What should be the procedure to displace oxygen in a long pipeline? 1

[jejaram]

Hi

I couldn't find a forum dedicated to gas, so i thought this would be the right one to post my question but if it isn't I apologize in advance.

I'm a loss control engineer working for the insurance industry where the NFPA codes are the most common, but for this specific question I think something else like API RP 2201 would be more suitable.

NFPA 67 says that to prevent detonation in a gas system, oxygen (that might have come into the pipeline due to some rupture of it) should be displaced with an inert gas. It also says that sometimes it is not economically feasable to do so. What should be the procedure then when there is need of repairing a gas pipeline with control valves every 25 km (15.6 mi), that has suffered some damage that requires cutting and welding new pipes?

Thanks.

 

[zdas04]

There are a few purging references, but they tend to be lame and dangerous. There is one on my Webpage that addresses the issues and has some pro-forma procedures. You could also purchase Chapter 6 of my book Practical Onshore Gas Field Engineering. The first is free (and darn well worth it), the second costs $31.50, but has a lot of information beyond purging.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[jejaram]

Thank you David, that was helpful.

 

[LittleInch]

You might have been better off in the pipelines forum as it see more traffic but never mind.


Dave's paper is, as I would expect, very informative and useful, but as I'm sure he would acknowledge, is aimed at purging pipes which are "empty" or full of air.

You have an entirely different issue I think, where you want to repair an existing pipeline which is currently full of natural gas.

To a certain extent it depends on how long your new bit of pipe is compared to the main length. So even say 100m of pipe in 25km is basically negligible, but 5km isn't.

Assuming your pie doesn't have a hole in it, then there are many options.

1) Completely clear the line of methane by venting down or re-compressing gas ( much better idea) into the next section beyond your isolation valve / pig trap. The clear the pipeline with a pig pushed by lowish pressure inert gas ( usually Nitrogen). Vent down, do your repair, gas back up with a pig pushed by methane and vent the inert gas until methane appears.

2) If pigging is not possible then de-pressurize, but install two gas bag taps either side of your replacement section, insert gas bags and inflate then purge out the repair section, cut it out, weld it up then remove the bags via the bag holes, seal off the bag holes. Then just re-pressurize as the volume of air is very small and with all the turbulence and mixing you won't get an issue if the volume is relatively small ( say < 5%)

3) As above but for a big section, treat it as an initial purge and flow low pressure high velocity (>7-10 m/sec) methane and vent at the far end for a volume of 1.5 of the pipe or until you're getting 90% methane and then you're good to pressure up.

All depends on the details, the operating companies normal way of doing it, ability to pig or not, is the pipe ruptured or just needing repair, length of the repair section or sections...

Hope this helps.

LI



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

 

[shvet]

For pipe with flammable gas purging before/after maintenance/repair see NFPA 56 and CEN/TR 15281.

 

[jejaram]

David,

The information on the document from your webpage is excellent, thanks a lot. I have a question though, is the part about "purging 2.5 pipe volumes" derived from any code or is it derived from experience?

 

[georgeverghese]

From memory, the flammability range for natural gas is between 3-5% nat gas in air ( lower flammability limit) to about 96-98% natural gas in air (upper flammability limit). I would limit the permissible amount of air in a pipeline to 50% of these limits, assuming no other limits apply. So keep purging till you get down to this limit. Also check if there are more stringent limits for O2 in the gas stream from downstream user contract specs on feedgas.

 

[Compositepro]

George, those limits you give a much wider that any that I remember, so I had to check. Methane is 5% LFL to 15% UFL.

https://en.wikipedia.org/wiki/Flammability_limit

 

[georgeverghese]

Compositepro, thanks for the correction - think I got this muddled up with the UFL value for some other gas I came across recently.
BTW, these values on concentration refer to vol/vol or mole/mole.

 

[zdas04]

Jejaram,
There were a series of experiments done in the 1920's that found that the 99% confidence level of "oxygen free" occurred when 2.5 pipe volumes had been purged. These experiments were widely publicized within the industry and virtually everyone rounded that up to "3". Every document that I've ever found dealing with purging or dilution for the purposes of rendering a volume safe has had the number three prominently included (sometimes applied to the wrong quantity, but still there).

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

George,

The only gas I know that has that sort of range is Hydrogen. Basically it can explode as soon as you mix it with any amount of oxygen.



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

 

[Compositepro]

Two factoids that I have retained for several decades are that the flammability range of most fuels are surprisingly narrow and that the range is unusually large for diethyl ether (1.9-48) and hydrogen (4-75). For this reason ether is used as an engine starter fluid.

Silane is 1.5-98.

 

[georgeverghese]

Think it was silane (SiH4) I got it mixed up with - I'm going through a text on inorganic chemistry these days.