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

 

[hydtools]

Maybe you need to change brands. What does Equiflow think the problem is?
Maybe the problem is with the means of flow control and not the pump.
Ted

 

[EdStainless]

Have you tested problem pumps without your control system?
Pumping against a fixed restriction that will result in 11-12bar pressure for example?
This really sounds like a systems issue.
What pressure is the internal relief set at?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Liad]

We run the system for different flow rates, it works fine until we set backpressure of more than 5 or 6 bar, in that pressure the flow goes to zero.

 

[LittleInch]

It sounds like the pressure relief system either inside the pump or external may be at fault?

Can you post some pump details and a P &ID?

What is the fluid?

What sort of flow range are you looking at?

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

 

[TugboatEng]

Is the gear pump wearing out? What do the rotors look like?

If the pump is in mechanically good condition then how about some details on the system. It is important to know the order of items after the pump. Where is the relief valve located, pressure transmitter, mass flow meter, regulating valve, and load?

Are you trying to control mass flow and pressure at the same time? You can't do that with a regulating valve. You'll need variable speed for the motor as well.

 

[Liad]

Thank you, guys.

I will try to answer some of the questions:

1. please see a part of the P&ID of the system:

2. we run the pump without a PRV and the result is the same.
we also run the pump in our workshop after the fault (the customer send the pump to us) and we connected it straight to a tank to test it, the same happens when we made some backpressure the flow rate went to zero.

3. We did not receive satisfactory answers from the pump manufacturerץ

4. From the inspection of the pumps, it appears that there are some gears grinding, but it does not seem very significant.

5. we control the flow with a servo motor drive or a VFD (depending on the system).

Thanks

 

[LittleInch]

Only thing is whether the shaft is not connected properly to the gears.

So it is OK with no real power going it but then slips once the force goes up.

You need to somehow stop the pump down or lock the gears and then see how much torque the shaft takes.

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

 

[TugboatEng]

Your regulating valves are on the suction side of the pump. That's irregular.

 

[LittleInch]

The motor to pump shaft and shaft to gear apart to be keyed shafts.

Perhaps check the keys are still there?

Also what exact pump model do you have? There are a number of pumps that size by that vendor.

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

 

[3DDave]

Failed keys would make a lot of noise while still turning under low backpressure and produce flow up until the friction with the key fragments is overcome.

I would not be surprised if there is a down-stream condition that blocks flow from the pump and causes the keys to shear. It would take only a fraction of a second to do some damage; perhaps it's repeatedly pounding the pump until the key shears through.

Instrumentation wise there should be a recorder that buffers data that can be set to push that buffer to a more permanent memory in the case of a spike so that an infrequent event can be captured.

"Gear grinding" is always significant, however I agree with Littleinch - check the keys.

 

[pierreick]

Hi,
It should not take long to open the pump, take some pictures and share with us. This will save words.
BTW you did not answer simple question about the fluid and its physical properties, in particular viscosity. Any change of temperature during the transfer? This will affect the perf of the pump.
Note: Did you replace the gears to see the impact?

Waiting for your feedback
Pierre

 

[ronfrend]

If a gear pump is giving reduced flow (assuming no flashing of fluid) and the gears are keyed to the shaft then the cause MUST be slippage of fluid past the teeth. That can be due to wear of the casing to tooth clearance or (and I have seen this much more often) scoring or wear of the gear end against the wear plate. I strongly suggest you strip one of these pumps down and use plasti-gauge or lead to measure the actual clearances. Check there is no excessive scoring of the end plate.

Ron Frend

http://www.predicon.net

 

[LittleInch]

What do you mean by "servo motor"?? Servo motors are usually linear or movement type actuators, not pump type motors.

If there is some flow it seems the motor is actually running though.

What flow and pressure readings do you have? Does the flow just drop off or gradually decline? Can you post actual data?
Which pump exactly is this?

Sudden drop seems to indicate either incorrect set pressure relief
Motor stalling / insufficient torque
Motor / shaft slipping.
Excess wear internal to the pump.



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

 

[TiCl4]

If flow is reducing with increased backpressure, you are experiencing increased internal slip due to wear. Water is a poor lubricating fluid and is harsh on gear pumps (no idea how well the oil is mixed in or what the oil type is). The teeth in the gear, even if "slightly" worn, will produce a significant amount of slip on low viscosity fluids (less than 300 cP). I suggest you check pump clearances and teeth dimensions of the gears and compare to a new pump. My bet is you'll find them worn.

If that is the issue, then the solution, unfortunately, is changing to a different type of pump. Lobe pumps offer similar performances with no metal-metal interaction, meaning they won't grind down like gear pumps do in non-lubricating fluids. If changing to lobe pumps, just make sure to account for the natural slip that is present in lobe pumps due to them relying on set clearances.

 

[Liad]

We use a servo motor to control the flow of the pump by increasing or decreasing the RPM of the pump (not Hz like VFD).
we checked the shafts and coupling and there was no issue with that.

It's a standard gear pump that one gear moves the other.
The decrease in pump performance is like the RPM per flow is approx the same for several months and then the pump starts to increase the RPM to reach the same flow. for example:
on the first day and for 2 months of working 500 RPM is enough for 10 LPM, then in a 2 days 1000 RPM is not reaching the same 10 LPM.

Our thoughts are right now adding a pulsation dampener that might help us or changing the type of the pump to diaphragm/lobe/screw.

 

[LittleInch]

Well that looks like you are suffering from accelerated wear due to the fluid. Once you get 50% slippage it will rapidly fail to create any flow at any sort of decent delivery pressure.

I think a pulsation damper would be waste of money. Gear pumps provide a fairly smooth pressure compared to piston pumps or AODD so not sure what a pulsation damper would do for you.

As said above, gear pumps tend to be best used for high viscosity self lubricating fluids, think oil pumps in your car for instance or pumping epoxy.

Something like an AODD sounds like it might be better. It looks like there is something in your fluid which doesn't like metal on metal seals like you get in gear, screw and lobe pumps.

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

 

[3DDave]

"we checked the shafts and coupling and there was no issue with that."

Does that mean you disassembled the pump and compared all the parts and critical dimensions to the replacement pump components?

 

[LittleInch]

I think they mean that the shaft is correctly working and no keys are broken.

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

 

[3DDave]

Until it is apart they cannot tell if the keys inside have sheared. If they did take it apart they would see wear or other damage. Hence I am skeptical they have examined the pump itself.