Flow Control Valve, Hydraulic - Description 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 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. 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).
Posted on: Wed, 06 Aug 2014 17:33:24 +0000
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