Liquid Pump Problems

Hi Guys

I was working on a system that uses Viking liguid pumps ( LL 480 ) on one of thier LPRs and Cornell on the second one. The vessels have common suction and liquid make up. These vessels supply -30 F liquid to several plate freezers, an ice maker and a cold storage. The Cornell pumps have no issues at all, however the Viking pumps do.
When I arrived on site the complaint was, the Vikings had lower than normal pressure ( 20 psi rather than 40 psi ) and pressure would be lost when suction hit 3 psi. I found many of the hand expansion valves open too far so I adjusted them all. Set up capacity controls on compressors so the suction pulled down slowly. Checked the pump regulator, no issues there. Adjusted both pump clearances and got the pressure back up to 45 psi, but the pressure would still be lost at 3 psi. Pulled the pumps from the housing to check for oil but there wasn't any. The pumps are quite noisy so I suggested complete rebuid or replace. After 2 days the clearance needed to be adjusted again to keep the pressure up so I assumed the pumps were grinding themselves to pieces due to worn internal parts. The customer opted for the rebuild. New parts installed and pumps still loosing pressure when suction reaches 3 psi. Any thoughts or suggestions?

Thanks
CanadianIce

When you mention suction pressure is that the compressor suction or pump suction?

Second question: how long has this been ocurring on the Viking pumps?

I assume these are centrifugal pumps. The lowest pressure in the system is at the eye of the impellor.

If that pressure drops below saturation pressure/temperature the liquid flashes, forming bubbles.

As these bubbles move outward in the impellor, where the pressure rapidly increases, they collapse (implode) making a noise that sounds like marbles moving through the pump. This is called cavitation... and it can totally destroy the impellor.

The pressure entering the pump must be kept high enough to avoid cavitation... and/or the pump discharge must be restricted enough to raise the pressure at the eye of the impellor to avoid cavitation .

I thought they would be gear pumps as I too have had problems with Vicking pumps but not the problems your describing

Viking pumps are gear pumps, not centrifugal.

Quote:

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Originally Posted by US Iceman

Viking pumps are gear pumps, not centrifugal.

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I stand corrected then. The problem could still be cavitation, although this is far less likely in a gear pump.

I'm sure the manufacturers literature would mention cavitation and how to avoid it.

Cavitation is the Achille's Heel of liquid pumps.

Thanks for the quick replies.

The suction pressure I am referring to is compressor suction. This problem has been going on for about a month now.
Some more information for you. The suction head on these pumps is 6 ft. They pull the liquid off of a 8" liquid header that is fed from the drop leg of the vessel.

CanadianIce

What did you find wrong with pump before rebuild ?
Is the pump still noisy after rebuild ?
Did you find any traces of oil in pump or pipewok?

If pump looses pressure at 3 psi , can you unload compressor and see if pump stablizes correct pressure again .
Has the relief valve on pump been checked out , can you adjust ?
Is liquid makeup solonoid on or off when cavitation occurs ?
If manual expansion valve is open to far it can cause sudden pressure drop and cavitation when it closes .

Hi Ranger1

1. When the pump was opened, there was significant wear on the rotor carbon bushing and the idler carbor bushing, which in tern causes the contact surfaces of the rotor and idler to wear. The divider on the back plate also had signs of wear.

2. All of these parts were replaced plus the seal and bearing and the pump is still noisy.

3. The only oil was on the surfaces, no puddling.

4. When the pump looses pressure it will only pick it back up if the compressors are unloaded and the suction rises a couple of psi.

5. These pumps use BPRs to relieve excess pressure and they are working properly.

6. The liquid make up solenoid can be on or off when cavitation occurs. The hand expansion after the solenoid is throttled down so there is no significant pressure rise in the vessel during liquid make up, to avoid the sudden decrease in pressure when the solenoid closes. The pump pressure can drop even when the vessel hasn't called for in awhile. It seems to be all about the suction pressure and the internals of the pump. I can't see any problems with the system set up.

CanadianIce

CanadianIce ,
Does pump have suction strainer ? if so take it out .
Maybe its worth double checking end clearance again as your first post stated ot changed after your first adjustment .
When pump cavitates are any liquid feed solonoids open ?
Does pump have any type of vent line except bypass ?
Bypass line or BPR should be set up for minimum flow when no solonoids are feeding to avoid gas building up in pump .

You may also need to confirm" BPR "as it should be a pressure differential control valve that controls amount of pump bypass .
Otherwise suction pressure will influence everything .
Another test would be to jack open "BPR" manually and throttle stop valve to see if it makes any difference to cavitation .

Hi Ranger1

1. No suction strainer.

2. Clearance has been checked as soon as I noticed the problem still existed.

3. When the cavitation occurs, there could be one or both make solenoids open or closed on vessels. System feed solenoids only close for defrost in the storage ( Checked hot gas flow during defrost to make sure it wasn't loading the vessel ) The plates manually fed by opening a shutoof valve to feed liquid through a pre set hand expansion valve that have all been recently set by me. ( 1- 1.5 turns open)

4. Pump has a vent on the suction for pump out and to release vapour during cool down. Checked this valve to make sure it is holding, also tried running with it cracked open but this valve is normally closed when pump is running.

5. The BPR has been tested for proper operation and I have jacked it open and operated with the valve in the line throttled but still have the same problem.

Keep in mind that both pumps on this vessel are acting exactly the same way. Each pump has its own bypass and seperate piping. The system has been in operation for 25 years with no major issues regarding these pumps. :confused:

Thanks
CanadianIce

Hi Canadian Ice,
I am not familiar with Viking gear pumps, but by your description and wear problems, lack of pressure etc., certainly sounds like cavitation, a restriction before the pump creating the vortex and cavitaion. Perhaps the isolation vav before suction , or a frozen mixture of oil and gunge in the drop leg. Seems odd on a 25 year old system that it starts doing weird things. Are the cornell pumps off same drop leg? OK stupid question.. is pump rotation the problem.

CanadianIce what end clearance are you setting pump to ?
I would have guessed 2-3 thou .

Have you raised operating level of vessel to see if any difference .

When an apprentice a few years ago changed out a pump like yours to a semihermetic centrifugal pump on an ice tank .
Had to drain 600 litres of oil out before we could get it going .

Your pump probably would't like very cold oil building up in suction header .

If its common to both pumps surely it has to be oil in pump suction or level control column, liquid level issue or end clearance .

If pump going wrong way on positive diplacement pump it will never work , centrifugal maybe .

Don't forget us when you get to the bottom of it , good luck .