Gas size from small to big

[dazetti]

Hi,

I have asked people in the gas industry this question more than once and no one can ever give me a straight answer or a reliable one. I know the reason why gas pipe should be installed from big to small. But what happens if say for example you connect 22mm gas pipe to 15mm, in terms of flow, pressure and volume?

Thanks

 

[ShaunCorbs]

Doesn't work you will only get the flow of a 15mm, don't mess with gas if your not qualified

 

[chris watkins]

Look up & do some reading on fluid dynamics.

Pole's formula is useful for flow of gas in pipes.

Hope that helps.

 

[kirkgas]

Doesn't work you will only get the flow of a 15mm, don't mess with gas if your not qualified

I don't agree that is wont work, done it loads of times never has a problem,

 

[ShaunCorbs]

I don't agree that is wont work, done it loads of times never has a problem,

agree it gets done but its the wrong way and you should never do it (bad practice)

 

[kirkgas]

agree it gets done but its the wrong way and you should never do it (bad practice)

But you specifically said it doesn't work, which it does, and bad practice is something that doesn't work, in theory from new I would always fit big to small, but it isn't seen as wrong to do it small to big where necessary due to constraints , bottom line operational checks give enough gas it cannot ever be deemed wrong

 

[ShaunCorbs]

But you specifically said it doesn't work, which it does, and bad practice is something that doesn't work, in theory from new I would always fit big to small, but it isn't seen as wrong to do it small to big where necessary due to constraints , bottom line operational checks give enough gas it cannot ever be deemed wrong

Unless very short it won't work as most boilers require 28mm these days,

So you would connect onto a 15mm pipe running through a house and then up it to 22mm just by the boiler no you wouldn't

The op doesn't state length so took at worst case

 

[chris watkins]

Doesn't work you will only get the flow of a 15mm, don't mess with gas if your not qualified

I am confused the gas supply to my meter is 15mm' ish (liner through old steel), my meter has 3/4" (22mm) connections & outlet pipework then increases to 28mm, a metre or so away.

Is this wrong / incorrect?

 

[Howsie]

Expanding pipe size does work with gas, in fact it's something you're taught very early on...

 

[Scott_D]

Hi,

I have asked people in the gas industry this question more than once and no one can ever give me a straight answer or a reliable one. I know the reason why gas pipe should be installed from big to small. But what happens if say for example you connect 22mm gas pipe to 15mm, in terms of flow, pressure and volume?

Thanks

What are you trying to do?
Fit a combi onto the 15mm gas supply for and older open vented boiler?

 

[oz-plumber]

I am confused the gas supply to my meter is 15mm' ish (liner through old steel), my meter has 3/4" (22mm) connections & outlet pipework then increases to 28mm, a metre or so away.

Is this wrong / incorrect?

Not if you have correct gas supply to each appliance it isn't.

But if you do, you can bet the Gas Authority will tell you that you have an undersized fitting line.

It would never be their fault for supplying an undersized supply pipe.

 

[chris watkins]

Not if you have correct gas supply to each appliance it isn't.

But if you do, you can bet the Gas Authority will tell you that you have an undersized fitting line.

It would never be their fault for supplying an undersized supply pipe.

Thanks oz but it was more a rhetorical question.

 

[Millsy 82]

I did it not long ago, house has been rendered meaning removing the meter box would be a pain so I over egged the 28mm to the boiler and the connection to the meter had about .5 mtrs of 22mm with 2 elbows. Difference between wp at meter and boiler just under .5 mbar so it does work.

 

[ShaunCorbs]

I did it not long ago, house has been rendered meaning removing the meter box would be a pain so I over egged the 28mm to the boiler and the connection to the meter had about .5 mtrs of 22mm with 2 elbows. Difference between wp at meter and boiler just under .5 mbar so it does work.

yea but thats only a short run, but what happens if it was the other way round all in 22mm except the last 0.5m in 28mm (upto a boiler you can imagine)

 

[Howsie]

Not ideal but there are calculations for what will and won't work so it can be worked out easily enough.

 

[quality]

I did it recently with 15mm from meter straight into a concrete floor for about 800mm the rest in 22mm. If it didn't work I would have dug the floor up but it was OK

 

[mfgs]

yea but thats only a short run, but what happens if it was the other way round all in 22mm except the last 0.5m in 28mm (upto a boiler you can imagine)

I think you just need to hold your hands up and admit you may be wrong on this one haha.

I have also done it where I couldn't get to about 1-2m of 15mm gas behind a newly fitted hand made kitchen. I jumped it to 28 to the boiler and the drop between the meter and boiler was fine.

 

[GrahamM]

The key here is the length of for example 15mm. A few meters of 15mm and then expanding to 28mm just before boiler will have little effect on pressure drop and as you are jumping to 28mm you may still get volume for your gas rate.
Its to do with the pressure & volume relationship of gas.
The pressure will increase in 15mm but the volume of gas will decrease. Jump up to 28mm the pressure will now decrease slightly but volume will increase.

 

[Masood]

Doesn't work you will only get the flow of a 15mm

Sorry mate, you're wrong.

don't mess with gas if your not qualified

That, I agree with 100%

 

[joni os]

I am confused the gas supply to my meter is 15mm' ish (liner through old steel), my meter has 3/4" (22mm) connections & outlet pipework then increases to 28mm, a metre or so away.

Is this wrong / incorrect?

Not quite the same case, the pressure difference across the meter will explain why it works and will certainly exceed 1 mbar. Considering the case, meter to appliance, and specifically a given volume of gas m/hr. The pressure drop depends on pipe size and length of pipework.
Adding pressure drop in large pipework to pressure drop in smaller pipework gives the same answer which ever you calculate first.However, good practice would place large bore nearest to meter especially when visual inspection of total run is not possible, otherwise calculations based on," what we see is what we have", could be incorrect.

 

[Pssst]

I can see why some struggle with this because it doesnt seem logical that you can connect a larger bore pipe to a smaller bore where the smaller bore one is the "live" end but yes it can an does work and why?

Well fluid flow is about a lot of different things. Pipe diameter, differential pressure, resistivity of pipe wall,atmospheric pressure etc.

Think about this...in a 20 storey block of flats, you test the standing pressure at the meter in flat 1 ground floor. Its 18mb.

You then have another job in flat 150 on the 19th floor. The standing pressure there is 23mb..why?

You have a meter with only a bit of 15mm cu coming off it and buried in the wall going upstairs. You want to fit a boiler upstairs but the customer doesnt want new piping/decor ripping apart. You do some sums and you realise that at the outlet of the 15mm pipe you can get the discharge rate you need for your boiler maintaining 1mb or less pressure drop. You cant run any more 15mm as the discharge rate/pressure would drop. You can though, connect on some 22mm which would maintain the discharge rate and keep drop below 1mb.