What Are the Roles of Pilot Ratios and Relief Valves in Hydraulic Systems?

[Treyzian]

I have a few questions and was hoping you would have a few answers.

What's the purpose of pilot ratio in counterbalance valves? Is it vital, or necessary to include in a catalog description of counterbalance valves?

What information should be included when designing a product code for cartridge valves (ie solenoid valves, check valves, pressure valves, flow valves, counterbalance valves)?

 

[Pkruse]

It is very important. I've had million dollar equipment jumping around violently, only after much testing and troubleshooting was I able to fix the problem by installing valves with different pilot ratios.

A CB valve is also a high pressure relief valve. Pilot pressure is a fixed fraction of that, called the pilot pressure ratio. You can adjust the relief setting, but not the ratio. If you use a CB valve for sequencing, it must have a ratio higher than all the valves down stream of it.

 

[Pkruse]

A CB valve is also often called a holding valve. They are exactly the same and they come off the same production line. The difference is that the ones that test "zero leak" get labeled "holding valves," and they are sold for more money. The CB valves might only leak a couple of extra drops at full pressure in a ten minute test, so they pretty well function the same. Since they will lock the fluid in the cylinders, you have a concern that if the system heats up the fluid will expand and fracture the cylinder barrel. I've seen that happen simply because it was a cloudy day and the sun came out and warmed up the cylinder. This is especially important because of the pressure intensification between the blind and the live ends of the cylinder. You might have five or six times as much pressure on one side of the piston as on the other. So they design a relief valve into the holding valve to prevent this. On the schematic, it is often called the "thermal relief valve." But on many schematics, it simply does not show the relief feature. Perhaps the draftsman was lazy.

So you need to identify two things in selecting the right valve. You want to know what your relief pressure needs to be, and you need to know how much pilot pressure you need to open the valves. Maybe you want the relief at 4500 psi, and you want the valves to open at 500 psi. Divide the two you you know what pilot pressure to order.

In selecting the required pilot pressure, you want to pick a number that is well below the operating range of the machine. Remember that the pump does not make pressure. The load generates pressure. So figure your pressure at your lowest load, and maybe pick a number about half of that for your required pilot pressure.

Be careful in selecting a relief pressure for applications where you need a holding valve, like on a mobile crane outrigger jack cylinder. The cylinder will have two holding valves, one on each port. If the piston seal leaks, then you will have a very high pressure on the rod end of the cylinder, also called the live end. In this type of cylinder, the intensification factor might be as high as 15 to one because the rods are so big. So you need to take this into account in designing every part of that system. Your cylinders might get very big, such that under normal conditions the pressure in them is very low. That can make finding a holding valve with a sufficiently high pilot pressure ration rather difficult, for the reason just stated and also because it takes so little pressure to retract the rod. You might end up with a ratio of 50:1; or it might be half that.