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Estimate Nitrogen for Purging Pipe Lines

07/02/2015 2:48 AM

Hi all,

I am incharge for purging of pipe lines for a new oil storage plant. The pipes to be purged are of following dimensions.

20" x 6 pipes of 1000m length

24" x 2 pipes of 1000m length

32" x 4 pipes of 1000m length

The pipes doesnt have any product inside. It is a new installation. We intend to get the atmosphere inert below 5% O2 content. How to calculate the volume of nitrogen required for this purging job??

We are planning to do pressure cycle purging.

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#1

Re: Estimate nitrogen for purging pipe lines

07/02/2015 3:17 AM

You would have to find out how the pressure cycle purge works.

Then you take the procedure and take it apart of how often you need to pressure up and from where and why and also how long and so on.

When you figured that out you will have a compression plan.

Now I hope I dont have to explain to you how to calculate the volume in your pipe. If so then all is lost and I do not see you succeed in this job. BTW we are not here to do your job.

Maybe you should formulate your question a little bit more specific. Like show us what you know already so we can guide you in the detail. I think if it comes to gas volumes you need to have an understanding of the PVT correlation and if you are asking gas mixture then maybe do so.

Technically you will need to bring in more and also Nitrogen than you already show and calculate. Do you buy Nitrogen in bottles?

Just asking because if you produce it on the fly . . .

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#3
In reply to #1

Re: Estimate nitrogen for purging pipe lines

07/02/2015 3:59 AM

I do know how to calculate volume and also know that you ppl are not here to do my job. I want to have feel of volume of nitrogen required for this project. I just want to know how many cycles of pressure purging is needed to get the atmosphere to below 5% O2 content. I thought ppl who have done this type of job before with lot of experience can give some numbers for me to work out. I want to know for one cycle of pressure purging how many times of actual volume should be compressed to get rid of moisture pockets and air pockets in the bends and orifices. From there i can calculate my numbers.

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#4
In reply to #1

Re: Estimate nitrogen for purging pipe lines

07/02/2015 4:07 AM

We bring in liquid nitrogen by ISO tanks not in bottles. I just want to know how many cycles of pressure purging is required to bring the atmosphere below 5% O2 content. People who have already such type of jobs before will be able to relate to this application and give me some approx numbers based on their experience.

Like for one cycle of pressure purging to how many times the actual the gas should be compresses to get rid of moisture pockets and air pockets in the bends and orifices.

I do know how to calculate volume for these pipes...

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#6
In reply to #4

Re: Estimate nitrogen for purging pipe lines

07/02/2015 7:43 AM

It sounds like you have a good start on finding your solution. I have never purged a new pipe, but was there when we purged old pipe for repairs. I like that you are considering pockets and bends, I was going to mention that and high points (because as you know N2 is heavier than air).

For the repair purge we used a gas analyzer to determine the output mixture until we were happy it couldn't burn.

As for moisture, unless your contract requires elimination I wouldn't worry too much about it (unless you think there might be hundreds to thousands of gallons of water). Even refined products will have some moisture content, that is why we use coalescer filters and drain water prior to dispensing or transfering fuel.

Drew K

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#35
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Re: Estimate nitrogen for purging pipe lines

07/14/2015 11:39 AM

"N2 is heavier than air"

How do you figure? N2 has a MW of 28.013 to Air's composite MW of 28.966. How does it wind up with a higher density?

Engineering Toolbox also shows Air as the denser of the two.

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#36
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Re: Estimate nitrogen for purging pipe lines

07/15/2015 8:37 AM

I believe it is because it is heavier than the part of air we need to support human life. Oxygen is lighter than N2 and is displaced. So the common phrasing is that nitrogen is heavier than air because it changes the mixture of 'air' we are referring to.

Drew K

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#37
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Re: Estimate nitrogen for purging pipe lines

07/15/2015 8:43 AM

Is that true? If it were, wouldn't the 78% of our atmosphere that's Nitrogen be down near the surface and thus keep the surface dwellers from access to the life giving Oxygen at a higher altitude?

