What is a Spool Valve? Spool valves are a common way of controlling hydraulic or pneumatic components by allowing, restricting or blocking the flow of the energy source by way of a simple spool mounted within an outer casing. A spool valve can be operated in a number of ways, we could use a button, a lever or by way of a solenoid. Spool valves are usually referred to by the nomenclature 3/2 or 5/3 etc., where the first number relates to the number of ports and the second to the number of differe