Help needed for gas pipe size

[D_Hailsham]

DH , friction loss is not linear , your calcs assume it is , this is why you have two different losses for the same gas flow rate (maybe).

I did realize that it was non-linear.

23.4 metres of 22mm delivering 3.37 m3 of gas would have a pressure loss of...

23.4/15 = 1.56 mb.

Where does the 15 come from?

15 metres of 22mm tube will deliver 3.4 m3/hr of gas at a loss of 1mb at each end. It seems your chart uses 3.6 m3/hr for 15m of 22mm

Presumably you are using different tables.

your calcs are also completely different to that of the chart you posted

I have found the same error on each of my tables I had summed 1-2, 2-3, and 4-5 to give the total at the boiler. It should be 1-2, 2-4, and 4-5.

However, I did not follow the instructions in the document as there seemed to be a flaw in the logic in that it assumes the maximum permitted flow is going down the pipe. To take just the section 1-2 as an example:

Assuming 28mm pipe, effective length of 23.4m and a flow of 3.37m³.

According to the instructions, the longest 28mm pipe which can carry 3.37m³ is 30m, so the drop will be 23.4/30 = 0.78mb.

But hang on a minute, a 30m pipe can carry 4.8m³ with a 1mb drop. But we are not putting 4.8m³ down the pipe, only 3.37m³, so the loss must also be lower due to the reduced flow. I assumed it was linear and multiplied 0.78 by 3.37/4.8, giving a drop of 0.54mb.

 

[doitall]

Dave,

If the meter is A to the branch, the cooker is C and the boiler is D, you'll find it much easier.

A-B. B-C, B-D

 

[Agile]

Be aware that gas pipe sizing is often poorly understood.

A Gas Safe Inspector ( now retired ) thought that the pressure loss is measured by setting alight three burners on the hob and comparing the pressure at the meter with that at the hob!

That is totally wrong, but if a G-S Inspecter can get it so wrong what hope is there for Joe Installer?

Tony

If there is just a hob thats how I would test the pressure drop, you use 3 burners because the 4th has your manometer on it (if there is no test point)

Thats kind of OK with just a hob. But will still give a lower than actual pressure loss.

However, where this Inspector went wrong is that it should have been tested at the maximum load with both hob and boiler on at max powers!