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#39
In reply to #37

Re: Estimate nitrogen for purging pipe lines

07/15/2015 8:58 AM

Well, we do have the sun to keep things mixed up for us...

I did a quick google search but couldn't find what I was looking for...google is getting worse about giving up what you want, it gives me what it wants...or what people pay google to show me.

I recall being informed with regard to evaporated liquid nitrogen to be cautious when the winds are calm because it will pool in low lying areas. That could be due to temperature density, but the details have faded to the fog of memory.

I need a better google.

Drew K

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#40
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Re: Estimate nitrogen for purging pipe lines

07/15/2015 9:03 AM

Ah, evaporated liquid nitrogen, that hasn't heated up yet. Lower T, identical P, R, and n, means lower V. A denser vapor, until T catches up - and one with no real impetus to diffuse, since what's around it is mainly itself, anyway. Very good. Thank you.

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#41
In reply to #39

Re: Estimate nitrogen for purging pipe lines

07/15/2015 9:03 AM

I know what you mean about google. I stopped using it for most of my internet searches.

You may be a bit low on Oxygen yourself.

I've never heard of having to be careful in calms winds because of risk of lack of Oxygen (or too much Nitrogen). While that doesn't mean it's not true, it just doesn't fit with organic life on Earth. Most of the life is on the surface or in close proximity to it.

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#38
In reply to #36

Re: Estimate nitrogen for purging pipe lines

07/15/2015 8:45 AM

Oxygen is heavier, MW 31.9988, 1.331 kg/m3.

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#2

Re: Estimate nitrogen for purging pipe lines

07/02/2015 3:57 AM

Some literature to read. At least google says there is something in there for the subject.

Instrument Engineers' Handbook, Third Edition, Volume Three

and because I want to be nice today:

web.mst.edu/~dludlow/classes/che258/inerting.doc

Its reading time

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#5

Re: Estimate Nitrogen for Purging Pipe Lines

07/02/2015 7:41 AM

they should have put someone in charge that knows this

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#7

Re: Estimate Nitrogen for Purging Pipe Lines

07/02/2015 12:18 PM

You could use vacuum/pressure purging, that would take care of the water, if the pipes collapse, you know that something is not right.

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#8

Re: Estimate Nitrogen for Purging Pipe Lines

07/02/2015 3:36 PM

There are codes and/or standards that dictate purging times/methods. I know they exist but not much more than that.

Try a search of your local online standards library and purchase the appropriate standard(s) for your application.

As a very rough guide calculate the pipe volume and use an initial figure of 21% for atmospheric oxygen 78% for atmospheric nitrogen. Calculate how much nitrogen you have to add to drop this to 5% by displacing the atmosphere with pure nitrogen. Get a suitably qualified Engineer to check your work and sign off on it.

Codes and standards may require multiple purge cycles, oxygen measurements or other things I have not mentioned above. Best get a copy of the applicable codes and standards.

BE WARNED!!!! Most of my knowledge on this particular subject comes from a CSB report on a company in America that didn't know what they were doing, did it wrong and there was an explosion and people died. I had better not see a CSB report on you in the future.

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#9

Re: Estimate Nitrogen for Purging Pipe Lines

07/02/2015 11:47 PM

There are numerous sources of information of purging/drying of pipelines. Although it is common in the USA to clean a new pipeline of the water used for the hydro testing, there are several other steps. The actual purging is often done by a contractor with these particular skills but there are many internet sources of information on drying pipe that will carry liquid petroleum ( gases are done differently).

