HYDRAULICS & PNEUMATICS Presented by: Dr. Abootorabi Flow Control Valves 1

HYDRAULICS & PNEUMATICS

Presented by: Dr. Abootorabi

Flow Control Valves

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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

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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.

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Flow Control Devices

Circuit containing a restrictor-type flow control valve

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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.

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Flow Control Devices

Circuit containing a bypass-type flow control valve

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Flow Control Devices

Conceptual operation of a flow control valve may be traced to a basic

orifice.

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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

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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

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Flow Control Devices

Noncompensated and compensated flow control valves

may have:

Fixed flow rate

Adjustable flow rate

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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

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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

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Flow Control Devices

Basic adjustable flow control valve:

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A simple needle valve

without check valve is

called also metering

valve.

Flow Control DevicesAdjustable restrictors:

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Flow Control Devices

Adjustable restrictors:

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Flow Control Devices

One-way flow control valve:

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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.

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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

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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.

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Flow Control Devices

A basic pressure-compensated flow control valve

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Flow Control Devices

Pressure compensator operation:

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Flow Control Devices

Pressure compensator operation:

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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.

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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

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Flow Control DevicesTemperature compensation using sharp-edged orifice:

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Flow Control Devices

Temperature compensation using a heat-sensitive metal rod:

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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

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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

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Flow Control DevicesCircuit containing restrictor-type, compensated flow control valve

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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

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Flow Control Devices

Typical bypass-type

flow control valve:

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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.32

Flow Control Devices

Operation of a bypass flow control valve during increasing or

decreasing load:

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Flow Control Devices

Operation of a bypass flow control valve during steady load:

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Flow Control Devices

Operation of a bypass flow control valve with stalled actuator:

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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 36

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.

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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.

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Flow Control Devices

Typical priority valve:

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Flow Control DevicesCircuit containing a priority divider valve

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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.

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Flow Control DevicesA typical use of proportional flow divider valve:

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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.

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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

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Design and Operation of Basic Flow-Related Circuits

Basic meter-in circuit

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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

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Design and Operation of Basic Flow-Related Circuits

Basic meter-out circuit

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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

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Design and Operation of Basic Flow-Related Circuits

Basic bleed-off circuit

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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.

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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

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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.

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