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Final Control ElementFinal Control Element



Final Control ElementFinal Controlm v

emencontroller

output

manipulatedvariable

(actuating signal)⇓

Actuator

Control

valve

displacement

or position 'x'

mv

Actuator rovides an out ut osition ro ortional to the

input signal

Control Valve adjusts the value of the manipulated

variable



Final Control ElementFinal Control

Element

m v

controller

out ut

manipulatedvariable

(actuating signal)⇓

ActuatorControl

displacement

' '

mv

,

response, moves the valve to a fully-open or fully-closed position, or

a more open or a more closed position (depending on whether

on o or con nuous con ro ac on s use .



Actuators

Pneumatic

spring –

diaphragm

actuators

piston

motor

Actuators

ec ro-pneuma c

actuators

Motorized-

Electric

actuators

rotary or linear

Solenoid

Hydraulic

operated

actuators



Pneumatic Actuators

–

springinput

m(3-15 psi)

diaphragm

backing plate

stem

output position

constantthrust force

x

Diaphragm actuators have compressed air applied to a flexible

membrane called the diaphragm.



Pneumatic Actuators

–

springinputressure

At equilibrium, (assuming no

m(3-15 psi)

diaphragm

stem): mA = Kx

backing plate output position m = the change in input pressurestem constant

thrust force

x A = the effective area of the diaphragm,

K = the spring constant (including

diaphragm),

=

Note: The actual value of ‘x’ (stroke length) is limited within

″ ″an ″ epen ng on e s ze o e ac ua or.



Spring – Diaphragm

type ctuators

Another Variation:



Direct-Acting and Reverse-Acting



Direct-Acting and Reverse-Acting

--

( spring-to-extend )

( spring-to-retract)

The diaphragm is pushed

upwards, pulling the spindle up,

and if the spindle is connected to a

This actuator is designed with

the spring below the diaphragm,

having air supplied to the space

rect act ng va ve, t e p ug sopened. With a specific change of

air pressure, the spindle will move

above the diaphragm. Theresult, with increasing air

pressure, is spindle movement in

through its complete stroke.e oppos e rec on o e

reverse acting actuator.



Pneumatic Actuators

Why Positioners are provided with Actuators ?

To overcome high static friction forces in the actuators.

To improve response time.

To improve linearity and to reduce hysteresis.

o re uce oa ng on con ro er ou pu .

n case o us ng ac ua ors, we ave non- near es ue o ap ragmarea and the spring constant. So the change in position due to change

in controller output is non-linear. With the use of positioners we can

- .



Pneumatic Actuators

Features

A positioner ensures that there is a linear relationship between the signal

input pressure from the control system and the position of the control valve.

A positioner may be used as a signal amplifier or booster.

Some positioners can also act as basic controllers, accepting input from

electrical input (typically 4 - 20 mA) can be used to control a pneumatic valve.

sensors.



Pneumatic Actuators

–

n

Relayn

gain = 1

restriction

Air supply

spring

Bellows

m

b

c

a0

Feedback

lever

in ut ressure

n

b x

an nozzle back pressure

x

stem

22 a y b ⎟

⎠⎜⎝

−=−



Pneumatic Actuators

–

ngain = 1

n At equilibrium, Baffle-Nozzle

se aration is:e ay

Air supply(Ps)

spring

m

BellowKbm

c

0

Feedback

lever

restriction

n

2 2bK mb x y

a⎛ ⎞= + −⎜ ⎟⎝ ⎠Bellows

c ba

b x


input pressure y

x

stem

22

x

a

bmK y b ⎟

⎠

⎞⎜⎝

⎛ −=−

b

Nozzle back pressure: n = – Kn. y

K is the nozzle gain

The change in output pressure is related to the back pressure as:

Kx = nA, K is the spring constant and A is the effective area of diaphragm.

Kxn A

=n n n

n nA Kx yK K A K A

= − = − = −⎜ ⎟⎝ ⎠ 2 2

b

n

K m b K xa K A

⎛ ⎞= +⎢ ⎥⎜ ⎟⎝ ⎠⎣ ⎦

Now, , 0, and 0.n

n

K A K yK A

>> ≈ ≈ bK m xa

≈ ⎜ ⎟⎝ ⎠



Pneumatic Actuators

–

ngain = 1

n

Air supply(Ps)

spring

BellowKbm

c Feedback

restriction

Bellows

m

c ba

0lever

b x


input pressure y

n

x

stem

22

x

a

bmK y b ⎟ ⎞⎜

⎝ ⎛ −=−

Conclusion:

,

only feedback lever ratio and the bellows stiffness factor, and it is not dependent

(if KnA >> K) on the spring-diaphragm non-linearities.

