Gear pump failure against backpressure

[Liad]

Hello All,
We are dealing with a phenomenon of a very rapid drop in pump performance shortly after the system is activated.
To go deep into the details:
We have a tank with an agitator inside, there is a liquid inside(a mixture of water and oil) with level control. there is an outlet pipe going to a gear pump (Liquiflo gear pump), a pressure transmitter, and a mass flow to control the flow coming from the pump with a servo motor.
The flow going into the customer production line with a backpressure of 10 to 11 bar fluctuates the backpressure in this range.
NPSHa is higher than the required.

In some factories we can see that the pump has not reached the SP flow after 3-4 months; in other factories, it can happen after a few weeks. If we try to run the pump to an open tank (without the backpressure of the line) and in the same flow it works just fine.

I am starting to think that maybe gear pumps need to work 24/7 against high pressure is not ideal, and maybe another type of pump is the solution.
I will be happy to hear your thoughts.
Thanks
Liad

 

[LittleInch]

It is a really small pump....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[3DDave]

Yeah - were I in the OP situation and the company didn't want to pay the pump maker to analyze it the first post would be "what caused this" with photos of whatever was broken. Are the keys broken, the gear teeth worn, the side plates gouged, the coupling failed?

 

[georgeverghese]

Mixture of oil and water in the tank ? That sounds like trouble.. What is this level transmitter - level controller really ? A dp cell - if so what is it calibrated for; oil or water? The impulse lines to the LT should be remote sealed for oil-water immiscible mix; and I think it ought to be calibrated assuming water.

From your replies, it looks like level controller cascades on to flow controller (which gets its signal from mass FT) and FIC output sets the speed of the pump. If the LT impulse lines are not remote sealed, the LIC output may call for the pump to speed up when it ought to be ramping down ( and vice versa). On consequent loss of level, the gear pump would be running dry with a collapse in delivery pressure.

 

[pierreick]

Hi,
To add to this thread, are you sure your emulsion is stable (single phase) all along the transfer process? You may have some phase separation if your mixing device is not design for this purpose. This can affect a lot of physical properties thus the gear pump performance, the level measurement if Delta P.
Mouvex eccentric disk pump is a good equipment to generate stable emulsion, generally installed on a recirculation line.

Pierre