Pressure-compensated flow control valve applications
When pressure drop across an orifice changes, flow through the orifice also changes. As pressure drop increases, flow increases, and as pressure drop decreases, flow decreases. Because of this fact, if pressure drop across an orifice were constant, regardless of upstream and downstream pressure fluctuations, then flow through it would stay the same. A pressure-compensated flow control valve (such as the one shown in Figure 13-2) automatically maintains a constant pressure drop across the orifice. There is a short discussion on pressure-compensated flow control valves on page 13-1, but a valve in cutaway form is applied to a bleed-off circuit in Figure 13-10.
In the bleed-off circuit, fluid from the directional control valve is sent to the cylinder to start it extending. Because the circuit has a fixed-volume pump and needs speed control, a bleed-off flow control is used to save energy. Instead of controlling flow to or from the actuator, excess flow is bled to tank across a pressure-compensated flow control at whatever pressure it takes to move the fluid. A meter-in or meter-out flow control circuit would send excess flow to tank across the relief valve at maximum pressure – wasting a lot more energy.
The reason for using a pressure-compensated flow control is that pressure will fluctuate as the actuator moves toward the workpiece and the flow to tank from a non-compensated flow control would change continuously. As a result, actuator speed could vary considerably while it moves. With a pressure-compensated flow control, flow to tank is constant, but actuator speed could still change due to pump efficiency as pressure increases or decreases. Any speed change from pump efficiency is present but practically imperceptible.
In the Figure 10-13 circuit, a 10-gpm pump sends 7 gpm to the cylinder and 3 gpm to tank. Fluid entering the pressure-compensated flow control passes by the compensator spool and flows on to the variable knife-edge orifice, which is set at 3 gpm. The variable knife-edge orifice restricts flow and creates backpressure in the incoming fluid. When backpressure reaches (and attempts to exceed) 125 psi, fluid in the inlet-pressure pilot line forces the compensator spool to the right. This restricts flow at the compensating orifice. After the compensator spool settles in at its 125-psi bias-spring setting, pressure at PG3 reaches 125 psi and stays there. This means that pressure drop across the variable knife-edge orifice is 125 psi. As the cylinder continues to move and pressure at PG1 and PG2 increases or decreases, pressure at PG4 stays at 125 psi and flow is constant. The cylinder moves at the same speed whether pressure is at or above 125 psi, and as much as 125 psi below the maximum pressure setting.
Figure 13-11 shows a pressure-compensated flow control in a meter-in circuit. Fluid from the valve enters the flow control and is restricted. Backpressure from restricted flow goes through the inlet-pressure pilot line and shifts the compensator spool to the right, restricting flow to the variable knife-edge orifice. Backpressure from cylinder resistance acts on the right end of the compensator spool through the outlet-pressure pilot line and adds to the 125-psi bias-spring force. This action and interaction always keeps pressure 125 psi higher at PG5 than at PG2. A constant pressure drop across the orifice maintains a constant flow to the cylinder.
Figure 13-12 shows a pressure-compensated flow control in a meter-out circuit. Fluid from the cylinder rod end enters the pressure-compensated flow control and is restricted at the variable knife-edge orifice. Backpressure through the inlet-pressure pilot line shifts the compensator spool to the right and restricts flow to the variable knife-edge orifice. Pressure at PG5 settles in at 125 psi and flow stays the same across the variable knife-edge orifice. Any backpressure from tank flow adds to the 125-psi bias-spring force and increases pressure at PG5 so it always stays 125 psi above PG4.
Pressure-compensated flow control valves are as much as five times more expensive than non-compensated models, so they should not be specified when accurate flow control is not required.
Changes in fluid viscosity also cause flow fluctuations. Thick fluid flows more slowly than thin fluid. A flow control valve without temperature compensation allows varying flow from cool oil at startup to oil running at normal or high temperature. The most common fix for viscosity variations is to use a knife-edge orifice. Knife-edge orifices have no flats to slow fluid flow, so they produce little change in flow between thick and thin fluids. Other devices to obtain constant flow with viscosity variations are available, but they can be complex and may cause malfunctions.
A flow control in a hydraulic circuit always generates heat. Some pump and flow control combinations produce a lot more heat and should be avoided if possible. The following examples show different pump and flow control combinations and suggest how much heat can be expected.
The fixed-volume pump and meter-in or meter-out flow control combination in Figure 13-13 is the worst-case situation. The example shows a cylinder stroking to the workpiece with flow controls set at 3 gpm. A 10-gpm pump driven by a 5-hp electric motor powers the circuit. Because it only takes 100 psi to move the cylinder while traversing, a lot of heat-generating energy is wasted. This example is somewhat exaggerated, but is not at all unheard of. Note the example only shows energy wasted on the extension stroke. With a reduced-speed retraction stroke, heat generation could almost double the figures shown.
The main generator of heat is the excess pump flow going across the relief valve at 1000 psi. The two circuits in Figure 13-14 show how to eliminate such wasted energy with a different flow control circuit or a different pump. While the energy wasted across the flow control valve is much less at these low flows, it still adds heat to a system. Also, the amount of pressure drop may be lower than indicated here because some actuators require more pressure to move them to and from the workpiece. Energy loss across a flow control cannot be eliminated. The amount of loss depends on pressure drop and flow rate across the orifice.
The circuits in Figure 13-14 show a fixed-volume pump with a bleed-off circuit and a pressure-compensated pump with a meter-in circuit. Both of these combinations save a lot of energy (although not as much as the load-sensing circuit that was shown in Figure 8-27). This type of flow control circuit wastes the least energy possible when using flow controls for speed control.