As a >> b, large position change is obtained with a small change in bellows

position, thus ensuring linear characteristic of the bellows.



Pneumatic Actuators

m

iston

spring

x

They are generally used where the stroke of a diaphragm actuator would

be too short or the thrust is too small.

They are used for long strokes.



Pneumatic Actuators

m

+AirMotor

spring

x

They are used for large thrust forces. Large torques are generated

from motor-gear arrangements to balance large thrusts.



Electro-pneumatic Actuators

When the controller output is electrical and a suitable air supply is available,

using an electro-pneumatic actuator, a large output power may be obtained from

a ow power con ro s gna .

cascading an electro-pneumatic

converter an a pneumat c actuator

-




-

Electro-' '

pneumatic

pneumaticConverter

signal(current4-20mA)

input

output(3-15 psi)

'm'




-

input

S

i

Nozzle

former (support system)

pneumaticoutput (m)

air su l

S

NCoil

Motora

(Ps)

Restriction

balance beam m

b

voice coil motor

- linear motor

Feedback bellows




-

S

inputcurrent

i

Nozzle

former (support system)pneumaticoutput (m)

air supply

voicecoil

used inloud speakers

S

N

o ceCoil

Motora

balance beam m

b

(Ps)Restriction

voice coil motor- linear motor

m

S

+ Force

Feedback bellowsN

= BlNi

B flux density

l mean length / turnN no. of turns

Effective force

ex erienced b the

former is a linear one.

force




-input

currenti

former (support system)pneumatic

output (m)

At equilibrium, for a change in input current

‘ ’ ‘ ’

S

N

S

Voice

Coil

Motor

a

Nozzleair supply

(Ps)

Restriction

voice coil motor

m

,

( )bmAbiln B.a =balance beam

m

b

- linear motor

Feedback bellows

b

a small Baffle-Nozzle separation.

b

m ibA= ⎜ ⎟

⎝ ⎠




-

Relay

m

diaphragm=

Air supply(P

s)

Restriction

spring

inputcurrent

i

N

o cecoil

motor Nozzle

a

outputposition

xS

cFeedback

leverb

0

dFeedback

spring(K

s)

a ancebeam




-

Relay diaphragmgain = 1 At equilibrium,

m

spring

Air supply(P

s)

Restriction

inputcurrent

i ( ) ⎥⎤⎢⎡ ⎟ ⎞⎜⎝ ⎛ = s

K . xab.d iln Bc

S

N

S

Voicecoil

motor Nozzle

a0

Ks

= spring constant of the

feedback spring (for a small

ou puposition

x

c

d

Feedbacklever

Feedbackspring

(Ks)

balancebeam

b-

Conclusion: output ‘x’ is independent of the

c arac er s cs o ap ragm an spr ng.



Electric Actuators

Erroramplifier

input Positionsensor

Low inertia

outputposition

servo motorgear train

to increase torque

low inertia servomotor - fast response



Electric Actuators

Rack and pinionout ut osition

+

Servo Motor



Electric Actuators

Armature

AC or DC

Coil

Spring Output position

A spring return type electric solenoid is used to actuate an iron cored armature.

A shading band type armature is used with AC supply to create unidirectional

pull.



Hydraulic Actuators

ey use ncompress e u o . ese are

used for high power and high speed applications.



Control Valves

A control valve in a pipeline acts as a variable restriction. An

actuator controls the ‘lift’ of a control valve to alter restriction.

All control valves have an inherent flow characteristic that

e nes e re a ons p e ween va ve open ng an owra e

under constant pressure conditions.

e ree ma n ypes o con ro va ves ava a e are:

quick/fast opening

linear

equal percentage



Control ValvesThe physical shape of the plug and seat arrangement, sometimes referred to

as the valve 'trim', causes the difference in valve opening between these valves.

