What should be the procedure to displace oxygen in a long pipeline?
What should be the procedure to displace oxygen in a long pipeline?
(OP)
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.
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.





RE: What should be the procedure to displace oxygen in a long pipeline?
David Simpson, PE
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
RE: What should be the procedure to displace oxygen in a long pipeline?
RE: What should be the procedure to displace oxygen in a long pipeline?
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.
RE: What should be the procedure to displace oxygen in a long pipeline?
RE: What should be the procedure to displace oxygen in a long pipeline?
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?
RE: What should be the procedure to displace oxygen in a long pipeline?
RE: What should be the procedure to displace oxygen in a long pipeline?
https://en.wikipedia.org/wiki/Flammability_limit
RE: What should be the procedure to displace oxygen in a long pipeline?
BTW, these values on concentration refer to vol/vol or mole/mole.
RE: What should be the procedure to displace oxygen in a long pipeline?
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).
David Simpson, PE
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
RE: What should be the procedure to displace oxygen in a long pipeline?
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.
RE: What should be the procedure to displace oxygen in a long pipeline?
Silane is 1.5-98.
RE: What should be the procedure to displace oxygen in a long pipeline?