Pressure Compensated Vs Load Sensing

Try my explanation in the Basic Fluid Power training book I use. Start in Ch. 8 and go to Page 14.

Can you have give a reference to where you read about Varying Loads and Pressure Compensated Pumps. I've used and have seen NUMEROUS circuits operating quite satisfactorily that way.

It seems the Fluid Power world is often so little understood by the average Mechanical Engineer and Maintenance person that a lot of undesirable information floats around in almost every location it is operating.

The main story I hear from students is, "A Hydraulic Circuit Always Needs a Heat Exchanger or it will OVERHEAT."

Hydraulic circuits only OverHeat when they are of an inefficient design that is wasting energy. A well designed circuit will operate at 10-20 Deg.F. above Ambient while running 24/7 in any condition. That means a Heat Exchanger is unnecessary in all but those atmospheres that are at elevated tempeatures.

I highly appreciate your knowledge about hydraulics so that please take my comment only as an additional one.

You are right if the reservoir convective area is big enough to permit the transfer to the air of the appearing losses at a limited temperature difference. If the reservoir is too small even if the losses are small an over heating can occur.

Seems like I missed the link to the Basic Ebook on the Hydraulics & Pneumatics magazine web site, so here it is.

http://www.hydraulicspneumatics.com/200/eBooks/

Hi,

There are significant differences between PC and LS. We are talking variable displacment hydraulic pumps.

PC first :- The typical pump is usually a piston unit which has a control to vary the output(volume) which is set at a fixed pressure. When that pressure is reached the pump will destroke to match the volume that is reqired by the circuit. Thus the power consumed is the pressure (it will be the maximum for that particular circuit) times the volume being suppiled. For example if it were 50 lpm maximum output pump and with PC setting of 200 bar. Then the sum is 50 x 200 divided by 600 = theoretical power required in kw's. ( you would need to factor this for inefficency by adding around 15%. If the demand is only 25 lpm then you would still be at 200 bar but the flow would decrease and you would save 50% of the power (slightly less in practice due to basic inefficiences of the pump). A graph is a very easy way to show this but I do not know how to do one in this reply. Often this type of pump will obviate the need for a cooler.

LS:- With this type of control fitted to a similar pump we can expect to not only match the flow demand but we can match the pressure demand. Of course there is small price to pay in that an LS control has minimum setting, usually around 15/18 bar.

However, used with a LS directional valve(all the mobile manufacturers do them) you can match the flow demand with the pressure demand. So taking the example above but where the flow requirement was only 25 lpm and say the pressure demand was 100 bar you would use a quarter of the power required by a fixed displacement pump and half the power of a PC. Again graphically this is very easy to show.

If you go to a Vickers or Rexroth web site/catalogue you should find these pumps illustrated.

I hope this helps you.

Regards

Oliver Dunthorne

Hydraulic Engineer

Oliver,

Thank you Oliver, I am a bit confused here...looking back at your calculations: If lets say i have 3 different pumps. One is a fixed displacement pump, one is a PC pump and the other is LC pump. Lets assume that the PC pump is set at 200 bar and the maximum flow rate of the three pumps are 50lpm. Lets also assume that the system requirement is 100 bar and a flow requirement of 25lpm. Can i say that for the three different cases the HP requirements will be as follow:

Fixed displacement pump ( even though the system requirement is 25lpm, the pump will consume 50lpm and relieves 25 lpm back to the reservoir and delivers 25lpm to the system): 50lpm*100/600 Kw's

PC pump: 25lpm*200/600 KWs

LC pump: 25lpm*100/600 Kws

Could you please clarify on this. Thank you.

Then again there are Load Sensing Fixed Volume Pumps that will only be relieving the excess at 100 Bar.

Sorry Bud, i'm not fully understanding ur answer. if a fixed displacement pump will relieve the excess at 100 bar, doesnt a fixed displacement pump without any load sensing capability also does the same thing? Whats the difference. I was also not aware that a fixed displacement pump could be have load sensing capability...It would be helpful, if you could give further explaination on this. Thank you.

Hi Tim

I am sorry I haven't replied earlier but I did not pick up your question until yesterday.

