Air in gas main

[Comgas25]

Hi guys, running out of ideas for a site I'm currently working on.
In brief, got a 4 year old school with air in the gas main in the science labs. For 4 years the teachers have been purging the gas in the science labs for about 10mins before they can use the bunsun burners.
The next day they have to do the same.
There are x6 labs all with black technigas proving systems. All set and working right. All labs doing the same.
Carried out soundness test on whole site (1.1 m cubed) did not drop at all.
Sized up all gas pipe feeding labs, kitchen and plant room - all correct.
Tested each lab separate and found very small drops. Capped off gas taps (only suitable up to 25mbar ), carried out strength test on Trac pipe feeds in labs - all good.
Repaired all leaks/faults with gas taps.
Every lab now sound.
Still air in labs the next day. (Proving system off through the night).
Any ideas
Thanks in advance

 

[ShaunCorbs]

Have you had the networks out to check it’s not the supply ?

 

[Comgas25]

Have you had the networks out to check it’s not the supply ?

Not yet, but I will to at least eliminate a possible issue on their side.
Forgot to add, they have no problems with boilers/water heaters, or any problems with the kitchen appliances. Just the labs.

 

[ShaunCorbs]

Have you tried putting the science labs on a vacuum test ?

 

[Comgas25]

Have you tried putting the science labs on a vacuum test ?

Sorry mate, I've been doing industrial and commercial gas for about 20 years now, but I've never carried out a vacuum test on a gas main. How is that done? Could you do that on Trac pipe? Thanks

 

[ShaunCorbs]

Sorry mate, I've been doing industrial and commercial gas for about 20 years now, but I've never carried out a vacuum test on a gas main. How is that done? Could you do that on Trac pipe? Thanks

In theory your line is sound as you’ve done a strength test

What’s the mop ?

Instead of putting air / gas in your pulling it out so negative pressure test

 

[Comgas25]

The mop is 23mbar
What's throwing me is even if there are some small leaks which I can't trace, how is air getting back into the pipework in the labs. When the solenoid in each lab is closed over night and also all gas appliances on site are off.

 

[ShaunCorbs]

Strange one the only thing I can think of is either gas is getting out and being replaced with o2 or o2 is getting in
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@Last Plumber you had anything like this before?

 

[chris watkins]

It has got to be something to do with the black technigas proving systems hasn't it ?
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How much pipework is there after the unit to the point they are purging from ?

 

[Comgas25]

It has got to be something to do with the black technigas proving systems hasn't it ?
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How much pipework is there after the unit to the point they are purging from ?

I've tested the proving systems, which all work as per MI. The nearest gas tap on the tables is about 2 meters from the proving unit, up to about 12 meters to the furthest table. Both need about same time to purge

 

[Ch4 plumbing]

What proving unit is it?

When they shut off the gas do they open the gas taps up in the labs? Wouldn't take 10 mins to purge but would explain a lag in gas?

 

[ShaunCorbs]

Have you tested the vent isn’t passing on the unit ?

 

[Ch4 plumbing]

Have you tested the vent isn’t passing on the unit ?

Or that the gas solenoid valve is letting by?

 

[Comgas25]

The proving unit is a black teknigas Labsafe PGLS001 Gas Pressure Proving System.
I don't think they open the gas taps, it seems a strict organised school, which only the lab technician has the key to operate the proving systems and she does seem to know what she's doing.
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There is a ECV either side of the solenoid also with a test nipple. I've done a soundness/let by on the valves - all ok.
Also checked with gascoseeker around valve all ok

 

[Chuck]

Is there anything, like a furnace or blow-torch, somewhere on the site that is connected to both gas and a compressed air supply?

 

[Comgas25]

Is there anything, like a furnace or blow-torch, somewhere on the site that is connected to both gas and a compressed air supply?

No just boiler room, kitchen and science labs.
I have also checked with gascoseeker on a test nipple before the solenoid and gas concentration is at 90% straight away, so it doesn't seem to be on the main gas lines, just the labs.

 

[ShaunCorbs]

Only way forward is to valve off and test at each point and record lv then test a day later and find which ones drop and see what’s on that circuit

 

[Comgas25]

I'm back there tomorrow with another engineer. Will go through everything again just in case I've missed something or try something else. Thanks for all replies, I will keep you informed if I find something. Please let me know if anyone has anything else I could try.

 

[ShaunCorbs]

Is there bypasses to the interlocks if yes turn the valves off to the interlocks and try that

 

[Comgas25]

Is there bypasses to the interlocks if yes turn the valves off to the interlocks and try that

No bypass valve, just a solenoid with a electronic pressure switch mounted on the side of it.

 

[ShaunCorbs]

No bypass valve, just a solenoid with a electronic pressure switch mounted on the side of it.

Valve before solenoid ? If so turn that off and cap all the burner points on the benches

Also cap the vent off the proving unit

 

[Last Plumber]

Strange one the only thing I can think of is either gas is getting out and being replaced with o2 or o2 is getting in
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@Last Plumber you had anything like this before?

No. It sounds strange. As @Chuck said, my first thoughts going off what is written were, has someone connected into the gas line with compressed air? Things like that can happen.
I need to clarify something too so excuse me for asking.
when you say it has air in the Gas pipe work, is that due to you not being able to ignite the flow of gas for ten minutes? Have you used a Gasco Seeker or similar to tell you the contents of the Gas flow i.e. what percentage Gas?
Or is it that it takes ten minutes for the system to spring to life?
Could the proving equipment be faulty?
Just a few thoughts.

 

[snowhead]

Does each lab hold pressure overnight?
Have you been there first thing when they switch on and watched what they do?
Are they shutting off the supply at the end of day with burners in use, leaving them open till next morning /session?

 

[Comgas25]

Hi lads. Update from today.
Now we have repaired all the leaks, and capped off the faulty/passing gas taps, it seems to have sorted the problem and we had gas within seconds at the outlets in all the labs. I'm still not 100% this is the problem, but it was all working today.
I'm back next week to replace the faulty taps and will check again as the system will sit there with no use for a few days.
Will keep you updated next week after I've been. Hopefully it's ok as I'm all out of ideas.

 

[PETE 123]

Gas is turned off then kids play turn taps on no gas comes out but as gas is lighter than air the gas will rise in pipework and taps and fill with air ,when turned on again theres air in pipework? maybe a cause.