Area Classification Boundary/Physical Barrier

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nhee2

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We have a project site which uses American Gas Association guidelines for area classification - in this case flanged connections create a 15' radius Class I, Division 2 location. The process is high pressure (>2000 psig) natural gas. There are other standards which define smaller radii/zero radii around flanged connections, but in this facility, the AGA standard applies.

Are there any guidelines or standards which allow for a physical barrier (wall, solid partition) between a source (flanged connection) and non-hazardous device/location, to shorten this distance? For instance - if an 8 foot wall is placed adjacent to a flanged pipe - would this barrier reduce the horizontal radii? As natural gas is lighter than air, the source leakage would need to travel up, over and down to reach the adjacent general purpose device (currently located <15 from the flange). The wall seems to be a reasonable approach, but it is nice to have some referenced standard to back this up.
 
A solid / non-porous barrier is usually recognized. See API RP500 Appendix D, Figure D-3 as an example of the most common way to handle the hazard-radius cutoffs, Be sure to document [Section 500.4(A)] any applicable extensions of the hazard radius.
 
Figure D.3. did not really provide what I was looking for. However - NFPA 497 Figure 5.9.9(a) (2012 edition is what I have) provides a conceptually similar situation - in my case - our installation looks like that figure minus the enclosed, adequately ventilated shelter. We have lighter than air gas, and a source within solid wall partitions and open roof.
 
I had to look up NFPA 497[ 2012] online. Clarify for me why Figure 5.9.9(b) isn't even closer to the installation you have described. Both (a) & (b) are lighter-than-air applications.
 
rbalex said:
I had to look up NFPA 497[ 2012] online. Clarify for me why Figure 5.9.9(b) isn't even closer to the installation you have described. Both (a) & (b) are lighter-than-air applications.
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5.9.9(b) shows a non-adequately vented shelter over a flammable gas source. I don't have any enclosure over the top of my source - just walls on the side. I am thinking my Div-2 area is shaped like a tee (or a mushroom). Inside the walls is the stem, above the walls is my mushroom. But the div 2 area does not extend down below the outside of the walls, similar to what is shown in 5.9.9.(a).

Maybe I can try to get a picture on here.

Thanks for the response.
 
OK, I get it. Nothing that collects gases above the release point. I no longer have a good CAD app so I will try to describe this as best I can.

Your "mushroom" description is pretty good except the Division 2 location will extend below the edge of the outside walls, lighter-than-air or not. Determining how much below the edge is the question. That's why I referred to API RP500 Fig D-3.It probably won't extend too far.

Establish a vertical line from the release point. Take the horizonal radius from the vertical line to the top of the wall. Subtract that from the "standard radius". That will create the new hazard radius from the edge of the top of the wall. Lighter-than-air gases can still go down just as heavier-than-air gases can go up. Down will be no more than the difference between the standard radius and the distance to the wall. It's a dispersion model that is being created,
 
rbalex said:
OK, I get it. Nothing that collects gases above the release point. I no longer have a good CAD app so I will try to describe this as best I can.

Your "mushroom" description is pretty good except the Division 2 location will extend below the edge of the outside walls, lighter-than-air or not. Determining how much below the edge is the question. That's why I referred to API RP500 Fig D-3.It probably won't extend too far.

Establish a vertical line from the release point. Take the horizonal radius from the vertical line to the top of the wall. Subtract that from the "standard radius". That will create the new hazard radius from the edge of the top of the wall. Lighter-than-air gases can still go down just as heavier-than-air gases can go up. Down will be no more than the difference between the standard radius and the distance to the wall. It's a dispersion model that is being created,
Click to expand...

Thanks. So if I my source is 3' from a 8' high wall, your method would say my Div 2 area will still extend horizontally 12' from the top of the wall. Additionally, it will extend down to grade (15' standard radius minus 3' = 12', but my wall is only 8' high). It sounds like I get zero benefit from the wall unless distance between source and wall is > 7' (standard radius minus wall height). Does this sound right? My API RP500 is a hard copy, back in the office, so I can't re-read Figure D-3 for a few days.

Thanks again.
 
Well now you're dealing with an entirely different animal. In fact, if the wall is significantly higher than the diameter of the enclosure, you may even need to consider there may be a Division 1 in the interior - again lighter-than-air or not. It would depend on how fast the gases could diffuse. you may need to do a full fugitive emissions calculation.
 
rbalex said:
Well now you're dealing with an entirely different animal. In fact, if the wall is significantly higher than the diameter of the enclosure, you may even need to consider there may be a Division 1 in the interior - again lighter-than-air or not. It would depend on how fast the gases could diffuse. you may need to do a full fugitive emissions calculation.
Click to expand...

Thanks.

The wall is not significantly higher than the width of the enclosure, I don't believe we are Class I, Div 1 inside the walls. However, I was less confident on how to treat outside the walls - appreciate the insight you have provided.
 
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