1. Actuators By themselves, valves cannot control a process
Actuators By 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 NH
BEE4523 Chapter 3 : Actuator

2. Actuators 1 Electrical actuator 2 Pneumatic 3 Hydraulic 4 Fluid valveNH
BEE4523 Chapter 3 : Actuator

3. Electrical Actuator NH
BEE4523 Chapter 3 : Actuator

4. Electrical Actuator 2 types of electrical actuators : Solenoid
Electrical Motors
Dc Motor
Ac Motor
Stepping motor NH
BEE4523 Chapter 3 : Actuator

5. Solenoid
Solenoid : converts an electrical signal into mechanical motion, usually rectilinear
Used when a large, sudden force must be applied to perform some job NH
BEE4523 Chapter 3 : Actuator

6. Solenoid Consist of a coil n plunger (freestanding / spring loaded)
Coil will have voltage or current rating in dc or ac NH
BEE4523 Chapter 3 : Actuator

7. Solenoid NH
BEE4523 Chapter 3 : Actuator

8. Solenoid
Solenoid used to change the gears of a two-position transmission
Used SCR to activate solenoid coil NH
BEE4523 Chapter 3 : Actuator

9. 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 NH
BEE4523 Chapter 3 : Actuator

10. armature toward the center of the coil
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    NH
BEE4523 Chapter 3 : Actuator

11. than pneumatic or hydraulic actuators
  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. NH
BEE4523 Chapter 3 : Actuator

12. Figure 38    Electric Solenoid ActuatorNH
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13. 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 NH
BEE4523 Chapter 3 : Actuator

14. 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 NH
BEE4523 Chapter 3 : Actuator

15. 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 NH
BEE4523 Chapter 3 : Actuator

16. DC Motor NH
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17. DC Motor NH
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18. DC Motor NH
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19. 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 NH
BEE4523 Chapter 3 : Actuator

20. DC Motor Field coil : coil used to produce static field
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 NH
BEE4523 Chapter 3 : Actuator

21. Series field Large starting torque Difficult to speed control
Good for starting heavy, where speed control is not important NH
BEE4523 Chapter 3 : Actuator

22. Shunt field Smaller starting torque Good speed control
Good for application where speed is to be controlled NH
BEE4523 Chapter 3 : Actuator

23. Compound field The best features of both series & shunt
Large starting torque
Good speed control NH
BEE4523 Chapter 3 : Actuator

24. 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 NH
BEE4523 Chapter 3 : Actuator

25. Synchronous ac motor AC voltage applied to field coils (stator)
Armature (rotor) is PM or dc electromagnet
Rotor follows stator
Speed of rotation: NH
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26. 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 NH
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27. 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 NH
BEE4523 Chapter 3 : Actuator

28. 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 NH
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29. 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 NH
BEE4523 Chapter 3 : Actuator

30. 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 NH
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31. 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. NH
BEE4523 Chapter 3 : Actuator

32. 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. NH
BEE4523 Chapter 3 : Actuator

33. The motor moves the stem through the gear assembly
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 NH
BEE4523 Chapter 3 : Actuator

34. 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. NH
BEE4523 Chapter 3 : Actuator

35. Figure 39 Electric Motor Actuator*
07/16/96
Figure 39    Electric Motor Actuator NH
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36. Pneumatic NH
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37. Pneumatic
Pneumatic : based on concept of pressure as force per unit area
Net force acts on diaphragm ; NH
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38. 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 NH
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39. 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 NH
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40. Pneumatic
Shaft position is linearly related to applied control pressure
Shaft position; NH
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41. Reverse Pneumatic Moves shaft in opposite sense from direct actuator
Obeys same operating principle
Shaft is pulled by application of control pressure NH
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42. 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. NH
BEE4523 Chapter 3 : Actuator

43. 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. NH
BEE4523 Chapter 3 : Actuator

44. 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 NH
BEE4523 Chapter 3 : Actuator

