Nitrogen purge Nat. Gas line

[SHOULDBFISHING]

Hi,

Does anyone know of a sizing chart that's available or the formula to determine how much nitrogen (Cu./ft)is necessary to purge a natural gas line? In the next few weeks, we will need to purge and permanently cap off an existing 6" dia, 17psi. natural gas line that is app. 600' long and is installed through one building, underground through an intersection and re-enters into another building.

After some research, I've "learned" that one method is to first purge the line and then provide a 10-15 psi static charge of Nitrogen in this abandoned line. Any opinions on this approach is welcome. Thank you.

 

[BigInch]

To fill it you'll need about 2 x the pipe's volume and some extra for overpurge volume. You need to PURGE it with a velocity that pushes the air/gas ahead of the N2. Just charging the line to 15 psi will wind up with half natural gas/some air (N2,O2, CO2, water vapor) and half N2... NOT a good mix for long term safe sealing.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[dcasto]

just purge at a rate equal to 3 feet/minute.

 

[zdas04]

You are really going to leave a line in the ground pressurized with 10-15 psig? Not the best idea I've heard today. If anyone ever hits this bomb with a hoe it will get very exciting.

If I was doing the job that I think you are trying to accomplish, I'd blow the line down to zero flow, pressurize it to about 15 psig with inert gas, then blow the line down to 3-5 inches of water and seal it. Going to 2 atm with inert gas gives you a pretty good assurance that the partial pressure of any residual gas is less than the LEL.

If you want to see the differences between a clearing purge, a dilution purge, and a displacement purge you can read about them at

http://www.muleshoe-eng.com/sitebuildercontent/sitebuilderfiles/s4Ch19purge.pdf

the purge that BigInch is talking about is a "Clearing Purge", but I'm not sure that a high-velocity purge is the best choice here.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[SHOULDBFISHING]

zdas04: "You are really going to leave a line in the ground pressurized with 10-15 psig? Not the best idea I've heard today. If anyone ever hits this bomb with a hoe it will get very exciting."

Just to reiterate, I did say 10-15psi of Nitrogen. This suggestion was given by a PE in our area. His reasoning was "to prevent any condensation and/or deterioration." You feel that a 3-5" WC is sufficient?

dcasto: How do you confirm you are purging at a rate of 3ft/min??

 

[BigInch]

I don't think you need to meter it. I'd just calculate the volume lost from the N2 container by its pressure drop over any given time and convert that to velocity in the pipe you're filling.

The guys are right, probably no need to fill to 15 psig. Bleed it down to 15 psia.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[zdas04]

You need to ask yourself "what is the purpose of sealing pressure in the line?" Once you answer that question, the next question is "what is the minimum pressure that will accomplish that goal?" When I go throgh that exercise I get a number very close to atmospheric pressure.

David

 

"to prevent any condensation and/or deterioration."


I am puzzled by the approach being taken. If this line is being abandoned, I presume it will be disconnected from C/P. That being the case, and you have "normal" soil moisture, then corrosion will compromise the the line integrity within 1-3 years. The nitrogen will not stop this or help in any way. Do you intend to reuse the line at some point in the future or are you trying to abandon it safely?
Gord

 

[zdas04]

Gaspro,
This discussion comes up pretty often. My preference is to remove any harmful liquid accumulation from the line to be abandoned with pigs and then leave it open ended to become part of the soil. Some jurisdictions have decided to "help" by passing laws that require the line to be filled with something and sealed. One state requires an abandoned line to be filled with concrete (in that state it is rare for an operator to ever declare the line "abandoned"). Several states require filling with an inert gas and sealing. I heard of one politician who allegedly said that if it was up to him there would be a requirement to annually record the pressure on all the abandoned lines (I assume that his intention is to make you fix the line if the pressure goes to zero).

I'm not sure where your 1-3 years for a line to fail after CP is removed came from, but I think it may be a bit over stated. I know of thousands of miles of gathering systems with no CP at all that have lasted in service for decades. Soil is never universally moist at pipeline depths, so to get pipe-to-soil corrosion you must have a holiday in your coating at the same location as a moist cell. It happens, but with a properly coated and jeeped line it seems to be acceptably rare.

David

 

[vzeos]

SHOULDBFISHING,

Retiring a natural gas pipe should be done with as much care as gassing it in. You say that the pipe passes through (or terminates in) two buildings as well as being buried underground. Let me point out that abandoned underground pipes have been found to provide migration paths for future gas leaks. For this reason it is important to cut and cap the pipe at each building wall. Also, if you have branches on your pipeline, you may need to install vents on each branch to affect a complete purge. It is not necessary to leave the pipeline with a nitrogen charge at elevated pressure. The only time I have ever needed to do that was after hydro testing a pipe segment that was not ready to tie-in yet. The nitrogen charge would prevent oxidation of the pipe interior. Anyway, if you retire a steel pipe underground, it would probably corrode through in a couple of years and eventually fill with water.

When you purge the pipe, you should have a written protocol. Your description sounds like you may need to introduce nitrogen from each building and vent somewhere in the middle. There should be a technician with a combustible gas indicator (CGI) at the vent looking for readings of 4% gas or less. Remember that a nitrogen release in a confined space such as a building will displace oxygen and could result in asphyxiation.

The definitive work on this subject is Purging Principles and Practice published by the American Gas Association. You should consider referencing this book. (

http://webstore.ansi.org/RecordDetail.aspx?sku=AGA+XK0101)

I have attached some material from this publication that may help you.

 

[zdas04]

vzeos,
The original Pipeline Purging Principles and Practice is absolutely the worst document that the AGA ever published. I didn't know that they had released a new version in 2006 until just now. The original document is filled with procedures that are either wasteful of gas (with zero net benefit) or downright dangerous. I'll have to get a copy of the new one and see if they got it right this time. Slamming that book is one of the cornerstones of my Pipeline Purging Class.

David

 

[Latexman]

I suspect after the line is purged and abandoned, everyone who was involved will eventually move on and the memory of what was done will fade. Therefore, while you are purging I'd recommend purging it more than less than the LFL. If the line is ever broken into and someone gets just a hint of the skunk gas odor, it could set off an unnecessary panic. So, I'd recommend purging it to practically 0% NG. The way I've done this in the past is when I think it is close to being finished to purge from one end and measure the effluent out the other end with one of the portable LEL/O2 meters in a plastic bag with a hole in one corner that is tied to the end of the pipe so the effluent flows into the bag, past the detector, and out the hole. The hole needs to be smaller than the detector so the detector doesn't fall out. Being in a chemical plant, we have many of these detctors laying around. They may not be readily accessable to you though.

Good luck,
Latexman

 

[vzeos]

zdas04, I dusted off my original copy (1954) of Purging Principles and Practice. Let me know which sections you have a problem with.

 

[zdas04]

I'm out of town right now, I'll get the details when I get back next week.

David