Typical trim shapes for spindle operated globe valves



Flow-Lift Characteristic of Control Valves

max

% flow 100vv

= ×⎜ ⎟⎝ ⎠

Position x(or lift)

Manipulated

variable v(or flow)

on ro a ve

From laws of fluid dynamics,

v = K√h x

K overall coefficient

h difference in head acrossvalve

max

% lift 100 x x

= ×⎜ ⎟⎝ ⎠




Quick/Fast Opening Control Valve

Here, the valve sensitivity at any flow decreases with increasing

flow. The fast opening characteristic valve plug will give a large change in flow

rate for a small valve lift from the closed position.

dx

Fast opening valves tend to be electrically or pneumatically actuated and

used for 'on / off' control.




Linear Control Valve

The linear characteristic valve lu is sha ed so that the flow rate is directl

proportional to the valve lift, at a constant differential pressure. Here the valve

sensitivity is (approximately) constant.

ow- c arac er s cfor a linear valve




Equal Percentage Valve

These valves have a valve lu sha ed so that each increment in valve lift

increases the flow rate by a certain percentage of the previous flow. The

relationship between valve lift and orifice size (and therefore flow rate) is not

linear but lo arithmic.




For these valves, sensitivity increases with flow. As the valve sensitivity at

Equal Percentage Valve

any g ven ow rate s a constant percentage o t e g ven ow rate, t e term

equal percentage is used.

dvdv

, . ,

i.e. sensitivity expressed as percentage of flow (= 100K %) is constant.dx

=v

=

( )dvdx

v0 v

minv

max

working range




Rangeability of a Control Valve

)maxmaximum controllable flow vR =

minminimum controllable flow v

The minimum controllable flow of a control valve depends on

its construction. The ran e of R is usuall between 20 and 50

under constant pressure drop across the valve (equal to constanthead). It is typically 50 for a globe type control valve.




e a on e ween ow, an rangea y un erconstant pressure drop (or head) across the valve

near a ve

vmax min

minv vv v x

x⎛ ⎞−= + ⎜ ⎟

vmax

vmin

max minminv vvv x⎛ ⎞−

= +⎜ ⎟

xmax

x0

max max max max

( )1 1 1

1 1 1v x x

Rv R R x R x

⎡ ⎤⎛ ⎞= + − = + −⎢ ⎥⎜ ⎟

max max max

1v x⎡ ⎤ 1 vdv R −⎛ ⎞Valve

max maxv R x= + −

⎣ ⎦ maxdx R x

=⎝ ⎠

constant





qua ercen age a ve

max

1 x

xv

R

⎡ ⎤−⎢ ⎥

⎣ ⎦=Differentiating,

max

11 1

.ln .

x

xdv R R

v dx x

⎡ ⎤−⎢ ⎥

⎣ ⎦⎛ ⎞⎛ ⎞

=⎜ ⎟⎜ ⎟max max max

1.ln .

v R=dv

max max

max

n,dx

v x= ⎢ ⎥⎣ ⎦

characteristic

Valve Sensitivity is proportional to ‘v’

max

ln,

dv Rv

dx x

= ⎢ ⎥⎣ ⎦





qua ercen age a ve

x

= volumetric flow through the valve at lift H,

,

V &

=max

V R

V && = R = valve rangeability,

H = valve lift (0 = closed, 1 = fully open),

&= maximum volumetric flow through the valve.maxV





qua ercen age a ve According to our previous notation: vV =& and

max x x H =

Therefore,maxV

R

eV

x

&& = can be written as,

( )lnmax

ex

x R

⎥

⎤

⎢

⎡

= or

( )

⎥

⎤

⎢

⎡

= evx

x R

max

ln ( )

⎥

⎤

⎢

⎡

=⎟ ⎞

⎜⎛

ev

x

x R

max

ln

lnlnmax

R⎥⎦

⎢⎣

⎥⎦

⎢⎣

Rvmax ⎥⎦

⎢⎣

⎝ Rvmax

xv ⎤⎡ ⎞⎛ ⎤⎡−=

⎞⎛ xv

xv maxmax

−

⎦⎣ ⎠⎝ ⎦⎣ ⎠⎝ maxmax xv

or⎥⎥⎦

⎤

⎢⎢⎣

⎡−

=⎟ ⎞

⎜⎛ 1

max

lnln x

x

Rv ⎥

⎥⎦

⎤

⎢⎢⎣

⎡−

=1

max x

x

Rv

or

maxvmaxv



Classification of Control Valves

Control Valves

Sliding Stem Valves Rotating Shaft Valves

Single -SeatPlug Valves

Double-SeatPlug Valves

Lifting GateValves

i l lidi C l V l



Single-seat Sliding-stem Control Valves

Stem

Packing

openx

close

v

(fluid flow)

v

plug

, , -

vary.

