how pressure compensation and load sensing works?

[guptara]

I would apprecaite if someone could explain in simple and plain words how pressure compensation and load sensing works in variable displacement pumps.

I would request if you can use imaginary pressure and flow values to make it simple and clear to understand how the contorl works.

Also, please point out if there are any resources to read about pressure compnsation and load sensing in pumps.

thanks in advance

guptara

 

[hydromech]

IN BASIC TERMS...

A pressure compensator attached to a variable piston pump will limit the pressure of the pump by "de-stroking" the pump at a set pressure.

With a compensator set a 200 BAR, the system pressure will rise to until the set pressure is reached. Then the compensator basically connects the system pressure to the servo piston in the pump. The servo piston moves, adjusting the swash angle, reducing the stroke distance of the pistons and therefor the volumetric output.

When the pump is de-stroked it will only provide enough oil to maintain 200 BAR. This is why its a "pressure compensator"...it limits pressure.

On a leak free system, the pump will only need to replace oil lost through the pump casing. The power consumption will be small.

A pump with a dead head leakage value of 1 litre per minute will consume 0.333Kw.

1 x 200
------- = 0.333
600

A system with several pilot operated valves will take more power because the pump is having to supply more oil to maintain the 200 BAR set pressure.

Lets say the system has 10 pilot operated valves with a combined leakage of 4 litres per minute, add the 1 litre for the pump leakage, that gives 5 litres per minute.

5 x 200
------- = 1.666 Kw.
600

Lets say the pump displacement is 50 litres per minute.
A fixed displacement gearpump on the same system will consume 16.666Kw when idle. 10 times more than the pump with a compensator.

Pressure compensators work best where the pump needs to be on standby or idle for a long period of time.

Pressure compensators have their limitations. They will give full flow upto the set pressure and then almost nothing, in terms of useable flow.

Not very good if you have variable load.

On a compacting machine where the load increases as the actuator extends, the actuator speed would be very high until the load reached 200 BAR. Then the actuator would stall.

Load sensing pumps work by taking two pilot signals from either side of a fixed orifice. This means that irrespective of the load on the actuator or the viscosity, the differential (pressure drop)across the orifice remains constant.

On most pumps this differential is 10 BAR.

This 10 BAR signal is then fed into the load sensing compensator.

The load sensing compensator will only see and respond to this 10 BAR reference signal.

Variable input speed and varying load do not have an effect on the pump output because the reference signal is always 10 BAR.

This makes load sensing pumps ideal for use on mobile equipment.

Lots of load sensing pumps still have pressure compensators fitted to them to limit the pressure/power when an actuator reaches full stroke.

A load sensing compensator represents mainly a flow control for the pump output flow, because the compensator keeps the pressure drop at the orifice constant.

When you get your head around this..we can move onto horsepower limiting compensators.

 

[Jacc]

I disagree!
Yes that is a correct description of a pressure limited pump but i don't think that is what guptara really meant.

Let's assume a proportional valve block with two valves operating one cylinder each. The block is fed by a load sensing pump.

Now open valve 1 and try to lift 3 tons with that cylinder. Just like hydromech said the pump will only be intrested in the pressure drop across the valve and will regulate the flow to keep that pressure drop at for example 20bar.

Now open the valve 2 to the second cylinder (loaded with 15 tons). System pressure will instantly increase 5 times. This has nothing to do with the pump but is merely a fact that the lines are now both connecte to the pump.

5 times higher pressure will make valve 1 squirt out some 2 times more flow, an unpleasant surprise for anyone operating the machine.
The pressure drop across the valve will also be some 5 times higher but the pump is now only intrested in regulating valve 2 which has the highest load pressure.

This is where the pressure compensation starts. A extra valve built in to valve 1 starts to throttle the oil to keep the pressure drop across the "abandond" valve 1 to increase more then a few bar.

 

[guptara]

Thanks for your comments guys...

Although i got some clarification to my orignal question...however i am still unclear the basic operation of a pressure compensated and load sensing pumps.

 

[hydromech]

Jacc...I disagree.

A single pump pushing 2 seperate cylinders will only generate enough pressure to move the lightest loaded cylinder...the path of least resistance.

In cases where there are multiple valves, the reference signals are combined via a shuttle valve so that all parts of the circuit have the benefit of load sensing.

Guptara...

None of it is easy to get your head around as both types of pump controllers do pretty much the same thing...they adjust the pump flow. They also look very similar.

You could also check this site. It has some information about the controllers.

http://www.parker.com/euro_hcd/new_web_page/pdf_2500uk/pv.pdf

Try this also...this one doesn't tell how they work. It just surveys the options.

http://www.fluidpowerjournal.com/2003/JA_03/FPJJA03_PumpControls.pdf

You might be better asking specific questions about how they work rather than general questions. You need to know whats inside the pump to start with.

