HYDRAULICS & PNEUMATICS Flow Control Valves Presented by: Dr. Abootorabi

Flow Control Devices Flow control devices produce the desired rate of actuator operating speed by controlling the volume of fluid allowed to reach the actuator. Flow control devices can be divided into two general types: Restrictor Bypass

Flow Control Devices Restrictor-type flow control valves limit the volume of fluid through the valve. Excess pump output is forced to return to the reservoir through the system relief valve.

Flow Control Devices Circuit containing a restrictor-type flow control valve

Flow Control Devices Bypass type flow control valves use an integral control port to return excess pump output to the reservoir. The returned fluid is at a pressure less than system relief valve pressure.

Flow Control Devices Circuit containing a bypass-type flow control valve

Flow Control Devices Conceptual operation of a flow control valve may be traced to a basic orifice.

Flow Control Devices The flow rate through a simple, sharp-edged orifice depends on: Area of the orifice Pressure difference between the inlet and outlet sides of the orifice Viscosity of the fluid, which varies with fluid temperature Simplified formula:

Flow Control Devices Flow control valves may be noncompensated or compensated: The flow rate through noncompensated valves varies as the load or fluid viscosity changes Compensated valves automatically adjust for fluid pressure variations to produce a consistent flow rate under varying load and temperature conditions

Flow Control Devices Noncompensated and compensated flow control valves may have: Fixed flow rate Adjustable flow rate

Flow Control Devices The simplest restrictor-type flow control valve is a simple orifice: Basically a calibrated hole Serves as a noncompensated, fixed-rate flow control device

Flow Control Devices A needle valve is the simplest restrictor-type, noncompensated adjustable flow control device: Consists of an orifice fitted with a tapered needle machined on a threaded stem Turning the threaded stem changes the effective area of the orifice, which adjusts the flow rate through the valve

Flow Control Devices Basic adjustable flow control valve: A simple needle valve without check valve is called also metering valve.

Flow Control Devices Adjustable restrictors:

Flow Control Devices Adjustable restrictors:

Flow Control Devices One-way flow control valve:

Flow Control Devices When using a restrictor-type, noncompensated flow control valve, actuator speed varies when system loads change. Caused by the change in pressure drop across the control valve, which varies the flow rate through the valve.

Flow Control Devices A pressure compensator maintains a constant pressure difference across the metering orifice of a flow control valve: Senses pressure on the inlet and outlet sides of the orifice These pressures generate forces that act on the end surfaces of a sliding spool that is preloaded by a biasing spring

Flow Control Devices Force generated by the biasing spring establishes the constant pressure difference across the orifice. This constant pressure difference maintains constant fluid flow through the valve even when system loads change.

Flow Control Devices A basic pressure-compensated flow control valve

Flow Control Devices Pressure compensator operation:

Flow Control Devices Pressure compensator operation:

Flow Control Devices Temperature compensation is necessary in flow control devices if an accurate, consistent flow rate through a valve is needed. This is due to the fluid viscosity changes that occur as fluid temperature changes.

Flow Control Devices Temperature compensation is typically accomplished in flow control devices by: Specially designed, sharp edged orifice Heat-sensitive metal rod that operates a needlelike control device in the metering orifice of the valve

Flow Control Devices Temperature compensation using sharp-edged orifice:

Flow Control Devices Temperature compensation using a heat-sensitive metal rod:

Flow Control Devices In a circuit using a restrictor-type, pressure-compensated flow control valve: Pressure drop across the internal flow-control device in the valve remains constant, which produces a constant flow rate through the valve Actuator speed will not vary when system loads change

Flow Control Devices In a circuit using a restrictor-type, temperature-compensated flow control valve: Internal flow-control device is adjusted for viscosity variations that occur during fluid temperature changes Flow remains constant as system operating temperatures change

Flow Control Devices Circuit containing restrictor-type, compensated flow control valve

Flow Control Devices Bypass-type flow control valves: Provide partly accurate flow to actuators Direct any excess flow from the pump directly to the reservoir through an integral port

Flow Control Devices Typical bypass-type flow control valve:

Flow Control Devices The operating pressure of a system using a bypass-type flow control valve is determined by the load on the actuator plus the pressure needed to overcome the force of the biasing spring. The relief valve functions only when actuator loads are great enough to increase system pressure above the cracking pressure of the relief valve.

Flow Control Devices Operation of a bypass flow control valve during increasing or decreasing load:

Flow Control Devices Operation of a bypass flow control valve during steady load:

Flow Control Devices Operation of a bypass flow control valve with stalled actuator:

Flow Control Devices The bypass flow control design provides an efficient operating flow control circuit Pressure in the system is only as high as needed to move the load and operate the valve compensator This reduces system heat generation and energy consumption Care must be taken to accurately determine actuator loads and the cracking pressure of the system relief valve

Flow Control Devices Priority and proportional divider valves are designed to divide one fluid supply between two circuit subsystems. Many of these valves can also be used to combine the flow from two different circuits.

Flow Control Devices Priority divider valves provide flow to one port before providing flow to a second port. Often used in mobile equipment where pump output is controlled by engine speed.

Flow Control Devices Typical priority valve:

Flow Control Devices Circuit containing a priority divider valve

Flow Control Devices Proportional divider valve splits input port flow into two proportional output flows. Ratio between the output flows may be fixed or variable. Ratio of 50-50 is most common.

Flow Control Devices A typical use of proportional flow divider valve:

Design and Operation of Basic Flow-Related Circuits Three basic flow control circuits are used in hydraulic systems: Meter in Meter out Bleed off These basic circuits meet the varying flow-control demands for systems with positive and negative loads.

Design and Operation of Basic Flow-Related Circuits The meter-in flow control design places the flow control valve between the pump and the inlet of the actuator: Should only be used for positive loads Cannot provide accurate control under a negative load condition The prime mover is always operating against the maximum pressure setting of the system relief valve

Design and Operation of Basic Flow-Related Circuits Basic meter-in circuit

Design and Operation of Basic Flow-Related Circuits The meter-out flow control design places the flow control valve between the actuator outlet and the reservoir: Can provide accurate control for positive and negative loads The prime mover is always operating against the maximum pressure setting of the system relief valve

Design and Operation of Basic Flow-Related Circuits Basic meter-out circuit

Design and Operation of Basic Flow-Related Circuits The bleed-off flow control design places the flow control valve in a tee in the working line between the directional control valve and the actuator inlet: Outlet of the flow control is directly connected to the reservoir Measured flow is diverted to the reservoir while remaining flow operates the actuator The prime mover operates against a pressure only high enough to move the load

Design and Operation of Basic Flow-Related Circuits Basic bleed-off circuit

Design and Operation of Basic Flow-Related Circuits Meter-in and meter-out circuits provide the most accurate actuator speeds. Both meter fluid flow delivered directly to or from the actuator. The meter-out circuit is the best method for negative loads that may pull the actuator.

Design and Operation of Basic Flow-Related Circuits The bleed-off flow control circuit is less accurate than either the meter-in or meter-out system: Flow is metered back to the reservoir while the remaining pump output establishes actuator speed The remaining flow can vary because of pump efficiency and system leakage Flow control valve metering accuracy under varying load conditions is also a factor

Design and Operation of Basic Flow-Related Circuits The bleed-off circuit is the most energy-efficient design. This is due to the fact the prime mover operates at a pressure only high enough to move the load.

The end.