Hazardous Area Classification of Storage Tank

[timm333]

I am trying to figure out the hazardous area classification of outdoor storage tank. It looks that the area within 10-feet of the tank is Class 1 Division 2. After 10-feet the area is still Class 1 Division 2, but it is only up to a height of 3-feet from the floor. The classified area continues until it touches the dike; after the dike the area is unclassified.

NEC exhibit 515.4 gives some information which is for cryogenic material; but I am looking for flammable Class-1 Group-C material. Also figure 5.10.4 (a) of NFPA-497 does not give much information.

My question is that what happens if the height of the dike is less than 3-feet, will the classified area still end at the dike, or will it extend beyond the dike? Also in case there is no dike, then how far away from the tank will the classified area extend horizontally? What do you guys think about it? Thanks

 

[powerpete69]

Per the figure 5.10.4(a), if you look at the right side, it says "tank in open (undiked) area" This shows the division 2 bubble only extending out to 10 feet. So with no dike, it clearly shows a max of 10 ft.

The left side of the photo is unclear. It almost looks like the dike makes the situation extend out past the 10 ft. I agree with the 36" height thinking, but of course it conveniently leaves that measurement out of the figure.

Looks like there might be more info in Figure 5.11.10(a) and 5.11.10(b).

 

[rbalex]

First and foremost, if the installation is within the Scope of Article 515, you must classify per Table 515.3 (See Sections 515.1 and 515.3) it is not limited to cryogenic material. BTW I don’t know what NEC exhibition 515.4 refers to. I imagine you are referring to the NEC Handbook; personally, I find it to not be too accurate with respect to hazardous locations.

That said, NFPA 497 is a good alternate if Section 515.1 doesn’t apply. Powerpete’s observation about NFPA 497 Figure 5.10.4 (a) is well taken about applying the left-right side of the figure; however, the 36“ height limit is purely imaginary. It isn’t fixed and is, in fact, set by the height of the dike or containment wall. While it does apply to cryogenic applications, See Figure 5.10.10(a) for a bit better application diagram. That is, it explains that the containment height restraint is determined by the distance from the container. This actually “works” for virtually any heavier than air vapors.