Final Control Element
Final Control Element m
Final Contro l emen
controller output
m
⇓
Ac A c t u ato at o r
Ac Actu tuat ator or
displacement or position 'x'
v manipulated variable (actuating (actuating signa sign al)
Control valve
v
rov ovid ides es an ou outt ut os osit itio ion n ro or orti tion onal al to th thee
input signal Control Valve
variable
adjusts the value of the manipulate manipulated d
Final Control Element m controller out ut
m
Ac tu ator ato r
Final Final Control Element
displacement ' '
v manipulated variable (actuating (actuating signal)
Control
v
, response, moves the valve to a fully-open or fully-closed position, position, or a more open or a more closed position (depending on whether on o or co con nuous con ro ac ac on s use .
Actuators Pneumatic actuators
spring – diaphragm piston motor
ec ro-pneuma c actuators Actuators Electric actuators
Hydraulic actuators
Motorizedrotary or linear Solenoid operated
Pneumatic Actuators – spring
input m (3-15 psi)
diaphragm
backing plate
output position stem
x
constant thrust force
Diaphragm actuators have compressed air applied to a flexible
membrane called the diaphragm.
Pneumatic Actuators – At equilibrium, (assuming no spring
input ressure m (3-15 psi)
stem): mA = Kx diaphragm
backing plate
output position stem
x
constant thrust force
m = the change in input pressure = 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-retract)
( spring-to-extend )
This actuator is designed with the spring below the diaphragm, having air supplied to the space above the diaphragm. The result, with increasing air pressure, is spindle movement in e oppos e rec on o e reverse acting actuator.
The diaphragm is pushed upwards, pulling the spindle up, and if the spindle is connected to a rec ac ng va ve, e p ug s opened. With a specific change of air pressure, the spindle will move through its complete stroke.
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.
.
n case o us ng ac ua ors, we ave non- near es ue o ap ragm area 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. electrical input (typically 4 - 20 mA) can be used to control a pneumatic valve .
Some positioners can also act as basic controllers, accepting input from sensors.
Pneumatic Actuators – n gain = 1 n Relay restriction spring Air supply
Bellow
s
b
c m in ut ressure
n nozzle back pressure
n
0
Feedback lever
a b a
x x
stem
− y =
b
2
−⎜ ⎟ ⎝ a ⎠ 2
Pneumatic Actuators – At equilibrium, Baffle-Nozzle se aration is:
n gain = 1 n e ay restriction spring Air sup ply Bellow (Ps) Bellows
K bm
c m input pressure
c
n y b
n nozzle back pressure
a
0
⎛ b ⎞ x Kb m ⎟ − a 2 ⎝ ⎠2
y = + ⎜
Feedback lever
a
b
b
x x
− y =
K b m
2
⎛ b ⎞ x −⎜ ⎟ ⎝ a ⎠ 2
stem
Nozzle back pressure: n = – K n. 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.
n= Now,
Kx
y = −
A
K n A >> K ,
Kn A
n
nA
⎛ b ⎞ K = x ⎢⎜ ⎟ + ⎥ 2 ⎣⎝ 2a ⎠ K n A ⎦
Kx
Kb m
= −⎜ ⎟=− Kn Kn A ⎝ Kn A ⎠
≈ 0, and y ≈ 0.
Kb m ≈ x ⎜
⎟ ⎝a⎠
Pneumatic Actuators – n gain = 1 n restriction spring Ai r s upp ly Bellow (Ps ) Bellows
K bm
c m input pressure
n nozzle back pressure
c
n y b a
0 b
Feedback lever
a x
x
− y =
⎛ b ⎞ x −⎜ ⎟ 2 ⎝ a ⎠ 2
K b m
stem
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
Air Motor
+
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 Actuators -
'' signal input (current 4-20mA)
Electropneumatic Converter
pneumatic output 'm' (3-15 psi)
Electro-pneumatic Actuators input i
former (support system) pneumatic output (m)
S Nozzle Coil Motor
N
Restriction
air su l (Ps )
a S
balance beam
voice coil motor - linear mot or
m b
Feedback b ellows
Electro-pneumatic Actuators input current
i
former (support system) pneumatic output (m)
S
Coil Motor
Nozzle
N
Restriction
air supply (Ps)
voice coil
used in loud speakers S
+
a S
balance beam
Force
voice coil motor - linear motor
m b
Feedback bellows
Effective force ex erienced b the former is a linear one.
