13 replies to this topic

[AZIZ_MN]

HELLO EVERYBODY,

I WANT TO KNOW YOUR OPINION ABOUT TOPIC GIVEN BELOW,
NORMALLY FOR CONTROL VALVES WE R USING DRIED INSTRUMENT AIR, INCASE OF INSTRUMENT AIR FAILURE I CAN USE NITROGEN FOR CONTROL VALVES OR NOT?
IT WILL MAKE ANY DIFFERENCE IN OPERATING CONTROL VALVES. ( FOR MY PLANT CONSIDER THAT NITROGEN IS ALWAYS AVAILABLE EVENTHOUGH UTILITY PLANT IS TRIPPED)

[daryon]

As long as the nitrogen is at the same pressure and clean I don't think it will make any difference to control valve modulation. Nitrogen can be used for this purpose but it is not economic to do so. Instrument air is cheaper, all you are doing is compressing, filtering and drying air. Rather than buying nitrogen bottles, or separating nitrogen from air using relatively expensive membranes or abosrbents.

If your instrument air plant trips you wish to switch to nitrogen? I guess you could do this but it is likely to mean that your nitrogen generator is greatly oversized for normal operation (will cost significantly more) or you have to store a lot of nitrogen bottles (not practicle). Don't forget, you also need to supply all SDVs and BDVs with nitrogen when the instrument air fails (as well as other users like air driven pumps, panel purges etc.).

Usally you will have duty/standy air compressors so if one fails the other takes over. Its not uncommon to have duty/standby dryers also. Then you have the air recievers to provide a buffer volume to supply critical users in case of a trip. If the cause of the trip isnot rectified bythe time the air in the recievers is used up the whole plant trips. That's unavioable. But normally the instrument air package are reliable and suprious trips don't often occur. I've not seen seen instrument air supply backed up by nitrogen before

Edited by daryon, 01 October 2009 - 07:58 AM.

[Padmakar Katre]

Dear,
The answer is yes as well no. The yes is because as such there is no problem of using N2 in absence of the instrument header as its a normal opertaing practice and everyone is aware. But use of N2 in the instrument air header is quite dengerous as instrumet air headers are have multiple tap points from where the air used. In case some one opens the N2 and exposure to N2 is dengerous. I haven't seen N2 as instrument air substitute as N2 is salebale product where as instrument air is compressed and dry air.

[matt1127t]

I work in a plant that operates chemical reactors. For crtical valves, we utilize compressed air cylinders as a back-up to our plant air. As an additional safeguard, we have the ability to switch over the entire plant from compressed air to nitrogen in the event of major outage.

As was mentioned before, the switch over must be controlled and the point of use sources for the compressed air (which would then be nitrogen)must be understood.

Matt

[sheiko]

Hello,

As Padmakar said, there is a potential hazard with the use of nitrogen as a temporary replacement for instrument air. Pneumatic panel board instruments use several cubic feet per minute of instrument air. It is entirely possible to displace air from a control room with nitrogen when the instrument air compressor trips off. Lack of oxygen does not cause any breathing discomfort but does cause death without prior notice.

Truly.

Edited by Art Montemayor, 07 October 2009 - 05:40 AM.

[AZIZ_MN]

HELLO EVERYBODY,

I WANT TO KNOW YOUR OPINION ABOUT TOPIC GIVEN BELOW,
NORMALLY FOR CONTROL VALVES WE R USING DRIED INSTRUMENT AIR, INCASE OF INSTRUMENT AIR FAILURE I CAN USE NITROGEN FOR CONTROL VALVES OR NOT?
IT WILL MAKE ANY DIFFERENCE IN OPERATING CONTROL VALVES. ( FOR MY PLANT CONSIDER THAT NITROGEN IS ALWAYS AVAILABLE EVENTHOUGH UTILITY PLANT IS TRIPPED

hellow friends,
thanks very much for sharing your knowledge regarding this question.
In our plant we don't have separate aircompressor for instrumentair, we r geting it from airseparation unit moleseive dryer outlet, and when ASU tripps we do have backup from buffer vessel and plantair dryer but some cases happened in past that same time both r not available so only in extreme case we want to use N2 as back up.
thanks again for reply
best regards
AZZU

[gvdlans]

so only in extreme case we want to use N2 as back up.

The fact that you will use nitrogen as back-up for use "in extreme case" makes it even more dangerous since operations or maintenance personnel will not expect the instrument lines to be filled with nitrogen. People have been killed in the past because of such attempts to keep the plant running in case of an instrument air failure.

This topic has been discussed here before, see http://www.cheresour...instrument-air/
Please read the contributions from Art Montemayor in this tread.

Please also check the following safety bulletin from the US Chemical Safety Board: http://www.csb.gov/a...gen-6-11-03.pdf

Edited by gvdlans, 06 October 2009 - 03:05 AM.

[shan]

When you know the risk of something, it is not that dangerous anymore.

Believe me or not, we even use natural gas as instrument gas in the offshore platform in Gulf of Mexico.

