Pressure-compensated flow control valves

Figure 6.35 illustrates the operation of a pressure-compensated valve. The design incorporates a hydrostat which maintains a constant 1.4kg/cm2 (20 psi) pressure differential across the throttle which is an orifice, whose area can be adjusted by an external knob setting. The orifice area setting determines the flow rate to be controlled. The hydrostat is normally held open by a light spring. However, it starts to close as inlet pressure increases and overcomes the spring tension. This closes the opening through the hydrostat, thereby blocking all the flow in excess of the throttle setting. As a result, the only amount of fluid that can flow through the valve is that amount which a 1.4 kg/cm2 (20 psi) pressure can force through the throttle.

To understand better the concept of pressure compensation in flow control valves, let us try and distinguish between flow control in a fixed displacement pump and that in a pressure-compensated pump. Figure 6.36 is an example of flow control in a hydraulic circuit with fixed volume pumps.

In this system, a portion of the fluid is bypassed over the relief valve in order to reduce flow to the actuator. Pressure increases upstream as the flow control valve, which in this case is a needle valve, is closed. As the relief pressure is approached, the relief valve begins to open, bypassing a portion of the fluid to the tank.

Flow control in a pressure-compensated pump as illustrated in Figure 6.37, is different in that the fluid is not passed over the reUef valve. As the compensator setting pressure is approached, the pump begins the de-stroking operation, thereby reducing the outward flow.

The design of a pressure-compensated flow control valve is such that it makes allowances for variations in pressure, before or after the orifice. In a pressure-compensated flow control valve, the actuator speed does not vary with variation in load.

Globe Valve

This is the simplest form of a flow control valve. The globe valve gets its name from the disk element ‘globe’ that presses against the valve seat to close the valve.

A simplified view of a globe valve has been illustrated in Figure 6.29.

The fluid flow through the valve is at right angles to the direction of flow in pipes. When this valve is opened, the entire surface of the globe moves away from the valve seat at once. Due to this action, a globe valve provides an excellent means of throttling the flow. In a hydraulic system, the globe valve can be operated either manually by means of a hand wheel or mechanically by means of an actuator.

Hydraulic Flow-Control Valves

Flow-control valves are used to control an actuator’s speed by metering flow. Metering is measuring or regulating the flow rate to or from an actuator. A water faucet is an example of a flow-control valve. Flow rate varies as a faucet handle is turned clockwise or counterclockwise. In a closed position, flow stops. Many flow-control valves used in fluid-powered systems are similar in design and operation to a water faucet’s.

In hydraulic circuits, flow-control valves are generally used to control the speed of hydraulic motors and work spindles and the travel rates of tool heads or slides. Flow-control valves incorporate an integral pressure compensator, which causes the flow rate to remain substantially uniform regardless of changes in workload. A nonpressure, compensated flow control, such as a needle valve or fixed restriction, allows changes in the flow rate when pressure drop through it changes.

Variations of the basic flow-control valves are the flow-control-and-check valves and the flow-control-and-overload relief valves. Models in the flow-control-and-check-valve series incorporate an integral check valve to allow reverse free flow. Models in the flow-controland- overload-relief-valve series incorporate an integral relief valve to limit system pressure. Some of these valves are gasket-mounted, and some are panel-mounted.

a. Gate Valve. In this type of valve, a wedge or gate controls the flow. To open and close a passage, a handwheel moves a wedge or gate up and down across a flow line. Figure 5-30,shows the principal elements of a gate valve. Area A shows the line connection and the outside structure of the valve; area B shows the wedge or gate inside the valve and the stem to which the gate and the handwheel are attached. When the valve is opened, the gate stands up inside the bonnet with its bottom flush with the wall of the line. When the valve is closed, the gate blocks the flow by standing straight across the line where it rests firmly against the two seats that extend completely around the line.

A gate valve allows a straight flow and offers little or no resistance to the fluid flow when the valve is completely open.
Sometimes a gate valve is in the partially open position to restrict the flow rate. However, its main use is in the fully open or fully closed positions. If the valve is left partly open, the valve’s face stands in the fluid flow, which will act on the face and cause it to erode.

b. Globe Valve. A disc, which is screwed directly on the end of the stem, is the controlling member of a globe valve. A valve is closed by lowering a disc into a valve seat. Since fluid flows equally on all sides of the center of support when a valve is open, there is no unbalanced pressure on a disc to cause uneven wear. Figure 5-31 shows a globe valve.

c. Needle Valve. A needle valve is similar in design and operation to a globe valve. Instead of a disc, a needle valve has a long, tapered point at the end of a valve stem. Figure 5-32 shows a sectional view of a needle valve. A long taper allows a needle valve to open or close gradually. A needle valve is used to control flow—

• Into delicate gauges, which could be damaged if high-pressure fluid was suddenly delivered.
• At the end of an operation when work motion should halt slowly.
• At other points where precise flow adjustments are necessary.
• At points where a small flow rate is desired.

d. Restrictor. A restrictor is used in liquid-powered systems to limit the movement speed of certain actuating devices by limiting flow rate in a line. Figure 5-33 shows a fixed restrictor. Figure 5-34 shows a variable restrictor, which varies the restriction amount and is a modified needle valve. This valve can be preadjusted to alter the operating time of a particular subsystem. Also, it can be adjusted to meet the requirements of a particular system.

e. Orifice Check Valve. This valve is used in liquid powered systems to allow normal speed of operation in one direction and limited speed in another. Figure 5-35 shows two orifice check valves.

f. Flow Equalizer. A flow equalizer (flow divider) is used in some hydraulic systems to synchronize the operation of two actuating units. An equalizer divides a single stream of fluid from a directional-control valve into two equal streams. Each actuating unit receives the same flow rate; both move in unison. When the two streams of return fluid operate in opposite directions, a flow equalizer combines them at an equal rate. Thus, a flow equalizer synchronizes the actuating
units’ movements during both operational directions.

Figure 5-36, shows one type of flow equalizer; the valve is in the splitting (divided-flow) position. Fluid, under pressure from the directional control valve, enters port 3. This pressure overcomes spring tension and forces plug 4 down and uncovers the two orifices in sleeve 2. The fluid then splits and should flow equally through side passages 1 and 5. The fluid flows through—
• Splitting check valves 7 and 15.
• Metering grooves 10 and 14.
• Ports 9 and 13.
• The connecting lines to the actuating cylinders.

Any difference in the flow rate between the two passages results in a pressure differential between these two passages. Free-floating metering piston 11 shifts to equalize the internal pressure, equalizing the flow.