Pressure regulation uses a servo to control flow area and hence pressure drop through the valve to control outlet pressure. A simplified schematic is shown in Figure 1.
Figure 1 Simplified Schematic of a Pressure Regulation Valve
In the pressure regulating valve shown in Figure 1, the outlet pressure, P2, is ported to the servo. The position of the servo and hence outlet flow area is based on a force balance between Pc and the spring force plus Prtn. The spring preload needs to be adjustable (adjustment device is not shown in Figure 1) to properly calibrate outlet pressure. Flow rate through the valve will change with changing inlet pressure conditions, depending on the flow area through the servo. With no applied pressure, the spring will push the valve completely open. The relationship between inlet pressure, outlet pressure and flow rate is governed by the orifice flow equation. Manufacturers of pressure regulating valves can provide charts of inlet pressure, outlet pressure and flow rate for a specific part number valve. These charts can be used to assess performance of a particular pressure regulator over the desired range of operating conditions. The flow control valve shown in Figure 1 only works when there is a flow demand on the system. With no flow, the regulator may close off the flow area trapping hydraulic fluid in the line downstream of the pressure regulator unless a stop on the spool is provided (if there is no stop a relief valve downstream of the pressure regulator may be necessary in some applications).
One application of a pressure regulating valve shown in Figure 1 would be control of a motor, where the torque is a function of the Δp across the motor (inlet motor pressure is controlled by a pressure regulator and downstream motor pressure is constant at the return pressure). A pressure regulator would ensure that motor Δp is sufficient to produce a necessary torque. However, this would lead to changes in flow rate with changes in inlet pressure. Another application is in a landing gear system where it is desirable to maintain a high pressure level and some variation in landing gear actuator rate is tolerable. Many landing gear installations are such that the landing gear actuator loses mechanical advantage as the gear is retracted. If there is a small margin in the hydraulic pressure required to pull the gear in the wheel well, the gear can stall out prior to full retraction. A pressure regulating valve will reduce flow to maintain pressure. Generally speaking, pressure regulators of the type shown in Figure 1 are used sparingly in aerospace applications. This is because a properly sized hydraulic pump will regulate to design system pressure with minimal variation (see Pumps, Hydraulic – Description for pump flow characteristics) and actuation components (actuators and valves) are designed for system pressure. In effect eliminating a pressure regulator is not absolutely required saves cost and weight.
The pressure regulator shown in Figure 1 provides pressure regulation when there is flow through the valve. Another type of pressure regulator bleeds off pressure to return to ensure the downstream pressure is constant under all flow conditions (see Figure 2). In this valve, as the pressure, Pc, increases the spool is pushed to left against the spring and the flow opening increases. Note that this valve, when regulating, wastes energy since flow directed back to the reservoir does no work.
Figure 2 Bleed Off Pressure Regulator
The type of pressure regulating valve shown in Figure 2 is common in brake control systems, where the goal is to maintain a commanded pressure at the brake under no flow conditions. The brake valve essentially acts as a pressure regulator where a proportional servo (or direct mechanical linkage) is used to control spool position and hence downstream pressure.
Pressure regulating valves can be devised in a multitude of ways. Another example of a pressure reducing valve is shown Figure 3 below.
Figure 3 Pressure Regulator
This valve closes down the inlet opening as outlet pressure increases, which is similar in operation to the valve in Figure 1. The spring preload needs to be adjustable (adjustment device not shown in Figure 3) to properly calibrate outlet pressure. Also, if outlet pressure gets too high then the piston chamber pressure (force) would be able to sufficiently compress the spring so that outlet pressure would be ported to return. This acts as a load limiting device.
When considering the use of a pressure regulating valve, the following factors should be evaluated
Pressure Rating – make sure valve is rated for your system pressure
Regulated Pressure Rating – should be in the range you desire
Regulation Range – what is the minimum and maximum inlet pressure required for regulation. Also, manufacturer performance charts should be evaluated for output pressure and flow rate variation over the range of input pressure conditions.
Regulation Tolerance – Need to ensure tolerance on pressure regulation is sufficient for you application. Tolerances result primarily from friction forces on the servo.
Temperature Rating – valve should be rated for fluid temperatures and applicable environmental temperatures
Leakage – leakage rate of the valve should also be evaluated. Leakage becomes wasted energy and reduced efficiency, however, some leakage will ensure fluid temperatures are warm during cold soak conditions.
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.
Failure Modes – the main failure modes in the flow control are servo valve jamming in any position from full closed to full open and contamination. It may also be possible for the adjustable device on the metering orifice to fall out of adjustment leading to lower or higher flow regulation settings.
Chattering – 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 damping and friction levels. See Friction - Hydraulic Components for further discussion of friction characteristics.
Pressure Regulating Valve Qualification
See Qualification - Hydraulic Components for discussion on control valve qualification and required certification testing.