Extent of Class 1 Div 2 area for 550 PSIG Natural Gas

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I stumbled on to this forum doing some internet searching on the subject. I was hoping that someone could help enlighten me on this subject.

My application is a combined cycle power plant in California. I have natural gas that is 500-550 psig as the fuel source. I have been tasked to generate the electrical classification drawings. I have used 25 feet out of API RP500 Figures 104 and 105 to justify my areas around leak sources. This is having a dramatic impact on our current layout. Does anyone have experience with high pressure (>500 psig) in California? What was your design basis (code, recommended practice, etc.) and what government agency reviews this information?

Any help you can provide would be greatly appreciated.
 

rbalex

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API-RP500 (RP500) is one of the two ?general purpose? documents for electrical area classification for Division classified locations. NFPA 497 is the other general document and covers both Divisions and Zones; however, because of the capacities involved with your application, it would generally re-direct a user to RP500.

I am just becoming familiar with another document, American Gas Association AGA XF0277. Its stated scope is ??gas utility areas?which flammable gasses are produced, manufactured, stored, transmitted and distributed.?

In other words, RP500 is the appropriate document.

There is a great temptation to simply look at the figures in RP500. They need to be applied consistent with the appropriate text. In the case of Figures 104 and 105, Section 14.3.8. Basically, the figures apply to the compressors or gas handling equipment.

Personally, I prefer RP500, Appendix D for a bit more analytical approach. It is time consuming. Developing a proper electrical area classification document for your application could take some time, but the end result will result in a more ?compact? electrical area classification.
 
rbalex

First, thank you very much for your prompt and informative reply. I had looked at Appendix D sometime ago, but was confused how to implement Figure D-4. I assumed with my Category 1 (Methane) volatily classification in combination with my high pressure (>500 PSIG) that I would have a large hazard radius (possibly 30ft, since Fig 104 and 105 indicated 25ft for pressures in excess of 275 PSIG). I could not figure out what hazard radius to use from Fig D-4 and why. Can you explain? D.6.2 indicates a radius of 15 ft for compressors, but does not quantify pressures. Does D.6.2 infer that the largest radius for natural gas applications should be 15ft?

I appreciate your patience and help.

Best regards,

Patrick
 

rbalex

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Section D.6.2 does indeed infer the largest hazard radius is 15 feet for compressors ? regardless of the pressure for lighter than air materials.

Note though that ?hazard radius? has special meaning in Appendix D. It probably would have been better to call it the ?hazard radius index? rather than ?hazard radius? since it is more the notation to tell you which set of radii to use.

Finding the other hazard radii for other lighter than air applications is generally a matter of determining which other Section applies similarly for heavier than air materials. For example, it would be reasonable to apply a 10 foot hazard radius for valves and flanges to Figure D-4, from Section D.5.4 and Table D-4. Note methane is typically classified as ?Category G.?

I was an author/editor for the last three Editions of RP500. I think I?ll suggest revising the term in the next Edition.
 
Relocating or replacing improperly classified equipment.

I am a bit confused why Figures 104 and 105 clearly indicate pressures in excess of 275 psig warrant a 25 radius for the classified area, while Appendix D only requires 15 feet. Appendix D does not qualify what pressures they are addressing. Also Figure D-4 even indicates larger hazardous areas in some instances, but does not give clear diretion on how to find the Hazard radius.

What are others experiences? When a code official reviews your hazardous area drawings, what standards, recommended practices, etc. are they using?

Your help and opinions are greatly appreciated.
 

rbalex

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With regard to Appendix D, you must look at the appropriate Sections, Tables, Diagrams, Figures, Etc. For example Table D-1 lists appropriate pressures and flow rates as they apply to pumps. Section D.5.5 makes a simple application statement about compressors by type/design with no regard for flow rates and a “break-point” at 20 bar and shaft diameter of 2” or less.

You need to be willing to take the time to analyze each application. Proper determination and documentation of classified locations is a time consuming process.

Truthfully, in nearly 40 years of doing this, I have never seen a municipal agency that has the requisite skill (or time) to check electrical area classification documentation that doesn’t come directly from application of Articles 510 to 516. This was one of the reasons a Licensed Professional Engineer was originally required to make the assessment for Art 505 applications since there was no US Standard at the time.

The NEC recognizes API RP500 as a authoritative Standard. See Section 500.4(B) and FPN No. 1&2.
 

natfuelbill

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Just a point. Section D5 is for heavier than air gases, section D6 is for lighter than air gases.
 
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hazloc2002

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Brazil
Area classification is a process for evaluating the risk of an explosion due to ignition by electrical devices. The key point is: if a flammable gas leaks, until what distance it will reach its lower explosion limit? So, it becomes clear that beyond this limit there won't be any danger of ignition.
To develop such study, you need to build a model. This will be only a mathematical model. To develop one, you need to know: some equations of gases behavior; the amount of gas that you guess that can leak; the gas pressure; the natural ventilation influence.
The first point is to define the diameter of the hole where the gas will leak (this is required by equations). All this process is not completely given in API RP-500/505 standards, because it is impossible to establish the "perfect" model, or a unique model that fits all needs.
So, API standards give for professionals some "illustrated clues". The figures help us to see what is expected to happen, not what will effectively happen in our installation.
API RP-500/505 clarifies that. As an example, see fig 26 at API RP505: there is a note saying that "Distances given are for typical refinery installations, they must be used with judgement, with consideration given to all factors discussed in the text. In some instances, greater or lesser distances may be justified."
If you take a carefully look at the figure, some data are not given, as the gas pressure, the air velocity, etc. so, how those distances in the figure were defined?
Or, you can ask: What does the expression "typical refinery" mean?
So, I recommend that you create a team of professionals to work in developing the mathematical model of your installation - see Appendix E1 Note on API-RP-505 (area classification should involve process engineers, fire specialists, electrical engineers, etc. It is not a one man band task). This will answer all your doubts about impacts on layouts.
 

rbalex

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Actually D5.6 is heavier than air also. Section 6 deals with lighter than air gasses.

The point is Section 6 does not deal directly with too many specific classes of equipment except compressors. However determining appropriate hazard radii for other classes may be found using similar heavier than air applications within the Appendix.
 
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