7 replies to this topic

[koteswari]

Dear all,
I have read the previous posts related to above topic.
I am working in a cryogenic plant in which we are venting out nearly 3800kh/hr of Gaseous nitrogen at 303K and at 0.24barg.
we are using instrument air from mail air compressor at 5.5 barg.

Is it feasible to install a compressor to increase the pressure of gaseous nitrogen to use as instrument air?

my instrument air requirement is only about 160kg/hr.

thanks

koteswari

[breizh]

Hi ,
You have to consider the savings !
In my company we are using N2 as a back up in case of Compressed air failure .

Breizh

[latexman]

You also have to consider possible asphyxiation hazard too. This will be instrument, layout, and ventilation dependent.

[Pilesar]

I see few advantages to using your waste nitrogen as instrument air. Your nitrogen pressure is low so compression cost will be roughly equivalent to air so there is little operational cost savings. Your nitrogen supply is only available while the plant is running so is less reliable than air and therefore you will still need the air compressor. The only advantage I see is the nitrogen is already dry. So the only savings comes from fewer regenerations of your instrument air dryer? What am I missing? I know it seems a shame to separate out good, pure, nitrogen and then just vent it... but low pressure gaseous nitrogen does not have much economic value.

[Art Montemayor]

koteswari:

Many years ago I wrote a rather long post about this very subject and how I (as a young project manager) almost caused the deaths of several of my chemical plant operators. This happened in Geel, Belgium in 1970 and it was a lesson that I have never forgotten.

I also designed for using nitrogen as backup for my pneumatic instrument system. Two weeks after startup, our air compressor failed and the operators switched to nitrogen during the night shift. Fortunately, I visited the night shift that night to checkup and see if everything was running normally. We had a control building, across the street from the process plant and almost all of our instrumentation was pneumatic at that time. After spending about half an hour in the control room, I started to feel dizzy and since we had double-doors I had to go outside.

One of my operators also joined me saying that he also had sort of headache and was dizzy. I then realized what was happening and ran inside to get everyone together and ask them if the backup nitrogen system was being used. The answer was positive.

We immediately shut down the entire operation in order to stop the instrumentation. Our control room was being slowly flooded with vented nitrogen from our control instruments inside the control room. We could easily have caused the asphyxiation of our operators with the use of the nitrogen in our pneumatic system. Since that time, I have never allowed this type of design. Nitrogen gas is a silent, stealth killer. Many persons have been killed by this gas - especially within enclosed areas.

[breizh]

thanks Art and Latexman for your valid point about safety !
Breizh

[Bodhisatya]

As mentioned LP nitrogen will hardly be of any use.Installing a Compressor and compressing will just burden up your Financial Side.

I too work in a Cryogenic Plant.And the LP GAN produced by the column comes out at a pressure of .23 bar(g) which is then compressed and charged in the header as Product.We too have an additional arrangement of using this GAN as instrument air.The GAN obtained substitutes the dry air available from our front end purification unit .As because all these arrangements came with the Project implementation schema,we didn't find it quite hard to excute.But in your case it doesn't seems to be economically attractive.

[Neelakantan]

Avoid use of N2 as air unless there are specific information passed all around; i know plant people using "air" for purging vessels and as "air supply" for man-entry; so be doubly careful; the second point is what about the emergency IA supply? i understand the N2 venting happens when the plant is stable; but do you have standby air system (diesel driven or highpressure supply pot?); these technical points will cover the advisability of the scheme.

Commercially, you are sending out 3.8T/hr? are you having a N2 production unit? or is this a purge N2 for perlite insulation or coldboxes? if you are selling N2 look at the cost of compression and reusing it; if it is purge N2, see why that much purge is happening and if it can be reduced.

regards
neelakantan