Only suggestion is to tie the nitrogen line into the air receiver with pressure control. The mixture of air and nitrogen may win you some time to inform the situation to the operators.

[Satyajit]

Hi Azzu,
I appreciate your question . It is a very important and common question for any process plant.
My answer is YES YOU CAN! You can connect N2 as a last resort to save the plant.
I have done it in Malaysia with some degree of care during plant emergency and also during commissioning when no Instrument air was available.
As many already highlighted the hazards involved in it, I would suggest following action points :

1. First of all improve the reliability of instrument air supply- a vital utility for the plant.
2. Do hazop study / job safety analysis for hooking up N2 to Inst air header.
3. Use energy isolation system or disconnect the N2 hook up when emergency is over.
4. Analyse N2 for presence of any O2 in N2 due to back flow.
5. Install double check valve with Double bock and bleed on possible N2 utility point which you may need for future emergency use.

Kind regards,

Satyajit

Edited by Satyajit, 06 October 2009 - 03:18 PM.

[Dacs]

Much have been said in this thread so what I can say is I think it's more economical just to have a buffer vessel for IA.

I've encountered a project before that has a buffer vessel that is good for 30 min continuous supply of IA.

Besides, if you were to use N2, you have to deal with its purity as well.

[ashetty]

Hi Shan,

In my experience natural gas is used as instument air in offshore platforms which are normally unmanned. This is usually done because you dont have a power source for the instrument air compressor motors. Wellhead gas with sufficient pressure can be used in these cases.But in large installations the risk may be different and might have to be assesed.For example in these remote offshore platform PSV relief and blowdown is normally "cold vented". We normally dont have that on an onshore installation.

Also the risk is quite different for natural gas release and nitrogen release into open air. Remember that you do need ignition for natural gas to cause damage (apart from the right air/fuel proportion). Dont need nothing for N2 to do damage in an enclosed enviornment.

The risk has to be assesed.

Regards.

[gvdlans]

Also the risk is quite different for natural gas release and nitrogen release into open air.Remember that you do need ignition for natural gas to cause damage (apart from the right air/fuel proportion).Dont need nothing for N2 to do damage in an enclosed enviornment.

I do not agree with this part of your post. Both nitrogen and methane (natural gas largely consists of methane) are considered "simple asphyxiants". So also for natural gas you don't need ignition to cause damage in an enclosed environment. See text on simple asphyxiants. If the natural gas contains high concentrations of H2S it is even more dangerous...

Edited by gvdlans, 07 October 2009 - 03:36 AM.

[ashetty]

Also the risk is quite different for natural gas release and nitrogen release into open air.Remember that you do need ignition for natural gas to cause damage (apart from the right air/fuel proportion).Dont need nothing for N2 to do damage in an enclosed enviornment.

I do not agree with this part of your post. Both nitrogen and methane (natural gas largely consists of methane) are considered "simple asphyxiants". So also for natural gas you don't need ignition to cause damage in an enclosed environment. See text on simple asphyxiants. If the natural gas contains high concentrations of H2S it is even more dangerous...

Hello gvdlans

"If the natural gas contains high concentrations of H2S it is even more dangerous" Completely agree..and silly of me to miss out on that,
but in open offshore installations, for sweet gas, it is sometimes safe to cold vent into safe location as the mol wt of natural gas (assuming predominatly metane) is much lower than the surrounding air and would tend to rise quickly.Nitrogen would probabely localize as it has a mol wt very close to that of air and would take time to dispese.

Point i`m trying to make is that every risk should be assesed individually.

Thanks.

[AZIZ_MN]

HELLO EVERYBODY,

I WANT TO KNOW YOUR OPINION ABOUT TOPIC GIVEN BELOW,
NORMALLY FOR CONTROL VALVES WE R USING DRIED INSTRUMENT AIR, INCASE OF INSTRUMENT AIR FAILURE I CAN USE NITROGEN FOR CONTROL VALVES OR NOT?
IT WILL MAKE ANY DIFFERENCE IN OPERATING CONTROL VALVES. ( FOR MY PLANT CONSIDER THAT NITROGEN IS ALWAYS AVAILABLE EVENTHOUGH UTILITY PLANT IS TRIPPED

hellow friends,
thanks very much for sharing your knowledge regarding this question.
In our plant we don't have separate aircompressor for instrumentair, we r geting it from airseparation unit moleseive dryer outlet, and when ASU tripps we do have backup from buffer vessel and plantair dryer but some cases happened in past that same time both r not available so only in extreme case we want to use N2 as back up.
thanks again for reply
best regards
AZZU

THANKS VERY MUCH TO GVLANDS
I HAVE READ BOTH LINKS GIVEN BY YOU AND ALSO OTHER FRIENDS COMMENTS, BUT AFTER READING Mr.ART MONTEMYOR EXPERIENCE, I THINK I SHOULD NOT GO AHEAD WITH THIS.
BEST REGARDS