Hazardous Area Classification vs T Code

[haze10]

Few questions regards Hazardous area classification.

I understand the need for T code limits in Class II area as dust have a much lower auto ignition temperature than gases.
But how are T codes assigned to Class I in Groups A,B,C,D. I know you would look up the auto ignition temperatures of all the chemicals and then pick the lowest one. But what is you are designing a new installation and not sure what chemicals would be added in the future. So say you choose Group C and D, and it includes almost all the chemicals used in industry, but not sure what T code to assign. What T values are you seeing for Group C and Group D vapors in typical applications.

If I remember correctly, there was one time in NEC history in which each Group was assigned an associated T code. Might have been 20 years ago but I do believe it existed. Does any else remember this, and of know of a source. I'm thinking those assigned value would be a good adoption point for a newly design plant.



Any and all advice appreciated.

 

[Gr8blu]

A very typical temperature code specification for most classified (gas) locations is T3, which limits surface temperatures to a maximum of 200 C. For some specific gas combinations that have lower autoignition temperatures, an occasional usage is T3C (max 160 C surface). Note that T3C does not directly correspond to an IEC classification - it's only present in North America. Every now and then I see a really low T-code value (T6 = 85 C max surface), although not usually for gas hazards.

These "blanket" attempts at defining a hazard are just that, though - blankets. Sometimes it overestimates the hazard, and sometimes underestimates it. Better to have a clear grasp of each section/apparatus within the process environment.

Converting energy to motion for more than half a century

 

[haze10]

Would you have any input on why the T rating for gases would be so low. I fully understand why it is that low for dust which have much lower auto ignition temperatures, but gases have much higher auto ignition temperatures, typically in the 400C range. Is it just to cover those very few chemicals with low auto ignition temps?

Do you happen to know the most common T value that manufacturers of EX equipment design to? I'm referring to Class 1, Div 1, group D areas.

thanks for any suggestions.

 

[Gr8blu]

haze10: There are a total of 125 Group D gases. Of these, more than a third (37 percent, actually) fall well under the nominal 400 C Limit you originally quoted. These low-temp gases include: turpentine (253 C), tetrahydronapthalene (385 C), proprionic anhydride (285 C), 1-pentene (275 C), 1-pentaol (300 C), pentane (243 C), octene (230 C), octane (206 C), nonane (205 C), naptha (288 C), monoisopropanolamine (374 C), methylcyclohexanol (296 C), methylcyclohexane (250 C), methyl n-amyl ketone (292 C), methanol (385 C), kerosene (210 C), isoprene (220 C), isoamyl alcohol (350 C), isoamyl acetate (360 C), hexenes (245 C), hexane (225 C), heptene (260 C), heptane (204 C), gasoline (247 C), fuel oils (210 C), ethylene glycol monomethyl ether (285 C), ethylene glycol monobutyl ether (238 C), ethylamine (385 C), 2-ethyl hexyl acrylate (252 C), 2-ethyl hexanol (231 C), ethanol (363 C), dodecene (255 C), dipentene (237 C), 1,4-dioxane (180 C), diemthyl sulfate (188 C), diethylene glycol monomethyl ether (241 C), diethylene glycol monobutyl ether (228 C), decene (235 C), cyclohexene (244 C), cyclohexane (245 C), cyclohexanol (300 C), butylene (385 C), butylamine (312 C), 1-butanol (343 C), butane (288 C), and acetic anhydride (316 C).

The T-code represents an upper limit on allowable surface temperature. By definition, it must be below (AIT * 0.80) for whatever hazardous chemical is present. Since a fairly large proportion of Group D gases have AITs between 250 and 300 C, the predominant "default" limit is equivalent to T3 (200 C). For the few exceptions with even lower AITs, the code - at lest in North America - becomes T3 C (160 C). For Europe, there is no direct equivalent to T3C, so the next available code would be T4 (135 C).

For gases, there are two conditions that could designate a specific T-code. One is the autoignition temperature itself, as noted above. The other is the temperature at which the gas becomes flammable (which can be much lower, since the density of gas(es) is so much lower than comparable liquids or solids.

Converting energy to motion for more than half a century

 

[mparenteau]

We set a standard where I used to work (T3C). The process engineers were aware of this and when new solvents were introduced into the plant, or during a new design, it would be assessed. Did it capture 100% of the new solvent add's? No. But during my tenure, we only had a few times where one slipped through the crack and picked up on process audits.

Mike

 

[haze10]

this haa been an excellent response. Special thanks to GR8blu for explaining this in a way that even I can understand.