Natural Gas - calculating pressure drop

[cardigan]

The pipe from my gas meter to my Worcester combi boiler is 22mm. It is 16m long and has 6 right angle bends after the meter.

Does anyone know the formula for calculating pressure fall from coming into themeter at 21mb to arriving at the boiler?

 

[Scott_D]

Just curious or do you think its undersized?

 

[cardigan]

I thought I was adequately sized but like they say 'size isn't everything.' I have a Worcester35CDi currently running off the end of my 22mm NG pipe. I was playing with the idea of replacing it with a Worcester38CDi but the British Gas salesman says it will not run off my 22mm pipe. Having formed the view that this should be an easy calculation and that someone somewhere must have produced a formula and that every CH engineer must be in possession of and using this formula everyday I am now wondering whether the formula in common use is the one where we suck our finger and hold it aloft to the wind.

 

[Reg Man]

From the figures you have given with the boiler on maximum rate at 4.12m3/h. The pipe run of this length will only deliver approximatley 3m3/h. So it looks undersize in its current state.

 

[cardigan]

The manufacturer specifies a degree of pressure not volume. Worcester state that the bolier needs NG delivered at a minimum of 16.5mb. The calculation must involve pressure arriving at the meter (MP) minus loss of pressure caused by friction in the pipe (L) The question is - how do you calculate the loss of preesure from the meter to the boiler?

 

[mfgs]

Reg Man has just worked it out for you, in terms of gas rate. This shows that BG are right and the pipework would be undersized and would need to be upgraded for the 38CDi to be fitted.

 

[Rob Sale]

Cardigan are you gas safe registered?

 

[cardigan]

Cardigan are you gas safe registered?

I am a retired mathematician and sometime DIY'er and although I would not attempt to tinker with anything involving my gas boiler I do like to understand what I am being told.

 

[Simon F]

the answer to the question you are asking is easy to find on-line, example listed :-
http://www.ukcopperboard.co.uk/lite...ion-Tips/Domestic-gas-planning-and-sizing.pdf

but there are so many other variables you need to understand.
the fact that the boiler manufacturer specifies 16.5mb does not mean that you can calculate the pipe to accommodate that drop from the meter.

 

[Scott_D]

Some info in the link.
http://www.ukcopperboard.co.uk/literature/pdfs/Installation-Tips/Domestic-gas-planning-and-sizing.pdf

 

[Rickster123]

And you're very welcome.....

 

[cardigan]

Thanks and thanks for the link to the copper pipework doc there should be enogh infor inthere for me to saiisfy my desire for a formula

 

[Scott_D]

You could post here, see if there is anyone local to quote against British Gas
[DLMURL="http://www.ukplumbersforums.co.uk/im-looking-plumber-gas-engineer/"]I'm looking for a Plumber or Gas Engineer[/DLMURL]

 

[Reg Man]

The manufacturer specifies a degree of pressure not volume. Worcester state that the bolier needs NG delivered at a minimum of 16.5mb. The calculation must involve pressure arriving at the meter (MP) minus loss of pressure caused by friction in the pipe (L) The question is - how do you calculate the loss of preesure from the meter to the boiler?

21mbar working pressure at the meter then should have 20mbar on the inlet to the boiler. BS6891

 

[chris watkins]

I am a retired mathematician and sometime DIY'er and although I would not attempt to tinker with anything involving my gas boiler I do like to understand what I am being told.

What you really need is a "man's formula" not these ready-reconer's you will be requiring Pole Formula.
Please see attachedView attachment Sizing Of Gas Pipework Poles.pdf

 

[Nostrum]

More to the point, why bother with a 38kw combi?

 

[cardigan]

Hi

Can anyone tell me the flow rate in cubic meters per hour of natural gas from the meter in an oridinary residential domestic dwelling house

TIA

 

[cardigan]

More to the point, why bother with a 38kw combi?

To better ensure that two people taking a shower at the same time (in separate showers!) both get emough hot water

 

[chris watkins]

Hi

Can anyone tell me the flow rate in cubic meters per hour of natural gas from the meter in an oridinary residential domestic dwelling house

TIA

No, cos it depends on what appliances are connected. (there is a maximum that each type of meter will pass).