N
= BlNi B l N
flux density mean length / turn no. of turns
force
Electro-pneumatic Actuators input current
i
former (support system) pneumatic output (m)
S Voice Coil Motor
Nozzle m N
Restriction
air supply (Ps)
a S
balance l beam
m
voice coil motor - lilinear motor
At equilibrium, for a change in input current ‘ ’, ‘ ’
a . B ln i = b(mAb ) b
a small Baffle-Nozzle separation.
b
Feedback bell ows
m = ⎜
⎟i ⎝ bAb ⎠
Electro-pneumatic Actuators Relay
diaphragm
Air s up pl y (Ps) m
Restriction input i current
o ce coil motor
spring
Nozzle
N
a S
0
c a ance beam d
b
Feedback lever Feedback spring (K s )
x
output position
Electro-pneumatic Actuators Relay
gain = 1
diaphragm
Ai r su pp ly (Ps ) m
Restriction input i current
Voice coil motor
spring
At equilibrium,
⎡⎛ b ⎞ ⎤ c( B ln i ) = d .⎢⎜ ⎟ x.K ⎥ ⎝ a s
S N
Ks = spring constant of the feedback spring (for a small
Nozzle a
S
0 c
balance beam d
b
Feedback lever
x
ou pu position
-
Feedback spring (K s )
Conclusion: output ‘x’ is independent of the c arac er s cs o ap ragm an spr ng.
Electric Actuators
input
Error amplifier
Position sensor output position
Low inertia servo motor gear train to increase torque
low inertia servo motor - fast response
Electric Actuators
Rack and pinion out 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
They use ncompress ble lu d o l . These 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
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 Valves The 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 ×100 ⎟ % flow ⎜ = v v max ⎝ ⎠ Manipulated variable v (or flow)
Position x (or lift) on ro
a ve
From laws of fluid dynamics, v=K h x K overall coefficient h difference in head across valve
×100 ⎟ % lift ⎜ = x x max ⎝ ⎠
Flow-Lift Characteristic of Control Valves Quick/Fast Opening Control Valve
Here, the valve sensitivity
dx 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. Fast opening valves tend to be electrically or pneumatically actuated and
used for 'on / off' control.
Flow-Lift Characteristic of Control Valves 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.
owc arac er s c for a linear valve
Flow-Lift Characteristic of Control Valves 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.
Flow-Lift Characteristic of Control Valves Equal Percentage Valve For these valves, sensitivity increases with flow. As the valve sensitivity at
any g ven ow rate s a constant percentage o t e g ven ow rate, t e term
dv
equal percentage is used. ,
dv dx
=
.
,
v
=
i.e. sensitivity expressed as percentage of flow (= 100K %) is constant.
( dv dx )
0 v mi n working range
v v max
Flow-Lift Characteristic of Control Valves Rangeability of a Control Valve
R=
maximum controllable flow v max ) minimum controllable flow v min
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 constant head). It is typically 50 for a globe type control valve.
Flow-Lift Characteristic of Control Valves e a on e ween ow, an rangea y un er constant pressure drop (or head) across the valve
near a ve
⎛ vmax − vmin ⎞ v = vmin + x ⎜ ⎟
v
x
v max
v
=
v mi n x
0
max
x max
v vmax
v vmax
=
1⎡ R⎣
+
−
vmin
x ⎤
dv
xmax ⎦
dx
=
⎛ vmax − vmin ⎞ x +⎜ ⎟
max
=
1 R
max
max
x ⎤ 1⎡ ⎛ 1⎞ x + ⎜1 − ⎟ = ⎢1 + ( R − 1) ⎥ R xmax
⎛ R −1 ⎞ v ⎝ R ⎠ xmax
R
xmax
Valve constant
Flow-Lift Characteristic of Control Valves e a on e ween ow, an rangea y un er constant pressure drop (or head) across the valve
qua
ercen age a ve
v
= R
⎡ x ⎤ −1⎥ ⎢ ⎣ xmax ⎦
⎡ x
⎤ −1⎥ max ⎦
⎢ Differentiating, ⎛ 1 ⎞ ⎛ dv ⎞ ⎣ x = R ⎜ ⎟⎜ ⎟
vmax
max
=
dv dx = n ⎢ ⎥, v ⎣ xmax ⎦
dx
v
.ln R.
max
.ln R.
