The two methods of controlling flow rate in a hydraulic circuit are (i) using a fixed orifice and (ii) using a flow control valve.
For accurate flow control, a device that regulates to a Δp across an orifice is required – referred to as pressure compensated flow control valve. Figure 1 shows a simplified pressure compensated flow control valve.
Figure 1 Simplified Schematic of a Inlet Pressure Regulated Flow Control Valve
In this valve, the Δp across the flow metering orifice is maintained at a constant value producing a constant flow rate. The general orifice flow equation indicates that holding the orifice area and Δp constant, where the fluid properties (bulk modulus and density) are relatively constant, will yield a constant flow rate through the orifice. To hold Δp constant, the upstream and downstream pressures are ported to different sides of a servo (piston). An adjustable spring assists the lower pressure to hold the valve open at low input pressures. As the Δp (force balance) across the servo varies the flow opening by the servo, the metering orifice inlet pressure is regulated. Hence as P1 increases (or P2 decreases), the servo moves to the left and reduces the servo flow area. And as P1 reduces (or P2 increases), the servo moves to the right thereby increasing the flow area. The flow control valve shown in Figure 1 modulates P1 to control flow. Calibration of the flow control valve is obtained by adjusting the metering orifice. The spring preload may also be adjustable.
A second example of the flow control valve is shown in Figure 2. In this valve, the servo modulates P2. However, the overall function of the valve is similar to the valve in Figure 1. Regulating P2 may be an advantage over regulating P1 if the servo port in Figure 1 could become the controlling orifice (flow area becomes smaller than the metering orifice). In this case the servo port opening would be controlling flow.
Figure 2 Simplified Schematic of a Outlet Pressure Regulated Flow Control Valve
Fixed Orifice vs Flow Control Valves
A comparison between the use of orifices and pressure compensated flow control valves is shown in Figure 3. The orifice flow varies dependent on (Δp). The amount of variation seen in a practical application of orifice flow depends on the range of Δp seen over the operating range of the orifice in the system. As shown in Figure 3, the flow control valve holds flow constant over a wide range of Δp (from min regulation Δp to maximum Δp).
Figure 3 Notional Graph Showing Flow Characteristics for an Orifice and Flow Control Valve
For an orifice (see Figure 4), flow is governed by the orifice flow equation (see Orifice Flow - Hydraulic)
As can be seen in Equation (1), for a fixed Δp flow can be controlled by controlling area. In practice, P1 and P2 are never constant and therefore orifices do not provide constant flow rates over all operating conditions (system pressure, downstream pressures, temperatures, etc.). Nevertheless, in many applications, fixed orifices can be sized to limit flow under the worst case condition and the accuracy of a simple orifice is sufficient. As an example, simple orifices are common in landing gear actuator circuits, where the time to retract or extend the gear can be in the range of 6-10 seconds. In this case, an orifice can be sized to maintain the 6-10 second requirement under all operating conditions.
In Figure 3, note the flow rate for a flow control valve is constant over a wide pressure range. However, as the input pressure range varies so will the output pressure. This will affect the inlet pressure to a downstream component. So, while the flow to a component (actuator or motor) downstream of the flow control valve will be constant, the inlet pressure to the component will change. This will affect the Δp across the component and hence the power output of the component (Power = Δp x Q).
Sharp Edge Orifice
Short Tube Orifice
Figure 4 Sharp Edge and Short Tube Orifices
When considering the use of a flow control valve, the following factors should be evaluated
Pressure Rating – ensure valve is rated for your system pressure
Regulated Flow Rating – should be in the range desired for the specific application
Regulation Range – What is the minimum and maximum Δp required across the orifice for regulation? For example, one flow control valve manufacturer has a maximum Δp of 3000 psi and a minimum Δp of 100 psi across the metering orifice to maintain flow regulation within the flow tolerance.
Flow Tolerance – Ensure tolerance on flow regulation is sufficient for your application. For example, some valves will control flow to within Δ10% of the setting. Tolerances result primarily from friction forces on the servo. When using flow control valves in parallel systems, such as thrust reversers or ground spoiler systems, flow tolerances should be sufficiently tight to avoid undesirable “asymmetric” operation between the two actuators (this may also be affected by piping differences – lengths & bends - between the two actuators and source manifold).
Pressure Drop Across the Valve in the Regulation Range – This will affect design pressure available to a downstream component and will affect sizing of that component. Keep in mind that while flow is constant over a wide pressure range, the outlet pressure of the flow control valve can vary significantly. This will affect available power to the downstream component.
Temperature Rating – valve should be rated for fluid temperatures and applicable environmental temperatures
Valve Materials – valve material(s) should be sufficient to pass proof and burst testing, not be susceptible to corrosion and other environmental considerations, and operate properly 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 dominant failure modes in the flow control consist of the servo valve jamming in any position from full closed to full open and degraded performance due to 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 the damping and friction levels. See Friction - Hydraulic Components for further discussion of friction characteristics.
Hysteresis – how does the flow regulation change when approaching a control point from a low pressure condition or high pressure condition? Hysteresis affects flow control accuracy and contributes to chattering.
Flow Control Valve Qualification
See Qualification - Hydraulic Components for discussion on control valve qualification and required certification testing.