Pressure Drop of 3 Phase Flow in Pipeline

[e43u8]

Dear All,

Referring to the thread378-351722 (titled: 3 phase flow in pressurized pipeline) there is discrepancy between the two declarations included in mentioned thread as follows:

Post by BigInch on 13 Sep 13 14:11 "Generally the more gas you have in the pipelines, flowing, the friction loss is not so great...."

Post by zdas04 on 13 Sep 13 19:09 "In a significant gas flow scenario, you will probably have a higher dP in the line on the left."

I think it had been missed to clarify which concept is correct at that time; hence should be done now.

 

[1503-44]

the original thread was more to do with flow of 3phase fluids, however you seem to ask about only the static conditions of the question concerning the diagram. so ...

from the original thread.
Two identical pipes (same ID and length), same elevation, each of them has two phases in them. They are both static, and they are both pressurized. The gas and oil content (by volume) are not the same in each pipeline, but the gas density is the same in pipe one as pipe two and the oil density is the same in pipe one as in pipe two. The temperatures are also the same.

Summary,
Two containers are indentical in all respects.
They are both static (no motion of container or fluids within).
They have pressure.
Oil and gas volumes in each are different.
Gas and oil densities in each pipe are equal, as well as their temperature.

Is the pressure the same in each pipe (pressure is a function of the pipe surface area) or different (pipe pressure a function of the surface area of the gas)?

Answers:
Is the pressure the same in each pipe?
Yes. The pressure is the same.
From Avogadro’s law, equal volumes of gases at the same temperature and pressure contain equal number of moles or molecules. If the molar amount is doubled, the volume is doubled.

The pressure of the gas, from the ideal gas law, is P = nRT/V
Density of a substance is calculated from its mass/volume, n is the number of moles, in the ideal gas law equation, where it also appears the volume, hence, the all the relationships are directly proportional to density.
We now can see that, if density of each gas is equal, the temperature of each gas is equal, then the ratio of pressures to volume is constant, so if the volume of gas in each pipeline is different, then the mass of the gas within each must also vary accordingly, the amount of which varies directly with the volume of the gas in each pipeline, which is the amount needed to quarantee that the pressures are equal. It is much the same for the oil, except the volume of oil does not very much with pressure, so we will ignore that. It can vary with temperature, but since temperatures are the same, the oil density is the same, so the amount of oil is directly proportional to the volume of oil, which we just said isn't much to do with pressure at all. The amount of oil is irrelevant.

([Is] pressure is a function of the pipe surface area) or different (pipe pressure a function of the surface area of the gas)?

Surface areas of pipe, gas or oil have nothing to do with the pressure, except as how surface areas might define the volume of the gas. Really to determine the pressure we only care about the volume of gas, the number of moles of gas and the temperature of the gas.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[1503-44]

If you ask about the flowing pressure drop. There is no discrepancy between the two statements. Total friction loss is dependent on the friction of the liquid and the friction of the gas flowing within the pipeline. Liquids have viscosities 10 or more times those of gases, so it is obvious that if the flow of gas is 10X that of the liquid flow in the pipeline then they will have roughly an equal contribution to total friction loss. Liquids have far greater densities as well, so in everything but flat pipes, the net results (overall pressure drop in the pipeline) can also vary significantly with elevation. In some flow patterns found in 2 or 3phase flows, liquid pushes the gas, in others, gas pushes the liquids. Unless you look specifically at phase rates and changes in elevation along the pipeline, it really cannot be said which one has dominance.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[LittleInch]

Well it depend son your specifics.

The answers are couched as "in general" which is correct and "in significant gas flow".

I don't see any conflict.

Three phase flow is so dependant on the gas and liquid volume flow rate, the start and finish pressures and the topography that generalisations are next to useless.

As I said in my response the most efficient means of transport is to try for an ACTUAL gas velocity of 3-5 m/sec.

But a 10% gas volume is a lot different to a 90% gas volume.

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