I'm working on a landfill project involving Methane. I'm evaluating whether or not we should be classifying a compressor package that compresses the landfill gas (54% methane). The compressor will be located outdoors. The compressor outlet is roughly 50psi and the receiver tanks are 200 gallon tanks. I believe, based on the fact that we have pressure relief devices, that a Class I, Division 2 (at a minimum) is justified. My recommendation is to classify as Class I, Division 2 for a radius of 15'. Does anyone know of any other codes that may cover this application?
Methane Electrical Classification
- Thread starter thelton
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- Mission Viejo, CA
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- Professional Electrical Engineer
NFPA 497 is a good reference standard although I prefer API-RP 500 for analyzing lighter-than-air gasses. Your facility is not directly in the scope of either Standard, but I believe they are both general enough that they may be applied reasonably. I know of no other more appropriate Standards.
Are there other volatiles beyond methane in the off-gas? I would suspect there is also some hydrogen-sulfide.
Are there other volatiles beyond methane in the off-gas? I would suspect there is also some hydrogen-sulfide.
You are correct, there is also hydrogen-sulfide in the gas. I'm trying to answer a question regarding why one compressor skid (for gas conditioning) is non-classified while another compressor skid was classified. I think ultimately the answer lies with the engineers for each respective supplier but I'm trying to offer a 3rd party opinion. Does API-RP 500 make distinctions regarding the distances of classified of lighter-than-air gases based on pressure and/or flow? I'm struggling to convince myself that we'll have a 25' radius of Class I, Div 2 based on such low operating pressures. I'm trying to run down a copy of the API standard meanwhile.
We do indeed have some hydrogen-sulfide in the landfill gas. Regarding the API code you mentioend, does it indicate different distances based on pressure and/or flow? I'm trying to untangle why one vendor would handle the issue of electrical classification differently than another. It may be an issue of operating pressure, relief devices, compressor seals, or something similar. Have you run across something like this before?
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
API-RP500 does make a distinction. From "Appendix D," the envelope in open air from the identified compressor source of a lighter than air gas is 25? up, 10? below, and 15? horizontal. This is regardless of pressure.thelton said:
For heavier than air gases (such as hydrogen sulfide) the envelope is more complicated to describe. The recognized pressure ?break point? is 291psi (20bar). For less than 291psi, the general dimensions for the envelope are:
1. 20? up from the compressor source,
2. from that maximum height, a horizontal radius of 20?, and all the way down to grade.
3. drop a reference point from the source down to grade.
4. from the point at grade a horizontal radius of 25?,
5. from that extended radius, up 10?,
6. from the reference point at grade a horizontal radius of 45?
7. from that extended radius, up 2?.
The other consideration for hydrogen sulfide is that it is a Group C material, methane is Group D. They also have significantly different ?T-ratings.? In an incidental release they will separate fairly quickly.
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