C1D2 group C & D and steel Blast resistant lease buildings

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vandv

Member
I apologize ahead of time because I know the answer to these questions. I have an ongoing debate with a few entry level "Engineers / Project managers". No degree here just in the business 20+ years.
I believe many of you will find it interesting though.
Information sources to be used include but not limited to;
NFPA 496 "Purged and pressurized enclosures"
NEC Article (500-517)
API 752 & 753 (5)
And a little common sense
My motto is, "When in doubt, error on the side of safety".
Factors:
1. A temporary BRB (Blast resistant building) used as an office.
2. A C1D2 (condenser section only) C & D modified BARD wall mount unit.
3. Interior components non-classified
4. All exterior components with the exception of the a/c C1D1 (Seals, Exp. disconnect etc.)
5. No gas detection.
6. No pressurization.
7. No pressure seal dampers in buildings penetrations (Supply and return HVAC frame)
8. Fresh air box on the side of the a/c unit with 5' stack.
9. Fresh air box has 12"x12" motorized low leakage pressure seal damper with motor on the interior side of the boundary.
10. There no MARKINGS on the building regarding procedures for Evacuation / or Shelter in place procedures.

THE DEBATE:
1. The BRB office is suitable for operating in a C1D2 C & D location.
2. There is no need for low leakage pressure sealed dampers in the supply and return of the a/c frame because there is a damper in the fresh air box on the side of the a/c unit that can be closed.
4. The a/c unit can continue to operate in "recirculation mode" by closing the damper in the fresh air box in a C1D2 C & D area during an abnormal release of flammable gas.
5. They even want to argue the true meaning of "Shelter in place".

Engineers, too smart for their own good. (Humor)

Any input here is appreciated. If I can learn something I embrace it.
Thanks,
V
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
If it is a D2 area, and the AC unit is listed as being suitable for D2, I don't see an issue. There is no general requirement in a D2 area to shut off anything in the event of an abnormal release of flammable gas.

If the air intake is above the level that is classified as D2, why would that even be an issue? It is taking in air from an unclassified area.
 

vandv

Member
Thank you for your interest. Keep in mind there is no gas detection system, no markings, no visible escape procedures, none of interior components are suitable for C1D2 area. Before opening the enclosure the area must be tested for an abnormal release of ignitable concentrations of gas. Only then can you open the door and turn on the power. Once the door is closed for an undetermined amount of time, you would be required to test the area again before you could open it.
Also, the a/c unit only has 5' stack on top of a fresh air box with a positive seal damper and no means to control/regulate the amount of O/A. Per NFPA 496 it's only necessary to maintain a slight positive pressure with no specific flow rate, there is no indicator to prove there is in fact a slight positive pressure.
Lastly, the back panel on the a/c unit (Modified BARD wall mount) is only design against driving rain, not group C & D. Remove the panel and your looking into the interior of the building (Enclosure).
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I would suggest this.

One is required to provide only such things as whatever governing codes there are actually require. As far as I know, there is no code that requires a gas monitoring system in a C1D2 group C/D area. Now there may be a requirement to monitor for hazardous gases, but I don't know that it has anything to do with it being a classified area. For instance, places that generate H2S are required to monitor for it, but it is not because of the area classification.

If you are suggesting that the building they installed (does not matter if it is a blast resistant building or not) is only unclassified because it is pressurized, there may be some reason to be concerned that there is monitoring of the pressure. However, the monitoring does not have to be real elaborate.

I have not run across any similar situation where people cannot enter a small structure without first testing for the presence of hazardous gases. It would be kind of a pointless thing to set it up that way. I have seen many dozens of such small structures set up in areas that are otherwise C1D2 and have never run across a single one where you are required to test for flammable gases before entering.

if the A/C unit is being run continuously to pressure the little structure, what difference would it make if the back wall was unsealed? As long as adequate pressure was maintained, i don't know of any requirement to limit air leakage from the pressurization supply.
 
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