Hydro pump characteristics vs. RPM

[Jay4200]

Pressure is a function of restricting the flow. You will get maximum pressure at idle to full throttle if you restrict the flow.

Do these pumps actually create full rated pressure (or internal bypass pressures) at idle? The pump is just a spinning impeller, isn't it? I'd imagine that pressure capacity would have some sort of relationship to speed, at least from and efficiency standpoint - well, it must, but I guess it would be moot provided low-RPM pressures can exceed the internal blow-offs.

I don't have one of those flow/pressure meters - wish I did.

Thanks - JayC

 

[SPYDERLK]

The flow is linear. Say the flow is 8.2 gpm at 2600 RPM the flow will be 4.1 gpm at 1300 RPM and 2.05 gpm at 650 RPM. The pump efficiency will determine how much of that flow you will retain at a given speed and pressure. If the pump is rated for 8.2 gpm at 2600 RPM and 2500 psi that would be at 100% efficiency. At 90% efficiency you will see 7.4 gpm at 2600 RPM and 3.7 gpm at 1300 RPM at 2500 psi.

Brian

I know what youre saying and, on face, with positive displacement it has to be true. However, under pressure, fluid bypasses thru case/rotor clearances and must be pumped again. This amount should be mostly a function of pressure rather than rpm. So bypass at 2500PSI at 1krpm would be [~]nearly the same as at 2krpm at that pressure.That would lead to a conclusion that the bypass results in a higher proportion of low rpm flow being lost than is lost at the greater flow that accompanies hi rpm. This would say that, at pressure, net flow at 2krpm is more than twice that at 1Krpm. So one would think that pump efficiency must rise with speed until suction side issues develop.
larry

 

[mmurphy]

Jay, are you thinking centrifugal pump?

Not really a spinning impellar, unless you might be considering a vane pump in that regard. Most impellar or centrifugal type pumps are used in low pressure applications. (water)

I do not recall, but I think you have a open system. I think your question is then what is it doing at idle? If no valves are being used, it is under no pressure and returning to sump under no pressure, except for friction restrictions. Yes full pressure can be met at idle.
When using valves then you are under pressure, lot lower volume of course at idle.
Increased RPM of course will help build up pressure quicker cause of volume.

 

[wdchyd]

Do these pumps actually create full rated pressure (or internal bypass pressures) at idle? The pump is just a spinning impeller, isn't it? I'd imagine that pressure capacity would have some sort of relationship to speed, at least from and efficiency standpoint - well, it must, but I guess it would be moot provided low-RPM pressures can exceed the internal blow-offs.

I don't have one of those flow/pressure meters - wish I did.

Thanks - JayC

Kubota Flow/Load Meter Test - Tractor Videos - Kubota, John Deere, New Holland and More

You mean like this one...??

 

[Jay4200]

Kubota Flow/Load Meter Test - Tractor Videos - Kubota, John Deere, New Holland and More

You mean like this one...??

Exactly. Please drop one off the next time you're in the area. Thankssomuch...

JayC

 

[IceStationZebra]

Volumetric efficiancy is depending on pump design, viscosity and pressure. A gear pump and a gear motor have similar efficiancy properties. See manufacturer ratings for detailed info. Power equation need info about the mechanical losses too

example.
gear pump volumetric efficiancy 85%
gear motor volumetric efficiancy 85%
(Internal leakage in spool valve X%)
Total volumetric efficiancy 0.85 x 0.85 x (X) = 0.7225 x (X)=72.25%
If you want 10hp output power on motor, you need to add mechanical friction losses to equation to...so you might end up with almost twice input power than output power.....

and pump speed.

I have a pressure vs. speed efficiency chart for a specific gear pump and the efficiency goes up with the speed. Basically the gear clearance is a leak path that is fairly constant at a constant pressure, so it will leak Xgpm. As the pump speed increases the total flow is going up but the leak path is staying roughly constant - so the ratio of total flow to leakage gets better. At low speed the leakage can get pretty serious.

System pressure can affect you two ways. The obvious route is the leak path around the gears, and as pressure increases so does the leakage. But good gear pumps also have pressurized wear plates that work to close that gap and keep leakage to a minimum. From the data I had this wasn't a linear relationship. I suspect this is due to changing flow forces inside the pump.

So for high speed, low pressure applications your efficiency could be up to the mid-90's. Then for low speed, high pressure applications it could drop the the low 70's.

ISZ

 

[SPYDERLK]

The flow is linear. Say the flow is 8.2 gpm at 2600 RPM the flow will be 4.1 gpm at 1300 RPM and 2.05 gpm at 650 RPM. The pump efficiency will determine how much of that flow you will retain at a given speed and pressure. If the pump is rated for 8.2 gpm at 2600 RPM and 2500 psi that would be at 100% efficiency. At 90% efficiency you will see 7.4 gpm at 2600 RPM and 3.7 gpm at 1300 RPM at 2500 psi.

Brian

I know what youre saying and, on face, with positive displacement it has to be true. However, under pressure, fluid bypasses thru case/rotor clearances and must be pumped again. This amount should be mostly a function of pressure rather than rpm. So bypass at 2500PSI at 1krpm would be [~]nearly the same as at 2krpm at that pressure.That would lead to a conclusion that the bypass results in a higher proportion of low rpm flow being lost than is lost at the greater flow that accompanies hi rpm. This would say that, at pressure, net flow at 2krpm is more than twice that at 1Krpm. So one would think that pump efficiency must rise with speed until suction side issues develop.
larry

Volumetric efficiancy is depending on pump design, viscosity and pressure. A gear pump and a gear motor have similar efficiancy properties. See manufacturer ratings for detailed info. Power equation need info about the mechanical losses too

example.
gear pump volumetric efficiancy 85%
gear motor volumetric efficiancy 85%
(Internal leakage in spool valve X%)
Total volumetric efficiancy 0.85 x 0.85 x (X) = 0.7225 x (X)=72.25%
If you want 10hp output power on motor, you need to add mechanical friction losses to equation to...so you might end up with almost twice input power than output power.....

and pump speed.

I have a pressure vs. speed efficiency chart for a specific gear pump and the efficiency goes up with the speed. Basically the gear clearance is a leak path that is fairly constant at a constant pressure, so it will leak Xgpm. As the pump speed increases the total flow is going up but the leak path is staying roughly constant - so the ratio of total flow to leakage gets better. At low speed the leakage can get pretty serious.

System pressure can affect you two ways. The obvious route is the leak path around the gears, and as pressure increases so does the leakage. But good gear pumps also have pressurized wear plates that work to close that gap and keep leakage to a minimum. From the data I had this wasn't a linear relationship. I suspect this is due to changing flow forces inside the pump.

So for high speed, low pressure applications your efficiency could be up to the mid-90's. Then for low speed, high pressure applications it could drop the the low 70's.

ISZ

Yup. Makes perfect sense. :thumbsup: ... Actually, efficiency can drop to zero at low speed hi pressure. At lo idle when hot the 7520 pump wont pop the relief valve.
larry