Natural Gas - Calculate drop in pressure

Forum about flow calculators available on pipeflowcalculations.com and how to find solution for your fluid flow problem.
Cardigan
Posts: 7
Joined: Thu Jan 31, 2013 12:39 am

Natural Gas - Calculate drop in pressure

Post by Cardigan »

I can access, on this site, what looks to be an excellent tool to calculate the drop in pressure of natural gas over a length of pipe ( if only I knew how to use it! )

I have natural gas entering a domestic dwelling meter at 21mb, it goes down a 22mm (external diameter) copper pipe for 16 meters to reach the gas boiler. There are 6 elbow joints on route and no other appliances fed from the supply. What is the pressure of the gas when it reaches the boiler?

The formula asks for:

W
Q
q1
q2
P1
P2
P1-P2
V1
V2
L
D
T
p

I can see that P1 look like the starting pressure, I'd be most grateful if someone would explain the others and how I use this tool to calculate the drop in pressure.

Also I don't know the flow rate, can that be calculated from the starting pressure or does anyone know the flow rate for natural gas entering a domestic dwelling in the UK?

TIA
Cardigan
Posts: 7
Joined: Thu Jan 31, 2013 12:39 am

Re: Natural Gas - Calculate drop in pressure

Post by Cardigan »

Please - pretty please!

Come on guys, you can do it!
admin
Site Admin
Posts: 375
Joined: Mon Feb 08, 2010 7:47 pm

Re: Natural Gas - Calculate drop in pressure

Post by admin »

I suggest you not to use natural gas flow calculator as it is based on the Renauard equation which is suitable for long distance medium and high pressure gas lines.
Proper tool is compressible gas flow calculator or just pressure drop calculator.
The first one is more precize but requires many properties flowing to be known.
The other one is for non compressible flow and as pressure in domestic installation is not high and pressure drops are not higher than 2-5 % of available absolute pressure this calculator can give you very precize results.

Now, to calculate pressure in front of boiler, you must know flow rate and internal pipe diameter. The flow rate can be calculated based on the consumption and power of your boiler. So if you have 24 kW boiler, maximum flow rate is about 2.6 m3/s on standard conditions. Equation for this is like this:

Q = 3600 * P / Hd = 3600 * 24 / 33600 = 2.57 m3/h on standard conditions (t=15 C and p = 1013 mbar).
There you should add some 5-15 % for energy losses in the boiler, which makes about 2.6 m3/h.

If the power of your boiler is different that 24 kW you can calculate flow rate based on that and above calculated value. If the power of boiler is 48 kW than the flow rate is also double - 5.2 m3/h.

The internal pipe diameter is probably 18 mm if wall is 2 mm thick. This value is very important as small changes in diameter makes very big change in calculated pressure drop.

Resistance coefficient for elbow depends on the shape of elbow. If elbow radius is R = 1.5 D, than resistance coefficient is 0.378 - I get this using resistance coefficient calculator. If you have six elbows than total K = 6 * 0.378 = 2.268.

I have also used viscosity of methane as mi = 10.3 x 10-6 PaS.

Finaly resulats are like:

1. volume flow rate (q):
q : 2.6 m3/h


2. weight flow rate (w):
w : 2.0279996 kg/h

3. pipe length (L):
L : 16 m

4. pipe diameter (D):
D : 18 mm

5. pipe roughness (kr):
kr : 0.01 mm

6. density (ρ):
ρ : 0.78 kg/m3

7. kinematic viscosity (ν):
ν : 13.205129 mm2/s

8. dynamic viscosity (μ):
μ : 10.3E-006 Pas

9. K factor - minor losses coefficient (K ):
K : 2.268

10. velocity (V):
V : 2.8381555 m/s

11. cross section area (A):
A : 254.46902 mm2

12. friction coefficient (f):
f : 0.040890027

13. Reynolds number (Re):
Re : 3868.709

14. pressure on the pipe start (p1):
p1 : 1034 mbar


15. pressure on the pipe end (p2):
p2 : 1032.787 mbar


16. pressure drop (p1-p2):
p1-p2 : 1.2130803 mbar


Please note that pressure is in absolute value so for 21 mbar inlet pressure, it is 1013 + 21 = 1034 mbar absolute pressure (atmospheric + gauge).

So, the pressure drop of 1.2 mbar is very low and that value is only for 24 kW power.
Pipe flow calculations - since 2000
Cardigan
Posts: 7
Joined: Thu Jan 31, 2013 12:39 am

Re: Natural Gas - Calculate drop in pressure

Post by Cardigan »

Thank you Admin for a very helpful post I do much appreciate your help on this.

Just to make my own position clear; I am a retired Maths teacher in the UK looking at changing my central heating gas boiler from a 30kw to a 38kw boiler and getting quotes from plumbers telling me that my 22mm gas pipe needs changing to a 28mm pipe (at great expense) without providing me with any calculations or clear reasons why. The gas boiler manufacturer states that their boiler needs a minimum pressure of 17mb. The gas supplier, by law, must supply 21mb (plus/minus 10%) to the meters of all domestic dwellings in the UK. All the formulas I can find want the flow rate rather than the pressure. That how I arrived at this most excellent forum.

Sorry but I have more questions is you have time . . .

In terms of the symbols in the calculations are lower case and upper case characters used to denote different criteria or the same i.e. is P the same a p and is Q the same as q?

