Area Classification Above A Trench

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circuitrider

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I have a situation involving a trench below a Class I Div 2 area. Per NFPA 497 it must be rated Class I Div 1. However the trench also extends beyond the boundaries of the Div 2 area above it. My understanding is that the entire trench needs the Div 1 rating. Unfortunately NFPA 497 does not address this situation. Is it possible for the area directly above a section of a Class I Div 1 trench to be nonhazardous?
 
circuitrider said:
I have a situation involving a trench below a Class I Div 2 area. Per NFPA 497 it must be rated Class I Div 1. However the trench also extends beyond the boundaries of the Div 2 area above it. My understanding is that the entire trench needs the Div 1 rating. Unfortunately NFPA 497 does not address this situation. Is it possible for the area directly above a section of a Class I Div 1 trench to be nonhazardous?
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No. See Section 500.5(B)(2)(3). Philosophically, without an impervious barrier between them, you cannot have an unclassified location adjacent to a Division 1 location. However, the Division 2 hazard radius may be as little as 18”. It’s a judgement call.

One of the worst fuel gas explosions occurred when a trench transported propane several miles from a source.

This is one of the the more subtle differences between NEC Divisions (and Zones) and IEC Zones. IEC does not necessarily require a physical barrier between unclassified and Zone 1. For most practical purposes, IEC Zone 1 is a moderately upgraded NEC Division 2.
 
rbalex said:
No. See Section 500.5(B)(2)(3). Philosophically, without an impervious barrier between them, you cannot have an unclassified location adjacent to a Division 1 location. However, the Division 2 hazard radius may be as little as 18”. It’s a judgement call.

One of the worst fuel gas explosions occurred when a trench transported propane several miles from a source.

This is one of the the more subtle differences between NEC Divisions (and Zones) and IEC Zones. IEC does not necessarily require a physical barrier between unclassified and Zone 1. For most practical purposes, IEC Zone 1 is a moderately upgraded NEC Division 2.
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Don't know about the incident you mentioned, but unfortunately a gas pipe in a trench doesn't trigger hazardous location if under normal operation the gas is expected to remain inside the pipe. Gas leak in an underground pipe can follow trench for a long distance though especially if soil that got filled into the trench isn't as compacted as other soil around the trench.
 
rbalex said:
No. See Section 500.5(B)(2)(3). Philosophically, without an impervious barrier between them, you cannot have an unclassified location adjacent to a Division 1 location. However, the Division 2 hazard radius may be as little as 18”. It’s a judgement call.
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I believe you are right that Section 500.5(B)(2)(3) requires a Div 2 area separating Div 1 from nonhazardous. If you could justify 18" though, why not, say, 4"? Or is that what you mean by judgment call?
 
circuitrider said:
I believe you are right that Section 500.5(B)(2)(3) requires a Div 2 area separating Div 1 from nonhazardous. If you could justify 18" though, why not, say, 4"? Or is that what you mean by judgment call?
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18" is pretty much a historic minimum. "Judgment" could put it as much as 10' although it isn't likely.
 
rbalex said:
No. See Section 500.5(B)(2)(3).
One of the worst fuel gas explosions occurred when a trench transported propane several miles from a source.
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A few questions just out of curiosity.

Who has a trench that is several miles long without any breaks?

Would an 18 inch break in the trench have done any good? Several miles of propane is a lot of propane. It seems to me that much propane would just spill over an 18 inch separation at grade.
 
petersonra said:
A few questions just out of curiosity.

Who has a trench that is several miles long without any breaks?

Would an 18 inch break in the trench have done any good? Several miles of propane is a lot of propane. It seems to me that much propane would just spill over an 18 inch separation at grade.
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could be propane got into some other trench for whatever reason and followed it, like underground crossing
 
petersonra said:
A few questions just out of curiosity.

Who has a trench that is several miles long without any breaks?

Would an 18 inch break in the trench have done any good? Several miles of propane is a lot of propane. It seems to me that much propane would just spill over an 18 inch separation at grade.
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Propane is extremely heavy and can maintain an explosive mixture for a long time. As part of an API study, a slow leak with a relatively low wind a propane leak can remain explosive for at least 3 miles before it becomes dispersed.

I won’t name the city. Basically, the trench was an above ground water drainage system that fed into a catch basin. The propane collected in the basin. How long the initial leak occurred is unknown.
 
rbalex said:
A closed piping system in a trench (or anywhere else) does not require classification.
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Not sure if you consider 'closed piping system' but some AGA standards and gas utility meter/reg station standards define 15' from flanged connections as Cl I Div 2. Although this doesn't sound like what the OP has.
 
nhee2 said:
Not sure if you consider 'closed piping system' but some AGA standards and gas utility meter/reg station standards define 15' from flanged connections as Cl I Div 2. Although this doesn't sound like what the OP has.
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This isn’t the OP’s case. In the scenario discussed in the OP, the pressure differential between the Division 1 and unclassified locations is essentially zero and 18” is the common hazard radius.

Closed piping has no designed route to atmosphere, such as a relief valve. Other (non-catastrophic) potential routes may be evaluated based on pressure differential, flange pressure rating, anticipated flow rates, etc. Typically, a 300psi rated flange on a 100 psi rated piping system would still be considered “closed”.
 
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