Natural Gas Cross Country Transmission Pipeline Depressuring
Natural Gas Cross Country Transmission Pipeline Depressuring
(OP)
Does anybody know of any real world data for the actual depressuring times (to atmosphere) for real natural gas transmission pipeline projects? ASME 31.4 846.21 c) calls for a blow down "as rapidly as practical". I was wondering what is achieved in practice, say for a 34" line operating at 45 barg in a Class II section. Does it come near the API 521 recommendation (for refineries in fire case) of reaching 7 barg within 15 minutes?





RE: Natural Gas Cross Country Transmission Pipeline Depressuring
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RE: Natural Gas Cross Country Transmission Pipeline Depressuring
Now comes the nearly impossible part. Your line is at around 8 psig. Your flow rate is not choked and is very much dependent on upstream pressure. I've tried to recalculate upstream pressure and new flow rate every 0.1 second with really lousy results. On one line (16 miles of 12-inch at 150 psig), I calculated the time to the end of choked flow to within 15 seconds of actual (I think it was around 40 minutes, it has been a while). Then I figured 70 minutes more to zero--it actually took just over 2 hours--that is as close as I've ever gotten. The assumptions behind all the flow equations expect steady state conditions and when pressure changes whole percents in a minute they really are meaningless.
If a +/- 25% estimate of blowdown time is possible, it is certainly way beyond my abilities.
In terms of "reaching 7 barg within 15 minutes", you could reasonably calculate the vent size (or sizes) that would be required to reach that goal. It could easily end up being a 42-inch valve in the middle of your 34-inch line.
David
RE: Natural Gas Cross Country Transmission Pipeline Depressuring
I limited the blowdown to less than 12" to attempt to minimize noise. It still could probably reach 140 dB though at full open. As you say, its best to notify the neighbors and the local police dept, if you have time to do so.
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RE: Natural Gas Cross Country Transmission Pipeline Depressuring
One trick I learned the hard way is if the barrel-isolation valve is pneumatic it is necessary to have an alternate pressure source (either another nearby pipeline or a nitrogen bottle) in case you have to stop the blowdown to reconsider your isolation (it happened to me once and the only way to shut the 16-inch valve was with a hydraulic hand pump, took forever).
David
RE: Natural Gas Cross Country Transmission Pipeline Depressuring
I have 2 visions, one of a Saturn V the other of a clown flying out.
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RE: Natural Gas Cross Country Transmission Pipeline Depressuring
Do you have any experience with the development and dispersion of the unignited plume of natural gas from such a blow-down/depressuring vent? The project I am looking at now has some vent stacks and I want to see whether a venting could affect the air traffic in the vicinity.
Do you have experience with the use of a portable flare? Are these typically fitted to an already existing connection to the pipeline? Or does one typically hot tap? (There might not be enough time for the hot tapping solution.) Are there technologies (or Red Adairs) to fit one directly to a leak?
RE: Natural Gas Cross Country Transmission Pipeline Depressuring
I have done dispersion modeling on a blowdown and the results vary by the atmospheric conditions, size of plume, etc., but the exclusion area (for a choked-flow blowdown) tends to be a few dozen feet. This analysis has always suggested to me that a portable flare (which is real common on drilling wells) would not significantly add to the overall safety of the process.
David
RE: Natural Gas Cross Country Transmission Pipeline Depressuring
On a liquid line we had a portable flare trailer with 200 feet of 2" line and a set of burners. When a line was rutured, we would isolate the segment and dispatch the flare trailer and a crew would clear the area, set up the flare, and connect to the segment via the mainline block valve bypass line. We would burn the liquis (actually it was becoming a vapor as it boiled off). It would still take a few days to get this all done, but we at least minimized the release at the leak site (rapidly as praticle).
RE: Natural Gas Cross Country Transmission Pipeline Depressuring
http://virtualpipeline.spaces.msn.com