensive when used for relatively few joints.

 

[LittleInch]

Usually comes down to risk / impact of leakage.

Flanges are notorious for being sources of leaks. Welds much less so.


"flanged joints are not recommended. do we have any criteria of joining pips"

Please expand on what you mean? I'm confused by "criteria". In HC service if you don't flange it, you weld it instead.

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

 

[0707]

You only have two options or weld or flage. It depends on the service and the danger of a leak and maitenance purposes.

 

[MJCronin]

What has not been mentioned above are the skills, schedule and project costs required for field assembly of piping systems.

An unskilled field force can be quickly trained to turn wrenches and bolt piping sub assemblies together..... An all welded system frequently requires a welding school, qualified welders or the importation of welders from offsite.

A piping system that requires welded joints, where the cost of importing welders is high, will be much higher than an all flanged system.

The choice between a flanged piping system and an all welded system will also affect the schedule ....

MJCronin
Sr. Process Engineer

 

[LittleInch]

Errr, well flanges need to be welded to the pipes using qualified welders, QA etc and it needs two welds per joint.

You can make up welded spools in a central area, under cover, warm, free from rain etc and then bolt them up, but always need to have some field fit lengths.

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

 

[weldstan]

Flanged spools will be much more expensive to fabricate - flange costs and added welds. But will be somewhat less expensive to join in the field and may be much less expensive depending on the alloy material and post weld heat treatment required and NDE required plus potential for weld repairs.

 

[1503-44]

Wait. Those were mentioned in first reply.

 

[bcd]

Most pros and cons were previously mentioned. Another consideration is fire safety. Bolted flange joint is more likely to leak during a fire than welded joint. Bolts can heat up quicker than the flange, stretch, and permit the gasket to blow-out.

 

[KevinNZ]

We always used to have large pressure vessels supplied to site with flanges. But when it was thought about and the question asked, are we ever going to undo the flanges? It was obvious that the flanges were not needed. Now those vessels come with butt weld nozzles and there is no maintenance or leaks worry about from flanges we did not need.

 

[gsakamoto]

This question is posed in a way that incurs infinite arguments because we don't have a clear interpretation of what the author is asking for.

Ahmad, please elaborate on your criteria.

 

[1503-44]

I think that's what he's asking for.

 

[Ahmad Ishfaq]

what does standard state about using flanges for hydrocarbon service. since we use q-class material as well for hydrocarbon service. what are the international standard clause that advise use of flanged pipe to pipe connections

 

[0707]

If it is a letal service flages shall be avoided if not it depends on the service and the danger of a leak and maitenance purposes.

 

[mk3223]

what are the international standard clause that advise use of flanged pipe to pipe connections

It can be specified as a Company Standard, but not in any International Code.

 

[1503-44]

Typical pipe design standards do not make requirements for configurations primarily dependent on fabrications needed for fit-up and maintenance purposes. Generally they allow welded, flanged, mechanical, or screwed fittings connections to be used, in fact even on the same piece of pipe. It is the operator's decision to use what works best for any particular situation.

 

[MickMc]

In general minimise flange joints as they are considered potential leak path therefore use when connecting to equipment, valves or instrumentation and as mentioned for certain services such as lethal/toxic all connections may be welded type. For hydrocarbon service you may also be required to only butt weld pipe and fittings depending on client whereas some will allow socket connections in small bore. Typically in projects I have been involved in, mainly middle east, only time we use flanged spools are when galvanised in utility service or when lined where diameter and configuration are limiting.

 

[LittleInch]

Ahmed, We can't read your mind.

Which "standard" do you talk about? There are many

What is "q-class" material. Clearly means something to you, but nothing to me.

In theory flanged connections are the same as welded if installed correctly, but do have a higher risk of failure and give rise to a hazardous area around each flange.

"lethal service" in ASME B 31.3 bans slip on flanges, but otherwise allows flanged joints.

Why are you asking exactly?

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