Hydraulic Valve Sizing

By Gary Alexander

Proper valve sizing for a hydraulic system can be a balancing act between system efficiency and cost. Large valves reduce pressure losses in the system which rob power and create excessive heat, but the valves can be very expensive. Smaller valves reduce system cost but cause higher pressure drops. These pressure drops waste power, create heat that may require a heat exchanger, and increase cost. Remember – whenever there is a pressure differential in a hydraulic system that does not result in work being performed, that pressure differential is directly proportional to the heat loss and horsepower wasted.
Here are some basic guidelines for valve sizing when designing a hydraulic system. These rules are particularly true for directional control valves:

• The manufacturer’s valve flow rating may not always be the best guideline for sizing the valve. This is because each flow rating is based on a certain “standard” pressure drop. These pressure drops vary between manufacturers. For cartridge valves, the rule of thumb has been that the flow rating is based upon a “standard” 100 psi pressure drop, but that doesn’t always hold true.

• The maximum flow rate required to flow through a valve is not always equal to pump flow. When a cylinder retracts, the flow discharging from the blind end of the cylinder can be much higher than the flow entering the rod end. This means that a cylinder with a large diameter piston rod, retracting at full pump flow, can be trying to push as much as twice the pump flow rate through the directional valve.

• Four-way, three-position directional valves can be used in series to unload the pump flow when the valves are all in neutral. This circuit can help a system run more efficiently. However, pressure drops through series components are additive. If each directional valve is sized for the pump flow rate when the valve is in neutral, the pressure loss through 3 or 4 active, or shifted, valves in series may reach unacceptable levels. Manufacturers of mobile stack-type valves offer many combinations of series and parallel circuits to help reduce pressure losses when the valves are in neutral. The best valve configuration choice will depend on the operating requirements for the particular machine.

• Valve sizing is often dependent on how often the valve or its actuator operates. If the machine is used infrequently, or a particular actuator is cycled infrequently, smaller valves can sometimes be used without adversely affecting the system efficiency. Valve rated flows can be exceeded in certain cases like these, but always consult the manufacturer before using a valve at higher than its rated flow.

• In the case of servo valves, proportional valves, and some pressure-sensing valves, over-sizing can have adverse effects on the system operation. Servo valves in particular are more responsive when there is a high pressure drop across the valve. Be sure to use the manufacturer’s guidelines when applying these valves in any hydraulic system.