Help: Water and air in inclined pipe

I looking for help with a water & gas flow problem.

water & air runs in an inclined pipe from point A to B.
At point A, P1=200psi, T1=180°F, Qwater=300gpm, Qgas=30ACFM.

Calculate Qwater (gpm), Qgas (ACFM) and Qgas (SCFM)at point 2 assuming P2=50psi and T2=175°F.

How would the calculation change if oil was used instead of water?

 

[johnp]

This is a Two-phase calculation. Computer software is available to solve the problem.

To solve it by manual/spreadsheet you have to take it step by step.

I assume that the pipe is falling, which means that the liquid is accellerating.

1 - Calculate the velocity of the fluid in the pipe using actual sectional area (with deduction for gas). Assume it is 100% full, and work out the friction loss in the usual way (use water or oil).

2 - Now go to the next point. At this head calculate the velocity of flow. That will give you a cross sectional area occupied by the fluid.

3 - Assume the pipe is 100% full and do the friction loss calculation.

4 - Calculate how much space is left in the pipe for gas, and check out its pressure and temperature at that volume.

5 - Repeat the calculation in steps until you come to the end.

I would guess that the gas will contribute very little to the overall pressure loss. But it will contribute a bit to accellerate the fluid flow.

I have seen some slick looking computer programs which do the same thing - small step by small step. You can do your calculation in 2 steps, or 10, or 100 if you like. It's the same Bernoullis equation each time, plus the Gas Laws.

The same technique is used when handling fluid with froth, or steam with condensate.

 

[johnp]

If you use oil instead of water not much will change in the liquid phase, but the vapour conditions will be quite different and affect the two-phase calculations.

Another problem you may encounter is choked flow, where the duct acts as a nozzle to limit the amount of flow which can get through. In that case you have to work from the upstream end until you reach the choked condition. Then go to the other end and work back. You won't have enough information to accuately predict what happens at the discontinuity in flow.

There is a pile of literature on Two-phase flow starting with:

G.B Wallis "One Dimensional Two-phase Flow" McGraw-Hill 1969

Some assume that the liquid and gas phases are intimately mixed and moving at the same velocity. Others assume that the liquid and gas phases are to some degree separate and moving at different velocities.

Alan Taylor datayler@ozemail.com.au has developped a computer program called FFDS which will do the two-phase calculation. I guess they could get one of their clients to run the calculation for you for about $US 500. If you are going to be a frequent user it may pay to buy a Two-Phase program.

 

[ChasBean1]

kevin, the parameters at Point 2 are: Qwater = 300 gpm and Qair = 34 acfm. Assume constant density of water, assume ideal gas for air.