Actuators By themselves, valves cannot control a process. Manual valves require an operator to...
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Transcript of Actuators By themselves, valves cannot control a process. Manual valves require an operator to...
ActuatorsBy themselves, valves cannot control a process. Manual valves require an operator to position them to control a process variable. Valves that must be operated remotely and automatically require special devices to move them. These devices are called actuators. Actuators may be pneumatic, hydraulic, or electric solenoids or motors
1BEE4523 Chapter 3 : ActuatorNH
2BEE4523 Chapter 3 : ActuatorNH
Actuators
1 Electrical actuator2 Pneumatic3 Hydraulic4 Fluid valve
3BEE4523 Chapter 3 : ActuatorNH
Electrical Actuator
4BEE4523 Chapter 3 : ActuatorNH
Electrical Actuator
2 types of electrical actuators :
• Solenoid
• Electrical Motors
Dc Motor
Ac Motor
Stepping motor
5BEE4523 Chapter 3 : ActuatorNH
Solenoid
• Solenoid : converts an electrical
signal into mechanical motion,
usually rectilinear
• Used when a large, sudden force
must be applied to perform some
job
6BEE4523 Chapter 3 : ActuatorNH
Solenoid
• Consist of a coil n plunger (freestanding / spring
loaded)
• Coil will have voltage or current rating in dc or ac
7BEE4523 Chapter 3 : ActuatorNH
Solenoid
8BEE4523 Chapter 3 : ActuatorNH
Solenoid
• Solenoid used to change the gears of a two-
position transmission
• Used SCR to activate solenoid coil
Electric Solenoid Actuators A typical electric solenoid actuator is shown in Figure 38. It consists of a coil, armature, spring, and stem.The coil is connected to an external current supply. The spring rests on the armature to force it downward. The armature moves vertically inside the coil and transmits its motion through the stem to the valve. When current flows through the coil, a magnetic field forms around the coil. The magnetic field attracts the
9BEE4523 Chapter 3 : ActuatorNH
armature toward the center of the coil. As the armature moves upward, the spring collapses and the valve opens. When the circuit is opened and current stops flowing to the coil, the magnetic field collapses. This allows the spring to expand and shut the valve.A major advantage of solenoid actuators is their quick operation. Also, they are much easier to install
10BEE4523 Chapter 3 : ActuatorNH
than pneumatic or hydraulic actuators. However, solenoid actuators have two disadvantages. First, they have only two positions: fully open and fully closed. Second, they don’t produce much force, so they usually only operate relatively small valves.
11BEE4523 Chapter 3 : ActuatorNH
12BEE4523 Chapter 3 : ActuatorNH
Figure 38 Electric Solenoid Actuator
13BEE4523 Chapter 3 : ActuatorNH
Electrical Motors
• Electrical motors : accept electrical
input and produce a continuous rotation
as a result
• Motor styles and sizes vary as demands
for rotational speed (rpm), starting
torque, rotational torque, etc
14BEE4523 Chapter 3 : ActuatorNH
DC Motor
• Rotation of dc motor :
produced by the
interaction of two
constant magnetic fields
• Employs permanent
magnet (PM) to form one
of the magnetic fields
• 2nd magnetic field formed
by current through coil of
wire within PM
15BEE4523 Chapter 3 : ActuatorNH
DC Motor
• Coil of wire
(armature) : free
to rotate
• Coil is connected
to current source
through slip rings
and brushes
(commutator)
• Slip rings are split
so that current
reverses direction
as the armature
rotates
16BEE4523 Chapter 3 : ActuatorNH
DC Motor
17BEE4523 Chapter 3 : ActuatorNH
DC Motor
18BEE4523 Chapter 3 : ActuatorNH
DC Motor
19BEE4523 Chapter 3 : ActuatorNH
DC Motor
• Torque will drive
the N armature
from the N PM &
the S armature
from the S PM
• Armature rotate
counterclockwise
• Rotation will
continuous
20BEE4523 Chapter 3 : ActuatorNH
DC Motor
• Many dc motors use electromagnet instead of
PM to provide static field
• Field coil : coil used to produce static field
• Wound field motor : dc motor to produce
static field
• Current can be provided by placing the coil in
series or parallel (shunt) or else with two
windings (compound) with the armature
21BEE4523 Chapter 3 : ActuatorNH
Series field
• Large starting
torque
• Difficult to speed
control
• Good for starting
heavy, where
speed control is
not important
22BEE4523 Chapter 3 : ActuatorNH
Shunt field
• Smaller starting
torque
• Good speed
control
• Good for
application
where speed is
to be controlled
23BEE4523 Chapter 3 : ActuatorNH
Compound field
• The best
features of
both series &
shunt
• Large starting
torque
• Good speed
control
24BEE4523 Chapter 3 : ActuatorNH
AC Motor
