Flow is controlled by either throttling or diverting it. Throttling involves reducing orifice size until all of the flow cannot pass through the orifice; bypassing involves routing part of the flow around the circuit so that the actuator receives only the portion needed to perform its task. If the flow inlet to an actuator is controlled, the circuit is said to be a "meter-in" system. If actuator outlet is controlled, it is called a "meter-out" circuit. When that part of the fluid being diverted to the reservoir or another part of the circuit is controlled, it is said to be a "bleed-off" system.

Noncompensated flow controls are simple valves that meter flow by restricting or throttling. The amount of flow that passes through an orifice and the pressure drop across it are directly related. As pressure increases, valve flow increases.

Common noncompensated valves are adjustable needle valves; flow through them varies with fluid viscosity and pressure across the valve. Usually, a needle valve is paired with a check valve that offers resistance to flow in one direction only. The combination permits flow to be adjusted in one direction, with free flow upon reverse. This type of two-valve combination is typically called an adjustable restrictor valve.

For some tasks, adjustability is either unimportant or potentially harmful. For such tasks, a fixed resistor valve can be used. Basically, it consists of a check valve with an orifice embodied in the valve. Some fixed restrictors make provision for disassembling the valve and changing the orifice; others have no such provision. In either type, the orifice is not changed during circuit operation so the valve is considered nonadjustable.

Both fixed and variable restrictor valves are simple, reliable, and inexpensive. They do not accurately control flow if load or viscosity changes. They can be used in any circuit, using any metering method. Experts recommend these noncompensated valves when accuracy is not important, when heat generation through power loss can be tolerated, and in such circuits as gravity lowering, where they can be used efficiently.

Pressure-compensated flow controls maintain nearly constant flow despite variations in circuit pressure. Like the noncompensated units, they incorporate a metering orifice. Flow pressure drop across this orifice is used to shift a balanced spool against a control spring. This spool movement is used to maintain a constant pressure drop across the orifice, which in turn, produces a constant flow. Pressure drop across the orifice is relatively low.

Check valves use a ball or poppet to prevent flow in one or more directions. In two-port valves, the ball or poppet is usually lightly spring loaded against one of the ports. In three-port valves, or shuttle valves, internal ridges guide the ball between ports.

Restrictive flow regulators work like an automatic variable orifice to control flow by throttling or restricting. Compensator spool movement blocks fluid flow through the valve. Flow passing through the metering orifice is accompanied by a pressure drop that is applied to each end of a balanced spool. The resulting force imbalance moves the spool against the control spring. Spool movement progressively blocks off flow area restricting or throttling flow through the valve.

Restrictive flow regulators are ideally suited to constant-pressure closed-center circuits and meter-out situations. Experts say they are the only pressure-compensated flow control that can be used in these applications. They are also recommended for gravity lowering devices where uniform lowering speed is required regardless of the load.

Bypass flow regulators control flow by diverting excess pump output to the reservoir. The same basic control orifice and compensator spool are used as in the restrictive flow regulator. But, instead of restricting flow through the valve, spool movement diverts or bypasses excess flow to the reservoir.

These regulators are used exclusively in variable-pressure open-center circuits, and only as a meter-in device. The resulting pump or supply pressure is slightly higher than that required to do the work, and automatically changes with load. Bypass flow regulators cannot be used as meter-out devices in any circuit or as meter-in devices in constant-pressure circuits. The ability of these valves to accept all flow supplied to them excludes them from these applications.

Combination bypass and restrictive flow regulators are a combination of the first two mentioned. They control flow by both restricting and bypassing, permitting full use of both regulated and bypass flow. Flow through a controlled orifice produces a pressure that shifts a compensating spool. Movement of the spool first uncovers the bypass-flow area. If bypass-circuit pressure is greater than regulated-circuit pressure, the spool moves farther to restrict or throttle the controlled flow. Regardless of pressures in either circuit, the combination flow regulator maintains a constant controlled flow. Full pump flow is at the higher of the two pressures.

Combination flow regulators are sometimes called "priority" valves. They establish priority flow to the control circuit and bypass to the secondary circuit only when the flow demands of the primary circuit are met. If pump supply is less than that required at the regulated port, all flow goes to the regulated port and none is diverted. This type of combination valve is ideally suited for meter-in speed control in open-center fixed-displacement pump circuits.