Help needed for gas pipe size

[doitall]

The 1mb is total all sections.

AS GW said if the Kw posted is outputs and the range is under estimated, you will need 35mm to the branch, although without doing the calcs it would be close if the last section to the boiler was 28mm.

So 28mm from the meter to the boiler and 22mm to the range. I may do the workings later, if I get bored.

 

[D_Hailsham]

I may be mistaken, but I always thought that the 1mb allowable drop was from the meter to the device, i.e meter to boiler, or meter to range.

That's right David.

The 1mb is total all sections.

You can't both be right!

 

[doitall]

The maximum allowable drop is 1mb for natural gas.

I think you need to understand what geewizz has posted, plus there's a slight misprint in his work.

And please don't take too much notice of Argile

 

[gaswizzard]

DH , the meter to first section could be .6mb and first tee to boiler .4mb (total 1mb) , the section from 1st tee to range could also be .4mb , total drop 1.4mb , considering both boiler and range are 'fed' from meter to 1st tee then with both aplliances operating each one only 'sees' a 1mb drop.

 

[D_Hailsham]

I used the OP's original figures (his calculations are way out) and got a drop of 1.49mb at the Range (5kW) and 2.42mb at the Boiler. So neither is within the 1mb limit.

Changing section 1 to 2 from 22mm to 28mm gives 0.58mb at the Range and 1.51mb at the Boiler. So the Range is within the 1mb limit, but not the Boiler.

Next, changing section 4-5 from 15mm to 22mm still gives 0.58mb at the Range, but the Boiler is now 0.73mB. So both are within the 1MB.

See the pic.

Untitled

• D_Hailsham
• 23 Jul 2012
• 1

 

[xr4x4]

I wonder what size pipe the supply to the meter is...!

 

[gaswizzard]

Your pressure losses are wayyyyy out DH


The other thing to bear in mind would be the boiler has an input rating of 3.3 m3/h , the range would more than likely be 15kw as opposed to 5kw.

 

[Agile]

I may be mistaken, but I always thought that the 1mb allowable drop was from the meter to the device, i.e meter to boiler, or meter to range.

That's right David.

The 1mb is total all sections.

You can't both be right!

DIA means the total addition of the pressure losses of all the sections in series to each appliance when all are operating at full power.

So all three are correct but DIA did not express it clearly.

Tony

 

[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

 

[doitall]

Calculators out

Based on the information given.

B-C is the cooker.

B-D is the boiler.

A-B load 3.76, corrected length 24m, pipe size 28mm drop 0.48.

B-C load 0.46, corrected length 4.3m, pipe size 15mm drop 0.0092.

B-D load 3.3, corrected length 8.15, pipe size 22mm, drop 0.48.

Total drop 0.97

 

[doitall]

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

Tony

BS 6891 says the size of the pipework shall be determined by the max gas rate of the appliances. And the pressure drop between the meter and the points to be connected shall not exceed 1mb, (natural gas)

 

[D_Hailsham]

Your pressure losses are wayyyyy out DH

I used the info given in Domestic Gas Pipe Sizing.

Using the data originally supplied by the OP, I calculated as follows for the existing 22mm section 1-2:

Current effective length: 23.4m; Flow rate: 3.37m³

According to Table 1 the maximum length for a 3.6m³ flow is 15M. So a 3.37m³ flow would give a drop of 3.37/3.6 = 0.936mb down 15m of 22mm pipe. But the actual length is 23.4, so the drop will be 0.936 x 23.4/15 = 1.46mb.

Another way of looking at it is as follows:

According to Table 1 a 25m run of 22mm can carry 2.6m³ with a 1mb drop. But we have a 23.4m run, so you will be able to carry 2.6 x 25/23.4 = 2.78m³ with a 1mb drop. But the actual flow is 3.37m³, so the drop will be 3.37/2.78 = 1.21mb.

Which is the correct way, and if neither, where have I gone wrong?

 

[doitall]

Not sure what you doing DH.

Assume a size pipe. max load, corrected length, div by max length in the chart.

Adjust pipe size to get the permitted drop.

You need to increase the first length to 28mm and try the calcs again.

Dave PM me on that other place if you need more info.

 

[gaswizzard]

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).

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

23.4/15 = 1.56 mb.

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 , your calcs are also completly different to that of the chart you posted (domestic gas pipe sizing).

 

[picasso]

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)