1. Hydraulic Flow Control • Metering Fluid Flow
Chapter 6
Flow Control
Hydraulic Flow Control • Metering Fluid Flow

2. A pressure-compensated flow control valve compensates for the changing pressures in a hydraulic system.
A pressure-compensated flow control valve compensates for the changing pressures in a hydraulic system. See Figure 6-1. The amount of fluid flow through an orifice is directly proportional to the pressure differential across the orifice. When there is a pressure differential in a flow control valve, piston speed is determined by applying the following procedure:
1. Calculate the pressure needed to move a piston when there is a load on the cylinder.
PF = F / Apf
where
PF = pressure created by the load on the cylinder (in psi)
F = load on the cylinder (in lb)
Apf = piston face area (in sq in.)
…Complete procedural list on page 182.

3. A variable orifice allows a variable amount of fluid flow.
A variable orifice is an orifice that allows an adjustable amount of fluid flow. See Figure 6-2. The diameter of a variable orifice can be adjusted by turning a handwheel, handle, or knob attached to the valve stem. The schematic symbol for a variable orifice is the same as the symbol for a fixed orifice but with a diagonal arrow through it. A variable orifice is a valve.

4. Variable flow control in a hydraulic system can be accomplished with globe valves, gate valves, ball valves, or needle valves.
When a flow control valve controls fluid flow, it causes internal resistance that increases pressure from the valve back to the pump. The increased pressure opens the relief valve, allowing excess fluid flow to return to the reservoir while metered (reduced) fluid flow continues to a branch system or hydraulic actuator. In some cases, the excess fluid flow is used in another part of the hydraulic system rather than returning to the reservoir. Noncompensated flow control in a hydraulic system can be accomplished with globe valves, gate valves, ball valves, or needle valves. See Figure 6-3. Most hydraulic systems use needle valves. Most hydraulic systems use needle valves to meter fluid flow instead of globe, gate, and ball valves, which are typically used as shut-off valves and only intended to start and stop the flow of hydraulic fluid. These valves have two connections, an inlet and an outlet, and are also referred to as two- way valves.

5. A globe valve has a disk that is raised or lowered over an orifice.
A globe valve is a valve with a disk that is raised or lowered over an orifice. See Figure 6-4. Globe valves are designed as shut-off valves for low-pressure service, typically no higher than 150 psi. They have seats that are slightly smaller than the means of transmission. They do not have a straight path for fluid flow, which must make two 90° turns to pass in one direction through the valve. The opening between the seat and the disk is controlled to meter fluid flow from no- to full-flow operations. An arrow placed on the side of the valve body indicates the direction of fluid flow through a globe valve.

6. A gate valve has an internal gate that slides over an orifice.
A gate valve is a two‑position valve with an internal gate between the two seats of the orifice. See Figure 6-5. Gate valves are generally used for full-flow or no-flow operations and are not designed for restricting fluid flow. Gate valves have ports that are the same size as the means of transmission. This causes the hydraulic fluid to flow in a straight path through the valve, allowing only a slight pressure drop in the system when the valve is fully open. Some gate valves tend to provide smooth fluid flow, however, vibration and erratic fluid flow can occur when a gate valve is used in a partially or completely open position.

7. A ball valve has a ball with a hole drilled through the center to allow fluid flow.
A ball valve is an infinite-position flow control valve with a ball that has an orifice through the center to allow fluid flow. See Figure 6-6. Ball valves allow hydraulic fluid to flow in a straight path, which causes less internal resistance and wastes less energy.

8. A needle valve has a narrowly tapered stem or needle positioned in-line with an orifice that is the same size as the stem or needle.
A needle valve is an infinite-position flow control valve that has a narrowly tapered stem or needle positioned in-line with an orifice that is the same size as the stem or needle. See Figure 6-7. Needle valves are flow control valves used in speed-control applications. Fluid flow takes two 90° bends through a needle valve, allowing the needle to create a pressure differential. Some needle valves are available with color-coded valve stems, which allow the technician to view any adjustments that are made. Needle valves are the most frequently used valves in industrial hydraulic applications.

9. A pressure-compensated flow control valve changes flow due to changes in pressure before or after the valve to keep flow constant from the valve.
A pressure-compensated flow control valve is a flow control valve that changes flow due to changes in pressure before or after the valve to keep flow constant from the orifice. See Figure 6-8. The size of the orifice in the valve is determined by a needle valve, while a spring and spool compensate for any change in pressure. Stable fluid flow occurs when the spool, assisted by the spring and controlled orifice, maintains a constant pressure differential across the valve.

