Flanges in Flare Line Header

[Headache2013]

Dears;

I'd like to get your advices regarding installation of 02 flanges in an existing 36" flange line header.

In the frame work of an on-going project, we have to tie in a 36" flare line header. We proposed to Owner to extend the existing 36" header by a 36" spool (cap in one side and slip on flange in the other side). Cutting the existing 36" flare line, weld a slip on flange then connect our 36" spool by bolting the two flanges.
Owner refused saying that flanges are not allowed in flare line header.
Could you please to let me know if there is any international code or practice that doesn't allow flanges in flare header line. ?


See Attached file


Regards

 

[Gator]

Why are you bothering with flanges when you have to do hot work (cut and fillet(s)) in the field anyway? And what's that N2 purge thing - a line plug?

Piping Design Central

 

[Headache2013]

N2 is nitrogen .
The 36" flare header if very long (around 1kilometer) and it is not so easy to do butt weld from safety point of view and shutdown limitation by Owner (24 hours).

This is the reason we proposed to proceed as follows:
a) cold cut the existing 36" flange.
b) put plaster joints inside the 36" and purging flexible to safe area.
c) weld at site a slip on flange while injecting N2 through the plaster joint to kill any coming gases.
d) connect the 36" prefabricated spool to the slip on flange welded in (c).

Owner refused and he pretended that flanges are not acceptable in flare header.

Again , is there any code dealing with such matters?

Regards

 

[racookpe1978]

Is the flare line stainless steel?

 

[Headache2013]

No, the flare line is carbon steel spiral welded pipe.

 

[BigInch]

They use spiral pipe in petrochemical plants and worry about putting in flanges?
Spiral pipe is often limited only to water pipelines and structural applications.

 

[LittleInch]

AFAIK, there is no "code" requirement - normally codes are not that prescriptive.

flanges are generally avoided so that there is no chance that anyone can either insert a blind flange or take it apart and leave the potential for the flare line blasting gas all over the plant. The consequences of a flare line essentially rupturing within the plant are so high ( basically huge explosion) that the remote possibility is enough for many companies to avoid flanges.

If I was you I would hot tap a couple of small flanges onto the existing header, insert a bag and then simply chop off the existing cap and weld on your new extension.

you will need to risk assess this and look at all potential, but if you were confident about hot work on the flare header to weld your slip on flange, then you should be ok for a single but weld.

BI has some views on spiral welded pipe and it has historically had a bad reputation, but I've seen it used for long distance trunklines and in theory at least is the same as any other seamed pipe.

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

 

[BigInch]

Its use on hydrocarbon pipelines has been extremely limited. In fact it is even very hard to find out anything about where it has been used.

 

[LittleInch]

You haven't been in Algeria then....

I think "extremely limited" is being rather unfair. See this from the largest mill I know which is Corinth in Greece, but there are a number of others around the world.

http://www.cpw.gr/Projects.aspx?CtgID=2

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

 

[Duwe6]

Might want to rethink the flare line work; a 'good' flare line has a 'whisper purge' of a few cubic-feet-minute of natural gas/methane. Prevents any air [oxygen] from entering the line and making the contents flammable/explosive. Do any work 'hot', ensuring that there is a small flame at the spot of work - proves that the gas purge is present and adequate. Yes, this looks scary. However, think about the reality of welding; the center of the arc is 20K degrees F! If the welder is dressed for 20K, a small soft flame is nothing.

Google for some videos of 'pipeline tie-ins' or 'gas purge tie-ins'; this is a standard practice in underground stuff. Just ask zedas or BigInch. When you've got a long line that cannot be completely cleaned, you purge it. And nat. gas does not make the flare 'sick' like nitrogen will, and the little flame is a real-time proof of a safe purge.

Back to the original question: flanges are a site-specific or owner-specific prohibition, probably in an unwritten, verbal grand-father 'specification'. No prohibitions in ASME, API, or NFPA that I know of. [installing a 'pancake' blind is another story]

 

[Headache2013]

Dear specialists;

Thank you for your answers. It seems I have to give you more details to get more advices.
Please be informed as follows:
* Owner doesn't accept hot tapping.
* total shut down of the facility for this tie in is limited to 24 hours. we have finish the job in 24 hours max.

I'm providing additional attachment which shows some details that can allow you to advise.

Regards

 

[Headache2013]

Hello everybody;
Is there any advice from you?

 

[LittleInch]

I thought we had given it...

Ok, the issue most of us can see is that if you are able to weld on a slip on flange onto the existing header, then it is difficult to see why you can't but weld this extension on, and address your clients somewhat pedantic attitude to flanges, but then he is the client.

If you need to just add another branch on your new extension and feed the bag through this extra valved connection, or use one of the existing new valved connections, have your shut down, cold cut the flare header, insert the bag, feed the bag line through the connection on the new header, inflate, position, weld, remove bag. There are a number of bag type isolation systems that are very good for low pressure applications like this.

I forgot to say before that erect scaffold and get crane, but would assume that is happening anyway.

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

 

[Headache2013]

Little Inch;

Thank you.

Before welding , do we need to inert the flare line or not? Is there any procedure to avoid inerting this long flare line.
Even bag inserted , there is always risk of leakage of remaining gas in the long flare line. This makes the welding (buttweld or slip on flange) dangerous.

 

[LittleInch]

headache, I can't make your decisions for you.

you could inert the last 100m or so before you cut it and bag it.

Yes there is the chance of leakage, but it will be low if any and that's what you need sensors for. Bags at low pressure are very effective at sealing. Welding anything in aplant is potentially dangerous, but take enough precautions and that risk becomes acceptable.

You could pressure tent off the welding area and pump in air from a safe location - this is what they do on oil rigs and refineries when they don't want to shut down.

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

 

[Headache2013]

Little Inch;

You said:
''You could pressure tent off the welding area and pump in air from a safe location - this is what they do on oil rigs and refineries when they don't want to shut down''.
It seems we can do this job without shutdown. This is very interesting for us. Would you please give more details about such procedure ''pressure tent off the welding area and pump in air '' because I don't understand it.

Regards

 

[BigInch]

You are right L.I. I haven't worked everywhere.... yet. I still haven't figured out how to be in two places at once.

Given the number of pipelines in the world and the number of spiral welded HC pipelines we can count together, I'd say extremely limited is a pretty fair summary.

 

[LittleInch]

Search for "welding pressure habitiat"

You find companies like this

http://www.safehousehabitats.com/Home.html

http://www.stsgroupuk.co.uk/ContentControl/DocumentControl/29126_thumb_STS Habitat Bro ONLINE.pdf

http://damini.com.my/positive-pressure-hotwork-encloser-system-habitat/

They are not cheap, but maybe cheaper than a shut down. Now if you can hot tap the piece of header you're going to cut off, insert your bag, inert your bit of the header and then test for gas etc, you might be able to do without a plant shutdown....

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

 

[Duwe6]

"Before welding , do we need to inert the flare line or not?" YES!

If there is any oxygen in the flare line, you will probably have a BOOM. Most folks cannot bring themselves to use a methane/ natural gas purge, so the ONLY other safe option is to inert it. With any oxygen, flare lines are very dangerous; without O2, it is just another piece of dirty pipe. You cannot ever get a flare line clean enough to be safe to weld on, baring actually going inside and scrubbing every dirty inch of it.

"you could inert the last 100m or so before you cut it and bag it." And as long as the purge pressure is [slightly] higher than the pressure beyond the inflated 'stop' bag, any leakage will be outbound, and you can maintain a good [and safe] purge.