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Help - Reading Axial Piston Pump Performance Curves

Help - Reading Axial Piston Pump Performance Curves

(OP)
Hi Members;

This is my first post on the forum.

I would be grateful your help to explain me how to read the attached variable displacement axial piston pump Performance Curves.
It graph corresponds to a P11V hydraulic pump.
We are having problems to reach the nominal flow and pressure values and maybe this chart would be good for us.

I will really appreciate your help.!
Thanks and best Regards.

Cordially;

Guillermo.



RE: Help - Reading Axial Piston Pump Performance Curves

That's an old pump design, so on the assumption that it's done lots of work, it is not surprising that you are struggling to get the flow and pressures you expect.

If you look close the curves tell you what they mean...

The more horizontal lines are the flow rates for a given speed and should be read on the left hand Y axis. They decay as the pressure increases, this is normal for a pump.

The diagonal lines from lower left to top right are the power required to drive the pump. To use them, select the working pressure on x axis, where the pressure line intersects the respective pump shaft speed, read across from the right hand Y axis to find the power.

The efficiency curves are at the top. Peak efficiency being at about 4000 PSI and also decreasing with pump speed. This again is typical of piston pumps and it is the overall efficiency, including volumetric efficiency, which is shown to drop with increasing pressure, and mechanical efficiency that drops with shaft speed.

It's all there to see, but this is for a new pump. The one you have likely to be worn out, so will not get near the values shown.

Cheers

Adrian

Adrian Wright CEng MIMechE
Engineering Specialist
Hydraulic Systems
Caterpillar

RE: Help - Reading Axial Piston Pump Performance Curves

(OP)
Dear Adrian;

Thank you for your reply.!

I know that our pump is very old, but it has a few running hours, so internally it is new.
Without pressure accumulated we are reaching 60 gpm, but when set up 3000 psi the flow goes down to 27 / 30 gpm approximatelly.
What do you think.? High internal leakage.? High case drain flow.? High case drain pressure.?
I need to reach at least 50 gpm running at 3000 psi.
The electric motor that moves the pump shaft is running at 1700 rpm approx.
The swash plate is going correctly to full angle when we demand flow. It is governed via Jupiter 500 Controller and a Servo-valve that provides control pressure to swash plate servo actuator.

Thanks again for your reply and advice.

Cordially;

Guillermo

RE: Help - Reading Axial Piston Pump Performance Curves

It sounds like the pump is reaching the pressure compensator setting and reducing flow to maintain the 3000 PSI. If it still takes 27 - 30 GPM to maintain this pressure, you have a very leaky system or a big open orifice somewhere. If the internal parts are not worn out, you wont have much leakage and certainly not a 50% reduction in flow.

If you need 50 GPM and 3000 PSI, you will need to wind the pressure compensator in a little bit.

Yes the pump is stroking to give full flow, but as I say, the system pressure is causing the compensator to open and it is backing the flow off.

The compensator must be set above the working pressure, but at a level that is not too high.

If your system working pressure is 3000 PSI, set your compensator to 3500 PSI, or lower if you don't have enough power to run at that pressure with 30 GPM.

Cheers

Adrian

RE: Help - Reading Axial Piston Pump Performance Curves

(OP)
Dear Adrian;

First all, I want thank you for you posts, which are very helpful for me.

Today, we runs the pump. When we achieve 27 gpm with 3000 psi system pressure, we observed that the stroke position indicator, remains in full (F) position, without any movement. It´s to say, how can I detect at which pressure our compensator is working.? According to technical info that we have, by one turn of the screw the working pressure rises up 2000 psi, but how can we know the actual pressure setting.?

If we can set correctly the compensator and fault continue, mayby is time to think that the pump has an exsecive internal leakage due to wear. We measure the case drain and the manommeter shows 75 PSI, the booster pump pressure is 140 psi.
According to pump technical manual internal leakage must be 2.5 gpm, so it is another important test to perform after check the compensator pressure.

