Pumping out of a vacuum: Will a pump do it?

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Discussion Overview

The discussion revolves around the feasibility of pumping liquid from a vacuum-sealed tank without compromising the vacuum. Participants explore various pump types and methods, considering the implications of pressure differentials and the physical properties of the liquid involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes a scenario involving a hot-liquid filled tank at approximately 100 mmHg absolute pressure and questions the possibility of using a pump to extract liquid without destroying the vacuum.
  • Another participant suggests that normally, to remove a nearly saturated fluid, one could blow it to a lower pressure volume, questioning the necessity of maintaining the vacuum.
  • A participant emphasizes the importance of the vacuum, explaining that the tank is part of a distillation process where high-boiling liquid needs to be pumped out while maintaining the vacuum.
  • Some participants propose using a second tank at a slightly lower pressure to facilitate liquid flow, while others mention the potential of using a peristaltic pump if temperatures are manageable.
  • Concerns are raised regarding the need for sufficient Net Positive Suction Head (NPSH) for various pump types, with some suggesting that a reciprocating pump might be more effective.
  • Discussion includes considerations about the pump inlet location and whether it requires lifting the liquid, with some participants suggesting that a flooded suction line could mitigate cavitation issues.
  • One participant proposes the use of a gas/vapor-rated pump or an eductor using condensed liquid as a motive jet as potential solutions.

Areas of Agreement / Disagreement

Participants express differing views on the best method to pump the liquid while maintaining the vacuum, with no consensus reached on a single effective solution. Various pump types and configurations are debated, indicating multiple competing approaches.

Contextual Notes

Participants discuss the implications of temperature and pressure on pump performance, highlighting the need for specific conditions to avoid cavitation and ensure effective pumping. The discussion remains open-ended regarding the most suitable pump type and method.

rollingstein
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I've a hot-liquid filled tank (approx. 5000 Litres) at ground level within which there's a considerably good vacuum. Approx. 100 mmHg abs pressure. Is it possible to pump out liquid from this tank using a pump (any kind) without destroying the vacuum in the tank? What kind of pump would work best?

Normally to remove liquid I (a ) either destroy the vacuum & then use a regular CFG pump or (b) connect to a second tank maintained at a lower abs pressure (i.e. better vacuum) & let the pressure differential do the sucking

A centrifugal pump doesn't work because it cavitates. The hot liq. is only about 10 C under its boiling boint (@100 mmHg) and the NPSH available isn't sufficient. That's my theory.

Would a reciprocating pump / dosing pump work better? What other options do I have? I've about 6 ft of total head available from surface of liq. till pump position. The discharge head isn't much. Approx. 10 ft & negligible pressure drop.
 
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Normally to remove this particular nearly saturated fluid you blow it to a lower pressure volume and tolerate that phase change? Is the vacuum essential, or provide an inert gas pressure for NPSH or blow it to your receiving tank.

I worked with comparably large volumes of de-oxygenated nearly saturated water by using N2 over pressure NPSH and sparging.
 
Thanks Doug! The vacuum is essential. I should explain more. e.g. In one situation the vessel in question is the reboiler of a distillation column. The entire batch is distilled under a vacuum & what's left at the end is high-boiling-liquid. It needs to be pumped out.

Breaking the vacuum & rebuilding it is what's currently done but that takes time. If I could pump out these "slops" while the column was still under a vacuum that's make the process easier.

PS. Your de-oxygenated water example was BFW? I'm just guessing.
 
LOL Well, It was going into a 'boiler', but it was a witch's brew of sludge removal stuff. A penny would disappear if left exposed to it too long.
 
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Just an idea, but a second tank at say 95 mm Hg should allow the liquid to flow to the lower pressure.
You may be able to have a greater pressure differential, but you stated the liquid was only 10 C from boiling.
If the temps are not too high a peristaltic pump might work also.
 
The delta P I was suggesting was positive from the source to the receiver.

A peri-pump requires some NPSH to re-inflate the intake volume.
 
Where are you taking liquid off of the tank? And do you have to lift the liquid through any distance (head) to raise it up to the pump inlet? Do you have a nozzle on the bottom that can feed the pump or do you need to draw the liquid up some distance to get it into the pump? Basically, if you have a line off the bottom of the tank, going directly into a pump, there shouldn't be any problem pumping it with 10 C of 'subcooling'. Your pump shouldn't cavitate. If the you don't have a flooded suction line, I can see there being issues with getting the pump started.
 
Q_Goest said:
Where are you taking liquid off of the tank?

Bottom head, centre nozzle.

Q_Goest said:
And do you have to lift the liquid through any distance (head) to raise it up to the pump inlet?

Nope. Pump inlet is always below level of liq. in tank.

I'll try it again with a CFG pump. I could always provide a short pipe-in-pipe subcooler to ensure the cooling.

But do you feel a reciprocating pump might work better?
 
johnbbahm said:
Just an idea, but a second tank at say 95 mm Hg should allow the liquid to flow to the lower pressure.

True. But that's inconvinient because it needs a second large reciever capable of taking a vacuum. And two additional vacuum build / release ops.

What'd be nice is to be able to pump from a tank under vacuum to a nearby atmospheric pressure tank.
 
  • #10
Sounds like only a gas/vapor rated pump will do. A fan, a turbine pump, most any vacuum pump. How about an eductor using condensed liquid phase as the motive jet?
 
  • #11
What's the saturation pressure of the liquid at the given temperature? And what pressure do you need to go to?
I assume CFG means "centrifugal"?
 

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