How to assess venting nitrogen for process preperations

[mrtangent]

Dear All,

When you vent down equipment either during process preps at the end of pressure cycles or on a re-start after shutdowns do you vent the "NITROGEN" into the open plant areas. IE FOR OUTSIDE BUILDINGS.

Take the case, Column has just been pressure tested to 6 barg nitrogen we now pressure cycle 3 times to 6 barg to remove all the oxygen ready for start up. In my place we have always vented the N2 locally as fast as praticaly as it does contain chemicals. Now I've always been told (when i was a younger engineer) that the dispersion is so effective that it dilute the nitogen to prevent a hazard - BUT HOW DO I KNOW THIS.

I was asked the same question by an engineer whom I mentor and I could not provide an adequate answer - so hense the question. So,

Does anyone test the air whist doing this ? use Air monitors, limits plant access while venting, uses dedicated vent stacks etc.
Are you aware of issues while venting this way?, is there a safety concern

I am well aware of the concerns to high N2 Concentrations from Confinded space entries and opening large manways but this seems a grey area to me where we dont have policys on whats acceptable so i'm looking for general advice on what else other companys do.

 

[jsummerfield]

Concerns exist. A high concentration of nitrogen is toxic in that it depletes oxygen. If you used nitrogen to inert a process line of vessel then the nitrogen would be laced with the hydrocarbon or other components. It is best if the nitrogen can be vented to a flare or "safe location". Consider oxygen monitors with low concentration alarms if the volume vented is high.

John

 

[epoisses]

It obviously depends on what line size you are venting through. In my experience with reactor (un)loading, when a reactor is swept with N2 through a 1" connection, the N2 from the top manhole is diluted very easily such that at a few ft from the manhole the O2 percentage measured is 20.
Yes, you will want to vent to a safe location if possible and you will want to limit access around the vent outlet to prevent people from being blown into their faces.

 

[Zoobie]

jsummerfield,

My understanding of the severity of nitrogen exposure is a little different than yours. Obviously oxygen deficiency is a real problem. In a lot of oxygen deficient atmospheres simply evacuating to a normal atmosphere can save a casualty. With nitrogen, if you get enough to collapse, their is a very good chance that you will die. One of the very real problems is that it is very difficult to resucitate a casualty with first aid after being exposed to nitrogen...it is very difficult to purge or displace the nitrogen from the lungs and the body can't do it itself once brought into a normal atmosphere.

I know my comment has very little to do with the original post but I thought I would point out the severity of exposure to a supposedly benign gas. I worked in a plant where I could be inches away from 150,000 amps of dc current, exposed to acid fumes, caustic, hexavalent chromium, etc. and the only real thing that scared me was exposure to nitrogen.

 

[epoisses]

Zoobie, certainly, you are right, I did not want to underestimate the consequences of exposure to N2 or rather lack of O2. I just wanted to highlight that the perimeter of the danger zone in certain situations in practice is not very large. In any case, it is compulsory when approaching the venting area to carry a O2 meter and to have a "buddy" nearby and at safe distance.

 

[mrtangent]

" it is compulsory when approaching the venting area to carry a O2 meter and to have a "buddy" nearby"

Is this your site standard or regulatory?
If regulatory which body ? by whom ? ...

 

[Ashereng]

Yes, nitrogen is certainly not "benign".

In my industry, nitrogen is used as a blanketing gas because for most oil and gas applications, it does not "REACT" with most process fluids (e.g. crude oil).

I have read in this site's other posting(s) that nitrogen does react with other process fluids in other industries.

 

[Zoobie]

Epoisses makes good points.

I believe it was only about 2 years ago when there were a couple fatalities at a BP Refinery in the UK at a manhole entrace to a tower.

Unfortunately sometimes the buddy system fails...actually it seems to happen far too often when the 'buddy' becomes a casualty as well in all types of confined space/poor atmosphere conditions. Unfortunately its human nature to run in and try to save your friend.

In regards to james1030bruce's latest question, I am not aware of any specific regulation. I believe that these are site/company standards that are influenced by industrial practice...I recently watched a safety video from one of the big 3 oil companies on nitrogen safety that is used around the globe. The content of the video drew on several well known refinery incidents. I think that you will find that O2 metering and a buddy system will be the bare minimum in a lot of policies/procedures.

 

[mrtangent]

Thanks Zoobie,

So, just to help me understand your point. During a normal process shutdown, clear the plant of chemicals purging equpment with nitrogen. You are now ready to vent the residual pressure down to atomsphere (say last 0.2 barg or 2-3psig if you like) would you use a buddy system here for typical open plant and use oxygen mointoring systems.

I am well aware of the buddy systems etc for confinded spaces etc but

 

[Zoobie]

How are your venting the residual pressure? Automatic valves, manual valves, cracking flanges? In the case of a 'first break' our procedure had gas testing and stand-by person requirements.

In the case of opening a valve to atmosphere (where you didn't have your face in it) we always relied on it rapidly dispersing into the atmosphere and took no special precautions. Its hard to make a blanket statement though as you need to consider location (indoors, outdoors, etc.), volume, body position, etc.

Now back to your first question....HOW DO I KNOW THIS...I'm probably in the same boat as you. I know it but I don't know how. I suppose mixing of gases comes into it. My broad look at it is, "well its nitrogen and air is 79% nitrogen" but that hardly sounds scientific. I have observed gas tests at areas of venting but again this pretty much site specific. I have never had concerns about dispersion of nitrogen in air except for the one nagging concern that if the nitrogen was substanially colder than the air that it would hang in low area for a period of time.

Maybe both of us will get lucky and an 'expert' can weigh in on the discussion.

 

[epoisses]

I am not sure what expert advice you hope to find. There are only a few options:

1. Spend a full day calculating the 3D O2 profile at a given distance around the vent at a given vent flow rate, N2 temperature, air temperature, wind speed....... good luck!
2. Spend half an hour to find the regulatory minimum, comply with it and go to sleep -- if anybody gets killed at least your @$$ is covered.
3. Take a few of your operators to the vent location and ask them their opinion. They should know all about what O2 levels you typically measure around a typical N2 vent, if your plant has not been constructed yesterday.
4. Discuss it with your HSE manager who will simpy require everybody within a 1 mile radius to wear breathing air, that could also be a solution.

Once again in my experience a small N2 vent into open air, even if the N2 is much colder than the air, does not require a huge safety perimeter. Yeah yeah yeah you still have to be careful, but it is completely different than N2 in a confined space which is very treacherous and requires extreme care.

 

[JOM]

James,

Is it possible to use odorised nitrogen, so people can detect its presence?

John.

J.

 

[Nitrogenn]

Has someone some experience (measuring values) of oxygen concentration in the surroundings of a manhole of a reactor that has an inertisation with a nitrogen blanket?
I'm especially interested in some values (or some good practices) when the manhole is opened for adding some extra chemicals.
Can the operator wear a breathing protection that is based on filtering the air in the surroundings of the manhole?

 

[Ashereng]

Nitrogenn,

You may want to start a new thread with this.

With regards to breathing source. If there is insufficient oxygen, filtering won't help - not enough oxygen. If it were you down there, would you prefer you own air, or a filter?

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