• Principle of ac motor : involves the
interaction between 2 magnetic
fields
• Both fields varying with the voltage
• Force between fields is a function of
rotor angle and current phase
25BEE4523 Chapter 3 : ActuatorNH
Synchronous ac motor
• AC voltage applied to field coils (stator)
• Armature (rotor) is PM or dc electromagnet
• Rotor follows
stator
• Speed of rotation:
rpmp
fns
120
26BEE4523 Chapter 3 : ActuatorNH
Induction ac motor
• Rotor is neither PM nor dc electromagnet
• Current induced from stator coils
• Ac field of stator
produces
magnetic field
changing through
closed loop of
rotor
27BEE4523 Chapter 3 : ActuatorNH
Stepping Motor
• Stepping motor : rotating machine that
completes a full rotation by sequencing
through a series of discrete rotational
steps
• Rotational rate : determined by the
number of steps per revolution and the
rate at which the pulses are applied
28BEE4523 Chapter 3 : ActuatorNH
Stepping motor
• 90° per step
• Rotor is PM driven by set of electromagnets
• Switches : transistors, SCRs or TRIACs
• Switch sequencer will direct the switches through a sequence of positions as the pulses received
29BEE4523 Chapter 3 : ActuatorNH
Stepping motor
• Pulse will change S2 from C to D
• Poles of electromagnet reversing fields
• Pole N/S orientation is different
• Rotor repelled and attracted, moves to new position
• Next pulse will change S1 from A to B – same kind of pole reversal and rotation of rotor
30BEE4523 Chapter 3 : ActuatorNH
Stepping motor
• Next pulse will change S2 from D to C– same kind of pole reversal and rotation of rotor
• Next pulse will change S1 from B to A– same kind of pole reversal and rotation of rotor, send back system to original position
• Continuous sequence as pulse come in
31BEE4523 Chapter 3 : ActuatorNH
Example 1
A stepper motor has 10° per step
and must rotate at 250 rpm.
Calculate the required input pulse
rate, in pulses per second.
Electric Motor Actuators Electric motor actuators vary widely in their design and applications. Some electric motor actuators are designed to operate in only two positions (fully open or fully closed). Other electric motors can be positioned between the two positions. A typical electric motor actuator is shown in Figure 39. Its major parts include an electric motor, clutch and gear box assembly, manual hand wheel, and stem connected to a valve.
32BEE4523 Chapter 3 : ActuatorNH
The motor moves the stem through the gear assembly. The motor reverses its rotation to either open or close the valve. The clutch and clutch lever disconnects the electric motor from the gear assembly and allows the valve to be operated manually with the hand wheel. Most electric motor actuators are equipped with limit switches, torque limiters, or both. Limit switches de-energize the electric motor when the valve has reached a specific position. Torque Rev. 0 Page 59 IC-07
33BEE4523 Chapter 3 : ActuatorNH
Limiters de-energize the electric motor when the amount of turning force has reached a specified value. The turning force normally is greatest when the valve reaches the fully open or fully closed position. This feature can also prevent damage to the actuator or valve if the valve binds in an intermediate position.
34BEE4523 Chapter 3 : ActuatorNH
35BEE4523 Chapter 3 : ActuatorNH
Figure 39 Electric Motor Actuator
36BEE4523 Chapter 3 : ActuatorNH
Pneumatic
37BEE4523 Chapter 3 : ActuatorNH
Pneumatic
• Pneumatic : based on concept of pressure
as force per unit area
• Net force acts on diaphragm ;
NforceF
mareadiaphragmA
Padifferencepressurepp
ppAF
:
:
:2
21
21
38BEE4523 Chapter 3 : ActuatorNH
Direct Pneumatic
• Condition in low
signal-pressure state
• Spring, S maintains
diaphragm and
connected control
shaft in position
• Pressure on opposite
side is maintained at
atmospheric
pressure by open
hole, H
39BEE4523 Chapter 3 : ActuatorNH
Direct Pneumatic
• Increasing control
pressure applies
force on diaphragm
• Forcing diaphragm
and connected shaft
down against spring
force
• Maximum control
pressure and travel
of shaft
40BEE4523 Chapter 3 : ActuatorNH
Pneumatic
• Shaft position is linearly related to applied control pressure
• Shaft position;
mNconstantspringk
mareadiaphragmA
Papressuregaugeappliedp
mtravelshaftx
pk
Ax
/:
:
:
:
2
41BEE4523 Chapter 3 : ActuatorNH
Reverse Pneumatic
• Moves shaft in
opposite sense
from direct
actuator
• Obeys same
operating principle
• Shaft is pulled by
application of
control pressure
42BEE4523 Chapter 3 : ActuatorNH
Example 2
A force of 400 N must be applied to open
valve. Determine the diaphragm area if a
control gauge pressure of 70 kPa (~10
psi) must provide this force.
