Po check or Counterbalance

The Pilot Operated Check and Counterbalance valve are not only different in construction they are also different in their use.
The POC is a normally closed valve that is primarily used for holding a load. It is a poppet valve held down onto a seat by a spring. The valve can be lifted off its seat by a small piston and the pilot pressure created in the in- put leg of the circuit. Like all check valves they can be sharp acting and unstable if fitted inappropriately.

The Counterbalance valve is a spool valve normally closed and piloted to open by pressure in the in- put leg of the circuit to the actuator. The big difference is that the valve will modulate (move back and forth), open or close in proportion to the pilot pressure. This valve is designed to stop the load running away. If the load tries to move ahead of the input flow to the actuator, the pilot pressure will reduce and restrict the output flow thus checking the speed.
Also the valve has an added feature of a relief valve that will prevent over pressure.
Comparing the POC with Counterbalance, the latter is far more controlled with a much smoother operation but I have found them to be more susceptible to failure than the POC and can often render a situation with a load stranded in the air.
Over the years I have experimented and replaced the CBV with a POC. To provide the same effect it is also important to install a relief valve on the holding side (Pressure to tank) and a flow control to stop the load from racing away ahead of the input flow. The only downside with this set up is that there is the danger of the relief valve failing open and it is not ideal for a varying load unless you use a pressure compensated flow control valve. Otherwise I found the system to be more reliable and less prone to leaking. This is not suitable for all applications (where maximum safety is required) but can have its place for some applications. Imagine where it is crucial that the actuator must not get stuck in a dangerous position.
If you have used these valves in a more unusual way or different setup or have comments regarding this subject I would be pleased to hear from you.
Cheers
Bob