Mystery gas

Our under stairs cupboard smells of gas. There is a "dead leg" of the old 2" iron gas main in the cupboard capped with a lump of putty. (The gas has been re-routed when the meter was moved outside and the street mains replaced with plastic pipe).
If I put a plastic bag over the pipe, it slowly fills with smelly gas (though not as smelly as neat gas). This gas does *not* burn however (I've tried).
It seems this pipe is definitely the source of the smell (rest of system recently tested and okay) but I'm puzzled as to what is coming through this pipe.
Any thoughts?
Tim
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It is probably a built up of the odorant put into the gas that is in the pipe. Natural gas has no smell, mercaptan is added so people report gas leaks.
mark
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It *could* be gas leaking from elsewhere that is tracking through the disused pipework.
A BG employee friend told me that (in the most serious cases) this is one possible route to gas explosions - a leak from the under street main, tracks through disused sewage pipes or whatever, until it finds somewhere to form a pocket.
In your case, it may be nothing. Personally, I'd get an expert opinion.
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Tim Downie wrote:

If you report it, it's likely to cost you money. If it were mine I would simply pour wet cement down it, filling it to about 150mm from the top, and then a few days later a tube of silicone sealant up to the top of the pipe. It's unlikely to be gas of any kind, and as one of the others have said, it's possibly something drifting along the pipe from somewhere else, maybe sewer gas, or a mixture of sewer odours and natural gas, or even coal gas, either way, block it up.
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Thanks, I'm inclined to agree. I'm satisfied that it's not flammable and I think the smell is down to residue in the old gas pipe. I suspect the flow through the pipe was induced by the winds yesterday (and the fact that we have an open chimney). creating a slightly negative pressure in the house.
Tim

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Phil L wrote:

Out of curiosity have you taken a bag of neat gas and tried to ignite it in the same way as a control. i.e can you easily burn a similar size sample of mains gas? I've no idea how easy this is. But I'd use a long taper just in case.

Well I'd tend to think that it is obviously a gas of one kind or another.
and as one of the others have said,

If it is sewer gas it could well be a problem.
Is the smell the same as that of the odourant in mains gas or is it more like rotten eggs?
If it mildy resembles rotten eggs then there could be some hydrogen sulphide present... often found in sewer gas. This can be a major problem.
You will smell it in low concentration 10-20 ppm but above about 90-100 ppm it dulls/kills your olafactory sensors and you no longer register it. Only a little above this level and you can lose conciousness and in the 1000 ppm region it is fatal.
Of course it may just be a dead leg from trapped mains as you say.
If it is a mains gas smell then I would still report it to Transco. It is very unlikely to cost you anything IMO and given a couple of tragic incidents over the years they do like to respond and investigate these issues.
cheers
David
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I would agree, twice when I have reported a potential leak to Transco, they have turned up very quickly and investigated. The first time, they ended up replacing the pipe into our house and it took all of one Sunday evening and they didn't leave until about 1am.
Second time they tracked it to a loose connection from the meter.
In neither case was there any charge. I think they have some sort of statuatory obligation to investigate this.
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On Tue, 22 Jan 2008 04:25:59 -0800 (PST) someone who may be Tim

If I have kept up to date the system operator does, free of charge for householders.
In the usual fashion, Transco no longer exist. If http://www.nationalgrid.com/uk/About/history/ is up to date they are partly now National Grid Gas plc. However, if the house is in "Scotland, the North East, Wales & West and the South of England" there is another organisation which does the same thing.
Here it is Scotland Gas Networks, a company held by Scotia Gas Networks http://www.scotiagasnetworks.co.uk
More work for lawyers with little or no benefit to "the real world", no-doubt.
--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
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...snip...

I seem to remember that one of the original proponents of gas, when faced with criticism because poor installations (dodgy installers using second hand gun barrels taped together if memory serves) were exploding, gave a demostration whereby he strode over to a gasometer, put a pickaxe through it and then proceeded to apply a match. A large yellow flame resulted but not the explosion expected by the observers.
Gas on its own doesn't burn dramatically, only when you mix it with air (for example leaving it to mix with the air under stairs!) so it is quite possible that a "bag full of gas" might not actually do much at all.
Paul DS
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Paul D.Smith wrote:

Which is why I connected it to a small bit of tubing and then "squirted" the gas slowly in the direction of an open flame. At slow gas speeds nothing happened. At higher speeds, it blew the flames out on my gas cooker! I'm happy it's not flammable. Air filtering through the ground and picking up residue from the stenching agent in the old iron pipework seems the likeliest explanation.
Given that the last gas leak we had in our house was *caused* by BG monkeys, I'm not in too much of a hurry to let them in again.
Tim
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On Tue, 22 Jan 2008 15:07:58 +0000 (UTC) someone who may be Gordon

I'm glad to hear it. Others haven't been so lucky.
http://www.journalonline.co.uk/news/1002177.aspx outlines the most well known case, but there have been others. The only remotely good thing in the whole episode was that Transco, as they then were, did not appeal against the fine.
--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
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Ha, I can cap that. I was working in Walsall in the mid 70s when someone digging in the road punctured a gas main which then caught fire. The flame was 3 or 4 storeys high and the fire brigade was called to damp down the closest buildings. ISTR the noise was quite impressive as well.
--
Roger Chapman

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