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Natural Gas Cross Country Transmission Pipeline Depressuring

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

No way!  To get something in the range of "practical" Calculate the volume of gas between your block valves at your maximum pressure then figure the blowdown time using the full open Cv of a 10" diameter plug valve.  Be sure to take your ear protectors with you.

http://virtualpipeline.spaces.msn.com

RE: Natural Gas Cross Country Transmission Pipeline Depressuring

I've tried to do this quite a few times.  I can calculate the mass of trapped gas pretty accurately.  When you open a big valve (I often use a pig launcher or receiver), you are flowing at sonic velocity (choked flow) so your mass flow rate is very well defined and the time to about 160% of an atmosphere can be calculated to within a few seconds (you need to recalculate mass flow rate every couple of psi, but that is pretty easy).  Ear protectors are pretty much a necessity, on the 12-inch line below the police got noise complaints from the nearest house--5 miles away.

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

For some strange reason I don't think I'd blow a major line down through a launcher.... intentionally.

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.

http://virtualpipeline.spaces.msn.com

RE: Natural Gas Cross Country Transmission Pipeline Depressuring

I thought about it a long time before I did the first one.  I couldn't come up with the "strange reason" not to do it.  The noise is a lot louder than using a 2-inch vent, but it doesn't last nearly as long.

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

Well most of the time there won't be a launcher in the segment.

I have 2 visions, one of a Saturn V the other of a clown flying out.

http://virtualpipeline.spaces.msn.com

RE: Natural Gas Cross Country Transmission Pipeline Depressuring

(OP)
Thank you both for the insight you have given me. My questions do not stop there, though.

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

People often call me an irresponsible cowboy because I regularly question conventional wisdom.  On this one I'm with the conventional wisdom, trying to hot tap within a few hundred feet of a leak in a 45 barg line is truly a dangerous, irresponsible thing to do.  The hot-tap involves a lot of welding, and welding is a really good ignition source, not a great combination.

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

I've always questioned the rapidly as practicle phrase.  One pipeline I worked at would have a blowdown sized at about 1/2 the line line size with a vball on it.  They would test the valve annually and blowdown a portion of line from the compressor station to the mainline.  After being sued by landowners for coating their crops with compressor oil, I said it isn't praticle to pay for crops so I put RO's in the blowdowns.

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

"Practical" is inversely proportional to diameter, length and beginning internal pressure.  I don't think it is necessarily an associative function.

http://virtualpipeline.spaces.msn.com

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