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Generally an open center valve comes with one return port open. There can be another port but it has a plug in it. That plug combines the oil passed through the valve when not actuated and the return oil from the return side of a cylinder and sends the oil to the hydraulic reservoir. When there are the two ports at the valve outlet side, that valve is usually capable of being converted to power beyond so you can use a series of valves. A power beyond conversion plug separates those two oil flows and has a thread in it that can supply oil to another valve. Each valve needs a line to the reservoir for the cylinder return oil, but they can be connected together with pipe Ts. For the open center hydraulic system, the oil flows through the valve(s) and returns all the time. The pump moves oil all the time, mostly at low pressure so there's not much heating. When you switch that flow to a cylinder pressure builds as the cylinder is loaded. That pressure rise requires there be a pressure relief valve in the system to vent excess oil to the return line or you can burst hoses or shear off the pump shaft. When you connect a single acting cylinder to a 4 way valve, you have only one hose. Works fine going out because the absent return hose is connected by the valve to the return pipe and there's no oil, no pressure and no problem. The trouble arises when you move the valve to return that single acting cylinder. Then the back side of the valve tries to put oil to the back side of the cylinder but the port is blocked and pressure hits the pressure relief valve. So long as the pressure relief valve is big enough for the full flow of the pump and you don't do that often you won't heat the oil too much, but you will heat the oil. You can plumb that cylinder return port on the double acting valve to the return line that goes back to the reservoir, or you can find a valve for open center that is MADE for a single acting cylinder. I happen to have one that I'm not anticipating using. I don't think it has the capability of being converted for power beyond, but the last valve in the line doesn't need power beyond. It came from Energy. Some tractor (8N and MF) conversion valves have a screw that opens the return line to prevent the pressure build up. So they "convert" for a single acting cylinder. Baum Hydraulics catalog has a few pages on hydraulics and how to hook them up. They do have a web page. I don't remember the URL. The sectional valves from Prince that are sold by Northern Tool are very handy when making up a small but complex valve and do include both open and closed center capability and single and double acting cylinders. On a closed center system, the power beyond port is plugged so oil doesn't pass back to the reservoir except oil returned from a cylinder. There the pump holds the oil at pressure. There's generally no pressure relief valve because the pump destrokes or contains the pressure relief valve. More recent tractors have a mix that runs like a closed center system but at only a couple hundred PSI until there's a load, then the pump pressure is let to rise to its limit. They claim the fast response of the closed center system with less energy wasted moving the oil. When you add valves to a closed center system you need a return line from each pump and a power beyond plug on all valves but the last one, there you need a closed center plug. Since the oil pressure is maintained high by the pump in a closed center system, letting a valve open to a blocked port is no problem and so you can use a single or double acting cylinder on a double acting valve with no changes. I do that all the time. Gerald J.
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