Si l t Slidi t C t l V l




Packinggland

v

(fluid flow)

position

open

close

x

Plug Types

v

plug

V-port equal percentage characteristic

Parabolic linear characteristicplug

Poppet quick opening characteristic

Si l t



Single-seat

Control Valves

Another

variation

F P A allowanceFriction =+Δ×

A= Valve seating area (m2)

· ΔP = Differential ressure

(kPa)

F = Closing force required (kN)

Si l t Slidi t C t l V l




Stem

Packinggland position

open

close

x

Features

v

v

(fluid flow)

plug

Can be shut off completely to provide zero flow – an advantage.

Requires large thrust force, thus a powerful actuator is necessary

– a disadvantage.

Solution ?

To overcome the disadvantage, double-seat arrangements are

used, which require a small thrust force to operate.

D bl t Slidin t m C nt l V l



Double-seat Sliding-stem Control Valves

xposition

vv

Double seat



Double-seat

Control Valves

Another

variation

Double seat Sliding stem Control Valves



Double-seat Sliding-stem Control Valvesx

position

Disadvantages

vv

It cannot be shut off completely because of the differentialtemperature expansion of plug and body (when hot fluid is flowing).

If plug expands more than body, it may cause breakage. If body

expand more, there will be significant leakage or offset in thesystem.

Double-seat valves are used where it becomes impractical to provide

sufficient force to close a conventional single seat valve.

Lifting Gate Control Valves



Lifting-Gate Control Valves

'x' position open

close

gate

vv

ng e-sea ng- a e a ve These are used to control flow of fluids containing solid matters

Here no change in direction of fluid flow takes place, due to the

control action.

. . .

Rotating Shaft Control Valves



Rotating-Shaft Control Valves

Rotating - shaft control valves

Rotary plug Butterfly Louver

used to control

li uid flow

used to control use to control

air flow at (e.g. oil to burner)

at low pressure

difference

low pressure

(draft control)

Rotary type valves, often called quarter-turn valves, include plugvalves, ball valves, butterfly valves etc. All require a rotary motion to

open an c ose, an can eas y e e w ac ua ors.

Rotary plug type Control Valves




rotationx

stem

(circular opening)

vv plug

plug (shown open)

When the pipe opening and plug opening completely align,

complete flow takes place. If they do not align at all, no flow takes

place.




Rotary plug type Control Valvesro a on

x

stem

(circular opening)

% ow

100

vv plug

0% rotation 100

x⎛ ⎞= ×⎜ ⎟

p ug s own open) max

plug opening

Circular

V-shapedfor different

flow-rotation

Rectangularcharacteristics

ere rotat on correspon s to an angu ar rotat on or

movement of 90o for x.

Ball Valves



Ball Valves

ea ures

It consists of a spherical ball

in a simple body form.

Rotatin the ball throu h 90°opens and closes the flow

passage.

Ball valves are an economic

means of providing control with

tight shut-off for many fluids.

Butterfly Valves



Butterfly Valvesvane

circular vane

rotation

xv v x

Vanercu ar

Rectangular

% flow

Circular vane

100 Here, 100%

rotation corresponds

to an angular

0 100

% rotation

rotation of 90o .

Butterfly



Butterfly

a ves

Another

variation

Louver Valves



Louver Valves

linkage

rectangular vane

x

vv x

rectangular duct

.

Hence considerable leakage takes place.

Louver Valves



Louver Valves

linkage

rectangular vane

x

vv x

rectangular duct

% flow

100

0 100% rotation

Three-port Valves



p

Three-port valves can be usedfor either mixing or diverting

service depending upon the plug

and seat arrangement inside the

valve.

Methods of Fluid Control



Methods of fluid control

throttling

(variable source)

re uires no se aratecontrol valve




inflow Headtank

Command

outflowto process

controlvalve

For fluctuating or intermittent inflow, closed-loop control is necessary to

ma nta n constant out ow. e c ose oop contro w generate necessary

command for the control valve to maintain constant outflow.




Head

tank

head

Process

restriction

by pass

Bypass is required when we cannot shut down the source, then the extra water is

Control valve

command

bypassed. the control valve employed will have inverse gain.

By pass is not economical, as a considerable portion of fluid is wasted.




Speed command

StorageanVariable speed

pump

Variable Delivery from a Variable Speed Pump

Final_Control_Element_anjan raksit

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