Electrical Area Classification of natural gas piping

[HiLevel]

I have a gas rack located a distance away from an enclosed duct burner. It's a typical gas rack with <5 psig natural gas. The design engineer has classified the area around the gas rack as Class I Group D Division 2. And, the result is a big change order and delay from the gas rack manufacturer. There are no other sources of flammable gas in the area, only the natural gas in the pipe. The design engineer is looking at codes like NFPA 497 and API 500 figure 104 to justify the classification. Their position is that the gas rack contains numerous threaded connections and devices such as SSOVs, and instruments that make it a high risk area. However, if this is the case, why don't the burner manufacturers design their systems to meet this classification? I know that NFPA and API both say you don't have to classify an area around a flame; duuuh. But, I don't have this typical condition to fall back on this time. I also know that the codes API 500 5.4.2 and others allude to lighter than air gases (natural gas) being less likely to accumulate; however, they don't go to the point of recommending classifications.
Why don't the burner and gas rack manufacturers build their systems to meet this classification if these standards are truly applicable.
Does anyone know of a reference? I also looked at AGA and they don't seem to get into electrical classification at all.
Thanks ahead of time!
Bill
If you think I'm in the wrong forum with this please let me know!

 

[stookeyfpe]

We discussed a similar issue a few weeks ago. See the attached thread:

http://www.eng-tips.com/viewthread.cfm?qid=342551

 

[chicopee]

Did the design engineer specify the radius of the classified area? Also you may want to discuss this situation with the design engineer. NFPA's are great sources of references but one thing that seems to be overlooked are the scopes of these codes.

 

[HiLevel]

Thanks for the reference to the similar issue; however, it was for a pipe passing through another building. I'm talking about a burner gas rack that has your typical valving and screwed connections. And, by the way, the NG pressure is only 5psig; I see no allowance mentioned for gas pressure until you get into compressor station ranges above 275psig.

The engineer has set a 15' radius around the gas rack as hazardous Div 2. He got this from NFAP 497 I believe. The figure he used from NFPA 497 was for flammable vapors heavier than air and up to 200 psig or something like that. A 2" radius around the gas valves and flanges would be a problem because the additional cost is in the SSOVs and instruments rated for the Div 2 classified area. So, the source itself is where the explosionproof requirements will be applied. It is telling to me that to obtain gas controls rated for a Class I Div 2 area they are non-standard. I know when I got into this issue several years ago, you couldn't even buy a SSOV valve rated for a hazardous Class I Div 2 area. At least now they do offer this option.

 

[stookeyfpe]

HiLevel:

A literal interpretation of NFPA 497 for the handling of Class I flammable liquids would require you and to classify the the bathrooms of our homes as Class I Group D, Division 2 because it could generate flammable vapor if I knocked over a bottle of isopropanol. It doesn't happen because the source is small, in-home ventilation dilutes the vapors and the loss history doesn't substantiate it.

Applying the engineer's logic, every home in the US and Canada would have a stove requiring a Class I, Group D, Division 2 valve on it because a fuel gas is piped into the dwelling and routed through a corrugated stainless steel tubing. Again, not realistic and its not applied in any dwellings or commercial businesses.

I suggest you read NFPA 497, Chapter 5 before specifying HAZLOC equipment that is not warranted. My problem with your scenario is that engineer calling out the HAZLOC area fails to understand the system is operating at < 5 PSIG and is equipped with a pressure regulator. Generally speaking, low pressure regulators fail close if a diaphragm or other mechanism suffers failure, stopping the flow of gas. The ignition system is most likely piloted. Finally, I'll bet the gas is odorized.

 

[HiLevel]

Stookeyfpe,
I was a little worried about where you were headed with that flammable gas in the bathroom analogy.

I agree with all you said, and I've poured over NFPA 497 Chapter 5. However, I have never found a standard specific to natural gas piping typical of low pressure burner gas racks. Without this, it becomes a matter of interpretation, and therefore the more stringent interpretation always wins. The alternative would be to do a detailed analysis of leak rates and dispersion, and the engineering required to perform this would cost as much as nonincendive equipment in the gas rack; and even then engineering judgement will still come into play.

I love the figures in NFPA 497 and API 500 because they're definitive and simple. However, the only figure I've found that could be stretched to relate to this is in API 500 Figure 104 and it's for pressures up to 275psig. So, applicability to low pressure burner gas racks is probably over kill.

I can't help but believe that with all the burner controls manufacturers out there someone wrote a standard for the electrical requirments of these controls at some point.

 

[stookeyfpe]

Hilevel

I did review NFPA 54, National Fuel Gas Code, and unfortunately it is silent on the issue. However, I did find that NFPA 54 is not referenced in NEC 500.4 (B) which one could interpret to mean that HAZLOC is not required for fuel gas systems.

 

[HiLevel]

Thanks Stookeyfpe,
I found the following reference to NFPA 54 in an ancient thread on the EE forum. I don't know that you can call a burner a gas mixing machine.. maybe.

7.12.5.2 Electrical Requirements. Where gas-mixing machines are installed in well-ventilated areas, the type of electrical equipment shall be in accordance with NFPA 70, National Electrical Code, for general service conditions unless other hazards in the area prevail. Where gas-mixing machines are installed in small detached buildings or cutoff rooms, the electrical equipment and wiring shall be installed in accordance with NFPA 70 for hazardous locations (Articles 500 and 501, Class I, Division 2).

I also see references to AGA XF 00277 the AGA electrical area classifiation standard. Sounds like it provides guidance on typical leak rates and dispersion modeling. My only concern with this is that it's probably written around gas compressors and high pressure natural gas situations. I find nothing specific to low pressure gas racks anywhere. I'll probably buy it just to see. Has anyone out there used it?

The British have a standard EN 60079-10 that may apply. While I'm reluctant to use a British standard to establish "good engineering practices" here in the US, I've found in the past that the European's are ahead of us in the area of hazardous area electrical.

I may try to copy this thread to the EE forum. Not sure how to do that. It looks like most of these type discussions have occured there.

Thanks again for you insights

 

[HiLevel]

Stookey fpe,
I posted this same toping under Electrical motors and controls issues.
Maybe we'll get some of their experience?
Thanks.

 

[DRWeig]

Here's a link to the new thread:

http://eng-tips.com/viewthread.cfm?qid=350691

Best to you,

Goober Dave

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[HiLevel]

Thanks guys,
I should have used a link.... didn't know about that!
Bill

 

[DRWeig]

No prob, Bill -- this is an interesting topic!

Best to you,

Goober Dave

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