Thanks for your help Adrian.!
Cheers;

Guillermo.

RE: Help - Reading Axial Piston Pump Performance Curves

Hello Guillermo

If you run the pump against a blocked valve, with a gauge on the pump outlet, you will be able to set the pump pressure compensator. As you wind the screw in, the pressure will rise.

Where are you measuring the flow from the pump?

The boost pressure looks good, so there will be no problem with keeping the pump full of oil. However, the casing pressure looks a little high. The casing should not exceed 43.5 PSI. At 75 PSI, it indicates high flow in the casing drain. A quick check here is to put your hand on the casing drain line. If it is hot, that is a good indication that there is a leak inside the pump.

Does the swash angle indicator ever move, are you sure it's working properly?

If you can adjust the open orifice on the outlet of the pump, you should see the pump respond and the angle indicator move. When you are setting the pressure, the pump should move to minimum displacement (at high pressure), then as you open the orifice, the pump should move to max displacement (at lower pressure).

with all that said, it is looking like the pump is more worn out than you might think. Check the casing drain, can you measure the flow in that line?

Let us know how you get on....

Cheers

Adrian

RE: Help - Reading Axial Piston Pump Performance Curves

(OP)
Dear Adrian;

Good Morning.!

Regarding to your question, I will comment you how pump is working;

It´s working in a Hydraulic Test Bench that supplies high flow and pressure for actuators, pumps and valve testing.
The test bench has two major parts;

Power Part: Here is Parker-Dennison Variable Displacement Axial Piston Pump model P11V, Booster Pump, Filters, Reservoir and Relief Valve.

Control Console: Here are all controls of the bench, as well as flow meters and manometers necessaries.

So, responding to your question about where we are measuring the output flow of the pump, we installed a turbine flow meter (0-80 gpm range) with digital indicator at the output of the pump directly. Then we set the Bench to maximum flow request at 3000 psi. Monitoring both flow meters (pump installed and control console), the reading was exactly the same. So based on it, we discard the possibility of system failure due to (for example) an uncontrolled relief valve draining.
I think that if we can not obtain maximum flow at the output of the pump, then the problem is the pump. Are you according with it.?

Regarding to your second question about the movement of the swash plate angle indicator, when we begin the start-up process, the arm is in 0 (cero) position. We starts the pump, without flow and pressure demand, it remains in the 0 position. When we demand full flow and restrict at 3000 psi, it moves to the F (full) position. The key is that we observed that once reached the 3000 psi, the swash plate angle indicator is not moving back showing the de-stroke necessary to maintain this pressure, it remains ever in the Full position.

We will measure the case drain flow tomorrow, I will stay here to say you results.

Once again, Thank you very much.!
Cordially;

Guillermo.

RE: Help - Reading Axial Piston Pump Performance Curves

Hi Guillermo...

Something is not right here.

On the pump, 0 is min flow and F is max flow.

The pump is set up to be in max flow position when not running. This is so that it starts with maximum angle and can easily prime (start oil flowing) upon start up.

When the pressure setting is reached, the compensator will send oil to the pump servo piston to move the swash to minimum flow setting, the indicator should move towards the zero.

To confirm - When the pump is switched off and not running, the indicator should be in the F position. It should move the 0 position when the pressure setting is reached.

I think it would be a good idea to check that the displacement indicator is in the correct position.

It would also be a good idea to check the displacement limiters, they can be used to set the min and max displacement.

It's also worth knowing if the pump suddenly went bad? Was it working OK, then go wrong? Or has there been a gradual decline in performance?

The pumps does seem to be moving, but not in the way we would expect. The 50% reduction in flow still suggests that the pump control is not working correctly, rather than it being a dramatic reduction in volumetric efficiency.

I noticed on the schematic diagram for these pumps that the boost pump relief valve drains in the main pump casing, so that will explain the higher case pressure. There is no point checking the case drain flow or temperature as the charge (boost) oil will cool it down.

Lots of questions for you...

Again, let us know how you get on.

Regards

Adrian

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