Directional Control Valve, Hydraulic - Description
Hydraulic valves function to control pressure, control flow or direct flow in response to external commands. Directional valves are valves that direct flow in response to external commands. Directional valves are usually servos (see servos) where the servo is positioned in response to solenoids, torque motors or mechanical input. Directional valves do not provide flow or pressure regulation and functional only to direct flow (much like a switch). Sometimes directional valves are packaged with other components such as orifices or check valves. This has the advantage of combining several functions into 1 assembly (or 1 part number) to simplify installation. Directional valves are either open or closed (in 1 position or another). Directional valves do not utilize a spool in a sleeve design, but quite frequently use this configuration. Also, the spool may be zero lapped or have underlapping or overlapping designs.
Directional valves are referenced by the number of positions the spool will take (2, 3 or 4 positions are typical) and the number of hydraulic ports in the valve (2 way, 3 way and 4 way are typical). Examples are shown below.
Two Way, Two Position (2/2) Valve
In a two way, two position valve, the servo can be in one of two positions and the two ways because there are 2 fluid ports in the valve (or, if you prefer, the valve housing). Although a spool arrangement is shown, any type of check valve could be considered a two way, two position valve.
(Two Positions Shown)
Three Way, Two Position (3/2) Valve
In a three way, two position valve, there are three inlet/outlet ports in the valve and the spool can be in one of two positions. A 3/2 valve would be used to allow fluid flow into or out of actuator or motor.
Four Way, Two Position (4/2) Valve
In a four way, two position valves there are four inlet/outlet ports in the valve and the spool can be located in one of two positions. For 4/2 valve fluid is always flowing through the valve with system pressure supplied to one of the two outlet ports at all times. The other port would then be ported to return. 4/2 valves would normally be used in hydraulic systems in conjunction with an upstream shut valve (or 2/2 valve). In this case a 4/3 valve usually makes more sense. However, 4/3 valves can be found in power control units (PCUs), where a shutoff valve is installed in the PCU where a shut valve is not packaged with the 4/2 valve due to other design considerations in the PCU.
Four Way, Three Position (4/3) Valve
In a four way, three position valve, the spool is in one of three positions and there are 4 inlet/outlet ports in the valve. In the midstroke position there is no flow through the valve. A good application of a 4/3 valve is actuator control, where the actuator control goal is to extend, retract or hold a position. 4/3 valves are used in servovalves, where the spool is controlled by a flapper valve or a jet pipe valve.
When specifying a directional control valve, the following parameters should be evaluated:
Pressure Rating – make sure valve is rated for your system pressure
Pressure Drop – this is the manufacturer’s pressure drop at a rated flow through the valve. There may be a tolerance on the pressure drop which may need to be evaluated
Pressure Drop Variance – how does the pressure drop change for non-rated flow conditions?
Flow Control – A directional flow may also incorporate flow control. This would normally be accomplished through port sizing or putting an orifice in the outlet flow. If flow control is part of the valve, then the method of flow control should be ascertained and the accuracy and tolerance of the flow for design and off-design conditions should be evaluated.
Temperature Rating – valve should be rated for fluid temperatures and applicable environmental temperatures
Actuation Time – time to move from open to closed may be important in some applications
Valve Materials – should be sufficient to pass proof and burst testing, not be susceptible to corrosion and other environmental considerations, and not cause any problems under temperature extremes
Seals/Clearances – affects overall reliability of the valve. Some valves may not use seals and will maintain tight clearances between spool and housing to minimize leakage across the servo pistons. The design characteristics can be affected by environmental conditions and aging/wear over time. See Seals - Hydraulic Components for discussion on seals.
Method of Spool Position Control – the directional valve can be controlled by a solenoid, a linkage, a torque motor, a pneumatic source or a hydraulic pressure source. This source needs to be evaluated under all foreseeable conditions to ensure if will open and close the valve as required. Additional specifications will likely be required for the control element.
Leakage – does the valve have high or significant leakage levels? Leakage is wasted energy.
Chattering – a directional valve should be evaluated for potential to exhibit chattering or limit cycle behavior under certain upstream or downstream conditions. This will be a function of the natural frequency of the servo as well as the damping and friction levels. See Friction - Hydraulic Components for further discussion of friction characteristics.
Failure Modes – the main failure mode jam in any position from full closed to full open and contamination.
Flow ratings (pressure drop for a rated flow) for directional valves are normally provided by the manufacturer. Directional valves may incorporate provide flow control through either outlet port sizing or an installation of an orifice in the valve outlet. Flow behavior would follow the orifice flow equation (see Orifice Flow – Hydraulic). For the affects of valve lapping on valve flow see servo section.
See Qualification - Hydraulic Components for discussion on directional valve qualification and required certification testing.
Pressure and Flow Characteristics for a 4/3 Valve
Reference the 4/3 directional schematic above, note the flow areas from Ps to PA and PB to Pr are equal (matched and symmetrical valve). Ignoring leakage through the servo piston, the flow rates are characterized by the orifice flow equations,
The flow areas A1 and A2 are functions of valve position, xv. The load pressure drop and load flow are given by
Since Q1 = Q2, equations (1) and (2) can be combined, such that
Since return pressure is small compared to system pressure, we can let Pr = 0, leading to equation (6)
Equations (3) and (6) can be combined to obtain
Equations (7) and (8) relate PA and PB to supply pressure and load pressure. If the load pressure is zero, PA and PB are equal to ½ of the supply pressure.
For the load flow rate, equations (7) and (8) along with equations (1) and (2) can be substituted into equation (4) to yield an equation for load flow in terms of supply pressure and load pressure. The final equation is
This equation assumes Pr is negligible. Also, note that A1 = A2.
The above relationships for PL and QL are important for understanding theoretical pressure flow characteristics of a four way, three position valve where the servo can be positioned in any position along the bushing (infinitely variable flow area). These equations are used is the discussion of hydraulic servovalves (see Servovalve, Hydraulic – Description).