Hazardous area classification - biogas condensate lute 2

[mcecasf]

Hello all, I have a query for the collective minds.

- UK installation
- very wet biogas pipework at low pressure (~35 mbarg)
- Several condensate collection pots in the piping system
- Each of these has a lute i.e. there is no automatic or manual control of liquid discharge, simply the condensate in the lute

I'm reviewing some hazardous area classification info and am not sure how the lute discharge should be handled.
- Pressure relief valves upstream should relieve before the lute is blown by any overpressure (and there is no chance of siphoning) so really there is no gas escape route

But it just doesn't seem right to have no zone here.... as there is basically an "open pipe" to the atmosphere. On that basis I've done the maths based on IGEM/SR/25 making the assumption that the lute acts like a pressure relief vent and get a hefty zoning requirement that I would rather avoid.

Does anyone have any better ideas as to how to handle this? Or am I stuck with that outcome? Given that the liquid seal won't leak, and overpressurisation needed to blow the lute is pretty "catastrophic", it has been sugegsted to me that no zoning would apply here.... On the other hand I have gas installation contractor telling me that we simply can't have that kind of lute seal on a gas line, but a quick internet search finds multiple places selling condensate accumulators that work on that basis with internal lutes.

Any comments welcome!

 

[mcecasf]

Just reread my post and it is not the clearest ;o)

Specifically, looking for any guidance or references as to how to handle zoning of a lute type arrangement, failing that opinions and/or any info on how other sites may have done this. Don't want to be chasing shadows OR missing a problem.

 

[LittleInch]

I had to go look up what a "lute" was and you'll probably get more response with a simple sketch.

It all goes on what the potential is for the collection pot getting a leak and draining the condensate out such that the differential water column in your "lute" reduces and can be "blown out" by your 35 mbar or what ever your design pressure is. If you can make the lute / pot deep enough so that it always has more liquid column than your max gas pressure even when empty then you should be able to say it will never vent.

However if you can't do that then then you could have gas blow by. Normally this can be reduced to a lower flow by installing an orifice plate which restricts the gas flow, but doesn't significantly impact the liquid flow. I don't know what your liquid flow is through the lute, but I can't see it being too much, so a relatively small orifice somewhere on the up side of the lute to reduce debris, should be able to reduce your gas flowrate.

You will still have a hazardous are, but you should be able to get this as a zone 2 area as it will be only occasional gas and hopefully not much.


My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[mcecasf]

Thanks Littleinch, I didn't think it was that rare a term!

Hadn't considered using an additional restriction, will have a think about that

sketch attached

 

[Hoxton]

Is this landfill gas?

In any case, as it is UK, then are you aware of the Environmental Services Association?

They have some Industry Codes Of Practice dealing with the hazardaous area implications of the extraction of landfill gas and leachate liquids etc.

The principles are also useful when dealing with digester gas.

They are available free of charge at

http://www.esauk.org/reports_press_releases/esa_reports/dsear_guidance.html

If you cannot find them from the link, please go to the website and search, as they keep moving the documents.

 

[mcecasf]

Cheers Hoxton, I've got the ESA info, sadly it doesn't cover this type of arrangement.

 

[moltenmetal]

I'm not from the UK and don't know your local rules at all. Take everything I say with an appropriate measure of salt- but it may help you analyze the situation. Or it may get you into trouble. Hence, keep the salt handy...

There is typically a Div 1 zone (typically considered to be a sphere) around any flammable gas discharge for several feet. I know your pipe isn't a gas discharge per se- it can only be one if for some reason your lute empties of condensate i.e. it leaks or dries out, permitting gas to discharge from the pipe. There is no concern about dissolved gas coming out of solution because your pressure is so low, so your concern is limited to loss of the water seal in the lute.

To me, this becomes a judgment call. I'd say that the pipe itself is likely to normally contain a combustible atmosphere so the inside of the riser pipe itself will be a Div 1 area, but the absence of flow of gas normally would mean that the spherical radius around the discharge of the pipe to a Div 2 area- it will be hazardous only in a rare upset condition (one that is possible but in your case improbable). The area outside that sphere would be general purpose- or might be rendered Div 2 by virtue of proximity to the tank or piping which is the source of the biogas, which itself when leaking can obviously also generate a Div 2 area.

The biogas itself is a mixture of CO2 which is denser than air and methane which is lighter than air. Does it have a tendency to accumulate near grade? If this is an outdoor location then I would imagine there is little credible concern for this given the comparatively small density difference between 50 mol % CO2/methane (30 g/mol)and air (29 g/mol). Indoors, it might be prudent to extend the zone 2/Div 2 area for some distance within a few feet of grade.

Your own local UK standards may be less tolerant of engineering judgment, but NFPA 497 is definitely tolerant of judgment. Some people would prefer to treat it as if it were a prescriptive code, which sets rules not requiring additional judgment- but in my experience this can lead to errors in both the excessively conservative AND the blantantly unsafe direction. Engineering judgment is definitely called for in such an analysis if the result is to be meaningful.

 

[chicopee]

Before investing in expensive electrical wiring for classified areas, do some air samplings for methane and hydrogen sulfide and any other flammable vapor constituents at the discharge point to find out their concentration levels. I doubt that the results would be detectable.

 

[moltenmetal]

Chicopee, those measurements would only get you out of considering the area Div 1 (normally hazardous). They wouldn't eliminate the area from being considered Div 2- though if you can demonstrate that a release of flammables is negligibly likely under any circumstances, you can argue for general purpose.