Start with:

American Petroleum Institute http://www.api.org/oil-and-natural-gas-overview/transporting-oil-and-natural-gas/pipeline

PC program developed for estimating pipeline drying time http://www.ogj.com/articles/print/volume-97/issue-7/in-this-issue/pipeline/pc-program-developed-for-estimating-pipeline-drying-time.html

Logical Pipeline Solutions http://www.logicalpipelinesolutions.com/Pipeline_Drying.html

Center of Excellence In Pipeline Services http://www.ceps-as.cz/en/services/pipeline-drying.html

Pipeline Drying http://www.kahn.com/hygrometers/PipelineDrying.pdf

Pipeline Rules of Thumb Handbook: A Manual of Quick, Accurate Solutions to ... https://books.google.com/books?id=2Dpn4DFJzd8C&pg=PA173&lpg=PA173&dq=pipeline+drying+procedures&source=bl&ots=DKc5cyk2e9&sig=CL_IXRGNWILAMDPeStAx9vIHb44&hl=en&sa=X&ei=Mv6VVYvqBYPu-AGc0q_wCw&ved=0CFsQ6AEwBw#v=onepage&q=pipeline%20drying%20procedures&f=false

These sources will most likely be much better than CR4 can. Some probably don't know what a pipeline is. Maybe a line of meerschaums on the shelf/counter of a smoke shop.

Good Luck, Old Salt

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#10

Re: Estimate Nitrogen for Purging Pipe Lines

07/03/2015 4:05 AM

Not sure how nitrogen purging will remove any water. Vacuum purging is the norm for this result.

As for reducing the O2 partial pressure to your stated 5% then it's a dilution process with pressure cycling not truly a purge or flush.

Pulling and holding a vacuum for some time and then filling the pipes with dry nitrogen at atmospheric pressure (0barg) or a little higher will ensure it's dry and full of nitrogen.

Say you fill the pipe to 1barg with nitrogen after a vacuum purge then you will need two times the pipe/vessel volume of Nitrogen at STP.

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#11
In reply to #10

Re: Estimate Nitrogen for Purging Pipe Lines

07/03/2015 5:02 AM

Start a fire and blow the exhaust down the pipe. That will get rid of both the oxygen and the water.

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#12
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Re: Estimate Nitrogen for Purging Pipe Lines

07/03/2015 6:41 AM

...except that water is normally a product of combustion.

But yeh, purge with an O2 free flow of flushing media and totally replace the pipe contents not pressurise and dilute.

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#19
In reply to #11

Re: Estimate Nitrogen for Purging Pipe Lines

07/04/2015 8:40 AM

Along that line of thought,just pipe the exhaust from a couple of diesel generators or big rigs into the piping.

Of course,any atmospheric moisture in the pipes will condense and run to the bottom of

the pipes as the temperature decreases furthur away from the source.

And he did state that they want to eliminate moisture also.

However,I like Old Salt's answer the best,especially the pig method is probably the

most efficient and least costly,and will result in a nearly pristine inner pipe system.

Live video can also be recorded and archived,ahead of and behind, as the pig travels

along the pipe, to detect any existing defects,which is useful if any future claims arise

from defects or procedure claims.

Remember:CYA

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#14

Re: Estimate Nitrogen for Purging Pipe Lines

07/03/2015 2:02 PM

First,perform a simple volume calculation using basic geometry 101.

I will not do that for you.

Use your own noodle.

This will tell you the volume of the pipes,and the volume of nitrogen required to fill

them to atmospheric pressure.

Then pull a vacuum on the pipes.

This will reduce moisture and most ambient air from the pipes.

This will also reduce the amount of nitrogen required to purge the system.

After filling to ambient pressure with oxygen,test the contents for percent of O2.

Also be sure to clarify whether the percent is to be by volume or mass of O2,and at

what temperature and ambient pressure the measurement is to be taken.

There are many more factors to consider,but this should at least point you in the right

direction.

They probably have a written procedure for their specified tests, so stay in

compliance with their requirements.

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#15
In reply to #14

Re: Estimate Nitrogen for Purging Pipe Lines

07/03/2015 11:59 PM

read your para 8 again mate.....getting your gases muddled.

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#17
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Re: Estimate Nitrogen for Purging Pipe Lines

07/04/2015 8:10 AM

Thanks!

You are absolutelty correct.I meant to say fill with nitrogen.

Somedays I couldn't hit a bull in the butt with a bass fiddle!

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#18
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Re: Estimate Nitrogen for Purging Pipe Lines

07/04/2015 8:15 AM

Correction to post #8:

It should have read :"After filling to ambient pressure with NITROGEN",not oxygen.

