Indoor Class I Division 2 location

[nhee2]

Can an indoor, non-ventilated location be classified as Class I Division 2? I would normally say no - if you classify it Div 2 then it's possible to accumulate gas, and if there is no ventilation, then the area should be Div 1. However, I am looking at NFPA 820 Figure A.4.2(d) which shows a drywell/vault physically separated from the wet well, and classifies as Div 2 if not adequately ventilated.

I'm trying to apply 820 to a landfill leachate riser building with no ventilation. Usually, there is ventilation and the building is conservatively classified Div 2. But maybe non-ventilated could be Div 2 as well?

 

[retirede]

D2 vs D2 is determined by whether or not the gas is present under normal conditions, or only after some abnormal occurrence. Ventilation may be a contributing factor, but is not determinant.

 

[rbalex]

I am also looking at NFPA 820 Figure A.4.2(d). Which specific row in Table 4.2.2 do you believe the figure applies to?

In reality, Class I, Division 2 is rather loosely defined. From the 2023 NEC Handbook. (I rarely cite the handbook but, in this case it's fairly good):

Determining the appropriate classification for an area requires an understanding of the following factors: (1) the type and amount of material present; (2) the potential for that presence under normal and abnormal conditions; and (3) the entire process that the material will or might undergo in given circumstances.

The NEC does not classify specific Class I, II, and III locations. Other standards and recommended practices of NFPA technical committees and other organizations such as American Petroleum Institute (API) with expertise in working with flammable and combustible materials can be used to classify locations.

If you read the NEC definition [500(B)(2) Class I, Division 2] carefully enough, Division 2 is defined in terms of possibility like normally or might.
Actually that normally component means the whole world is Division 2 because normally in context of classified locations is undefined. (Even Merriam-Webster’s Collegiate Dictionary, 11th edition doesn't quite cut it) Section 500.8(B)(5) has an interesting take on normal. (Starting a motor is normal)

That said, in the particular application under consideration; Table 4.2.2 and Figure A.4.2(d) represent the experience of the NFPA 820 Technical Committee.

 

[nhee2]

I am also looking at NFPA 820 Figure A.4.2(d). Which specific row in Table 4.2.2 do you believe the figure applies to?

I was looking at row 15b. Considering a vault with no ventilation (Case D)

Often on other projects we are applying guidance from AGA XL1001 in area classifications - there are many figures / recommendations within that document that consider potential sources (like a leaking flange) as a Div 2 source if outdoors, a Div 2 area if within an adequately ventilated building, and a Div 1 area if within a non-adequately ventilated building. In some cases we've applied these figures in non-pipeline applications, more to justify adding some ventilation to a room than to go with a Div 1 classification. But admittedly the AGA standard is considering (usually) higher pressure equipment/sources, with a higher likelihood and impact of leaks than an NFPA 820 application.

Thanks for responses.

 

[rbalex]

The issue now is, "How would the gas migrate from the wet well (not shown in diagrams) to the dry well.?"

This is where the Handbook is a bit anemic on the list of factors to consider. In this case, a major consideration is the physical properties of the gas/vapor in question.

For the most part, the primary (often only) gas involved is methane (CH₄). It's a lighter-than-air and dissipates very quickly. It isn't likely to enter the dry well. However, Hydrogen Disulfide (H₂S), another common sewer gas, is a heavier-than-air gas that could (remember that definition of Division 2) vent from the wet well and find its way into the dry well. It probably wouldn't, but who knows?

In almost every classified location evaluation, considering the classified envelope of the source is necessary.

 

[rbalex]

A curiosity just struck me. I was reviewing the NFPA 820, Annex A, location classification Figures and none of them seemed to consider lighter vs. heavier than air gases. I have worked with NFPA 820 for a while and never noticed that before,.

 

[nhee2]

However, Hydrogen Disulfide (H₂S), another common sewer gas, is a heavier-than-air gas that could (remember that definition of Division 2) vent from the wet well and find its way into the dry well. It probably wouldn't, but who knows?

In almost every classified location evaluation, considering the classified envelope of the source is necessary.

Thanks. In this application (landfill leachate) it is possible although unlikely for the gasses to be present. The enclosed area will be Div 2 (potentially conservative but not much cost impact in this instance). When the design had eliminated some natural ventilation louvers I had walked myself down a path to Div 1 using arguments applicable to higher pressure gas applications but really not relevant to this application.

Agree that 820 doesn't distinguish between heavier/lighter gases.