How to Calculate Vapor Density Against Lower Flammability Limit?

10/29/2020

I have a question regarding the need to provide electrical classification for a building using Class 1A flammable liquids.

For this building, 4 gallons could be out in use (located anywhere in the building). The MAQ is 10 gallons. I have discussed this with NFPA 30 committee, the manufacturer, and the AHJ (who is a knowledgeable FPE). They have all determined that even if the MAQ is not exceeded, electrical classification is required if a flammable/explosive mixture could be present.

The NFPA 30 staff directed me to a provision in NFPA 497, Section 5.5, that if the materials will not reach 25% of the lower flammable limit (LFL), this could be utilized to justify not providing classified electrical fixtures.

Does anyone know how to do this calculation, or know of firms that can be hired to do this?

I am an FPE and do not know how to perform it, and I have spoken with other FPE’s who also are not aware. The gentleman I spoke with at NFPA 30 agreed it would be by an industrial hygienist or a chemical engineer and not an FPE.

But does anyone know any that can be hired for a one-off calculation, or have a place where I could learn how to perform it myself (NFPA reference, SFPE article, course, etc.?).

Thank you!

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5 Comments

10/29/2020 08:31:27 am

Fortunately, vapor density and LFL are intrinsically related so this is only a two-equation problem.

Check out Section 6/Chapter 12 of the NFPA Handbook, page 6-202.

You'll first need to calculate the vapor equivalent of 4 gallons, convert it to a volumetric value in ft3, and then multiply that by the ratio of the relative percentage LFL of the liquid of interest, in this case, reduced by a factor of four to account for the safety factor stated by the technical committee.

Hope this helps!

FRANK J. HERRICK

10/29/2020 10:18:45 am

C=mass contaminant volume of contaminant

---------------------------- = ----------------------------------- x 10 to the 6

Volume air Volume air

Concentration units:
mg/m3 PPM

Ben Brooks link

10/29/2020 10:41:51 am

NFPA Fire Protection Handbook, Volume II, 20th Ed.
Chapter 12, 6-202 to 6-203

Shahpoor

10/29/2020 12:36:13 pm

You can definitely find the calculations in
Fundamentals of Fire Phenomena by James Quintiere.

I am not as experienced as everyone else here, so apologies if this answer sounds unrealistic, I think the better option is to get a lab to perform a closed cup or open cup test on it to determine LFL.

10/29/2020 07:40:03 pm

You problem is very straightforward and love to help you out here, The whole Purpose of Electrical Area Classification is to "Avoid Explosion if there is flammable mixture present and caught Ignition sources",. In any facility, Flammable mixture can not be avoided when air is freely available and storage of flammable products is your design intent. So the only option left for engineering to address this risk is to control the ignition sources. So how we do this? Avoid ignition sources as much as you can. However In any facilities either indoor or outdoors, Electrical Equipment/Instrument are unavoidable and poses for risk for sources of Ignition too. So next question is how to avoid ignition? select the Electrical equipment in such a way that it does not create ignition (or withstand-Explosion proof) or even if created, select suitable equipment which fits into that zone where it is to be located and sustained surface temperature lower than Ignition temp (these areas are classified as ZONE 0, Zone 1, Zone 2 or Division 1 & Division 2). Zone 0, No electrical/Instrument allowed.

In you problem, Given is Class 1A flammable liquids, which means Flash Point is lower than 37 Deg C (assuming you are following NFPA 30, if IP 15, threshold will be 55 deg. C). Since it is, flammable, Means this will potential to generate the flammable mixture, could cause for explosion if ignition sources are there, therefore has to be classified regardless of it's quantity otherwise say Unclassified Area. For Area Classification, Flash point is single most important parameter (in your case this is lower enough), second is Autoignition temperature (could be high enough) and last is Gas Group (in ur case it's IIA). LEL nothing to do much with Area Classification (just additional info). It's you are storing something above LFL and below UFL, you are in danger zone. Noting allowed to stored between LEL and UFL zones practically. It should be either Lower than LFL or higher UFL.

For this purpose, NFPA 497 is correct reference (NFPA 30 is only provides flammability/combustibility aspects of each fluids, nothing else in this regards), if you still need to more information to understand subject, check out API 500/505 and IP 15 (now knows is Energy Institute 15)..these are Codes for Area Classifications.

Hope this will Help

Regards
Sunil

How to Calculate Vapor Density Against Lower Flammability Limit?

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