More often lately, especially in regards to mobile hydraulic machinery, I hear about a concept called flow sharing, or post-pressure compensation, which I’ll just refer to as post-compensation from here out. Post-compensation doesn’t automatically imply flow sharing, because you can post-compensate with only one metering device. I know the term flow sharing is a little equivocal, but in our case it refers to equal sharing of flow between all branches.
The difference between pre- and post-compensation is where the pressure compensator is relative to the metering valve, which can be an orifice, needle valve, proportional valve, etc. A pressure compensator maintains flow across the metering valve by regulating the pressure drop across it. Without a compensator, flow could vary widely based on downstream pressure created by the load(s). Pre-compensation occurs before the metering valve, and post-compensation occurs after the metering valve. Either method will work fine if you have but one metering valve and actuator in the circuit.
As hydraulic systems become more complex, with various actuators being controlled by various valves, having one compensator per metering valve can still result in complications stemming from fluid taking the path of least resistance, especially with limited flow resources. Even with each metering valve having its own pressure compensator upstream of itself, this doesn’t guarantee each metering sub-circuit will have enough flow to create the pressure drop (flow) it wants.
In systems where the flow potential to operate all actuators simultaneously is higher than the flow provided by the pump, a pre-compensated system will allow flow to subordinate to the path with lowest pressure, leaving little or no flow for the higher pressure actuators. However, if most functions occur with singularity, pre-compensation should be adequate. On the other hand, if multiple functions require simultaneous operation, post-compensation would be beneficial.
Post-compensation with flow sharing places the compensator element after the metering valve, but also requires a pilot signal to be relayed to the master hydrostat (which is just a compensator dedicated to controlling pump flow) or to pilot port of the load-sensing pump. The post-comp circuit gets complicated when the actuators are all double acting, which requires that a compensator be used for each work port. For example, a double acting cylinder needs a compensator on the A port and a compensator on the B port, as well as reverse flow check valves for each work port on top of load sense check valves to relay load pressure back to the pump or master hydrostat.
Post-compensation sounds complicated, doesn’t it? I’m sure you’re wondering what the pay-off is, so I’ll tell you. With post-compensation, and when pump flow is too low to satiate the demands of every actuator in the system at once, instead of flow being defaulted to the sub-circuit with the lowest pressure, it splits the flow evenly amongst all actuators, albeit at a reduced rate to each branch. I would have liked to be in the lab for the “eureka moment” when this was first discovered, because it is brilliant! Yeah, I'm a nerd ... so what?
Post-comp circuits aren’t for the faint of heart, and are as complicated as most mobile hydraulic systems tend to get. This blog entry may have benefitted from a sample schematic, but I didn’t want an application example to muddy the waters of an already whitecapped lake. Just know that your multi-actuator hydraulic system would behave more predictably with integration of post-compensation, especially if pump flow is limited.
