Pipe sizing conundrum....

[Blumper]

Hello fella's, was wondering if anyone might take the time to confirm / refute the following....

The engineer that I am working with and I are starting a job on Monday, old three story building being converted into 4 flats, all old pipework etc being ripped out and everything going in new. The buiding has a cupboard outside the front door containing 4 new gas meters. I'm working out gas pipe runs and sizing for the installation and the first floor flat is an issue....

The only gas appliances going in each flat are Worcester Bosch 24i Greenstar combi's, all other appliances are electric. On that basis, I'm working out that I need a flow rate of 2.87m3/hour minimum, (I've not factored in any extra for possible future requirements) There is a total of 6mtr of 35mm pipe from the meter to run outside the building (inc fittings allowance) before it taps into the wall into the bedroom of the flat. From inside the bedroom, it will reduce down to 28mm and have an AECV and test point (another question there later) and it then has to run a total of 26.5mtr through the flat to the kitchen at the back. Then there is a final 2mtr run of 22mm up the wall into boiler

Using the flow rate tables, I'm calculating a drop in mbar of 0.20 for the 35mm run, 0.88 for the 28mm run & 0.1 for the 22mm......which is a total pressure drop of 1.18mbar....

However, firstly, am I right in assuming that the pressure drops are only applicable to horizontal pipe runs, in which case I can take out of the calculation 4.5mtr of the 35mm which, (without boring you with all the calculations) will give a revised total pressure drop of 1.03mbar.......

Either way, I'd be much happier if my total pressure drop was around 0.8mbar, as it is for all the other flats......so what I'm asking fella's, is in the 'real world', will this calculation prove to be precise in its figures and I have to extend the 35mm run for longer, or is it likely that the maths err on the side of caution and a calculated 1.08mbar drop is worth gambling on ?

Finally, I've got to put a test point downstream and adjacent to the AECV, anyone know if/where you can get a 28mm test point coupling ? I can get them in 15 & 22mm, but not bigger ? Its either that or a 26mtr length of neoprene hose for the AECV let by test...............

Would appreciate any comments, thanks fella's..

 

[ecowarm]

I could well be wrong here but my long hand maths have come to 34.2m of 35mm = 0.855mbar and .3m of 15mm 0.1mbar making a total pressure drop of .955mbar

Does not sound right but that's how it worked out, soon as you add 28 or 22 it goes over imo

 

[mikegas]

Just Checked it on the pipe size app an got:
6m 35mm = 0.12
26.5m 28mm = 0.53
2m 22mm = 0.10

*If bends are included

 

[Simon F]

am I right in assuming that the pressure drops are only applicable to horizontal pipe runs, in which case I can take out of the calculation 4.5mtr of the 35mm .

this is a new one to me. I thought the pipe sizing is to do with restriction due to pipe walls.
irrelevant of direction, vertical pipe going up - ok see some logic there with gas being less dense than air,
but what about vertically going down?
what are others thoughts on this?

 

[zzzjim]

Maybe we should have kept quiet about , apparent pressure change due to large height changes !


( Knowing when to ignore , which laws of physics ... for a simple answer )

 

[Howsie]

Not sure I agree with the gas height theory as there is no air for it to be relative to in a pipe...

 

[AWheating]

if your struggling to get the pressure drops correct how about 35mm into flat, aevc and then 28mm(or less) trac pipe to appliance? from what i can remember pressure loses are alot less with trac pipe. You would have to check with the manufactures data to confirm this tho.

as for test point you can use a 28 x 28 x1/2" end feed tee with bush and test nipple. (tee like BES part NO.10031) any decent plumbing merchant will get them within a few days or go to bss or pipe center.

 

[AWheating]

i would also add that when pressure drop's are likely to be near the limit its even more important to de burr/ream the ends of your pipework after cutting.

 

[stani]

Just Checked it on the pipe size app an got:
6m 35mm = 0.12
26.5m 28mm = 0.53
2m 22mm = 0.10

*If bends are included

Mikegas, how accurate do you rate the app for pipe sizing?

 

[ecowarm]

26.5m 28mm = 0.88 without an app

 

[Blumper]

It's density in relation to air is determined, as everything else is, by gravity, therefore it is always lighter than air, (0.6 relative density) whether it is trapped within a pipe or in the open air..... and as for it going vertically downwards, it would have to be subject to pressure to force it against its inclination to rise naturally......

 

[Howsie]

It's density in relation to air is determined, as everything else is, by gravity, therefore it is always lighter than air, (0.6 relative density) whether it is trapped within a pipe or in the open air..... and as for it going vertically downwards, it would have to be subject to pressure to force it against its inclination to rise naturally......

I'm still dubious because it's not relative in the pipe as there is no air in there. /scratch head.

 

[Blumper]

Think of it like this then Howsie, ..........Helium has a specific gravity of 0.13 (Air taken as 1.0)..if you fill a balloon with Helium, it floats, .......and the Helium is trapped in the balloon with no air in there with it just like gas in a pipe, so like gas, because it is lighter than air, it rises regardless.............

 

[Mike Jackson]

That analogy doesn't follow because the gas pipe isn't floating away. When you initially install the pipe if you allow some gas into it the gas will rise and displace the air. Once the pipe is full of gas the contents of the pipe all have the same relative density so it will not rise. It doesn't matter if pipework is vertical horizontal or corkscrew the resistance for a set length is the same so the pressure loss will be the same.