flow equation for biogas line

[LeahLee]

Hallo,
I am trying to choose proper equation to calculate pressure drops in biogas grid (50km) (CH4-0.6, CO2-0.4), pressure range is from 1 to max.20 bars, material is PE100. I thought about Spitzglass (>1psi).
Anybody can help?

 

[BigInch]

I think Spitzglass will be fine for low pressure work, while Colebrook-White, or Churchill should do well for higher pressures. I would not be too concerned with compressibility effects for pressures less than 15 bars, but be aware that you should start considering those at higher pressures.

Independent events are seldomly independent.

 

[LeahLee]

Thank you BigInch! One more question, by compressibility effects you meant compressibility factor Z or compressible flow?

 

[BigInch]

Yes, the Z factor. That's what is meant by compressible flow, considering the effects of the compressibility factor when converting mass flows to/from volumetric flows.

Independent events are seldomly independent.

 

[zdas04]

Spitzglass is a simplification of the compressible flow calculations. It doesn't really work for any flow, but will occasionally be within 25% if pressure never gets above 2 bara. The Colebrook equation defines the lines in the Moody Diagram and doesn't really have much to do with calculating pressure drops in a pipe. The Churchill equation is a convenient way to estimate Nusselt Numbers.

With a 50 km line you probably want a gas flow equation. Just a thought. You could use
[ul]
[li]Panhandle A if your Reynolds number is between 5E6 and 1E7[/li]
[li]Weymouth if your pressure never exceeds 9 barg and you can ignore the assumption that the gas has zero CO2 or H2S[/li]
[li]AGA Fully Turbulent if you want to run concrete pipe (all of the modern pipes have an effective roughness outside the fully-turbulent region of the Moody Diagram)[/li]
[li]You could use the Isothermal Gas Flow equation if you never go below 6000 Reynolds Number and you have no condensation, and downstream density never drops below 90% of upstream density, and you don't have any heat transfer to or from the environment, and no work is done on or by the fluid[/li]
[/ul]

The only one that has any chance of matching measured pressure drops is Isothermal Gas Flow, but you may have to break up the line into fairly short segments to satisfy the incompressible assumption.

I find that if I ignore compressibility I get farther and farther and farther from the calculation matching reality. Let's say that over your 50 km you drop from 20 barg to 1 barg at 30 C (which means that you had to break the line up into something like 1 km segments and solve it 50 times), the value of the compressibility of your gas increases about 4%. So to get +/-10% everything else has to be within 6%. Not a handicap I am going to give up easily. When I do this kind of problem I recalculate average pressure, friction factor, and compressibility for each step.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[LeahLee]

Actually it is possible that pressure would not exceed 9 bars, I'm trying to design possibility of providing larger line pack inside and looking and materials durability I chose 20 bars but probably i would not be that much. So Weymouth should be the most proper.
Thank you for all help!

 

[LittleInch]

When you say "biogas grid", what do you mean? If this is a multi input system or grid, then you really need to use a network analysis program or it can go horribly wrong, ditto the large pressure range if this is all in a single pipe or set of pipes as the volume expansion effects can cause you to be a long way out as my esteemed other posters point out above. There are many simple analysis programs and calcualtions around, but tend to be for short sections or where the pressure doesn't change much from one end to the other or you need to split your line into lots of sections and have difficulties in coping with a network situation. For a 50km long network or pipe you really need to get this right and doing it "manually" will take a long time and may give you signficant errors.

First off are you sure you don't have or wont have any liquids in this system as that will screw up any calcuations you have as they are all based on single phase gas.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[LeahLee]

By biogas grid I mean that only biogas will be pumped into the polyethylene pipes (no blending with NG), from few biogas plants and delivered to CHP unit. Considered length of the network isn't large, it is small area with biogas production units (agricult.) combined and sending gas to utilisation point (CHP). Only very little amount of water are possible that shouldn't have impact, because biogas will be dried before inlet point, so calculations are indeed based on single phase gas.

 

[BigInch]

Where'd the 50 km come from?
Compressibility can be ignored with little loss of accuracy, when pressures are less than 200 psig.
Do NOT use Weymouth! That's for 20" + diameter, high pressure and long length.

Independent events are seldomly independent.

 

[LeahLee]

50 km of grid including pipes with intermediate deliveries- it is a system of connected biogas production plants with CHP units. In this range of pressure (1-10bars, saying 20 bars before I take into account more the material durability than the cost-effectiveness of compressing that is lower for biogas) proper equation makes me really confused.
Thank you BigInch.

 

[LittleInch]

I think you really need to look at your range of pressures. 10 to 1 barg is a huge range for gas and will result either in a vastly oversized pipe or horrendous friction losses / lack of capacity in a small one. The most efficient way to move gas (like electricity) is the densest (highest voltage) you can get away with. It's not too difficult to compare a smaller higher pressure pipeline with the addition of compressors at the inlets and a pressure regulating skid at the end versus higher pipe costs. Higher pressure nearly always wins over any sort of decent distance if there is only one exit point.

PE pipe pressure rating already allows a large element for aging and long term effects so don't double dip and end up with thicker pipe than you need.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[LeahLee]

Well, in general operating pressure shouldn't exceed 1 bar, but the point is that I have to design in some point og the grid the individual segment of pipe when line pack flexibility will be provide, simply storage by increasing of pressure. It already works well in natural gas grids but for biogas this is new idea, but if it would work then the cost savings on building storage tanks will be huge. And for this lenght I'm trying to match a proper equations/calculations.

Thank you once more.