Question about how my open center FEL valve with power beyond

[wdchyd]

Having two control valves in series without PB and both actuated to full relief will double the pressures of both control valves.
The diagram and video show only a few hundred psi to show theory, but using control valves with thousands of psi is dangerous.
I personally was called in to troubleshoot a faulty system at a wood chip manufacturer, their Gresen SP monoblock valves (without BP plugs) had a catastrophic housing rupture on an upstream control valve, the operator had a bungy cord tied on a valve handle on a downstream valve, he operated the upstream valve and the housing split and shot oil into the cab of his truck.
So, this does apply to this thread topic as the OP was asking why it’s needed.

 

[oldnslo]

I believe just to demonstrate how pressures in series are additive which is same concept as having valves in series and operating the last valve now pressurizes everything upstream or if using tank port vs power beyond pressurizing the tank port.

 

[oldnslo]

Oops wd typed faster than me

 

[npalen]

Okay, I see what you are saying. The operator apparently had both valves, each with its own relief valve, deadheaded simultaneously against their respective loads.
If both reliefs were set at, say, 2500 psi then the actual was 5000 psi, right?

 

[WranglerX]

Okay, I see what you are saying. The operator apparently had both valves, each with its own relief valve, deadheaded simultaneously against their respective loads.
If both reliefs were set at, say, 2500 psi then the actual was 5000 psi, right?

Don't think so, you can not multiply pressure beyond what system is set up for.... IF system pressure relief/control valve is set at say 2500 psi you can never go beyond that.... But you could deadhead 2500 psi against both valves if you have valves operated improperly... And it is customary to have pressure relief/control valve after pump but before or in first valve in system...

 

[npalen]

But if the only relief valves in the system are in the two directional valves and both set at 2500 PSI and I operate both valves at the same time and each has a cylinder attached that reaches end of stroke----and the pump is capable of 5000 psi before it explodes or kills the engine????

 

[etpm]

Having two control valves in series without PB and both actuated to full relief will double the pressures of both control valves.
The diagram and video show only a few hundred psi to show theory, but using control valves with thousands of psi is dangerous.
I personally was called in to troubleshoot a faulty system at a wood chip manufacturer, their Gresen SP monoblock valves (without BP plugs) had a catastrophic housing rupture on an upstream control valve, the operator had a bungy cord tied on a valve handle on a downstream valve, he operated the upstream valve and the housing split and shot oil into the cab of his truck.
So, this does apply to this thread topic as the OP was asking why it’s needed.

I'm confused. It sounds like you are saying the pressure can be double of what the pump is supplying. That can't be right. If the pump can only supply, for example, 2000 PSI, then it is not possible to get 4000 PSI, right? Am I wrong? I think I am misunderstanding you.
Thanks,
Eric

 

[ptsg]

I've had my homemade backhoe setup with no power beyond fitting ever since I made it, and it has easily over 1000 hours without a single hydraulic issue. I used about the simplest and cheapest valves I could find, the Badestnost ones that Summit sells for instance.

The travel valve feeds the backhoe valve through the Tank port and I even had a pressure gauge on the PB port of the travel valve. However, the only reason I didn't have any issues is because I have a separate main relief valve right after the pump so I wasn't relying on the built in ones of the valves. Even when dead heading both valves at the same time as I've done multiple times by using the travel motor along with the backhoe to push/pull stuff..

The valves didn't really care about pressurizing the tank port and no external leaks existed even though those are rated for like 300 psi at the Tank port.

Now, I did finally stopped procrastinating two weeks ago and finally installed the PB fitting I made also years ago to "fix" my hydraulic system and mainly to be able to get set the travel pressure a bit higher than the backhoe one.

For anyone reading this, DON'T DO IT! It's not the correct way to do it and it will damage something eventually, depending on how the hydraulic system is setup.

 

[LD1]

I'm confused. It sounds like you are saying the pressure can be double of what the pump is supplying. That can't be right. If the pump can only supply, for example, 2000 PSI, then it is not possible to get 4000 PSI, right? Am I wrong? I think I am misunderstanding you.
Thanks,
Eric

Most tractors have a positive displacement (gear) pump. There are only 3 things that can limit its pressure...
1. A pressure relief.
2. Engine HP
3. Pump failure

With a positive displacement pump....it WILL move fluid if you have the power to spin it. Up intil the point it either fails, engine stalls, or a pressure relief opens.

IF the only means of pressure relief is in the valves.....and you have two valves in series using the tank passage of the first valve to feed the second valve.....the relief settings are indeed additive.

So if your loader valve is max'd out, like cylinder full extend or trying to lift something too heavy (lets say 2500psi).....you are seeing full pressure of that relief on the supply from pump to that valve. But the oil coming out of the relief and going down the tank line now has 0 psi......and if you use that to feed another valve with a 2500psi relief.....and use it to the MAX......you now have 5000psi on the line from the pump to the first valve.

Thats why PB is important, because its basically feeding the second valve straight from the pump as well. And the max pressure will never exceed the highest relief setting.

All tractors are different so I dont dare say "most tractors"......but all the tractors I have dealt with have a main system relief that can never be exceeded. So you pump will never be allowed to go over the main setting regardless of how the valves are hooked. So in most cases......requiring a PB to connect valves in series is simply to protect the valve's tank passages from overpressure....and keep it from cracking a casting or blowing seals....or even some goofy things going on with hydraulics.

 

[npalen]

Where do you typically find that main system relief valve located on tractors? Is it usually adjustable?