45. 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 NH
BEE4523 Chapter 3 : Actuator

46. 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 NH
BEE4523 Chapter 3 : Actuator

47. 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 NH
BEE4523 Chapter 3 : Actuator

48. necessary adjustments
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 NH
BEE4523 Chapter 3 : Actuator

49. 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. NH
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50. 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   NH
BEE4523 Chapter 3 : Actuator

51. This causes the valve to move upward and open
  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 NH
BEE4523 Chapter 3 : Actuator

52. 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 NH
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53. Figure 36 Pneumatic Actuator with Controller and PositionerNH
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54. Hydraulic NH
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55. Hydraulic Hydraulic : used when large forces are required
Hydraulic pressure ; NH
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56. Hydraulic Basic idea same as pneumatic
Except incompressible fluid used to provide pressure
Pressure will be very large by adjusting area of forcing piston NH
BEE4523 Chapter 3 : Actuator

57. Hydraulic allows the lifting of a heavy load with a small force NH
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58. Hydraulic Pressure is transferred equally throughout the liquid
Force on working piston;
Working force in terms of applied force; NH
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59. 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. NH
BEE4523 Chapter 3 : Actuator

60. Pascal’s Principle NH
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61. Example 3
Determine:
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
The hydraulic pressure NH
BEE4523 Chapter 3 : Actuator

62. 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. NH
BEE4523 Chapter 3 : Actuator

63. 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. NH
BEE4523 Chapter 3 : Actuator

64. A typical piston-type hydraulic actuator is shown in Figure 37
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 NH
BEE4523 Chapter 3 : Actuator

65. 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 NH
BEE4523 Chapter 3 : Actuator

66. 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 closed The 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. NH
BEE4523 Chapter 3 : Actuator

67. Figure 37    Hydraulic ActuatorNH
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68. Fluid valve NH
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69. 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 ; NH
BEE4523 Chapter 3 : Actuator

70. 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 NH
BEE4523 Chapter 3 : Actuator

71. Control-valve principles
There will be a drop in pressure and flow rate varies with square root of pressure drop
The volume flow rate ; NH
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72. Control-valve types
Control-valve characteristic : different types of control valves depends on relationship between valve stem position and flow rate through valve NH
BEE4523 Chapter 3 : Actuator

73. Control-valve types Control valve using pneumatic actuator
To drive stem and open or close valve NH
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74. 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 NH
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75. 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 NH
BEE4523 Chapter 3 : Actuator

76. Control-valve types 2. Linear
Flow rate varies linearly with stem position
Ideal situation, valve determines pressure drop; NH
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77. 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
Rangeability;
Flow rate; NH
BEE4523 Chapter 3 : Actuator

78. Control-valve types
Three types of control valves open differently as function of stem position NH
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79. 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); NH
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80. Control-valve sizing
Valve size (inches) Cv
0.3 3 1 14 1½ 35 2 55
108 4
174 6
400 8
725
Control-valve flow coefficients for different size valves NH
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81. Example 5
Alcohol is pumped through a pipe of 10 cm diameter at 2 m/s flow velocity. Calculate the volume flow rate NH
BEE4523 Chapter 3 : Actuator

82. Example 6
A pressure difference of 1.1 psi occurs across a constriction in a 5 cm diameter pipe. The constriction constant is m3/s/kPa1/2. Determine:
(a) The flow rate in m3/s
(b) The flow velocity in m/s NH
BEE4523 Chapter 3 : Actuator

83. 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. NH
BEE4523 Chapter 3 : Actuator

84. 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 at maximum pressure of 50 psi
(b) The required valve size NH
BEE4523 Chapter 3 : Actuator

85. Summary The important information in this chapter is summarized below
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%.   NH
BEE4523 Chapter 3 : Actuator

86. 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.   NH
BEE4523 Chapter 3 : Actuator

87. 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. NH
BEE4523 Chapter 3 : Actuator