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Discuss Capping off redundant pipework question. in the Gas Engineers Forum area at Plumbers Forums

Messages
7
Hi all.

I done some work for someone a few years ago and I've been getting paranoid that things haven't been done the right way.

I capped off a gas cooker supply that was no longer in use, left the redundant pipework capped off at the appliance isolation valve in the kitchen.

I'm getting paranoid that if someone later down the line for some reason, was to drop the pressure in all of the pipework, say during a boiler service or for a boiler appliance shut off valve change over, such that when the pressure in all of the pipework is dropped to atmospheric pressure, this will also include the branch that went into the kitchen to feed the old gas cooker. When the engineer then goes to purge air to gas again for what he might believe is the only branch of pipework in the house (meter to boiler) that he doesn't realise there is another branch going to an old gas cooker, and then the new gas mixes with the air in the gas cooker branch and creates an explosive mixture in the pipework?

Now I know there must be loads of people all over the country who have just capped the pipework at the appliance shut off valve when removing an old appliance, and they will not have capped at the tee in the main branch since not always feasible and is it even required in a small domestic system? With an IV of less than <0.035m3

The branch of redundant pipework I'm talking about is probably no more than 0.003m3 (15mm copper of no more than 20meters run max)

My training booklet from college IGE UP 1B edition 3(small installations) doesn't say anywhere that redundant pipework should be left full of air, only that the appliance which is not commissioned must be disconnected from gas supply and sealed with appropriate fitting.

My training booklet for IGE UP 1A (commercial installations of up to IV 1m3) does state that "where a section of pipework is to be removed permanently, the pipe ends onto which it had been originally connected shall be isolated physically by spading or blanking off the ends. De-commissioned pipework shall be left full of air"
 
I'm not GSR anymore so can't return to fix the work myself, it is necessary for me to arrange for this old client of mine to get a GSR engineer to go in and cap it off where it tee's off from the main branch?
 
It’s fine wouldn’t think anything about it
 
It’s fine wouldn’t think anything about it
Hi Thanks for reply mate.

Are you saying basically because it’s such a small system and small run of pipe work that it doesn’t matter? Because I’m also paranoid that I’ve somehow managed to let air in that leg anyway. I know the installation is 100% leakage free as I remember doing my tightness tests after capping and they were fine but I don’t remember checking that the valve at the gas cooker was sound. Maybe the system dropped to atmospheric pressure without me realising while I was disconnecting the pipe work from the valve to the cooker? Pretty sure the boiler fired up first time though… it was so long ago and it was the only gas job I done outside of college after getting my acs, haven’t done any since, can’t quite remember everything I done and how I done it. Just getting anxious something could go wrong. If someone cut into that pipe and it had an air/gas mixture, wouldn’t it explode?
 
Hi Thanks for reply mate.

Are you saying basically because it’s such a small system and small run of pipe work that it doesn’t matter? Because I’m also paranoid that I’ve somehow managed to let air in that leg anyway. I know the installation is 100% leakage free as I remember doing my tightness tests after capping and they were fine but I don’t remember checking that the valve at the gas cooker was sound. Maybe the system dropped to atmospheric pressure without me realising while I was disconnecting the pipe work from the valve to the cooker? Pretty sure the boiler fired up first time though… it was so long ago and it was the only gas job I done outside of college after getting my acs, haven’t done any since, can’t quite remember everything I done and how I done it. Just getting anxious something could go wrong. If someone cut into that pipe and it had an air/gas mixture, wouldn’t it explode?

Correct think how many runs have been capped off for fires/ back boilers etc

As you purged the line when you did it to the working appliances they will run fine

No as the mixture would be off because of the room volume
 
Correct think how many runs have been capped off for fires/ back boilers etc

As you purged the line when you did it to the working appliances they will run fine

No as the mixture would be off because of the room volume

Correct think how many runs have been capped off for fires/ back boilers etc

As you purged the line when you did it to the working appliances they will run fine

No as the mixture would be off because of the room volume
Aye you’re right I’m sure there’s loads of scenarios like this where people might not have access to go all the way back to the main branch to disconnect, just thought the possibility of having any gas/air mixture in a pipe was a big no.

Sorry can you explain what you mean by the mixture will be off because of the room volume? Isn’t there a chance the mixture in the dead leg could be within 5-15% gas in air within the pipe? If the whole system was dropped to atmospheric pressure for some reason and then the gas gets turned back on? It would purge the gas/air mix through and out the boiler but wouldn’t the dead leg that tees off from the main branch and then goes off to the old gas cooker be a mixture of gas and air still?

Cheers appreciate the advice.
 
Aye you’re right I’m sure there’s loads of scenarios like this where people might not have access to go all the way back to the main branch to disconnect, just thought the possibility of having any gas/air mixture in a pipe was a big no.

Sorry can you explain what you mean by the mixture will be off because of the room volume? Isn’t there a chance the mixture in the dead leg could be within 5-15% gas in air within the pipe? If the whole system was dropped to atmospheric pressure for some reason and then the gas gets turned back on? It would purge gas/air mix through and out the boiler but wouldn’t the dead leg that tees off from the main branch and then goes off to the old gas cooker be a mixture of gas and air?

Cheers appreciate the advice.

Asoon as it’s exposed to the room volume eg cap end removed there’s not enough residue gas left in a 15mm pipe as you’ve already purged what’s in the boiler

Or the other way eg being purged via boiler eg air to gas boiler would fire once all the air until correct / % mix etc
 
Asoon as it’s exposed to the room volume eg cap end removed there’s not enough residue gas left in a 15mm pipe as you’ve already purged what’s in the boiler

Or the other way eg being purged via boiler eg air to gas boiler would fire once all the air until correct / % mix
 
Last edited:
Asoon as it’s exposed to the room volume eg cap end removed there’s not enough residue gas left in a 15mm pipe as you’ve already purged what’s in the boiler

Or the other way eg being purged via boiler eg air to gas boiler would fire once all the air until correct / % mix etc
Ok I think I see what your saying mate,

So basically when the boiler fires up then you know there’s no air in any of the pipes, not even in the dead legs that branch off from the main pipe?

I thought each leg had to be purged separately from the end of each leg
 

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