With regards to the below statement:

Q = 3600 * P / Hd = 3600 * 24 / 33600 = 2.57 m3/h
on standard conditions (t=15 C and p = 1013 mbar).

1. Can I know where 3600 and 33600 comes from?
2. Can I know what P and Hd is (is P is pressure in millibars how come it is shown below as power output (24 as in 24kw))?

With regards to the below statement:
Please note that pressure is in absolute value so for 21 mbar inlet pressure, it is 1013 + 21 = 1034 mbar absolute pressure (atmospheric + gauge).

Is there any simple way of explaining how 21 gets up to a massive 1034?

Thanks again
Cardigan
Posts: 7
Joined: Thu Jan 31, 2013 12:39 am

Re: Natural Gas - Calculate drop in pressure

Post by Cardigan »

Sorry about this but . . . . . Flow Rate . . .

In calculating the flow rate surely the starting point is what volume of pressure is supplied rather than what volume of pressure is required by the appliance?
admin
Site Admin
Posts: 375
Joined: Mon Feb 08, 2010 7:47 pm

Re: Natural Gas - Calculate drop in pressure

Post by admin »

Ok, let me try to clarify more:
3600 stands for conversion from sec to hours in flow rate which is in m3/h
33600 is lower heat power of natural gas. It is the value that maybe you have on your bill. The unit is kJ/m3. That represents how much energy you have in kJ from 1 m3 of gas.
P - is for Power of boiler and p is pressure.
q = Q
Flow rate only can be calculated if you know pressure on the start and on the end of pipe. Once pressure difference is established, based on that avaialable energy in terms of pressure, flow rate is set to some value. If you decrease pressure on the end of pipe, more energy will be transformed to higher flow rate. So pressure on the pipe end is as important as pressure on the start.
Calculation of flow rate is based on the difference of absolute pressures, at least in calculators on pipeflowcalculation.com. Absolute pressure means atmospheric + gauge pressure that gauge meter is showing. Gauge on your installation is only measuring how much is pressure in pipe higher than pressure of atmosphere pressure which is 1013 mbar. So to use equations in calculators, you must enter absolute pressure 1013 + 21 = 1034 mbar.
Hope this helps.

Anyway I think you are on the edge with the 22 mm pipe for 38 kW, but very probably it will be ok. But in case if you change the boiler only, you will see if pipe of 22 mm is ok. For sure, once you use full power of boiler, if pressure drop is to high, and pressure on the boiler inlet is not sufficient, boiler has its own protection system and it will simply stop.
Pipe flow calculations - since 2000
Cardigan
Posts: 7
Joined: Thu Jan 31, 2013 12:39 am

Re: Natural Gas - Calculate drop in pressure

Post by Cardigan »

Hi

Thanks once again . .

You state:

P - is the Power of boiler and p is pressure.

And the below formula includes P and not p

Q = 3600 * P / Hd = 3600 * 24 / 33600 = 2.57 m3/h

Please forgive my ignorance but can you confirm that the above formula is not using / does not need the pressure 'p' (1034) as a starting point in calculating the flow rate?


Thanks
admin
Site Admin
Posts: 375
Joined: Mon Feb 08, 2010 7:47 pm

Re: Natural Gas - Calculate drop in pressure

Post by admin »

P in that formula is power of boiler 24 kW which I used as possibile one. If the power of your boiler is different also flow rate will be.
For example - for 38 kW it is Q = 3600 * 38 / 33600 = 4.07 m3/h
Pipe flow calculations - since 2000
Cardigan
Posts: 7
Joined: Thu Jan 31, 2013 12:39 am

Re: Natural Gas - Calculate drop in pressure

Post by Cardigan »

Hi,

Thank you for your patience I do appreciate it, I do hope my ignorance is not becoming an irritant.

My requirmenets are possibly less exacting and I wonder what your views were on Poles formula which I found on the web site of the University of Hong Kong (see link here below (page 10)). I would much appreciate your opinion. I am presuming I've done something wrong because in this formula the drop in pressure increases is I increase the flow rate (surely a greater flow rate would give a lesser drop in pressure). Also the answer that comes out showing the pressure drop in millibars does not take into account atmospheric pressure (as we discussed above).

I have transformed the given formula to h = (s * l * q^2) / (0.0071^2 * d^5). I have added half a meter to the pipe length to compensate for each bend

0.58 (s) 19 (l) 36 (q^2) = 396.72
0.00005041 (.0071^2) 3200000 (d^5) = 161.312 = (h) 2.459333466

http://www.mech.hku.hk/bse/MEBS6000/meb ... _steam.pdf
Cardigan
Posts: 7
Joined: Thu Jan 31, 2013 12:39 am

Re: Natural Gas - Calculate drop in pressure

Post by Cardigan »

• q = flow (m3/h)
• d = diameter of pipe (mm)
• h = pressure drop (mbar)
• l = length of pipe (m)
• s = specific gravity of gas (density of gas / density of air)

h = ( q^2 * s * l ) / ( 0.0071^2 * d^5 )

q = 6 m3/h
d = 20mm
l = 19M
s = 0.58

The result is 2.459 i.e. the pressure drops by 2.2459 mb, which on the face of it looks fine, the problem is that if I reduce the flow rate the loss of pressure over the length of the pipe drops. Which in theory means that if I start with 21mb gas pressureat the meter and have a lower flow rate than 6m3/h I end up with a higher pressure at the end of the pipe; that can't be right can it?
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