making a Class I, Group D, Div II into a safe area

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kenlpeters

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I have a metal enclosure (9' wide by 26' long x 10' high) which houses a generator and a gas turbine fueled by natural gas. Under normal operation, the natural gas is contained. However, it may be possible under abnormal conditions (broken line, leaking connection, etc) for natural gas to escape into the enclosure. Hence, the area is classified as a Class I, Group D, Div II area.

When operating, the enclosure is continuously purged with air from a safe area and the enclosure is equipped with a gas detection system for alarming and shut down at LEL %'s well below the combustible limts.

Questions:
1. If I have a spark free fan for the enclosure purge, I purge the enclosure say for 10 air changes and verify the LEL's are below the alarm or trip settings, prior to powering up the rest of the elctrical system within the enclosure does all the electrical equipment within the enclosure (generator, transmitters, igniter systems, lights, receptacles, etc) have to be Div II rated?
2. What is the nuimber of air changes and the rate of air changes required to make a Div II area a safe area?

Thanks

Ken
 

jmargolis79

Member
Location
minneapolis
Why is this class 1 div 2? i have gas lines in my house and they could break or leak but my house isn't classified. I also don't think that a purged air system is intended for a this large of an area. at 26 by 9 feet this is a room.
 

99cris

Member
1.No.This is the nice fact about the purged/pressurizied enclosure.
Anything inside is like, lockated in an unclasified area.
NEC-2007 aart500 and up.(500.8(A3)) Nobady care what you have inside if you have this kind of enclosure (or XP)
2. Usually is per manufacturer book.In your big(realy big) encloosure, I am guessing 10 changing? cople of hours?
 

KentAT

Senior Member
Location
Northeastern PA
Generally, I would say:

If your enclosure is rated as a Div 2 area without the purge, adding a purge to the enclosure does not make it an unclassified area. It is still a Div 2 area, with provisions for allowing unclassifed electrical equipment to operate within the area as long as there is adequate ventilation and a gas level that is monitored by fixed gas detection equipment, and the area remains below 20% LEL.

If you have a gas detection system malfunction, or it the gas level reaches 40% LEL, you need to automatically disconnect all electrical equipment not suitable for a Div 2 area. If doing this would present an increased hazard, you scrap the idea of using mechanical ventilation as a protection technique to begin with.

(Pressurizing the enclosure serves no purpose, since the hazardous fluid would originate from within the enclosure to begin with.)

With these point in mind, I believe the answer to your Q1 is "no" based on your conditions listed, but the installation as a whole might change that, especially as far as what actions are automatically taken upon gas being detected.
 
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Johnny Crab

Member
Location
south Texas
Similar application to an analyzer house with gases piped into the analyzers inside(pressurized house, alarms on LEL's & fire/smoke detectors, shutdown PLC brick, etc.) EXCEPT you have a probably very hot device inside too.

What I've done for 20 years in similar situations is make sure all my instrumentation is rated for the Hazardous Classification and looked at the purge/pressurization/alarm system as more of a personnel protection system. With a nice leak and hot generator you are likely to have a kaboom. The alarms and such are to keep operators informed and, if the alarm is off, provide a reasonably safe area for workers to enter.

Real world answers if you want to protect folks and equipment:

1) Yes. It will cost more but it is good insurance.

2) Purge info here: http://www.bebcoeps.com/
More references with room or enclosure type:
http://www.ofeo.si.edu/safety_health/docs/safety_manual_pub_07/ch_27_att1.doc
 

bobgorno

Senior Member
Location
Colorado
NFPA 37 (2010) - Installation and Use of Stationary Combustion Engines and Gas Turbines


4.5.2 Engine rooms or other locations shall not be classified
as hazardous locations as defined in Article 500 of NFPA 70,
National Electrical Code, solely by reason of the engine fuel, lubricating
oil, or hydraulic fluid.

There is much more in this standard that may be applicable.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
True, however in this case from the description it seems like a useless excercise. The engine and the exhaust pipes are inside the enclosure and already presenting an ignition source. The place should have been unclkassified to begin with.
I agree with that. I have never understood an area that has normally occurring ignition sources being a classified area.
 
Generally, I would say:

If your enclosure is rated as a Div 2 area without the purge, adding a purge to the enclosure does not make it an unclassified area. .

That is incorrect.

NFPA 496
3.3.8 Pressurization.
The process of supplying an enclosure with a protective gas with or without continuous flow at sufficient pressure to prevent the entrance of a flammable gas or vapor, a combustible dust, or an ignitable fiber.​

3.3.8.1​
Type X Pressurizing.
Reduces the classification within the protected enclosure from Division 1 or Zone 1 to unclassified.​

3.3.8.2​
Type Y Pressurizing.
Reduces the classification within the protected enclosure from Division 1 to Division 2 or Zone 1 to Zone 2.​

3.3.8.3​
Type Z Pressurizing.
Reduces the classification within the protected enclosure from Division 2 or Zone 2 to unclassified.​

 
Similar application to an analyzer house with gases piped into the analyzers inside(pressurized house, alarms on LEL's & fire/smoke detectors, shutdown PLC brick, etc.) EXCEPT you have a probably very hot device inside too.

What I've done for 20 years in similar situations is make sure all my instrumentation is rated for the Hazardous Classification and looked at the purge/pressurization/alarm system as more of a personnel protection system. With a nice leak and hot generator you are likely to have a kaboom. The alarms and such are to keep operators informed and, if the alarm is off, provide a reasonably safe area for workers to enter.

Real world answers if you want to protect folks and equipment:

1) Yes. It will cost more but it is good insurance.

2) Purge info here: http://www.bebcoeps.com/
More references with room or enclosure type:
http://www.ofeo.si.edu/safety_health/docs/safety_manual_pub_07/ch_27_att1.doc

It's a waste of good money. No wonder we have manufacturing shipped off-shore.
 
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