10. A temperature- and pressure-compensated flow control valve compensates for changes in hydraulic fluid temperature and pressure.
A temperature- and pressure-compensated flow control valve is a flow control valve that compensates for changes in hydraulic fluid temperature and pressure. See Figure 6-9. Hydraulic fluid viscosity changes as temperature changes, which affects flow rate and piston speed. The higher the temperature, the easier it is for hydraulic fluid to flow. The lower the temperature, the more difficult it is for hydraulic fluid to flow.

11. When calculating the piston speed of a cylinder with no load, the two variables that are taken into consideration are the flow rate and the piston face area.
When calculating the piston speed of a cylinder with no load, the two variables that are taken into consideration are the flow rate and the piston face area. See Figure Depending on the length of the rod, piston speed can be measured either in feet per minute (fpm) or inches per minute (ipm). Piston speed in fpm is calculated by applying the following formula:
Ps = (Q × 19.25) / Apf
where
Ps = piston speed (in fpm)
Q = flow rate (in gpm)
19.25 = constant
Apf = piston face area (in sq in.)

12. Metering fluid flow in one direction is accomplished with a combination of a flow control valve and a check valve.
In many hydraulic systems, fluid flow is metered in only one direction. This is accomplished with a combination of a flow control valve and a check valve. See Figure By placing a flow control valve and a check valve in reverse parallel with each other, it is possible to meter fluid flow in one direction and allow unrestricted or full flow in the opposite direction. This combination is usually referred to as a flow control valve with a check or internal check. Flow control valves with checks are used in most applications where a flow control valve is installed. Anytime full flow is required in the opposite direction of metered flow, flow control valves with check are used.

13. A single-acting cylinder uses metered fluid flow for extension, and spring, gravity, or other mechanical means for retraction.
A single-acting cylinder is a cylinder that uses fluid flow for extension of movement and spring, gravity, or other mechanical means for retraction. See Figure When hydraulic fluid flows into the cylinder, the piston and rod extend. When hydraulic fluid flows out of the cylinder, the piston and rod retract. A flow control valve with a bypass is used to control one direction of piston movement and speed in a single-acting cylinder.

14. A single-acting cylinder uses a meter-in system and meter-out flow control valve to control piston speed in both directions.
Another common method of controlling piston speed during extension and retraction is to place the meter-in flow control valve between the cylinder and the directional control valve. The meter-out flow control valve is placed between the directional control valve and the reservoir. See Figure 6-13.

15. Either port of a double-acting cylinder can function as an input or output, depending on whether the cylinder is extending or retracting.
Either port of a double-acting cylinder can function as an input or output, depending on whether the cylinder is extending or retracting. See Figure During extension, fluid flow enters the cap end of the cylinder with enough pressure to extend the piston and rod. Fluid flow then exits the rod end of the cylinder without restricting the movement of the piston and rod. During retraction, fluid flow enters the rod end of the cylinder with enough pressure to retract the piston and rod. Fluid flow then exits the cap end of the cylinder without restricting the movement of the piston and rod.

16. To control piston speed in both directions using meter-in, two flow control valves with check valves are placed between the directional control valve and the cylinder.
To control piston speed in both directions using meter-in, two flow control valves with bypasses are placed between the directional control valve and the cylinder. See Figure One is attached to the cap-end port and the other is attached to the rod-end port.

17. On a schematic diagram, the bypass check valve arrow points toward the cylinder for meter-in systems and away from the cylinder for meter-out systems.
Meter-out systems are typically used when a change from high pressure to low pressure may occur, such as a drill press breaking through a workpiece. Meter-out systems help prevent uncontrolled movement of a cylinder by metering hydraulic fluid that is exiting the cylinder. Meter-out systems are more commonly used in industrial hydraulic applications than meter-in systems. On a schematic diagram, the bypass check valve arrow points towards the cylinder for meter-in systems and away from the cylinder for meter-out systems. See Figure 6-16.

18. A bleed-off system uses a flow control valve without a bypass.
Bleed-off meters fluid flow to and from a cylinder by returning (bleeding off) some of the hydraulic fluid to the reservoir. Flow control valves used for bleed-off do not require a check valve. See Figure The flow control valve is connected to the line entering the cylinder and to the reservoir. Bleed-off systems take excess fluid flow from the pump and direct it through the flow control valve to the reservoir. For example, if there is a flow rate of 12 gpm at the pump and 4 gpm is sent to the reservoir, then there is a flow rate of 8 gpm to the cylinder. Bleed-off systems can be used in applications such as reciprocating grinding operations and the vertical lifting of a load.