Centrifugal vs Gear Pump for lube system

[TangoCleveland]

We are design a lube system for compressor bearings for 60 gpm and 100 psig. In order to scavenge oil and air, our reservoir will operate at 5 psia pressure, partial vacuum. Vendor says a gear pump needs 16' NPSH, we have 13' NPSHa. Vendor has a centrifugal pump that only needs 3' NPSH, and we're considering using the centrifugal. I've not seen any lube systems with centrifugal pumps, only gear pumps. What are the disadvantages of using centrifugal pumps in a lube system?

Larry

 

[ccfowler]

OldSohioEngr,

I would not be very much inclined to use a centrifugal pump for such an application, and I'm presuming that the proposed gear pump is of the common two meshing external spur gear configuration.

Instead, I suggest that you consider a "Gearotor" pump configuration. This is an internal-external gear pump type that has some very nice characteristics. The void cavities open and close with relatively gentle rates of change in the void volume. (Flow into and out of the void spaces is parallel to the gear shaft axis.) As a result, the NPSHr is much smaller, and the operating noise level is usually much lower.

I've used the "Gearotor" style pumps very successfully, and where appropriate, I consider them to be a preferred type even though they may seem a bit more expensive initially. The pumped fluid serves as the pump lubricant, so your compressor lube oil system should be an excellent application.

 

[smckennz]

Three disadvantages:

Efficiency in the smaller sizes like yours.

Difficulty at startup when the oil may be below normal temperature and much more viscous. You could eliminate the suction strainer with a centrifugal, but there could still be difficulties

For scavenge duty, the presence of air is almost inevitable. This can stop a centrifugal from pumping; a decent gear pump will handle it OK.

I prefer screw pumps, but for your light duty, a gear pump, as the vendor and ccfowler suggest appears the best solution.

3 out of 3 recommend gear pumps.

Cheers

Steve

 

[TangoCleveland]

I gave it a little thought and looked at pump curves. Assume your filter gradually plugs up and adds backpressure to the pump discharge. A centrifugal pump will move back on its curve and flow will decrease. A positive displacement pump will give constant flow regardless of backpressure. I'd guess that you would not want flow to decrease in a lube system, so a positive displacement pump would have to be used.

Larry

 

[25362]

Manufacturers of rotary pumps would ask for higher NPSHa values for fear the pump may encounter in practice conditions that limit their performance such as gas entrainment and temperature fluctuations resulting in viscosity changes and consequent line losses.

OldSohioEngr tells us the vendor said, I quote, "a gear pump needs 16' NPSH". This is a generalization difficult to accept in view of the fact that the majority of rotary pumps operate with suction lifts.

One is inclined to ask whether there is a way of improving the NPSHa by 3' (23%) so the proposed gear pump becomes acceptable. Otherwise it is probable another vendor could offer a gear pump suitable for the available NPSH.

 

[TangoCleveland]

Thanks for your responses. I found out that NPSHr for a gear pump depends on pump speed, with lower speeds giving a lower NPSHr. In my case, an 1800 rpm gear pump requires 16' NPSH, and a 500 rpm pump requires 2' NPSH. We're getting a motor driven pump with a speed reducing gear.

Larry

 

[25362]

Larry, OK, but what about flow rate at lower rotating speeds ?

 

[TangoCleveland]

Gear pumps are sized based on cubic inches pumped per revolution, so for our case we used a larger pump at a slower speed. The cost went up because of the larger pump and the speed reducer, but it will work.

Larry

 

[1623]

I am looking for a pump with following main spec. If any one knows some pump manufactures or engineering firm which they could desing and maufacutre the pump.

Majore spec data:
1. Flow rate: 5l/min @ 40psi
2. Brushless moter
3. Variable speed moter controller
4. 24vdc operating voltage
5. Operating life: >30,000hrs
6. Fluid type: water

Thank you.

 

[automatic2]

In a scavenging system, it is not unusuall to find both styles of pumps at work. The centrifugal does the actual scavenging, sending it's output directly to a gear, or other positive displacement pump. As the action of scavenging typically includes large volumes of air, a much higher flow rated pump is required to deliver the fluids, thus the centrifugal.

 

[abeltio]

the pump mentioned in the original posting is indeed small so a pd pump should do the job...
regarding the use, in general, of centrifugal vs. rotary pumps in lube oil service:
I am familiar with the GE industrial type gas turbines and they evolved in the following way:

1. frame 5/6/7/9 std design (i.e. not F class)
1.a main lube oil pump: mechanical (pd) type driven by the auxiliary gearbox (i.e. driven by the gas turbine)
1.b auxiliary lube oil pump (100% capacity): submerged centrifugal type pump with AC motor

2. frame 6/7/9 F class units
2.a lube oil pump: redundant submerged centrifugal type pump with AC motor in separate (off base skid)

3. all frame sizes
emergency lube oil pump: submerged centrifugal pump with DC motor

so... as you can see there are as many designs as opinions...

saludos.
a.


saludos.
a.