NFPA 497----Open Transfer System Vs Closed Transfer System

[IntrinsicSafety]

This figure only has the hazardous area defined around the leak point, and I am surprised to see that the hazardous area does not exist at grade level. Is this because of the presence of the vapor hood? Any leaking or fugitive emissions are sucked up through the hood and do not make it to grade level?

No, it is because the leaking is presumed to be small, and the flammable vapors will dilute to below LFL by the time they travel 3' from the leak point.

3' (1m) is a fairly common rule of thumb for Div 2 hazardous locations. That distance comes up in a lot of standards.

I am trying to think of other methods that could potentially limit the hazardous area to the radius defined around the leak point, and eliminate the classified area at grade level...

To get a hazloc extending all the way down to grade, you'd need more emissions than would be expected for a normally sealed vapor recovery line.

Put another way, it would be a pretty crappy vapor recovery line if it didn't recover all of the vapor - right?

A catastrophic failure of the fill and/or vapor recovery line(s) would probably cause a much larger Div 2 area, and it would probably extend down to grade. Area classifications don't normally take such catastrophic failures into account because (a) they don't happen, and (b) if you assumed they did happen, everything would be a classified area and nobody would be able to do anything, ever.

Given this, most area classifications involving sealed systems presume fugitive emissions + some kind of "small", as-yet-undetected-but-will-be-found-and-fixed-soon leak. Which obviously depends upon the process, pressure, etc.

For example, if I have a charging station for a mildly flammable heavier than air liquefied gas, would placing the vessel being charged at grade level on a grated platform still require a hazardous area (up to) 3 feet above the grade level extending 15-25 feet even through the grade level is a grated structure above a pit which is below grade?

Yes, absolutely.

OK, >maybe< not 15-25'. Though that is what I would start with until I'm convinced otherwise.

The reason is you don't >know< that the leaking vapor is going to go straight down into the pit. Maybe there's a little bit of air movement that pushes it horizontally. Meaning you can never count on the released vapor >only< falling, nor always staying in the pit after it gets in there.

This is why Fig 5.10.4(c) and 5.10.4(d) show hazardous locations extending >above< the leak/emissions points, as well as below. Just because the vapor is a little bit heavier than air doesn't guarantee it's going to >always< settle.

I guess - depending on the situation - that you could argue for a reduced grade-level distance.

I would be worried that the pit would fill up, meaning my default would probably be an additional Div 2 area extending 3' from the edge of the pit, at grade level, 1-3' high. Given the pit is likely at least 3'-6' wide, these additional at-grade border areas would make the total distance 9' to 12'. So I'm not sure you would gain much, if anything, in terms of the height or horizontal distance along grade level.

Yes, it is worst-case. But worst-case is what we should be dealing with, even for Div 2 locations.

I know the pit below the grate would need to be considered a classified area, and would need to be monitored and ventilated, but would the hazardous area exist also above the grating above the pit or trench, or just in the trench itself? Would it be reasonable to say the area above the grating is not classified, but the area below the grating, ie the pit itself, would be the classified area?

Per the above, I would have real problems with accepting a "catch pit" for vapors/gases.

Liquids, sure - any released liquids will fall into the pit. But it's the vapors that we are worried about.


This is rather contrived, and the final result will obviously depend upon the actual situation.

If you were willing to put in the work, and depending on the situation, you >may< be able to prove to a reasonably (i.e. very high) level of confidence that the at-grade distance is less than 10'.

I, personally, would probably not bother. It is highly unlikely that anyone would put any electrical equipment anywhere near a tank hazloc that is not dead-smack in the middle of said hazloc. This is because all the action happens either in the tank (where level, pressure and temperature monitors will live) or at the fill points (where transfer, flow, pressure, fuel quality and cross-contamination instruments live). As reducing the hazardous area is unlikely to make any practical difference to anything, why put in the work?


A couple of additional things, just for interests sake:

- The pit would be a classified area, but it would not (usually) need to be monitored nor ventilated unless you want to put electrical equipment in there.

- Ventilating a pit implies forced-air ventilation, in which case (perhaps) the pit is no longer considered a classified area at all.

[rbalex]

Yes, it’s interesting, what about it? You do realize this is a virtually impossible scenario since the NFPA 497 Figure is based on a stationary railroad loading/unloading rack and the 505 example you fantasized about would not be a realistic application for a railroad tank car? In fact, look over the NFPA 505 Table 4.2(a) very carefully for examples of trucks likely to be approved for your scenario. Reading the NFPA 505 Scope carefully wouldn’t hurt either.

If this doesn’t clarify the issue for you, contact David Wechsler. He is a member of CMP14 and both NFPA 497 and 505 Technical Committees. He has retired from Union Carbide and I no longer have his phone number so finding it is an excercise left for the student.

[fifty60]

That is interesting. What about the situation where the grade level flooring is grated above a pit? Would the hazardous area still potentially exist 3 feet (for example) above the grated grade level? Or would the grating potentially eliminate the hazardous area at grade? For example, if the floor was grated could an electrically powered truck of just Type E (according to NFPA 505) be driven across the grating?

[rbalex]

Basically, you're dealing with a situation similar to a gasoline fuel pump nozzle. These are sealed connections and no emissions should occur in the first place. For the most part, the vapor recovery system should eliminate potential leaks; this is also why the hazard radius is only 3'.

[fifty60]

Looking at figure 5.10.4(b), which is a Tank Car Loading and Unloading via a Closed Transfer System, material transferred only through a dome, the material being transferred is a flammable liquid.

This figure only has the hazardous area defined around the leak point, and I am surprised to see that the hazardous area does not exist at grade level. Is this because of the presence of the vapor hood? Any leaking or fugitive emissions are sucked up through the hood and do not make it to grade level?

I am trying to think of other methods that could potentially limit the hazardous area to the radius defined around the leak point, and eliminate the classified area at grade level... For example, if I have a charging station for a mildly flammable heavier than air liquefied gas, would placing the vessel being charged at grade level on a grated platform still require a hazardous area (up to) 3 feet above the grade level extending 15-25 feet even through the grade level is a grated structure above a pit which is below grade?

I know the pit below the grate would need to be considered a classified area, and would need to be monitored and ventilated, but would the hazardous area exist also above the grating above the pit or trench, or just in the trench itself? Would it be reasonable to say the area above the grating is not classified, but the area below the grating, ie the pit itself, would be the classified area?

[fifty60]

Thank you Rbalex, as always.

[rbalex]

I'm going to answer backwards. The "dome" is the auxiliary vessel on the top of the tank car. It houses the tank car's part of the vapor recovery system.

"Open" and "closed" refer to the method the loading, unloading, and vapor recovery lines are connected to the tank car before loading or unloading occurs. A "closed" system makes all sealed connections before the external processes begin. An "open" system may have a brief moment where the connections aren't completed before the external processes begin.

In modern nations, only closed connections are permitted.

[fifty60]

I am studying NFPA 497. I am looking at figure 5.10.4(c) and 5.10.4(e). (c) uss the terminology "Unloading via a closed transfer system", and (e) uses the terminology "unloading via an open transfer system". What do the terms open/closed transfer system mean in this context? Also, (e) refers to a "dome". What does "dome" mean in this context?