Pneumatic Actuators A simplified diagram of a pneumatic actuator is shown in figure 35. It operates by a combination of force created by air and spring force. The actuator positions a control valve by transmitting its motion through the stem. A rubber diaphragm separates the actuator housing into two air chambers. The upper chamber receives supply air through an opening in the top of the housing.
43BEE4523 Chapter 3 : ActuatorNH
The bottom chamber contains a spring that forces the diaphragm against mechanical stops in the upper chamber. Finally, a local indicator is connected to the stem to indicate the position of the valve. The position of the valve is controlled by varying supply air pressure in the upper chamber. This results in a varying force on the top of the diaphragm. Initially, with no supply air, the spring forces the diaphragm upward against the mechanical stops and holds the valve fully open. As supply air pressure is increased
44BEE4523 Chapter 3 : ActuatorNH
from zero, its force on top of the diaphragm begins to overcome the opposing force of the spring. This causes the diaphragm to move downward and the control valve to close.With increasing supply air pressure, the diaphragm will continue to move downward and compress the spring until the control valve is fully closed. Conversely, if supply air pressure is decreased, the spring will begin to force the diaphragm upward and open the control valve. Additionally, if supply pressure is held constant at
45BEE4523 Chapter 3 : ActuatorNH
some value between zero and maximum, the valve will position at an intermediate position. Therefore, the valve can be positioned anywhere between fully open and fully closed in response to changes in supply air pressure. A positioner is a device that regulates the supply air pressure to a pneumatic actuator. It does this by comparing the actuator’s demanded position with the control valve’s actual position. The demanded position is transmitted by a pneumatic or electrical control signal from a controller to the
46BEE4523 Chapter 3 : ActuatorNH
positioner. The pneumatic actuator in Figure 35 is shown in Figure 36 with a controller and positioner added. The controller generates an output signal that represents the demanded position. This signal is sent to the positioner. Externally, the positioner consists of an input connection for the control signal, a supply air input connection, a supply air output connection, a supply air vent connection, and a feedback linkage. Internally, it contains an intricate network of electrical transducers, air lines, valves, linkages, and
47BEE4523 Chapter 3 : ActuatorNH
necessary adjustments. Other positioners may also provide controls for local valve positioning and gauges to indicate supply air pressure and control air pressure (for pneumatic controllers). From an operator’s viewpoint, a description of complex internal workings of a positioner is not needed. Therefore, this discussion will be limited to inputs to and outputs from the positioner. In Figure 36, the controller responds to a deviation of a controlled variable from set point and varies the control output signal accordingly to
48BEE4523 Chapter 3 : ActuatorNH
correct the deviation. The control output signal is sent to the positioner, which responds by increasing or decreasing the supply air to the actuator. Positioning of the actuator and control valve is fed back to the positioner through the feedback linkage. When the valve has reached the position demanded by the controller, the positioner stops the change in supply air pressure and holds the valve at the new position. This, in turn, corrects the controlled variable’s deviation from set point.
