Choked Flow Analysis for a Purge

[MechEng176]

Hi Folks,

I'm working on a project where we have a natural gas line that needs to be purged using Nitrogen gas. Can't use a pig because of different size pipes and can't use a cycle purge because of balloons (for lack of a better word) we have in the line. I've attached a pic, the balloons are what separate the system that needs to be purged from the in-service system. They have a hold strength of 2psi, so if I pressure my system to over 2psi while purging they will move. For safety I'll try to go to 1.5psi max. I need to determine 2 things, 1 total venting capacity (keeping in mind the 1.5psi threshold), and 2 the Nitrogen flow rate required (these 2 should be equal right?).

We will pump Nitrogen from a tie-in point on the left side of the drawing and all the vents will be open. From my understanding for the choked flow calculations, P0 is the max pressure on the upstream side of the vents, so 1.5psig (or would it be the pressure at which N2 comes out of our Nitrogen pump? Because that is 6MPa, likewise I'd change density0 depending on if it's 1.5psi or 6MPa). I also added up all the area of the vents and made the calculator think it is just one big vent, will that affect anything?

One last thing, does Reynolds number come into play here?


Thanks!

 

[zdas04]

Your line pressure upstream of the vent is limited to 1.5 psig. Let's assume you are pretty close to sea level and 1.5 psig is 16 psia. To get choked flow you need to satisfy the choked flow equation which would say that with 14.5 psia downstream of the vent you need something like 25 psia upstream. You are short by 9 psia or 36%. You are never going to get choked flow.

With a dilution purge I want to get the system pressure to about 150% of atmospheric pressure (call it 22 psia) before I start venting. You can't do that.

With a clearing purge I need to inject at about a Reynolds Number of 50,000 and I want pressure upstream of the vent to be over 15 psig (call it 29.5 psia), you can't do that either.

With a displacement purge you need to get a flow rate in the largest pipe cross section in the 8000 to 12000 Reynolds Number range, which is the only option you have. This is the weakest of the three kinds of purges so I would probably purge 9 pipe volumes at 1.5 psid. There isn't a basis for this number since no one has done experiments at that low a differential pressure.

This project would scare the hell out of me. I don't know what technology the "bags" are, but they are the wrong technology (I'm picturing something like a Foreman Plug, but those are good to closer to 20 psid).

You might want to take a look at that purge document on my web page under samples. It covers many of the key points that your project seems to be missing.



David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[MechEng176]

Thanks for the link to that document, I've added it to my Resources folder.

I don't know much about the bags either, customer just told us they're good till 2psig.

For the actual purge procedure rather than have all vents open the whole time, I'm thinking first I close all the vents except for the one furthest away (making all other vent points being on deadlegs) and start the purge (put a gauge on one of the other vents), once a gas tester on it says LELs are good, I'll close it and one by one work my way to the deadleg vents closest to the injection point. Then I'll have them all open and once all the LEL readings are good call it a day (a long day since we won't be pumping that fast).

 

[BigInch]

I'd tend to purge to the nearest vent, then the next. Otherwise you run the risk of the dead legs continuously contaminating the N2 as it flows to the farthest, wasting a lot of N2 and time.

Purging flows should be in the turbulent range if possible to minimize any cross contamination of the purge gas and gas being purged as it flows down the pipe, especially when there is a significant difference in gas densities and when the purge gas is lighter than the gas being purged.

Independent events are seldomly independent.

 

[zdas04]

Be dang careful with the "gas tester". They are all looking for a quiescent stream, not a flowing stream. Sticking one in the vent will give you truly random numbers. You need to flow into a bag and sample the bag gas or the numbers will lie to you.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"