Hydraulic filter and cooler placement

[bmklawt]

Long time lurker first time poster, can not find an answer to my question.
This is on a mini excavator, it is my understanding that the return line to the hydraulic tank should ideally be zero PSI but could be up to 100 PSI, does this mean my filter and cooler should be no higher than the return line output on control valve and no lower than the Return line into tank?

 

[Harry in Ky]

That's a confusing question, and why would you be concerned about it? I would expect to see zero AT the tank, but depending on a number of variables the pressure after the last control valve (but before any coolers or filters) would of course be slightly higher. 100 PSI? I rather doubt that. I would think 30 to 50 more likely.

 

[bmklawt]

I was reading on the internet that the return line should ideally be zero PSI but in some cases could be up to 100 PSI, i really have no clue about this.
I was thinking if the return line has no pressure, gravity feed, then all the plumbing the cooler and filter should run down hill from control valve to tank. I guess I would like to know can the cooler and/or filter be above the outlet on control valve or below the tank, will it push the fluid uphill?

 

[Harry in Ky]

I think you're way overthinking this. The system is no doubt capable of 2000+ PSI on the pressure side (when the load requires it) so the return side (again, after the last valve) needs to move oil uphill a few feet if/when necessary to reach the reservoir that's not going to create a problem. After all, it's that same pump pushing the oil isn't it?

 

[WranglerX]

THE pressure on return line to filter and cooler has no relationship as to height/location as to outlet of control valve or height of inlet to tank .... IT will either have pressure or not, pressure is caused by resistance to flow (in the sense-Plumbing) ...There has to be "some" pressure to have flow, but its actually a "free flow" after last work control valve.... Be more concerned about getting good air flow through cooler...

 

[oldnslo]

I was reading on the internet that the return line should ideally be zero PSI but in some cases could be up to 100 PSI, i really have no clue about this.
I was thinking if the return line has no pressure, gravity feed, then all the plumbing the cooler and filter should run down hill from control valve to tank.

The tank line is not gravity feed. In designing the filter and cooler circuit you want components that will handle the maximum return flow with minimal pressure drop or loss. A good return filter will have a bypass valve usually in the 30 - 50 PSI range to handle surges like cold oil start up. A cooler should be designed the same way.

Most people try to design around a known pressure loss based on published pressure loss curves at a given flow with a given oil viscosity. Not positive but I believe 5 to 10 PSI is the target for each component

I guess I would like to know can the cooler and/or filter be above the outlet on control valve or below the tank, will it push the fluid uphill?

Cooler and filter can be above the valve.

Note: if you have components like pump or motor with case drain lines these typically must have less than 30 PSI back pressure to prevent seal damage and are frequently plumbed directly to tank on a separate line.

 

[rScotty]

I was reading on the internet that the return line should ideally be zero PSI but in some cases could be up to 100 PSI, i really have no clue about this.
I was thinking if the return line has no pressure, gravity feed, then all the plumbing the cooler and filter should run down hill from control valve to tank. I guess I would like to know can the cooler and/or filter be above the outlet on control valve or below the tank, will it push the fluid uphill?

Theoretically a return line should work best if it were at zero PSI, but of course there is always some resistance to flow. If nothing else, there is the fluid friction in the return line itself as well as the cooler and whatever filtration is there. But the lower the PSI in the return line the better.

You need to be thinking in terms of fluid FLOW and not PSI. Try to think of it this way: The hydraulic pump creates flow; any resistance to that flow is what creates pressure.

If there is no work being done by valves and cylinders, the flow just circulates from sump to pump and then back to the sump through the return. The only pressure is the frictional pressure through the hoses and fittings. But anytime you block the flow at all, pressure is created. Blocking the flow can happen be deliberately when a valve activates a cylinder, or it could happen by accident if say a pipe in the cooler was smashed flat.

Whenever there is a restriction to flow, the pressure at all points between the pump and that restriction will rise. The pump doesn't know where the restriction is. It doesn't know if it is working a control valve and cylinder like normal.... or working against a plugged return line component. The pump will just keep on pushing and the PSI will keep on rising untl something moves, breaks, or a safety relief bypass valve is activated.

BTW, that is why every system downstream of the pump will have it's own relief/bypass valve.

What we just said means that a return can have some resistance to flow and a sump doesn't really have to be the lowest point in the system. The hydraulic pump will just see that rreturn resistance as something to overcome and will create enough PSI to keep the fluid moving. The reason you want the return flow to be low resistance is that if the pump has to push hard to flow through the return, its doing so also makes the pressure everywhere in the system higher. That higher system "backpressure" can interfere with the smooth operation of some control valves.
Hope this helps,
rScotty