49BEE4523 Chapter 3 : ActuatorNH
For example, as the control signal increases, a valve inside the positioner admits more supply air to the actuator. As a result, the control valve moves downward. The linkage transmits the valve position information back to the positioner. This forms a small internal feedback loop for the actuator. When the valve reaches the position that correlates to the control signal, the linkage stops supply air flow to the actuator. This causes the actuator to stop. On the other hand, if the control signal decreases, another
50BEE4523 Chapter 3 : ActuatorNH
This causes the valve to move upward and open. When the valve has opened to the proper position, the inside the positioner opens and allows the supply air pressure to decrease by venting the supply air.positioner stops venting air from the actuator and stops movement of the control valve. An important safety feature is provided by the spring in an actuator. It can be designed to position a control valve in a safe position if a loss of supply air occurs. On a loss of supply air, the actuator in Figure 36 will
51BEE4523 Chapter 3 : ActuatorNH
fail open. This type of arrangement is referred to as "air-to-close, spring-to-open" or simply "fail-open." Some valves fail in the closed position. This type of actuator is referred to as "air-to-open, spring-to-close" or "fail-closed." This "fail-safe" concept is an important consideration in nuclear facility design
52BEE4523 Chapter 3 : ActuatorNH
53BEE4523 Chapter 3 : ActuatorNH
Figure 36 Pneumatic Actuator with Controller and Positioner
54BEE4523 Chapter 3 : ActuatorNH
Hydraulic
55BEE4523 Chapter 3 : ActuatorNH
Hydraulic
• Hydraulic : used when large forces are
required
• Hydraulic pressure ;
21
1
11
:
:
:
/
mareapistonforcingA
NforcepistonappliedF
Papressurehydraulicp
AFp
H
H
56BEE4523 Chapter 3 : ActuatorNH
Hydraulic
• Basic idea same as
pneumatic
• Except
incompressible fluid
used to provide
pressure
• Pressure will be very
large by adjusting
area of forcing piston
57BEE4523 Chapter 3 : ActuatorNH
Hydraulic
allows the lifting of a heavy load with a small force
58BEE4523 Chapter 3 : ActuatorNH
Hydraulic
• Pressure is transferred equally throughout the liquid
• Force on working piston;
• Working force in terms of applied force;
22
2
:
:
mareapistonworkingA
NpistonworkingonforceF
ApF
w
Hw
11
2 FA
AFw
59BEE4523 Chapter 3 : ActuatorNH
Automobile Hydraulic Lift
• A hydraulic lift for automobiles is an example of a force multiplied by hydraulic press, based on Pascal's principle. The fluid in the small cylinder must be moved much further than the distance the car is lifted.
60BEE4523 Chapter 3 : ActuatorNH
Pascal’s Principle
61BEE4523 Chapter 3 : ActuatorNH
Example 3
Determine:
(a)The working force resulting from 200 N
applied to a 1 cm radius forcing piston if
the working piston has a radius of 6 cm
(b)The hydraulic pressure
62BEE4523 Chapter 3 : ActuatorNH
Example 4
If the lift cylinder were 25 cm in diameter
and the small cylinder were 1.25 cm in
diameter, determine the force on the fluid
in the small cylinder to lift a 6000 N car.
Hydraulic Actuators Pneumatic actuators are normally used to control processes requiring quick and accurate response, as they do not require a large amount of motive force. However, when a large amount of force is required to operate a valve (for example, the main steam system valves), hydraulic actuators are normally used. Although hydraulic actuators come in many designs, piston types are most common.
63BEE4523 Chapter 3 : ActuatorNH
A typical piston-type hydraulic actuator is shown in Figure 37. It consists of a cylinder, piston, spring, hydraulic supply and returns line, and stem. The piston slides vertically inside the cylinder and separates the cylinder into two chambers. The upper chamber contains the spring and the lower chamber contains hydraulic oil. The hydraulic supply and return line is connected to the lower chamber and allows hydraulic fluid to flow to and from the lower chamber of the actuator. The stem transmits the motion of the piston to a valve
64BEE4523 Chapter 3 : ActuatorNH
Initially, with no hydraulic fluid pressure, the spring force holds the valve in the closed position. As fluid enters the lower chamber, pressure in the chamber increases. This pressure results in a force on the bottom of the piston opposite to the force caused by the spring. When the hydraulic force is greater than the spring force, the piston begins to move upward, the spring compresses, and the valve begins to open. As the hydraulic pressure increases, the valve continues to open. Conversely, as hydraulic
65BEE4523 Chapter 3 : ActuatorNH
oil is drained from the cylinder, the hydraulic force becomes less than the spring force, the piston moves downward, and the valve closes. By regulating amount of oil supplied or drained from the actuator, the valve can be positioned between fully open and fully closedThe principles of operation of a hydraulic actuator are like those of the pneumatic actuator. Each uses some motive force to overcome spring force to move the valve. Also, hydraulic actuators can be designed to fail-open or fail-closed to provide a fail-safe feature.
