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

[rollingstein]

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.

 

[Doug Huffman]

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.

 

[rollingstein]

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.

 

[Doug Huffman]

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.

 

[johnbbahm]

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.

 

[Doug Huffman]

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.

 

[Q_Goest]

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.

 

[rollingstein]

Where are you taking liquid off of the tank?

Bottom head, centre nozzle.

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?

 

[rollingstein]

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.

 

[Doug Huffman]

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?

 

[Q_Goest]

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"?