Liquid Ring Vacuum pump

[Covboy69]

Hi!

I’ve got an odd question and I’m not sure there is a definitive correct answer but I’d appreciate your postulations!

A customer uses a Liquid ring vacuum pump to create a vacuum in a vessel we supply.

Whilst carrying out a service on the vessel recently we were asked to inspect the pump as it was “noisy, and didn’t work well” (It did pull a vacuum but was incredibly noisy)

Even though it wasn’t our equipment, to placate the customer we examined the pump and found that:

a) the vacuum inlet (suction) line and water outlet (discharge)line we plumbed in reverse.
b) The pump was rotating in the wrong direction.

After a brief chat to the manufacturers to confirm our suspicions and we switched the pump direction and the mis-plumbed lines. Upon start up the pump now no longer pulled a decent vacuum, appeared to stutter and the motor got extremely hot to a point of smoking.

We undid our changes and tested the pump and it worked as prior to the changes.

The customer admits they’ve been using the pump in its current configuration on and off for six months, so my question is;

What is likely to have happened to the ‘backward’ operating pump during these six months that’s rendered it useless when set up correctly…and can it be fixed (I’m guessing that its probably terminal) but we'd like to advise the clients...

 

[micalbrch]

I think I remember that a liquid ring vacuum pump can used as a compressor, too (like a motor can be turned into a generator). That's what perhaps happened when you changed the in-/outlet and the direction of rotation. Just a guess.

 

[JJPellin]

The liquid ring vacuum pumps that I am familiar with have vanes on the rotor that are not straight radial vanes. They have a slight backwards sweep. The impeller may also be reversed on the rotor and when you corrected everything else, the vanes were now running with a forward sweep.

Another possibility is that you read the drawings wrong. I have worked with steam turbines from many manufacturers and thought that it was a universal truth that all turbine drawings and references were based on a view looking from the governor end toward the coupling end. However, I recently found a turbine manufacturer that has always referenced everything from the coupling end looking toward the governor. So, when we hooked up our counter-clockwise turbine to our counter-clockwise pump, the pump was spinning the wrong way.

Lastly, I would wonder if some of the external piping had been modified to accommodate the backward machine. I would compare the seal water piping, and controls to the P&ID to see if they changed something else.


Johnny Pellin

 

[rmw]

Liquid rings are compressors. They compress from a deep vacuum up to atmospheric or slightly over.

The rotating liquid ring is what does the compression, so if the rotor is capable of creating the liquid rotating ring, and the gas can flow through it, it will function.

I agree with JCPellin that it may be a case of two wrongs making a right if there was a wrong in the first place. I'd lean to a reference frame problem. It sounds like it was right (they will be noisy during certain operating conditions even when rotating in the right direction and plumbed correctly) and when you plumbed it to "right" it was wrong and pumping too much liquid, overloading the motor.

Keep investigating. Something out of sorts here.

rmw

 

[JohnGP]

Something indeed a bit odd. Are there any detailed manuals from the pump supplier to perform a check against?

Our liquid rings are two stage and I don't imagine that they would provide decent vacuum performance if plumbed and operated in reverse. According to their (generic) manual, such pumps can work as compressors, but in your case may have been overloaded due to the induced differential pressure.

"Noisy" can be due to cavitation. Lack of performance can be due to solids buildup within the vanes. Also incorrect seal water flow - either side of the ideal flow can see a drop off in performance. Interesting problem.

Cheers,
John

 

[Covboy69]

Many thanks for the replies. Heres some more info with regards to some of the queries posed;

The manufacturer’s manual is a fairly poor affair and not much useful info is obtained when chatting to their engineers either.

The pump casing has the 'function' of each port cast into the back of the casing (quite tricky to spot unless you're looking for them) so we know what should go in or come out of each port.

We checked the rotation of the motor (after our adjustments) which was rotating in the correct direction (In the direction of another directional arrow cast into the casing)

Have to admit, one thing we didn’t measure directly was the seal water flow. Other (cooling related) parts of the unit were seeing water flows of 6l/min and the unit needs about 4 to operate at its highest vacuum level so we assumed the mains water was sufficient to deliver the required rate but we did throttle it back to see if it made any difference.

The piping is a simple affair so no odd things could have been done; inlet water pipe straight from mains, vacuum line straight from vessel and water/air outlet pipe straight to drain.

We also added a vacuum breaker valve / bleed valve to the unit. (A suggestion from a totally different manufacturer whom we contacted in desperation) to try and reduce the noise.

 

[zdas04]

A vacuum breaker sounds like an odd bit of kit to put on a vacuum pump. This whole thinks seems terribly odd.

David

 

[ash9144]

Sounds very odd.
I once worked with liq ring vac pump installation that was piped backwards as well. It discharged all of the seal water back into the process and pressured up the process. It did not pull vacuum on the process but tried to pull vac on the vent system (normal pressure discharge vent). Perhaps all of your piping is reversed.

 

[rmw]

The vacuum breaker thing is a way to create a little flow through the pump. If there is no flow, and the seal liquid is water, the pump will create its own flow from the seal water.

But it is a band-aid in this case.

rmw

 

[chief]

It helps to have a wafer check valve on the suction and discharge side of your vacuum pump, prevents recycling the air and overheating, in the case of a worn pump. The tolerances on vacuum pipes are very tight, 4 thou ins if i recall.
The principle is quite simple, a water ring given a reciprocating action in an elliptical shaped housing.
So the 2 critical items are the water seal and rotor vertical clearance. Things to check, Rotation speed correct, seal water supply to spec.

Offshore Engineering&Design

 

[JJPellin]

An excellent point is brought up by the comments from chief. When you reversed the piping did you make sure to reverse all of the check valves? A liquid ring machine of this sort may have multiple check valves in both the inlet and outlet lines. If any check was left in place in the wrong orientation, that would explain why it didn't work.

Johnny Pellin

 

[Covboy69]

Yes. The two lines are straight forward affairs with no check valves or anything in line...

 

[Ironhead4248]

Covboy69,

I am wondering if you have solved your dilemma with your customer's LRVP?

If not, can you provide me with the make and model of the unit in question and perhaps I can help. These are very simple devices and when installed/operated correctly they are very reliable units.

JAK

 

[TheDruid]

Hi folks,

another thing: the noise is probably from cavitation, which is dependent on the temperatures in the pump.

What temperature is your "vacuum" and what temperature is your seal water ?

Important is, that the temperature of your seal water is as low as possible, which helps in volume flow and decreases cavitation.

 

[MortenA]

Old tread - but anyway: How could a LRP cavitate? The pump is not filled with a liquid, the liquid acts as a seal and to compressed together with an off-center impeller.

Best regards Morten

 

[TheDruid]

Hi,

the LRP i encountered (Gardner Denver aka Nash aka Elmo) do have liquid inside the casing, because it acts as the "piston" pumping the air.

Cavitation can occur, if the temperature of the air is much higher than the temperature of the liquid, in which case some local cavitation takes place.

best wishes

Alex. Slanina

 

[rmw]

LRVP's do have a liquid piston in them, and the motor (driver) does work on that fluid and adds heat to it and if it isn't constantly cooled through an external cooler or continuously replaced with new cooler fluid, it will boil. It is referred to as cavitation, but it is really the boiling of the seal fluid, IF that is, the vapor pressure of the fluid is in the vicinity of the operating pressure of the pump which in the case of the OP as it was water, it will boil at low temperatures associated with the vacuum in the vessel.

rmw