1 max
characteristic
Valve Sensitivity is proportional to ‘v’
dv
ln R
= ⎢ ⎥ v, dx ⎣ xmax ⎦
1 xmax
Flow-Lift Characteristic of Control Valves e a on e ween ow, an rangea y un er constant pressure drop (or head) across the valve
qua
ercen age a ve V & = volumetric flow through the valve at lift H,
=
x
&= V
R
&max V
,
R = valve rangeability, H = valve lift (0 = closed, 1 = fully open),
&max V
= maximum volumetric flow through the valve.
Flow-Lift Characteristic of Control Valves e a on e ween ow, an rangea y un e r constant pressure drop (or head) across the valve
qua
ercen age a ve
According to our previous notation: Therefore,
=
&= V
e
x
R
⎡ (ln R ) x x ⎤ ⎢e ⎥
&max V
⎢ ⎣
R
v
⎝ v max ⎠ or
or
v max
⎥ ⎦ ⎛ v ⎞
⎡
x ⎤
⎣
x max ⎦
⎛ v ⎞ ⎟ = ln R ln⎜ v max
H =
and
x x max
can be written as,
max
max
& =v V
=
⎡ (ln R ) x x ⎤ ⎢e ⎥ max
⎢ ⎣
R
⎥ ⎦
⎛ v ⎞ ⎟ = ln ln⎜ v ⎝ max ⎠ ⎢ ⎣
⎛ v ⎞
−
⎡ x ⎤ −1 ⎥ ⎢ x ⎣⎢ max ⎥⎦
⎡ (ln R ) x x ⎤ ⎢e ⎥
⎝ vmax ⎠ or
v v max
=
= R
max
R
⎡ x ⎣ x max ⎡ x ⎤ −1 ⎥ ⎢ ⎣⎢ x max ⎦⎥
⎥ ⎦
−
⎤ ⎦
Classification of Control Valves
Single-seat Sliding-stem Control Valves Stem Packing
open
x
close v (fluid fl ow)
v
plug
, vary.
,
-
Single-seat Sliding-stem Control Valves Packing gland
open
x position
close
Plug Types
v (fluid flow)
v
plug
V-port
plug
equal percentage characteristic
Parabolic
linear characteristic
Poppet
quick opening characteristic
Single-seat Control Valves Another variation
A × ΔP + Friction allowance = F
A= Valve seating area (m 2) · P = Differential ressure (kPa) F = Closing force required (kN)
Single-seat Sliding-stem Control Valves Stem Packing gland
open
x
position close
Features
v (fluid flow)
v
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.
Double-seat Sliding-stem Control Valves x position
v v
Double-seat Control Valves Another variation
Double-seat Sliding-stem Control Valves x position
Disadvantages v v
It cannot be shut off completely because of the differential temperature 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 the system.
Double-seat valves are used where it becomes impractical to provide sufficient force to close a conventional single seat valve.
Lifting-Gate Control Valves 'x' position open close gate
v
v
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
Rotary plug
Butterfly
Louver
used to control li uid flow (e.g. oil to burner)
used to control
use to control air flow at low pressure (draft control)
at low pressure difference
Rotary type valves, often called quarter-turn valves, include plug valves, 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 rotation x
stem
(circular opening)
v
v
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 Valves i
ow
x
100
stem
(circular opening)
v
v
plug
⎛
0
% rotation ⎜ =
plug l s own open
max
Circular plug opening
for different
V-shaped Rectangular
flow-rotation
characteristics
ere rotat on correspon s to an angu ar rotat on or movement of 90o for x.
x
⎞ × 100 ⎟
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 vane circular vane rotation
x
v
v
x rcu ar Vane Rectangular
% flow
Here, 100%
100
Circular vane
rotation corresponds to an angular rotation of 90o .
0
100
% rotation
Butterfly a ves
Another variation
Louver Valves rectangular vane linkage
v
v
x
x
rectangular du ct
Hence considerable leakage takes place .
.
Louver Valves rectangular vane linkage
v
v
x
x
rectangular duct
% flow 100
0
100
% rotatio n
Three-port Valves
Three-port valves can be used
for 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 arate control v alve
Methods of Fluid Control
Head tank
inflow
Command
control valve
outflow to process
For fluctuating or intermittent inflow, closed-loop control is necessary to
ma n a n cons an ou
ow.
e c ose
oop con ro w
genera e necessary
command for the control valve to maintain constant outflow .
Methods of Fluid Control
Head tank
head Process restriction by pass
Control valve command
Bypass is required when we cannot shut down the source, then the extra water is
bypassed. the control valve employed will have inverse gain . By pass is not economical, as a considerable portion of fluid is wasted.
