Guidance Regarding Liquid Holdup

[SuperG]

After conducting a search, I am afraid I could not come up with any answers, so please forgive me if this ends up being a simple question:

Regaring liquid holdup in gas pipelines, the liquid falls out and just sits in the pipeline, not moving...right?

If so, what about calculated holdup volumes, which are on an incline...won't those holdup volumes tend to move downhill, even against the flow of gas?

Thanks for the education!

 

[dbevil]

The effects of liquid holdup will be the result of a number of factors and is very complex in nature.

The ratio of liquid to gas is an important factor as well as the relative densities of the two phases, as well as the mixture velocity. Another important factor is the inclination of the pipe. Quite different phenomenom occur depending on whether the pipe is horizontal, inclined or vertical. In long pipelines in hilly terrain, severe pressure drops occur from liquid loading of all of the uphill runs under the right conditions, causing some lines to become 'liquid locked' and stop flowing.

At low velocities and large liquid ratios, the liquid can tend to fall out, separate, form slugs or any number of other scenarios (slug flow). At higher velocities, the liquid can run along the bottom of the pipe at one speed with the vapor at another (stratified flow). At higher velocities still and low liquid ratios, the liquid can form small droplets and move along at vapor velocities (mist flow). And there are a number of intermediate forms the mixture can take as well.

There are a number of technical articles and texts that define all the variables and discuss the different ramifications of the different scenarios, which would be far too numerous to discuss here.

Some references to search for are: Beggs and Brill, Dukler, Flanigan, Hughmark, Lockhart and Martinelli, Taitel, Duns and Ros, Orkiszewski, Gregory and Aziz.

 

[SuperG]

Thanks for the info...

I am using Pipephase to model a pipeline, which has liquid holdup. I am curious...at steady-state, I see a reported volume of liquid in each pipe segment, well does this mean that once that volume drops out, no more liquids will drop out until some or all is removed?

Thanks!

 

[dbevil]

You need to look at the difference between the slip and non-slip liquid. The difference is the volume of liquid that would be removed if you pigged the line. Also, if you did not include every elevation change greater than the OD of the pipe, the program will not give you an accurate liquid inventory in the pipeline (this includes the sag between pipe supports), unless the system is in mist flow regime.

 

[ggordonstewart]

The liquid hold-up you calculate is at steady state conditions. If the throughput changes, or the inlet pressure or temperature changes, then the calculated hold-up also changes. If the hold-up volume decreases between two operating conditions, the difference in volume will arrive as a slug.

G. Gordon Stewart, P.Eng.
Gas & Oil Process Engineering Consultant

http://www.ggordonstewart.com/

ggstewar@telusplanet.net