The function of a priority valve is to allow hydraulic fluid flow to certain functions within a hydraulic system when the pressure is greater than or equal to a specified level. In effect, the priority valve gives priority to certain components over less critical components. For example, in a 3000 psi hydraulic system, a 2000 psi priority valve may be installed such that components downstream of the priority valve are only supplied hydraulic pressure when the pressure in the system is above 2000 psi. Below 2000 psi, the priority valve will be closed.
A typical application of a priority valve is to preserve hydraulic pressure for primary flight controls. In this case, when pressure drops below a specified level, one or more priority valves will close leaving flow available to primary flight control hydraulic actuators. For example, in the F-14 hydraulic system, a priority valve isolates landing gear, brakes, nosewheel steering, ram air door servo actuator, overwing fairing and other components when the engine driven pump pressure drops below 2400 psi. When the isolation valve shuts at 2400 psi, the engine driven pumps provide hydraulic pressure to pitch servo, roll servo, yaw servo, rudders, speedbrakes and hook lift. In other applications, a hydraulic system is powered by a main system pump and an auxiliary pump. Under normal main system pump, all hydraulic services can be powered. However, should the main pump fail, the auxiliary pump takes over. The auxiliary pump may be powered through a power transfer unit or an electric motor. Auxiliary pumps are normally sized smaller than a main system pump since under normal operation there is a weight penalty when the system is not being used. If the auxiliary pump has a lower output pressure, than a priority valve can be used to isolate non-critical components at the lower aux pump pressure capability.
Priority valves have been designed in a number of ways. Two of the most common types are shown here. The simplest priority valve is similar in design and operation to a check valves (see Check Valves, Hydraulic – Description) or relief valves (see Pressure Relief Valves, Hydraulic – Description). Referring to Figure 1, flow through the valve only occurs when inlet pressure, P1 is sufficient to overcome downstream pressure, valve friction and spring force. By setting the spring force sufficiently high so that high pressure is required to overcome the spring force, the valve can act like a priority valve (reference figure below). Some priority valves allow free flow in the reverse flow direction (the priority valve shown in Figure 1 does not allow reverse flow).
Figure 1 Simple Priority Valve Schematic
A 2nd arrangement for a priority valve is shown in Figure 2. In this valve, flow goes through a single port and flows out through port A and port B. As the pressure at the inlet drops, the spring will push the piston to left. This will start to reduce the flow area through port B and eventually when the pressure is low enough, the flow area through port B will be zero and all available flow will go through port A. Hence at the outlet of this valve will be 2 sets of functions. The lower priority functions will receive flow through port B while the high priority functions will receive flow through port A. Also, note that flow rates through both port A and B can be tailored based on port sizes in the housing and by controlling critical flow related dimensions in the piston.
Figure 2 Priority Valve Schematic
A third type of priority has a single inlet and two outlets – primary and secondary. To control flow between the two outlets, a servo (spool) is monitors downstream pressure in both lines. When the pressure in the primary outlet drops a force imbalance causes the spool to move reducing the flow area to the secondary outlet. When pressure returns to nominal level, the spool moves back to the neutral position and normal flow occurs in both outlets. In this way, the valve acts a pressure compensated flow control valve on the primary outlet with excess flow going to secondary services or possibly back to return. This type of valve is more common in industrial hydraulic systems and is not used much in aerospace. There may be some military applications (cargo doors, ramps, etc.) where this type of valve may be useful.
When selecting a priority valve, the following parameters should be considered
Pressure Rating – make sure valve is rated for your system pressure
Pressure Setting – this is the pressure at which the priority valve opens and allows flow
Valve Pressure Drop – what is the pressure drop across the valve in normal operation (when pressure is sufficient to hold valve open). This will affect pressure available to downstream components and must be taken into account when sizing downstream components.
Reseat Pressure - what is the re-seat pressure (the pressure where the valve will close, once open)?
Flow Rate – what is the rated flow rate when the valve is open? Ensure the valve does not become a flow limiting device. If there are 2 outlets, what are the flow rate characteristics as a function of poppet position (or inlet pressure)? Are the flow characteristics acceptable from a system point of view?
Reverse Flow Capability – Is reverse flow capability a requirement? Is so, what is the pressure drop vs. reverse flow characteristics?
Leakage – What is the valve leakage rate when pressure is below priority pressure setting? What is the affect of leakage on downstream components? Can this leakage be tolerated?
Temperature Rating – valve should be rated for fluid temperatures and applicable environmental temperatures
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
– although there may not be seals on a piston or poppet, there will
be seals on the housing. The seals should be compatible with type of
fluid and the environment where the valve operates. For priority
valves with ball or conical poppets, damage may be possible when
re-seating against sharp edges (see Figure 3). Wear around the
poppet seats will lead to leakage. For relief valves with
elastomeric seals, see Seals - Hydraulic
Components for discussion on hydraulic
Failure Modes – the main failure mode is a jam of the poppet in the closed position or open position. A failure in the open position would be a latent failure. It may also be possible for the adjustable device on the valve spring to fall out of adjustment leading to lower or higher regulation settings.
Hysteresis – what is the difference between the relief pressure setting and reseat? This difference should be large enough to prevent any chattering of the valve. Chattering may occur in a situation where the valve shuts and system pressure increases quickly due to reduction in flow through the valve and the valve then re-opens. Then the pressure reduces and the valve closes, repeating the cycle at a very high frequency. This could happen if the priority valve setting and the “degraded” pressure level are close to each other.
Priority Valve Qualification
See Qualification - Hydraulic Components for discussion on control valve qualification and required certification testing.