Material Groups

[nollij]

I have another real treat for some of you folks out there with an opinion or two.

Background:

API RP 500 section 5.5.5 states:
"In mixtures of manufactured gases, the mixture should be considered Group B if the gases contain more than 30 percent hydrogen by volume."

NFPA 497 section 3.3.5, the definition of Comustible Material has the definition for the Groups of materials based on their Maximum Experimental Safe Gap (MESG; section 3.3.9) and Minimum Igniting Current (MIC; section 3.3.10). The breakup for the various Groups are as follows:
Group A: Acetylene
Group B: MESG < 0.45mm or MIC <0.40
Group C: 0.45mm < MESG < 0.75mm or 0.40 < MIC < 0.80
Group D: MESG > 0.75mm or MIC > 0.80

Now, on to the dilemna. Currently, there is a fuel gas line that runs the length of the refinery that was never considered part of the original plant classifications. This fuel gas line would be Group B per the API RP 500 definition. However, per NFPA 497 calculations, the mixture would fall into the Group C category. There is enough of an impact that going "eh conservatively, its Group B" is not an acceptable answer.

So far, we have gotten jumbled responses ranging from "NFPA 497 is actually law to API RP 500 is the common accepted industry standard."

My opinion: NFPA 497 is more in line with how the equipment is constructed for the hazardous location groups. It is also a more technical approach for what would actually cause an ignition. Therefore, it may be prudent to adopt the NFPA 497 method for defining material groups. However, if this is the method used for this process stream, the rest of the refinery should maintain that consistency (ie, you don't use NFPA 70E Arc Flash calculations when convenient and then use IEEE 1584 in other places).

Thoughts? Criticisms? Opinions?

Thanks.

 

[rbalex]

First, neither one is law; they are both recommended practices - read their titles and scopes. They are however both ANSI documents and could be used for citations under FedOSHA or possibly your State OSHA under the "general duty" clause; which, by the way, is the same clause NFPA 70E is cited, for those parts that weren't specifically adopted.

API RP 500 isn't updated as often as NFPA 497, but in the next edition, it will use the same Group definitions. NFPA 497 specifically references API RP 500 as the appropriate document for refineries in Section 5.7.1, so does the NEC in Section 500.4(B) FPN 2.

Having served on the Technical Committees for both documents (and CMP14 as well) - I have no strong preference. I choose whichever I believe is appropriate for the specific application. I agree you should be consistent though - for a given application. I will acknowledge I believe RP 500, Appendix D is the best overall analytical approach.

So the big question - why are you classifying the area around the pipeline? As long as the piping is the only "source" under consideration, both documents would leave the area around a closed piping system unclassified.

 

[nollij]

So the big question - why are you classifying the area around the pipeline? As long as the piping is the only "source" under consideration, both documents would leave the area around a closed piping system unclassified.

We are using Appendix D (for the most part) and identifying point sources as follows:
Primary Sources:
Pump Seals
Compressor Seals
MOVs
Control Valves
PSVs to Atmosphere

Secondary Sources:
Manually Operate Block Valves >=8"
Process Vents
Open Process Drains
Instrument Vents or Traps
Sample Points
Heat Exchanger Heads

There are a lot of point sources for this line throughout its routing. I would also think there might be battery limit block valves or MOVs for each unit it goes into.

API RP 500 isn't updated as often as NFPA 497, but in the next edition, it will use the same Group definitions. NFPA 497 specifically references API RP 500 as the appropriate document for refineries in Section 5.7.1, so does the NEC in Section 500.4(B) FPN 2.

Are you saying that API RP 500 is going to use the same Group definitions as NFPA 497 in the next edition? Do you have any more information on this? When will the next edition come out?

 

[rbalex]

...Are you saying that API RP 500 is going to use the same Group definitions as NFPA 497 in the next edition? Do you have any more information on this? When will the next edition come out?

I'm not sure when it will be released; I haven't been on the TC since 2003. My understanding though is it should be early next year and will be a major revision. Per API regulations a Standard is usually revised every 6 years, with a possible 4 year extension, but it can be revised or reaffirmed as often as the TC deems it necessary. The Group definitions will be the same as NEC Section 500.6(A)(1) through (4).

The NEC and NFPA 497 used the same Group definitions as API RP 500 until 1997 when NFPA 497 initiated the "new" definitions. The NEC followed suit in 1999. There wasn't a need to update RP 500 for Groups alone since, for the most part, the "Division" Groups are the still same effectively. For example NFPA 497, Table 4.4.2 hasn't effectively changed with respect to "Division" rated Groups for over 30 years. The biggest change was the addition of "Zone" rated Groups.

