Purged enclosures in Class I, Div 1 areas

[jscottelwood]

I have a question of the venting of a nitrogen purged (7.5 CFM/min.) enclosure in a Class I, Division 1, Hazardous environment. The prints say that the environment exists to a 15 foot radius around any/all equipment in this area. Will it violate code to vent a nitrogen purged enclosure 15 feet up into the air? I would think that the hazardous gases/fumes would be stronger at that elevation than at ground level. We are talking about a hydrogen plant with natural gas or natural gas + Refinery gas feeds. I also would like feedback on the difference between pressurized and purged enclosures.
Pressurized means sealed to the environments and purged has a vent which could allow gases into the enclosure in the case of an accidental leak or shutdown in the purging system or conveying means. Thoughts???

 

[petersonra]

I have a question of the venting of a nitrogen purged (7.5 CFM/min.) enclosure in a Class I, Division 1, Hazardous environment. The prints say that the environment exists to a 15 foot radius around any/all equipment in this area. Will it violate code to vent a nitrogen purged enclosure 15 feet up into the air? I would think that the hazardous gases/fumes would be stronger at that elevation than at ground level. We are talking about a hydrogen plant with natural gas or natural gas + Refinery gas feeds. I also would like feedback on the difference between pressurized and purged enclosures.
Pressurized means sealed to the environments and purged has a vent which could allow gases into the enclosure in the case of an accidental leak or shutdown in the purging system or conveying means. Thoughts???

The terminalogy is the only difference. years ago, this kind of protection system was called purged, now the term pressurized is used.

In any case, it is vented, and has a means of insuring a continuous flow of the pressurizing gas is being introduced into the pressurized enclosure. In a division 1 area, this usually takes the form of a D/P switch that shuts off power to the enclosure if flow or pressurization is lost, and a timer that prevents re-energization until a pre-determined amount of the pressurization gas passes through the enclosure to remove any potentially hazardous gases.

I am curious why you would want to extend the vent 15 feet up in the air.

 

[jscottelwood]

That's what our engineering department has called for. I have been reading NFPA 496 (...snore) and it doesn't specifically differentiate between pressurized (with seal) and purged (vented). I am under the impression that they "advise" a period of time to purge before putting elctronics into operation. I need to do some scientific discovery work on this one (type of gasses, ignition temps, types of controls, etc.). I'll keep you posted...

Jeff Elwood

 

[petersonra]

That's what our engineering department has called for. I have been reading NFPA 496 (...snore) and it doesn't specifically differentiate between pressurized (with seal) and purged (vented). I am under the impression that they "advise" a period of time to purge before putting elctronics into operation. I need to do some scientific discovery work on this one (type of gasses, ignition temps, types of controls, etc.). I'll keep you posted...

Jeff Elwood

It is more than advice. You do need to pass a certain amount of the non-hazardous gas through the enclosure before restoring power. This is usually done at a much higher than normal flow rate to speed things up.

 

[don_resqcapt19]

It is called "purged and pressurized" because first you have to "purge" the enclosure by flowing at least four enclosure volumes of the protective gas at a pressure not less than 0.1 inches of water. After the enclosure is purged then it must remain pressurized at not less than 0.1 inch of water. There is no specific flow required at this point...just what ever flow is required to maintain the pressure.
Also Bob is correct that the requirement to purge before energizing is more than advice...it is required in you application as you need Type X pressurization.
Don

 

[rbalex]

... I have been reading NFPA 496 (...snore) and it doesn't specifically differentiate between pressurized (with seal) and purged (vented)...

Jeff Elwood

If you read the definitions in Section 3.3 carefully, you will find the essential difference between purging and pressurization. You may be a little bit surprised.

Edit add: To answer your original question, the location of the exhaust vent port is pretty much irrelevant.

 

[justdavemamm]

To answer your original question, the location of the exhaust vent port is pretty much irrelevant.

On a non-electrical note, venting nitrogen into a room presents other concerns. We purge with air. On a project where nitorgen was used in a process and might escape into the room, an O2 sensor / alarm monitor (by MSI ?) needed to be installed.

 

[rbalex]

Excellent catch. ?Instrument Air? is generally my preferred purging/pressurizing medium too; especially for relatively large applications. Bottled Nitrogen is a pretty good alternate for static pressurized systems where the bleed rate is small and general ventilation of the location meets ASHRAE standards.

 

[petersonra]

Excellent catch. ?Instrument Air? is generally my preferred purging/pressurizing medium too; especially for relatively large applications. Bottled Nitrogen is a pretty good alternate for static pressurized systems where the bleed rate is small and general ventilation of the location meets ASHRAE standards.

Mostly I see it where people want to used bottles of nitrogen becasue they don't have clean, dry air available. I always thought it had to be very expensive.

 

[jscottelwood]

Thank you

Thank you

Thank you,

All of your input has been helpful.

On to the next mission.....

I just love the oil and gas industry!!!

We also build separators and cryogenic systems.

(I've been reading alot!!!)