BASIC Flow Control Pressure Compensated Valve

Flow Control Valves

Myounggyu NohDept. Mechatronics EngineeringChungnam National University

Spring Semester, 2004

PLC and Fluid Power

1

Check Points of this Chapter

1. Difference between flow restricting and flow diverting types of flow control

2. Difference between compensated and noncompensated types of flow control valves

3. Effects of pressure and temperature changes on flow through flow control valves

4. Difference between meter-in and meter-out circuits

PLC and Fluid Power Flow Control Valves

2

Introduction

• The only way to control the speed of actuators is to control the rate at which thefluid flows into or out of the actuator.

• This control can be accomplished

– by varying the speed of a fixed displacement pump– by varying the output of a variable displacement pump– by using a flow control valve to meter the fluid flow at the required rate.

• Two ways to control the flow

– restricting the flow rate into or out of the component– diverting fluid flow away from the component


3

Flow Restricting Valves vs. Flow Diverting Valves

1. Flow restricting valve

• placed in series with the actuator• constitues a major restriction in the flow path• allows a portion of the fluid flow to pass through it and blocks the remaining, or

excess.• The blocked fluid is usually returned to the reservoir through the pressure relief

valve.

2. Flow diverting valve

• diverts a portion of the fluid flow back to the reservoir.


4

Needle Valves

• A needle valve is the simplest and most common of the flow restricting valves.

• Although a needle valve restricts flow in both directions, there is a preferred direction.

• When the flow is in this direction, the pressure loss across the valve is slightly lessand the valve is self–cleaning.

Wide OpenPartially OpenClosed


5

Needle Valve with Internal Check


6

Orifice

• An orifice restricts the amount of flow that can pass through it.

• Three factors affect the flow rate through the orifice

1. the size (area) of the orifice (A)2. the pressure drop across the orifice (∆p)3. the viscosity of the fluid, which is a function of the fluid temperature.

p2

A

p1


7

Orifice Equation

• The flow rate through an orifice can be written as

Q = C ×A

√2g∆p

γ

where C orifice constantA the area of the orificeg the acceleration of gravityγ the specific weight of the fluid

• Therefore, the flow rate is dependent on the pressure differential.

• To compensate for the upstream or downstream pressure varations, a needle valve isused.


8

Flow Rate through a Needle Valve

• When a needle valve is used,

Q = Cv

√∆p/Sg

where Cv is the flow coefficient and Sg is the specific gravity of the liquid.

• Cv: the volume flow rate of water at 60◦F that will flow through the valve in oneminute at a pressure drop of 1 psi.

• The flow cannot easily be controlled, unless the pressure drop remains constant.


9

Pressure–Compensated Flow Control Valves

Outlet

Inlet

• A compensator creates a pressure drop that keeps the pressure differential across thevalve constant.

• Thus, a constant downstream flow rate.

• Pressure–compensated valve is more suitable, when a steady actuator speed isrequired.


10

Flow Diverting Valves

Inlet

Bypass Outlet

• Divert a portion of the fluid flow back to the reservoir.

• Alternative terms: bypass flow control valves and three–port flow control valves.

• Often temperature–compensated as well.

• Less heating of the fluid and savings in input power, compared to restrictor typevalve.


11

Priority Valves

• Priority valves: usually used to ensure that certain essential system functions areperformed at the expense of nonessential functions.

• Proportional flow divider: divides the input flow between two circuits. Used inapplications such as differentially actuated power steering systems.

SecondaryPriorityOutlet

Inlet

Outlet


12

Meter-in or Meter-out Flow Control Circuits

• Meter–in circuit

– used only when the load is always resisting the force of the actuator.– suspended load can drop uncontrolled, even if the meter–in valve is fully closed.

• Meter–out circuit

– preferred over the meter–in technique– adds stability and prevent the uncontrolled drop of the load.– possibility of pressure intensification– big pressure drop across the flow control valve, resulting in high rate of heat

generation


13

Meter-in vs. Meter-out

METER−IN METER−OUT


14

Other Flow Control Circuits

Speed Control Using Three−Port Flow Control Valve

Bleed−off Flow Control Circuit


15

Pressure Drop and Temperature Rise

• Power loss due to the pressure drop across a flow control valve

HPLOSS =∆p × Q

1714

where ∆p pressure drop in psi

Q flow rate in gallon per minute (gpm)

PLOSS =∆p × Q

60000where ∆p pressure drop in kilopascal

Q flow rate in liter per minute (Lpm)

• Temperature rise due to the power loss

∆T =HGR

cp × W

where cp is the specific heat of the fluid and W is the weight flow rate (γ × Q)


BASIC Flow Control Pressure Compensated Valve

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