If the system pressure requirement is only 100 bar then you would set all three pump controls to 100 bar, Fixed, PC and LS. If however the maximum required is 200 bar then all would be set at 200 bar. So your fixed displacement pump would be 200 x 50 / 600. Thus you would be using only 25 lpm of flow and but the relief valve would be relieving at 200 bars so you would be wasting 3/4 of the input power. (12 1/6 kws) i.e. it would cook instantly. I hope this is clear if not please come back to me.

The easy way to understand the differences is to draw a graph with pressure (bar) on the horizional axis and Flow (lpm) on the vertical axis. Most manufacturers do show some graphs in their literature. I will try and draw you some graphs to show you the differences. But probally not today as the sun is shining for a change and I have some jobs to do outside!

Regards

Oliver Dunthorne

vanuta wrote:

"Sorry Bud, i'm not fully understanding ur answer."

Don't fee left out, I reread my post and I don't understand it either.

I should have said,

A Load Sensing Fixed Volume Pump can save energy since it will relieve the excess flow at a 100-150 PSI above the actual pressure the work at any moment requires.

You can see my explanation of a Load Sensing Fixed Volume Pump in Chapter 8 starting on page 14 in this Ebook on the Hydraulics & Pneumatics web site.

http://www.hydraulicspneumatics.com/200/eBooks/

It is a book I used to teach Fluid Power Basics to Mechanical and some Electrical Engineers and Maintenace persons.

Sorry for the Non-Answer.

Oliver,

lets say, i have this particular situation...

I have a pump that is driving a motor that is driving an agitator to mix fluid of different density. The pressure demand for this agitator varies from laets say 100-200 bar, depending on the density of the fluid...Lets say, i have three types of pumps...

1) A Fixed displacement pump of 60lpm

2) A variable dsiplacement pump with pressure compensation set at 250 psi and max flow of 60lpm

3) A variable displacement load sensing pump with a pressure compensated setting of 250 psi with max flow of 60 lpm

Lets say i have two situations..

1) where the load demand is 100 psi and the flow requirement is 25 lpm

am i right to say...

Power used by fix displacement = 100x60/600?

Power used by PC = 200x25/600?

Power used by LS = 100*25/600?

2) Where the load demand is 200 psi and the flow requirement is 30 lpm.

Power used by Fixed displacement = 200x60/600?

Power used by PC = 200x30/600

Power used by LS = 200x30/600?

Could you please clarify...Thanks..

Hi,

I will do some sums on your figures and try and find out how to attach some graphs but it might take me a couple of days to get around to it as I have a lot on at present, but I will respond.

Regards

Oliver Dunthorne

Thanks for your response..you can mail me the graphs, if you have difficulty attaching them... my e-mail add is vanforum0407@gmail.com

Hi,

So in a PC pump, am i right in saying that irregardless of the load demand pressure, the pressure will always be whatever that has been set on the pump? Let me give an example.

Lets say a pressure compensating pump has been set at 200 PSI. During operation, my load varies between 100-200 PSI.. so am i right in saying that my system pressure will always be 200 PSI?

Thank you.

gd qn..

Fixed displacement pumps CANNOT have load sensing or pressure compensating systems. Both need to de-stroke the pump, being a swash plate in a axial piston pump or a cam ring in a vane or radial piston pump, this cannot be done if they are FIXED.

I think the terms need some explanation.

Today load sensing with a variable displacement pump fitted with a load sensing control and a directional control valve suitable for LS such as the Sauer Danfoss PVG then matching the load pressure and the flow demand is the norm. This is known today as full LS control

If the same set up is used with a variable delivery pump fitted with Pressure Compensated control then only the flow is matched as the pressure rises up to the RV setting (ie maximum).

The use of a fixed displacement pump again with the same valve cannot match the flow but it can match the load pressure (the excess flow goes over the RV at load pressure).

The use of a fixed displacement pump with a three port flow control (incorporates a relief valve) can do a pressure match but the flow is spilled to tank at load pressure.

If you goto the Sauer Danfoss web site and put in PVG32 and open the leaflet there is a set of four graphs which show the various options open to you. I hope this helps and does not add further confusion.

Oliver Dunthorne