Close Loop Hydraulic System

I can't recall about the reservoir sizing in hydraulic systems, but I think I used to use at least 3:1 ratio for total system flow. This would leave enough dwell time for the fluid to properly "settle".

As for heat exchanger sizing, "normally" 25% of your HP input is a starting point, but in some cases may be as high as 50% required.

A couple of basic equations for calculating are:

HP heat generated= PSI X GPM/1714

BTU/Hr heat generation=1.5 x PSI X GPM

Should be a starting point for you anyway.

Even that 1.25 times on oil reservoir seem pretty tight.

Where did you get those equations from?

Yes, hence my 3:1. Last big project I did converted 1200 HP diesel into all hydraulic power including open loop(for general use) and closed loop circuits for traction motors.

I dug out some of my old pocket data books that I still keep in my desk drawer. I haven't checked to see if they are still available, but mine are old and this one is by Womack. Sometimes it pays to be a packrat.

I had picked up some 8 x 8 Square Tube, with 1/2" Wall thickness for some small projects that I was working on...... (wood splitter) was one, and I thought that would be handy to have, for my oil reservoir.

Theirs more to it, it's almost like a whole processing center. where the resevior would also be running some other equipment.

I'm the same way too, fortunately, about 4 years ago, I started to organize my data information......... then I started adding what I would find off the internet, if I thought it would be useful some day....... I can't believe the info I had pick up in such a short time.

The square tube is a good idea for the reservoir, have done it myself. Just have to make sure it is well sealed(welded) and cleaned out really good before you use it as a reservoir. I filled it up and ran a separate pump with filter for quite awhile to make sure I got all of the contaminents out of the oil.

As for info, you don't have to know everything, just know where to look it up.

The cleaning was the biggest issue, .....

as far as looking it up, That's why I started to organize my stuff.....

Come pare to 20 years ago, but the internet helps......

But at work, you only use a small percentage consistently.

Yes, but more thoughts back to the original question,,, as I recall, on the closed loop drive systems I routed them directly through the coolers (I think anyway)and the real problem was finding coolers that would handle the high hydraulic pressure as normally the coolers would be in a return circuit at a lower pressure.

And as for at work, the small percentage gets used so often, you remember it just because of repetition. Stuff I could never remember on tests at university are now engrained.

and the real problem was finding coolers that would handle the high hydraulic pressure as normally the coolers would be in a return circuit at a lower pressure.

That would be a problem ......

It was way back then, but I do believe we did solve the problem, as we build the machine and sold it.

Put your heat exchanger in the return line.

Size the cooling for all pumps that run.

Good point, as reviewing the schematic the system is one way flow only.

You appear to have no way of regulating the flow to each of the motors. The motor with the highest load will cede flow and the one with the lowest load will run faster.

Your drawing is very small on my laptop but the pilot on your valve returning fluid to tank looks like it will be pressurized as soon as you turn on the main pump. If this is the case, the bulk of the flow would take the easiest path in a tank pump tank loop.

Hi,

As I see it you have an open loop system and you are considering the differences between open and closed loop with regards efficiency. To really answer your question you needs to provide more information on the various components and the application. The reason for going to closed loop would most probably centre around control rather than efficiency.

The easy answers are.

Yes you can use a smaller reservoir for closed loop than open loop.(In theory at least)

Yes take the inefficiency of all the units at the worst situation and that is the heat you have to get rid off.

You should take the drain lines from the main units and put them through the cooling system. Beware of pressure drops though.

In a close loop system you only have the charge pump flow available for cooling but this flow can be altered dependent on the application. i.e. Change the size of the charge pump.

The decision to go Open or closed is more to do with application in my view. But I assume the two motors are driving a common shaft and therefore it would seem that closed loop would work well. If all components are Piston units then it would appear that closed loop would be a good choice.

Maybe my comments do not help but with the information you provide it is the best I can do.

Regards

O Dunthorne

Hi Oliver, just some questions based on comments:

"You should take the drain lines from the main units and put them through the cooling system. Beware of pressure drops though."

Based on my circuit, do i have to cool my charge flow (26 GPM) +drain lines of the system ( drain from the main pump + 2 motors)?

I would also like to know why only for piston units, closed loop would be a good choice?

Hi Again

i have just had another look at the diagram (regretably it is very small and my eyesight is not good) I see that it is closed loop not open loop. However my comments still hold good.

Oliver Dunthorne

Dear Vanuta

I have expanded your drawing as it was too small to really see what you had drawn.

It is not quite a close loop nor is it an open loop!

Firstly if the units, particularly the pump are not piston units then forget closed loop as you will generate too much heat to cool by the charge pump flow.

You cannot put the return line from the motors back into the charge pump reservoir.(That makes it open loop)

I suggest you draw the schematic as a conventional closed loop system i.e. as though it could go into reverse even though you do not intend to. Then you will have a conventional spill valve and purge valve and all the components will be in the correct place. Just copy any Danfoss, Rexroth closed loop circuit there are loads on the web. Then at least it will work and all you have to do is calculate the losses and size the heat exchanger, filter etc.

What that small relief valve is in the top left and where it is going is beyond me.(I cannot read the name tag)

Your circuit looks like a cross between an open and closed loop but is neither.

If you are going to spend money on it get help now.

Regards

Oliver Dunthorne