>also, how do you build a practical system when the flow rate is not constant (AND doubles the flow here, for example)? You could probably make it constant. In

>also, how do you build a practical system when the flow rate is not constant (AND doubles the flow here, for example)? You could probably make it constant. In this case, at the output of the AND gate, put a pressure regulator valve, that will allow the extra pressure to leak out of the system (or into a return pipe). A similar problem would be duplicating the signal. The simplest solution I can see is a device that has two always pressurized inputs, and one control input. When the control is pressurized, it leverages a spring loaded switch to allow the two always on streams through.