6 replies to this topic

[halkeshhulk]

Why does high liquid velocity can't take off the air bubbles that gets trapped at the high points in the pipeline during the liquid flow, instead causing liquid flow gets lowered drastically.

Can someone help me to understand the how exactly trapped vapor in pipeline can reduce liquid flowrate?

Edited by halkeshhulk, 26 February 2023 - 04:57 AM.

[latexman]

Trapped gas occupies pipe flow area, increasing liquid velocity and pressure drop, thus reducing liquid flowrate..

[halkeshhulk]

Trapped gas occupies pipe flow area, increasing liquid velocity and pressure drop, thus reducing liquid flowrate..

 

Thank you for the reply sir.

 

Can I consider the trap air location as a reducer in the pipeline and pressure loss same as reducer? 

 

In case when the trapped gas quantity increases, can't the gas gets pushed off from the loop to the downstream vessel or column due to high velocity of the liquid?

[latexman]

Can I consider the trap air location as a reducer in the pipeline and pressure loss same as reducer? 

 

 

Probably not.

 

What is the exact problem/situation?  How is the air/vapor lock forming?  More details = better reply.

 

Is it a cooling water return header up on a high rack?

[halkeshhulk]

Can I consider the trap air location as a reducer in the pipeline and pressure loss same as reducer?

Probably not.

What is the exact problem/situation? How is the air/vapor lock forming? More details = better reply.

Is it a cooling water return header up on a high rack?

It's not the case at my work area,

I have gone through videos on air lock, and got the doubts.

[latexman]

Oh, okay. I just have a hard time visualizing an air lock that is represented as a lumped parameter like just a reducer. I guess I could visualize it as a reducer, a length of smaller horizontal diameter pipe, and then an expander. But it could be I just not visualizing what you are. A sketch would help.

[katmar]

As explained by Latexman, the vapor increases the liquid velocity like a reducer but there is another reason for the increase in overall pressure drop. Even if the velocity of the liquid is not enough to flush the liquid out of the pipe it will push the vapor in the direction of flow and this will generally mean that the vapor is partially forced down the down-sloping piping after the high point where the vapor is collecting. Having this static bubble of vapor means that the liquid cannot fill the cross section of the down-sloping piping and there is no (or less) pressure recovery in the liquid as it flows downwards from the high point.  So static head is consumed in lifting the liquid to the high point but is not fully recovered on the way down again.  This is known as slack flow and google will give you all the details.

 

In order to flush the vapor with the liquid you need a minimum Froude Number. Typically this is taken to be about 0.65 for "horizontal" pipelines.