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Pressure Drop Across A Vacuum Line

vacuum deaerator pressure drop

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#1 Sherif Morsi

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Posted 25 August 2016 - 10:55 AM

Hi,

 

I am currently performing some pressure drop calculations from a water filtration unit to a Deaerator (operating at vacuum). The Deaerator supplies water injection pumps.

 

The pressure at the control valve is 4 barg. The fluid from the valve flows 20 meters upward till it reaches a shutdown valve installed close to the inlet nozzle on the top of the tower.

 

How do I account for the pressure for the portion of the line downstream from the control valve?

 

See attached sketch

 

Thanks

Sherif

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#2 Bobby Strain

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Posted 25 August 2016 - 07:03 PM

To avoid any complicated analysis I always assume that the pressure at the vessel entry is at the bubble point pressure of the liquid. Then calculate the pressure drop accordingly. Including the static head with the vertical section liquid filled.

 

Bobby


Edited by Bobby Strain, 25 August 2016 - 07:49 PM.


#3 latexman

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Posted 26 August 2016 - 04:42 AM

This type of pressure drop calculation being done seems strange to me because it is "backwards".  Most pressure drop calculations proceed downstream; in the direction of flow.  This one proceeds upstream; in the opposite direction of flow. I have to admit these "backwards" pressure drop calculations cause me to pause and second guess myself.  They just don't feel "normal".  I guess it's because these "backwards" calculations are infrequent.  Then, I realize that all pressure drop calculations are from a known pressure to an unknown pressure, and I get over this small, mental barrier.



#4 Sherif Morsi

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Posted 26 August 2016 - 10:54 AM

Thank Bobby, I never thought about the bubble point pressure of the liquid filling the tower. However, I think this is gonna be negligible in this specific case since it is seawater at 20 C.

 

Latexman, I thought about the same backward approach, but I think the pressure drop gradient will stabilize across the line based on the difference between the 4 bar upstream pressure and the "almost" zero pressure downstream.

 

As of last night, we decided to take the approach of a normal line going to a pressurized pressure. It's a conservative approach but we needed to since there is no much room in the system to manipulate the pressure so we had to make sure the system is still running smoothly under the worst case conditions.

 

Thank you very much. I really appreciate your feedback

 

Regards,

Sherif 






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