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.

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.  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...

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.

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.

MORE NEWS