Better still, find someone who will let you set one up. Thats the only real way to learn how they work.

Let us know how you get on

Regards

Hydromech
Hydraulic systems Engineer

 

[budt]

guptara;

Try this explanation:

Very simply put a Load Sensing Pressure compensated pump can have the Compensator set at 3,000 PSI but never see that pressure unless an actuator stalls against a load

At pump start a pressure gauge at the pump outlet would read 175-300 PSI as adjusted on the Load Sense section of the pump Load Sense control and a flow meter at the pump outlet would read 0 Flow.

When an actuator is activated a Load Sense line from the actuators port that is being powered would notify the pump that it required say 800 PSI to move it at the rate of 6 GPM. A gauge at the pump out let would immediately raise to 975-1,100 PSI and the flow meter would show 6 GPM. As pressure at the actuator changed pressure at the pump would also change but would always stay 175-300 PSI higher than the load up to the stall point at 3,000 PSI.

When nothing is moving the pump compensates to no flow at Load Sense Spring setting which is usually adjustable. (Note the 175-300 PSI figure)

This means there is a lot less heat generation since pressure drop in the circuit never exceeds the Load Sense setting. Also case drain flow is much less when no work or less work is being done and horsepower in is slightly more than it takes to do the work when a single actuator is moving.

When more than one actuator is being powered multiple Sense line to the pump show the highest load requirement through Check or Shuttle Valves and lesser load circuits are wasting energy but usually at a lesser rate than with a pressure compensated only pump. Small price to pay for the overall energy savings.

Using Load Sense on a multiple actuator circuit often eliminates the need for a heat exchanger and pressure compensated flow controls.
_________________


Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[guptara]

Thanks guys for your time and input.

I got the basic gist of how these pressure and load sensing contorl works.

I guess it needs more reading or looking at more application where they are used and how they control the circuit.

In the mean time if you have anymore ideas to add some value to this topic, you are welcome to do that.

thanks once again

guptara

 

[danhelgerson]

Generally speaking, a pressure compensated pump is a piston pump, usually with 9 pistons. The pistons are housed in a chamber very much like an old fashioned revolver where six bullets are stored in a rotating chamber. Each of the pump pistons has a “foot” that has a swivel joint. The foot is forced against a circular plate (called a swashplate) that does not rotate but is capable of changing its angle in relation to the pistons.

The barrel that holds the pistons is attached to the input shaft on the pump so that when the prime mover rotates, the barrel with pistons rotates as well. If the swashplate is perpendicular to the pistons, there is no movement of the pistons within their respective cylinder cavities in the barrel. The result is that there is no displacement. If the swashplate is angled to its maximum away from perpendicular, the pistons are forced in and out of their respective cavities within the barrel as each piston “foot” is forced to slide around the angular swashplate causing maximum displacement.

The swashplate is typically spring offset to the maximum angle (maximum displacement) position. There is a piston opposing the spring and if high-pressure fluid is allowed to enter the piston chamber, it causes the swashplate to move toward perpendicular or zero displacement.

In a pressure compensated pump, there is a small valve spool that normally vents the chamber behind the swashplate piston. There is an adjustable spring at one end of this spool that is used to establish the desired pressure. The other end of the spool is exposed to the system pressure. This is usually done internal to the pump so there is no external plumbing.

When system pressure rises to the level established by the adjustable spring, the spool moves and allows pressurized fluid to enter the swashplate piston chamber. The fluid pushes against the swashplate spring and reduces the angle of the plate toward minimum.

If the fluid passing across an orifice causes the pressure at the pump, the pump will de-stroke (move toward minimum angle) until the pressure drops to the setting of the adjustable spring on the spool. The spool now returns to its original position and stops flow to the swashplate piston. The spool will now continue to modulate the flow to the swashplate piston to maintain a swashplate angle that will produce the desired pressure in the circuit.

When we add load sensing, we add an additional valve spool that responds to a differential pressure as opposed to some maximum pressure. One end of the spool is exposed to the outlet pressure on the pump (just like the pressure compensated system). The other end of the spool is offset by a spring that sets the load sensing pressure. This spring may require anywhere from 6 to 15 bar and may or may not be adjustable. A pilot line to some remote location that is downstream from the control orifice connects the spring chamber of the load sensing spool. When the differential pressure through the plumbing and across the control orifice reaches the spring setting of the load sensing spool, the spool shifts and allows pressurized fluid to enter the swashplate piston chamber. The spool now modulates to maintain the differential pressure just as in the case of the pressure compensated system. If the resistance to flow increases to the level of the pressure compensator, that system overrides the signal from the load sensing spool and pushes the pump to minimum displacement.

I hope this is helpful.


Dan Helgerson CFPS, AFPI, AJPP

http://www.cfpsos.com