66BEE4523 Chapter 3 : ActuatorNH
67BEE4523 Chapter 3 : ActuatorNH
Figure 37 Hydraulic Actuator
68BEE4523 Chapter 3 : ActuatorNH
Fluid valve
69BEE4523 Chapter 3 : ActuatorNH
Control-valve principles
• Flow rate in process control : volume per unit time
• If a given fluid is delivered through a pipe, the volume flow rate ;
smvelocityflowv
mareapipeA
smrateflowQ
AvQ
/:
:
/:2
3
70BEE4523 Chapter 3 : ActuatorNH
Control-valve principles
• To regulate flow rate of fluids through pipes
• Placing variable-size restriction in flow path
• Stem and plug move up and down
• Opening size between plug and seat changes, thus changing flow rate
71BEE4523 Chapter 3 : ActuatorNH
Control-valve principles
• There will be a drop in pressure and flow rate varies with square root of pressure drop
• The volume flow rate ;
Padifferencepressureppp
PasmconstantalityproportionK
smrateflowQ
pKQ
:
//:
/:
12
2/13
3
72BEE4523 Chapter 3 : ActuatorNH
Control-valve types
• Control-valve characteristic :
different types of control valves
depends on relationship between
valve stem position and flow rate
through valve
73BEE4523 Chapter 3 : ActuatorNH
Control-valve types
• Control
valve using
pneumatic
actuator
• To drive
stem and
open or
close valve
74BEE4523 Chapter 3 : ActuatorNH
Control-valve types
• Types determined by shape of plug and
seat
• Stem and plug move with respect to seat
• Shape of plug determines the amount of
actual opening valve
75BEE4523 Chapter 3 : ActuatorNH
Control-valve types
1. Quick opening
• Used predominantly for full ON/full
OFF control applications
• Small motion of valve stem, max
possible flow rate
• Allow 90% max flow rate, 30% travel
of stem
76BEE4523 Chapter 3 : ActuatorNH
Control-valve types
2. Linear
• Flow rate varies linearly with stem position
• Ideal situation, valve determines pressure drop;
mpositionstemmaximumS
mpositionstemS
smrateflowmaximumQ
smrateflowQ
S
S
Q
Q
:
:
/:
/:
max
3max
3
maxmax
77BEE4523 Chapter 3 : ActuatorNH
Control-valve types
3. Equal percentage
• Percentage change in stem position produces equivalent change in flow-equal percentage
• Not shut off the flow completely in its limit of stem travel
rateflowminimumQ
rateflowmaximumQ
Q
QR
:
:
min
max
min
max
max/max
SSRQQ
• Rangeability;
• Flow rate;
78BEE4523 Chapter 3 : ActuatorNH
Control-valve types
• Three
types of
control
valves
open
differently
as function
of stem
position
79BEE4523 Chapter 3 : ActuatorNH
Control-valve sizing
• Correction factor (valve flow coefficient), Cv : allow
selection of proper size of valve to accommodate flow rate
• Measured as the number of U.S. gallons of water per minute
that flow through full open valve with pressure differential 1 lb
per square inch
• Liquid flow rate (in U.S. gallons per minute);
gravityspecificliquidS
psivalveacrosspressurep
S
pCQ
G
Gv
:
:
80BEE4523 Chapter 3 : ActuatorNH
Control-valve sizing
• Control-valve
flow
coefficients for
different size
valves
Valve size (inches) Cv
¼ 0.3
½ 3
1 14
1½ 35
2 55
3 108
4 174
6 400
8 725
81BEE4523 Chapter 3 : ActuatorNH
Example 5
Alcohol is pumped through a pipe of
10 cm diameter at 2 m/s flow
velocity. Calculate the volume flow
rate
82BEE4523 Chapter 3 : ActuatorNH
Example 6
A pressure difference of 1.1 psi occurs
across a constriction in a 5 cm diameter
pipe. The constriction constant is 0.009
m3/s/kPa1/2. Determine:
(a) The flow rate in m3/s
(b) The flow velocity in m/s
83BEE4523 Chapter 3 : ActuatorNH
Example 7
An equal percentage valve has a
maximum flow of 50 cm3/s and a
minimum of 2 cm3/s. If the full
travel is 3 cm, calculate the flow at
a 1cm opening.
84BEE4523 Chapter 3 : ActuatorNH
Example 8
Calculate:
(a) The proper Cv for a valve that must
allow 150 gal of ethyl alcohol per
minute with a specific gravity of 0.8
at maximum pressure of 50 psi
(b) The required valve size
Summary The important information in this chapter is summarized below.Valve Actuator Summary Pneumatic actuators utilize combined air and spring forces for quick accurate responses for almost any size valve with valve position ranging from 0-100%.
85BEE4523 Chapter 3 : ActuatorNH
Hydraulic actuators use fluid displacement to move a piston in a cylinder positioning the valve as needed for 0-100% fluid flow. This type actuator is incorporated when a large amount of force is necessary to operate the valve.
86BEE4523 Chapter 3 : ActuatorNH
Solenoid actuators are used on small valves and employ an electromagnet to move the stem which allows the valve to either be fully open or fully closed. Equipped with limit switches and/or torque limiters, the electric motor actuator has the capability of 0-100% control and has not only a motor but also a manual hand wheel, and a clutch and gearbox assembly. End of text.
87BEE4523 Chapter 3 : ActuatorNH