Sorry 'bout that,and a "hat's off" thanks to Wal for spotting the error.

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#16

Re: Estimate Nitrogen for Purging Pipe Lines

07/04/2015 12:20 AM

Unless you are fully committed to the nitrogen pressure cycle purging of these pipes (which you shouldn't be at this stage of the project) step back and look at other more efficient, faster and cheaper methods to clean the pipes. At a total of 12 km of pipe nitrogen cycle purging is going to be expensive and long time wise.

A much better method would be to pig the lines with a pig(s) that is made to do what you want. If you only want to inert the atmosphere and clean out residual water, pig it with a unit that squeegees the inside surfaces and has a pig to inner surface seal on the rear sides. Put the pig in the line at one end and push it through with your nitrogen. At the other end bleed the air off as the pig gets closer, remove the water pushed ahead, remove the pig and your pipe is clean, dewatered and contains a 100% inert atmosphere. No O2! Make life easier on yourself and much cheaper for the corporation.

Good Luck, Old Salt

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#20
In reply to #16

Re: Estimate Nitrogen for Purging Pipe Lines

07/04/2015 11:46 AM

Hello Old Salt, As the only "pigs" I am familiar with (sort of) are GoGo girls, I have to ask, can pigs handle bends, as in minimum bend radius? Thanks.

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#21
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Re: Estimate Nitrogen for Purging Pipe Lines

07/04/2015 2:26 PM

GA! Don't know how that got OT'd but it is a very good question and one that more CR4 members should be aware of. These are very ingenious devices that are almost unlimited in what they can do from inside a pipeline. Besides that, in my younger more adventuresome years I never met an ugly "pig" after I had 5 or 6 shots of "Rebel Yell".

Pigs come in all sizes and capabilities. There is really no limit in the sizes that can be made. For simple cleaning of a 1/2" tubing it could be as simple as putting a cylinder shaped "slug" of PE foam in the tubing and putting some gas pressure behind it. On the large end of the scale, there are many 42"-48" and larger pigs in use. Yes there is a minimum radius bend dependent upon the flexibility of the pig, the length, the rigidity of it, what equipment is loaded on the pig, the purpose of the pig and several other factors. The minimum bend is dependent upon all of these factors. They can't do "90o elbows" but normal sweep elbows could be done for some tasks. "Long sweep elbows" shouldn't present problems very often. Basically, tell them what you want or want done and they will deliver it or you can purchase it.

A cleaning pig for a 28-inch oil pipeline. The blue plastic disks seal against the inside of the pipe to propel the device and to remove loose sedimentation or scale buildup. The black rectangles at the top and the circular disks in the center are magnets to attract and remove any loose metal objects in the pipe.

Pipeline pigSource: www.pipeline-pigging.com Inspection pigs, also referred to as in-line inspection pigs or smart pigs, gather information about the pipeline from within. . The type of information gathered by smart pigs includes the pipeline diameter, curvature, bends, temperature and pressure, as well as corrosion or metal loss. Inspection pigs utilize two methods to gather information about the interior condition of the pipeline: magnetic flux leakage (MFL) and ultrasonics (UT). MFL inspects the pipeline by sending magnetic flux into the walls of the pipe, detecting leakage, corrosion, or flaws in the pipeline. Ultrasonic inspection directly measures the thickness of the pipe wall by using ultrasonic sounds to measure the amount of time it takes an echo to return to the sensor Specialty pigs, such as plugs, are used to isolate a section of the pipeline for maintenance work to be performed. The pig plug keeps the pipeline pressure in the line by stopping up the pipeline on either side of where the remedial work is being done. A combination of gelled liquids, gel pigs can be used in conjunction with conventional pigs or by themselves. Pumped through the pipeline, there are a number of uses for gel pigs, including product separation, debris removal, hydrotesting, dewatering and condensate removal, as well as removing a stuck pig. Because there now exist multi-diameter pipelines, dual and multi-diameter pigs have been developed, as well. - See more at: http://www.rigzone.com/training/insight.asp?c_id=19&insight_id=310#sthash.E6cB1KQo.dpuf

