Flow Valves

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. 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