 

[nollij]

I received a response from a manufacturer on how their equipment is tested to NRTL standards. The two responses are as follows:

The significant driver is the equipment’s ability to withstand the internal explosive pressures of the actual gas present.
Group C and D gases explode with relatively little internal pressure as compared to Group B (Hydrogen) gases.
Plus NRTLs (UL, CSA-US, FM, ETL) require that the box not only be tested for the actual explosive pressure that it will see, they require a 4 times hydro pressure test to allow for a safety margin.
For example, if we take a GUAT26 box and certify it for Groups C and D we might find that the internal pressure measured is 500 psi (measured with an internal pressure transducer). We then have to pass a witness hydro test of 2,000 psi (4 times 500 psi) to get the box certified for Groups C and D.
If we take that same GUAT26 box and fill it with hydrogen, we might find the measured internal pressures to be 700 psi. Actually, the GUAT26 would withstand the pressure. BUT, because it has to be certified to 4 times the pressure, the hydro test is done at 2,800 psi (4 times 700 psi) and the box would fail (crack) during this hydro test. Accordingly we have to thicken the walls and the product ends up being our EABT26 (which is certified for Group B because it has thicker walls than the GUAT26).
Once again, the main differentiator is the equipment’s ability to pass the 4 times actual pressure hydro test.

There are 4 characteristics of Group B gases that we account for in designing equipment suitable for the area: 1.MESG 2. MIC ratio 3. MEP(max explosion pressure) 4. AIT(auto ignition temperature)
Gases fall into gas groups A,B,C or D based on MESG and MIC ratio. MESG uses a test device that has 2 chambers separated by 2 parallel metal plates, these plates can be adjusted for different gaps between them. A gas/air mixture is introduced on both sides of apparatus and ignited on one side. MESG is the maximum gap between these two plates that will not cause ignition of the gas in the volume without the ignition source. Enclosures for gases with a small MESG number will require smaller gaps in their joints and tighter tolerances than those designed for higher MESG numbers. MEP is the peak pressure produced inside an enclosure when the test gas/air mixture is ignited inside it. This design parameter is important since the housing has to survive a hydrostatic pressure test, without permanent deformation, to a pressure 4X the MEP of the gas. Explanation above covers this. The gas groups, from A to D, contain gases with MESG values and MIC ratios that go from smallest to largest.

So... to me, neither of the standards adequately describe the necessary information to apply the equipment. They should perhaps be combined into one definition so there is an equivalency between the way the equipment is manufactured and the means to classify the area.

 

[rbalex]

So... to me, neither of the standards adequately describe the necessary information to apply the equipment.

It isn't their purpose, read their scopes. The Standards are intended to determine electrical area classification not what equipment to use. Equipment application is in the NEC's scope [500.1 & 501.1]- after you have determined the electrical area classification.

They should perhaps be combined into one definition so there is an equivalency between the way the equipment is manufactured and the means to classify the area.

Not going to happen. If you want to try, you might make a formal Proposal (a "suggestion" won't do) to NFPA to change NFPA 497, you have until sometime in May 2013 to do it. A suggestion might work with API RP 500, but you will need a sponsor from the Interindustry Advisory Group (IAG) to do it - good luck. Stephen Crimaudo [crimaudos@api.org] appears to be the current Staff Liason.

 

[rbalex]

BTW here is a link to the latest API RP 500 Draft (bottom, right hand side). See Sectio 3.1.5.4. It isn't official and hasn't been published yet.

 

[nollij]

It isn't their purpose, read their scopes. The Standards are intended to determine electrical area classification not what equipment to use. Equipment application is in the NEC's scope [500.1 & 501.1]- after you have determined the electrical area classification.

I understand that and I realize that it appears that I am approaching this backwards. However, if this decision were up to me, I would have made it. However, the people who will be making this decision want to have all relevant facts on the table. How the equipment construction relates to process material information is one of those questions. It may not seem like the correct direction to approach this issue but nonetheless it is relevant information to be provided.

Thanks for all of the information and help that you have provided (once again!).

 

[rbalex]

You may want to review Art 500 again;especially Sections 500.7 and 500.8. Section 500.7 describes various Classified Location “Protection Techniques,” and defines the Classifications they are appropriate for. Section 500.8 describes several relevant application concepts,(A) suitability, (B) the basis for equipment and installation approvals and (C) the markings to look for on “suitable” equipment.

EDIT ADD: BTW done properly, Classification ALWAYS comes first - then selection of equipment.

 

[nollij]

The API RP 500 draft has the definitions as well as retains 5.5.5. Apparently these definitions should not be contradictory towards one another. Which, is part of the confusion on our part.

 

[rbalex]

Contradictory and inconsistent are not necessarily the same thing. If either applied, it would be prudent to consider some Group B source material. However, once the point sources are identified, identify the Group B envelopes and the Group C & D envelopes independently. In open air, gaseous hydrogen will separate and dissipate quickly.