 

[cardigan]

No, cos it depends on what appliances are connected. (there is a maximum that each type of meter will pass).

Thanks - can you tell me how I find out what the flow maximum flow rate I should expect to get from my meter?

 

[chris watkins]

Thanks - can you tell me how I find out what the flow maximum flow rate I should expect to get from my meter?

Look at it, Qmax = (if metric It is likely to be around 6 M3/h depending on the meter) but why ?

 

[stevetheplumber]

qmax is on the front of the meter i belive

 

[scorps]

i think he's asking for the caloric value,but i might be wrong

 

[Dotty]

Watching.

 

[cardigan]

Look at it, Qmax = (if metric It is likely to be around 6 M3/h depending on the meter) but why ?

Because I've found a formula to calculate drop in gas pressure and the formula requires the flow rate

 

[cardigan]

Watching.

That's nice to know

 

[chris watkins]

Watching.

Hello, it's the eye-in-the-sky, again !!! am I in the pooliverse? (Perishers)
(no gas seals being removed here, we are all taking care, info is freely out there, so not the sole preserve of GSR !)

 

[chris watkins]

Because I've found a formula to calculate drop in gas pressure and the formula requires the flow rate

Trick will be to find out how much gas each appliances will be using running flat out (kW input), then convert to amount of gas required to meet this. (clue = look at your gas bill for how much you get out of one one cubic Metre)

 

[vern]

21mbar working pressure at the meter then should have 20mbar on the inlet to the boiler. BS6891

On these figures 18.5 Mbar at gas valve inlet as you are allowed 1.5 Mbar, across the internal gas Pipe and isolation valve on WB boilers!

 

[TBServices]

To better ensure that two people taking a shower at the same time (in separate showers!) both get emough hot water

This will never happen with any standard combi, 38kw or otherwise.

And you shouldn't be concerning yourself with gas rates, pressures and formulas relating to such things

 

[Reg Man]

On these figures 18.5 Mbar at gas valve inlet as you are allowed 1.5 Mbar, across the internal gas Pipe and isolation valve on WB boilers!

Yes Vern thats what WB say. Thats why I said on the inlet and not at the boiler inlet test point. I put an inlet test point just before the boiler isolation valve when I feel necessary on marginal jobs, it certainly prooves the pressure drop through the boiler valves and pipework.

 

[vern]

Yes Vern thats what WB say. Thats why I said on the inlet and not at the boiler inlet test point. I put an inlet test point just before the boiler isolation valve when I feel necessary on marginal jobs, it certainly prooves the pressure drop through the boiler valves and pipework.

Yep good practice regards the test point just below appliance, had a 2 Mbar drop, from this point to gas valve inlet on an 837 combi, but some of this is the fan suction having an effect too, hence the tolerance.

 

[cardigan]

What you really need is a "man's formula" not these ready-reconer's you will be requiring Pole Formula.
Please see attachedView attachment 10554

Dr Poles formula can be found on page 10 of this doc and you will see that the root factor covers both the numerator and the denominator . [DLMURL]http://www.mech.hku.hk/bse/MEBS6000/mebs6000_1011_04_steam.pdf[/DLMURL].

And, transornation of the formula for H gives h = ( s * l * q^2 ) / ( 0.0071^2 * d^5) i.e. D to the power of 5 not D times 5

 

[cardigan]

shouldn't be concerning yourself with gas rates, pressures and formulas relating to such things

I shouldn't but you should. I expect a 'professional' to come to my house and show me a mathematical equation proving that I need to pay to put in wider pipe. I expect that someone who is charging £50-70 an hour can do that. Is that asking too much?

 

[mfgs]

I shouldn't but you should. I expect a 'professional' to come to my house and show me a mathematical equation proving that I need to pay to put in wider pipe. I expect that someone who is charging £50-70 an hour can do that. Is that asking too much?

What happened to trusting someone who clearly knows what they are talking about. If you have a problem with your car do you go down to the mechanic and make him prove his diagnostic skills in order to satisfy you that he isn't taking you for a ride? I know if I had someone ask me for a mathematical equation to prove that the gas supply needed upgrading I would tell them to stick the job up there arse. Anyone that even entertains having you as a customer is clearly a much more patient, understanding man than me. Either that or they are desperate for work.