Centrifugal Pumps on Vacuum Tower

[RotEquipEngr]

I am looking more for some helpful tips on explanation. I am working on building a training module for operators in refineries and chemical plants. I am putting together some information on centrifugal pumps and how they operate under certain conditions. I am currently working on a section about using centrifugal pumps on a vacuum tower. After talking to some operations personnel, there is a misconception about no liquid being in the suction line of a pump when the tower is under vacuum. They believe the pump has to "suck" all the air out of the line before it can pump. As we know, centrifugal pumps do not "suck"; the liquid is moved into the suction line by weight and static head pressure. I would like some ideas on communicating exactly how pumps connected to a vacuum system work. For example, is the suction line on a pump has a gauge reading of 11.5 psia, this line is under vacuum. It is hard for operations personnel to under stand how the line could be under a vacuum, and yet, liquid is still making it to the suction of the pump. I am hoping to gain more insite and a better way to communicate this detail to others.

Thanks in advance for any help.

 

[25362]

Just a thought: speaking about "absolute pressure" rather than vacuum may help. Also the introduction of Bernoulli's equation may give them an idea of different kinds of "pressure" and their interchangeability.

 

[Artisi]

Have a look in at this link - although I'm not sure if it addresses what you want.

http://www.mcnallyinstitute.com/index.html

 

[Scipio]

I think 25362 has the right idea about just speaking in terms of absolute pressure, though most operators I know just look at me strange if I mention anything like Bernoulli's equation.

You might want to prefix it with a discussion on priming pumps though, since it seems like that's where the confusion's coming from, and a centrifugal pump has to be primed before it can draw suction from a vacuum. It might also be worth discussing in terms of 'suction lift' instead of vacuum. From the pump's point of view, once it's primed it doesn't matter if the sub-atmospheric pressure at the suction flange is due to a vacuum in a closed tank or because the pump is mounted 20 feet over the surface of an atmospheric tank, and the latter is a little easier to visualize.

 

[Artisi]

Scopio's approach is spot-on.

Start your explanation with a pump pumping with 1 - 2 ft of lift on the inlet side - explain the pressure on the water surface and the coresponding pressure at the inlet (atmosphere minus the suction lift) lower the water level a bit further and go thru the explanation again - until the "suction lift" is at 34 ft and the inlet pressure to the pump is minus 34 ft (full vacuum) - then substitute the lift for operating from a vacuum vessel and go thru the same proceedure - ie 1-2 ft of vacuum and gradually increase this to full vacuum.

This should give your operators a good idea of what is happening.

 

[RotEquipEngr]

Thank you all for the valuable input! This has been a big help providing me with a way to improve my communication with guys out in the field.