PB vs. Tank

[kevincook]

I have a simple question that I'm sure has been explained before but I can't find the answer.

I understand that if I install a valve and there are other valves down stream that I need to have PB (power beyond).

What I don't understand is how is the PB different than the port returning to the tank. It seems to me that once the fluid leaves the valve it could be used by another valve regardless.

Can someone help me understand how the PB and Tank ports are different? What am I missing here?

Thanks,
Kevin

 

[J_J]

The PB port will allow the continuous supply of fluid through an open center hyd system.

The PB adapter will separate the hyd flow into cyl OUT fluid and pump fluid.

If you do not use the PB adapter, the valve is just an open center valve or could be converted into a CC valve.

You can use a valve with a PB port and plugged as the only or last valve in the hyd circuit.

PB port is designed for high pressure, and the OUT port is only rated to about 500 psi.

Every valve in an open center hyd circuit should have a PB port with adapter to feed the next valve.

Just about every valve manufacturer states that the OUT port should go to tank unimpeded, meaning no or very little back pressure. .

 

[Jay4200]

To clarify - the flow goes from IN to PB when none of the valves are opened, so pressure is available downstream. Pressure only builds when you open a valve, and as the cylinder gets pushed in one direction, the fluid in the other side of the cylinder gets pushed through the OUT port (to Tank or Sump), so there is no pressure involved. With the PB plug in place, that is the only fluid that goes to the sump. The last valve in the line doesn't need PB, since all of the flow is going to the sump, so everything flows out of the OUT ports.

 

[Ivan49]

I have some Prince valves here that need a PB valve to be hooked together. I have some cross valves that are older and I have 3 of them plumped together. What is the difference in the workings of these to makes of valves.

 

[J_J]

To clarify - the flow goes from IN to PB when none of the valves are opened, so pressure is available downstream. Pressure only builds when you open a valve, and as the cylinder gets pushed in one direction, the fluid in the other side of the cylinder gets pushed through the OUT port (to Tank or Sump), so there is no pressure involved. With the PB plug in place, that is the only fluid that goes to the sump. The last valve in the line doesn't need PB, since all of the flow is going to the sump, so everything flows out of the OUT ports.

You are correct except, the cyl OUT fluid always flows through the OUT port, as it is expended fluid.

PB fluid normally goes through the PB adapter or can go through the OUT port if the valve is not used for PB.

 

[CJONE]

I have some Prince valves here that need a PB valve to be hooked together. I have some cross valves that are older and I have 3 of them plumped together. What is the difference in the workings of these to makes of valves.

With SOME of the older valves you could get away with this. The outer return cores were strong enough to handle 2000psi without blowing the spool seals, but they were still not designed to do this. If you did this without a upstream relief you also risk blowing a pump apart or hose by using 2 or 3 valves at once by not providing a return path for the oil that went over the relief. 3 valves set at 2000psi would equal 6000psi if they were all bypassing over the built in reliefs! So it is just luck that the system works without damaging stuff. Or there is a upstream relief. Most all valves now either use a PB plug to isolate the pressure core from the return cores or they use a different outlet cover to do the same.. Energy still makes a valve that you can backpressure but you have to have the system designed correctly to use it. CJ

 

[kevincook]

To clarify - the flow goes from IN to PB when none of the valves are opened, so pressure is available downstream. Pressure only builds when you open a valve, and as the cylinder gets pushed in one direction, the fluid in the other side of the cylinder gets pushed through the OUT port (to Tank or Sump), so there is no pressure involved. With the PB plug in place, that is the only fluid that goes to the sump. The last valve in the line doesn't need PB, since all of the flow is going to the sump, so everything flows out of the OUT ports.

Jay, thanks for that. I think that is the part that I was missing.

1. If I partially open a valve some fluid is directed in that direction and the rest still passes through the PB and can be used by a different valve (assuming I have enough flow volume).
2. The fluid that is redirected does not return back to pass through the PB port but it instead directed back to the tank. this makes sense since there is no backpressure going to the tank.

If I partially open a valve, I will have pressure available on the that valve and I will also have pressure available on another downstream valve since the fluid came from the PB of the first valve. Do I have that correct?

Thanks,
Kevin

 

[J_J]

Yes, that is correct.

Each valve will allow the hyd components in that circuit to develop the pressure for the given supply.

For instance, if the pump flow is 10 GPM, and the first valve is using 5, then the other 5 GPM is usable by the second valve.

If the first valve is using all the pump flow, then there is none for the second valve.

I would set the relief pressure the same for both valves.

 

[skyhook]

Interesting, and i follow this.
But i would like to ask or add to this conversation this,....
If the PB flow to the next valve or remote, etc....
why can't this PB be "tapped" with a tee?
I mean, if and only if "one" of the downstream valves were used at a time,
and they both had returns to the tank, could it not be possible to tap?

 

[J_J]

Because the flow will take the path of least resistance.

So if you actIvate the lever for operation, you offer resistance, then the fluid will go through the other valve.