Gas Meter Area

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Nick

Senior Member
OK, I have exhausted my resources and am looking for opinions. This is the scenario. I am completing a small prime power generating station with natural gas generators. There is a dispute as to the classification around the in coming gas supply metering area. I asked the question weather or not it is class 1 Div 2 or not. The plant is in operation and you can usually smell gas in the area. It is most likely blow off from the pressure regulator. The engineers are saying that the area is unclassified. The CM (who works for the engineering firm) is saying it is. The issue is the cost of adding seals to the conduits where they come out of the ground. Back when the question was first asked, by my predecessor, it would have been easy. When I asked the question the conduit was installed, concrete encased and backfilled. Spacing is not adequate for seals. When I explained the difficulty and cost to add these seals now of course nobody wants to do it. My question is would you consider this a classified area? Here is a link to a picture so you can see what I am talking about. There is a 120V motorized gas valve (the termination box faces away from us in the pic) A flow meter, temperature meter and two pressure meters all of which will operate at 24V 4-20ma. The tall cylinder in the pic is a filter element that must be periodically changed. There is bypass piping so the plant would not have to be shut down. Thanks for any input.
BTW: This is an old picture. Now the area is complete so we would be starting over to add seals at this point.
 

rcwilson

Senior Member
Location
Redmond, WA
Re: Gas Meter Area

Your pictures show a lot of flanges, valves, filters and other devices with leak points on a natural gas system. Classification could depend on the operating pressure. If it is 800 psi, the area probably should be Class 1 Div 2, maybe even Class 1 with the vents. If it is 2-5 psi, like a typical commercial gas line, any leaks should dissipate quickly enough in that outdoor installation to prevent a build up to explosive levels. If the pressure is less than 100 psi it may fall in the latter category.

I?ve put in about 35 gas fired power plants and we would classify a metering/filter installation like this as C1 D2. But in some cases where the pressure was low, the piping was in the open to dissipate the gasses and there were no arcing components in the area, we have done engineering evaluations and decided that leaving the area unclassified was acceptable and reasonable. (Our client had us put in seals and XP pressure switches anyway.)

Other things the engineer might consider is whether propane is ever used to augment the natural gas. Propane is heavier than air and will get into the grade level enclosure and conduit. Gas is lighter than air and will not get into the UG conduit ? unless that pressure transducer bursts and gas goes down the conduit. Some engineers require seals on all conduits to pressure or flow taps on gas lines to prevent a leak down the conduit.

This type of analysis is described some in NFPA 487 and API500.

In your case, the engineer may have already done this analysis and decided it is unclassified. If the owner decides it is C1 Div 2, you need to pull wire and install seals.

IS the Motor Operated Valve rated Class 1 Div 2? It probably has some arcing components in it. Is the conduit to it sealed or is it a factory selaed device?

BTW - smelling the mercaptan that is added to the gas means there is a release of gas. It may not be enough to reach close to the LEL (Lower Explosive Limit)but it indicates there is gas present now, so there may be more in the future.

So lots of information, no decision. What do you expect? I?m an engineer.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Re: Gas Meter Area

I see a lot of tools in the area. Tools (especially power tools) some times cause sparks. Did you have a hot work permit? If you needed a hot work permit than it's a good bet someone considers it a classified area.

Really, the plant drawings should determine the extent of classified areas. If they are wrong, they need to be chnaged first.
 
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