Direct-operated proportional valves are used when flow through the valve is approximately 25 gallons per minute (GPM) or less. Two-stage valves, which incorporate a pilot valve and main spool, are used when higher flow rates are required.
In order to troubleshoot the valve and system, you must be able to read hydraulic symbols. In Figure 1, the symbol for a direct-operated proportional valve is shown. Notice the four squares in the symbol. These represent the number of positions in which the valve spool can be shifted. When there is no power to the valve coil, the spring will shift the spool to the position shown on the far left. This is known as the “fail safe” position. In this condition, all flow is blocked through the valve.