There are several sources of excellent information concerning pigs and their capabilities:

https://en.wikipedia.org/wiki/Pigging

http://petrowiki.org/Pipeline_pigging

http://www.ppsa-online.com/about-pigs.php

An ultrasonic leak detection Pig---

They also come in "dumb pig" configurations

Good Luck, Old Salt

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#22
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Re: Estimate Nitrogen for Purging Pipe Lines

07/04/2015 3:05 PM

I marked it OT, because it was a question. I have always wondered about pipe pigs since I saw a movie where smugglers used one. The nat gas scent injection station nearby has the chamber Nicor uses to send and receive. Thanks for the info.

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#24
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Re: Estimate Nitrogen for Purging Pipe Lines

07/07/2015 8:35 AM

Smart pigs are essential tools for pipeline maintenance. I spoke with the pig team that had done one of the laterals several times and reported an anomaly several times...before it started to leak. I didn't get an answer for why it had to leak before we dug it up to inspect the anomaly. Turned out someone had swiped the pipe with a digger and left a large scratch that had proceeded to corrode and eventually leak.

For simple cleaning, a foam or rubber pig is best though. I would worry about pushing it with nitrogen, the pig might tend to stick and surge ahead as pressure changed which might cause it to skip past small puddles and the friction with the walls might cause melting or wear of the pig leaving debris behind it.

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#23
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Re: Estimate Nitrogen for Purging Pipe Lines

07/07/2015 8:18 AM

What kind of gaseous pressure is required to push a squeegee pig? I have only ever pushed them with fuel in active lines. Most of my fuel pipeline experience is from the operator level, my repair project management experience is with water lines at a power plant.

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#25

Re: Estimate Nitrogen for Purging Pipe Lines

07/09/2015 8:52 AM

I think everyone is over complicating the issue: Simply use an air tester (like what you normally use for confined space entry permissive)....at the beginning of first purge, the reading exiting the pipe will be ~20%, and you need to find some way to purge all the way to the end of the pipe through a smaller valve, and even enclose the tester in its own little see-through box if you have to. You can also use the type of oxygen sensor used in combustion monitoring for %O2, as long as it reads precisely in actual %O2 (and not excess %O2, which is extremely confusing to some). Also you could use a bubbler tube to sparge the sample of gas from the pipe through a pure thermostated water sample with a submerged D.O. sensor that reads in ppm O2, and then back-calculate the the actual %O2 in the gas sparging the liquid. IT is not rocket science, maybe you need a chemist working there with some brains.

As each purge progresses, you will want to see the %O2 reading decrease. Keep going until the % O2 is well below the targeted 5%, then block the pipeline in (for each section purged) at a slight positive pressure, and make sure to maintain the cap pressure until the pipe is put in service.

If this contradicts any of the industry standards, then they need to re-write them. I am the Mighty Favog, and I have spoken.

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#26
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Re: Estimate Nitrogen for Purging Pipe Lines

07/09/2015 9:31 AM

Unfortunately this method will not work for most oxygen meters and 4 or 5 gas meters. Many depend upon a background gas of air and other gasses in high concentration will give erroneous readings. For example, one MSA LEL meter depends upon air being the background gas for detecting flammables. If this meter is used to detect the LEL's in an atmosphere of argon it gives a reading of about 40-50% LEL. As per MSA this was because the meter was being used with the argon background gas and not air. Equipment was normally used for condensing Benzene vapors but was shut down to facilitate brazing a copper tubing. Tubing was argon purged extensively and several samples taken for other methods of analysis with 100% argon being present and no explosives present.

Very good idea though.

Good Luck, Old Salt

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#27
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Re: Estimate Nitrogen for Purging Pipe Lines

07/09/2015 10:38 AM

Then use a Dissolved Oxygen probe (that is accurately calibrated, not hard to do), and sparge the gas through a water volume. Oxygen saturation is then controlled by the partial pressure of oxygen in the headspace (bubbles and top space) through and above the liquid. That will absolutely work.

