Placement of sealoffs

[Jon456]

I'm trying to get a better understanding of why sealoffs are placed in relation to classified areas.

Let's say we have a steel spill containment box for gasoline. Inside the box is a Class 1 Division 1 area. Next let's consider an electrical device inside that box. Conduit must pass through the wall of the box to bring power to the electrical device. I believe the sealoff is supposed to be the first fitting the conduit connects to on the inside of the box. But why on the inside and not immediately outside the Class 1 Div 1 boundary? It seems to me if there were an ignition inside the conduit, you'd want to arrest it outside the space that could possibly explode.

 

[don_resqcapt19]

Boundary seals can be on either side of the boundary.

 

[gadfly56]

I'm trying to get a better understanding of why sealoffs are placed in relation to classified areas.

Let's say we have a steel spill containment box for gasoline. Inside the box is a Class 1 Division 1 area. Next let's consider an electrical device inside that box. Conduit must pass through the wall of the box to bring power to the electrical device. I believe the sealoff is supposed to be the first fitting the conduit connects to on the inside of the box. But why on the inside and not immediately outside the Class 1 Div 1 boundary? It seems to me if there were an ignition inside the conduit, you'd want to arrest it outside the space that could possibly explode.

First, the containment box is going to be Class I, Div 2 unless gasoline is normally present during operations. Second, XP devices are specifically designed to prevent an ignition inside the conduit from exiting the device and igniting an explosive atmosphere that might be present. And Don is of course correct, you can put the seal on either side of the boundary. Please note, that depending on the design of the containment box, the boundary isn't necessarily at the wall of the box.

 

[nhee2]

First, the containment box is going to be Class I, Div 2 unless gasoline is normally present during operations.

Doesn't Table 514.3(B)(1) define space within and under dispenser pit or containment as Division 1?

 

[Strathead]

I'm trying to get a better understanding of why sealoffs are placed in relation to classified areas.

Let's say we have a steel spill containment box for gasoline. Inside the box is a Class 1 Division 1 area. Next let's consider an electrical device inside that box. Conduit must pass through the wall of the box to bring power to the electrical device. I believe the sealoff is supposed to be the first fitting the conduit connects to on the inside of the box. But why on the inside and not immediately outside the Class 1 Div 1 boundary? It seems to me if there were an ignition inside the conduit, you'd want to arrest it outside the space that could possibly explode.

It is a good question. The concept is easy, in that the conduit isn't going to explode, it will contain the explosion, so the intent is to keep the explosion confined to the immediate area of the classified area with any explosive force both minimized and vented in the classified area. However, I have always pondered that a seal inside the boundary still has one connection on the non-classified side of the conduit system that could allow gasses in. That being the threads on the seal itself. They are supposed to be made up wrench tight, but electrical threads aren't tapered so the are not water and pressure tight. So why would the requirement be outside the barrier period?

 

[don_resqcapt19]

It is a good question. The concept is easy, in that the conduit isn't going to explode, it will contain the explosion, so the intent is to keep the explosion confined to the immediate area of the classified area with any explosive force both minimized and vented in the classified area. However, I have always pondered that a seal inside the boundary still has one connection on the non-classified side of the conduit system that could allow gasses in. That being the threads on the seal itself. They are supposed to be made up wrench tight, but electrical threads aren't tapered so the are not water and pressure tight. So why would the requirement be outside the barrier period?

The conduit threads and the female threads on sealoffs and other explosion proof equipment are NPT (national pipe thread, taper). Conduit couplings are straight thread.

 

[gadfly56]

Doesn't Table 514.3(B)(1) define space within and under dispenser pit or containment as Division 1?

Well OK, if it is in fact a dispensing operation. And it looks like that still would only apply if the containment is below grade. I was thinking in a more general case.