We are designing a process which will have a section that will be classified as Class 1 Div. 2.
Where can I find some calculations to determine the size of this classification area around the process?
I would start with a review of OSHA 1910.307, Hazardous (classified) locations. Of particular interest is 1910.307(g)(4)(i)
I found the following link to be a pretty good read. It may not apply in your instance but I found it to be interesting.
Stephen Benton CESCP
Depending on the specific material and environment that is driving you toward identifying this classification, I would definitely start with NFPA 30 and 30A, and then possibly move on to NFPA 59A. These standards have illustrations of various tanks/structures/installations that can help you better understand the envelopes created by the boundaries.
And, of course, if you think you might be struggling with more than one interpretation and can't quite put your finger on the right answer, always remember that your local Fire Marshal is there to help you.
NFPA 30 got me on the right track.
It led me to NFPA 497, which deals with classification, but it doesn't mention how the size of the classified area is calculated.
It lists 25 ft for most situations. What factors affect this?
You would look at the particular use and the standard will tell you. For instance NFPA 59a for the storage of LP gas will tell you any refilling port or connection and the radius will be division one class one classification and anything around the entire perimeter of the tank will be Class one division two.
Many of the areas have been already determined. What is the particular use or hazard?
It is standard practice in the oil and gas industry to use API 500, "Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division 1 and Division 2" to determine the size of the classified area surrounding the hazardous process equipment. API 500 bases the classified area size on the process equipment containing heavier than air or lighter than air gases and whether the equipment is above grade or below grade. API 500 also contains figures to help with determining the classified area based on the type of structure or equipment.
In the Annex of NFPA 500, there is an alternate method for calculating the area of classification using the point source concept which takes into account the velocity at which the materials are released. This might be helpful for the calculations you need.
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