Capacity of a pipeline as a function of pressure differential.

[MrHexMesh]

Hi there,

I'm evaluating a gas transmission line where a specific capacity has been requested but the differential between the lines don't seem to be enough to achieve said capacity.

All in all, I was wondering if I could get a second pair of eyes to verify my approach. What I'm doing is using the General Flow Equation and colebrook-white friction factor (similar to using panhandle A/B or Weymouth) and solving for a delta-P as a function of Q capacity and a Inlet pressure P1. As a simple example, if I were to put a 20" lateral line in between two existing pipelines, lines 1 and 2:

Lateral Line Size: 20in
Line 1 operating pressures: (800-900 psig)
Line 2 operating pressures: (1100-1300 psig)

The worst case scenario for capacity would be when the lines are operating at pressures which give the lowest differential (line 1 @ 900 psig and line 2 @ 1100 psig).
Solving for a differential of 200 psig, I get a maximum capacity of around 230 MMscfd.

The best case scenario for capacity would be when the lines are operating at pressures which give the highest differential (line 1 @ 800 psig and line 2 @ 1300 psig).
Solving for a differential of 500 psig, I get a maximum capacity of around 380 MMscfd.

So, if it was said that 500 MMscfd is the desired capacity for this scenario I could easily say that it isn't possible. Correct? Is my approach valid?

 

[Latexman]

With the limited data you gave, yes and no. Simplistically, it gets you close, so that's the "yes" part. The "no" part is you have to model/do calcs of Line 1, Line 2, and the Lateral Line to see how the combination of the flow in the Lateral Line, more flow downstream of the tie-in in Line 1, and less flow downstream of the tie-in in Line 2 affects the "system".

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[BigInch]

As long as you are using the appropriate compressibility factors, you can use the general flow equation, calculating the average gas pressure (not the arithmatic mean), calculate viscosities, and Colebrook-White friction factor at the average gas pressure, the hydraulics will work out close. Pressure drops in each segment of pipe should be limited to 10% of the total to stay close to the averaged values. What you should find is that, at the same temperatures, capacity is higher at higher pressures, due to the increased density, but pressure drops are also higher due to the increase in viscosity.

What happens with the lateral included has quite a lot to do with where the lateral connects into the main lines, its length and diameter.

 

[bimr]

Another way to look at this is by calculating the gas velocities.

Operational velocity is typically 40-50% of the erosional velocity. In the first case, the gas velocity is 94% of the erosional velocity. In the second case, the gas velocity is 113% of the erosional velocity.

The velocities are also well above the range of 16.5-33 ft/sec for continuous operation recommended by major oil companies.

Experience has shown that the cost effective pipelines should have a pressure drop of 3.5-5.8 psi/mile unless pressure drop is of secondary importance. Because of the high compressibility in gas pipelines, as the mean pressure is increased the system pressure loss associated with flow is reduced along the pipeline because the actual gas velocity, the velocity calculated at the actual pressures and temperature within the pipe, is reduced. For a fixed mass flow, the efficiency of moving gas along a pipeline is increased (less system pressure loss) as the pipeline pressure is raised.

If there is additional gas flow in the laterals, it would make worsen the proposed situation.

No, it will not work.

 

[georgeverghese]

Assuming that your crossover flow range is correct, what happens to the line pressures on line 1 annd line 2 with the changes in flow both up and downstream of the crossover - are these line pressures still the same - how would that be the case?

 

[LittleInch]

You don't give the line length so it is difficult to work out whether the high velocity / high pressure drop is worth it or not.

My first reaction when I saw this was the same as georges - you seem to be basing this on fixed pressure sources and arrival sinks. In reality it is normally quite difficult for either to stay the sam when that amount of gas is moving. hence your actual cross flow will be lower, maybe by 50-60%.

You would seem to really need a network model to get close to reality.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]