Besides, who was talking about Argon, I thought we were talking about nitrogen purge?

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#33
In reply to #27

Re: Estimate Nitrogen for Purging Pipe Lines

07/09/2015 2:55 PM

Inert gases such as Nitrogen and the Noble Gases such as helium, neon, argon, etc. do the same thing. When working with a gas if you want to fill a container, pipeline or process unit process, argon has the advantage of density. It will not mix with the gaseous material being removed and stays on the lower level. This is an advantage when filling from the upper part, the lower nozzle or a side connection. Argon "sits" where you put it if there are no leaks. Due to Nitrogen's density being near that or air and oxygen they will mix and more has to be purged out to obtain an inert gas volume.

Good Luck, Old Salt

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#28

Re: Estimate Nitrogen for Purging Pipe Lines

07/09/2015 10:53 AM

Thanks to all of you for your valuable feed back and suggestions. We have finalised to go with pressure cycle purge to reduce O2 level to below 5%. Additional info is that the line is hydro tested and hot air blown to remove moisture inside. So the line is pretty clean. Our scope is to inert the atmosphere. Feedback from people who have done pipeline purging before is that we should be able to achieve 5% O2 content in 2 to 3 cycles. The MAWP of the pipe is only 2 bar.

So can I say that if the total volume of 1 pipe is say 100m3.

Volume required to pressure cycle purge per pipe is 100 x 3 ( no of cycles) x 2 ( inject twice the actual vol to get 2 bar ) = 600m3 of N2. Can some one help to confirm that this is the right way to calculate total N2 required.???? Appreciate ur help.

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#29
In reply to #28

Re: Estimate Nitrogen for Purging Pipe Lines

07/09/2015 11:19 AM

As far as the maths to your example calculation, you have that right. As to whether it will be sufficient, I do not know, and neither do you.

How long will you hold the pressure at 2 bar each time? I think that might matter as to the final result.

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#30

Re: Estimate Nitrogen for Purging Pipe Lines

07/09/2015 11:34 AM

We usually continuously purge without closing the outlet for a while. Then purge again and close the outlet to increase pressure to 2 bar and hold for a while say half an hour for the N2 gas to mix with O2 and then vent again. This happens for 3 cycles. We will measure the O2 content when venting gas each time to check if the level has reduced to below 5% O2 content. I want to be pretty sure with the N2 volume now.

Only thast a question now. Will 3 purgigng cycles be enough??? and twice the actual volume is enough to raise the pipe pressure to 2 bar????

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#32
In reply to #30

Re: Estimate Nitrogen for Purging Pipe Lines

07/09/2015 1:36 PM

To answer your questions: Probably yes, but check % level each time. 2nd question is definitely yes.

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#31

Re: Estimate Nitrogen for Purging Pipe Lines

07/09/2015 1:26 PM

You won't be able to work it out unless you have some idea as to the maximum pressure and the minimum pressure your pipeline can withstand and to know that you need to do a hydraulic test first.

How did that go?

And if you haven't done that yet then why not let the nitrogen in when you drain the water out. That way, you only need one pipe-full in each case!

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#34

Re: Estimate Nitrogen for Purging Pipe Lines

07/14/2015 9:02 AM

We usually continuously purge without closing the outlet for a while. Then purge again and close the outlet to increase pressure to 2 bar and hold for a while say half an hour for the N2 gas to mix with O2 and then vent again. This happens for 3 cycles. We will measure the O2 content when venting gas each time to check if the level has reduced to below 5% O2 content. I want to be pretty sure with the N2 volume now.

Only thast a question now. Will 3 purgigng cycles be enough??? and twice the actual volume is enough to raise the pipe pressure to 2 bar????

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#42
In reply to #34

Re: Estimate Nitrogen for Purging Pipe Lines

02/14/2016 7:15 PM

You need to do manual calculation if you dont want to measure after every cycle.

Use ideal gas law. How many kg of N2 is introduced and how many N2-O2 mixture is vented... (just use ideal gas). You can do it!

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