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Control System ComponentsTopic: Actuators and Valve Positioner
Prepared by :Prof. Rajesh Zadfiya Instrumentation & Control Engg.
Institute of Technology Nirma University
Actuators• A valve actuator is a
device that produces force to open or close the valve utilizing a power source. • This source of power
can be manual (hand, gear, chain-wheel, lever, etc.) or can be electric, hydraulic or pneumatic.
Contd..• Basic actuators turn valves
to either fully opened or fully closed positions. • But modern actuators have
much more advanced capabilities. They not only act as devices for opening and closing valves, but also provide intermediate position with high degree of accuracy.
Type of Actuators
• Two types of actuators are common: pneumatic and electric actuators.• Pneumatic:
• Pneumatic actuators utilize an air signal from an external control device to create a control action. These are commonly available in two main forms: • Diaphragm actuators and • Piston actuators
• Electric:• Electric actuators are motor driven devices that utilize an electrical input signal to
generate a motor shaft rotation. • This rotation is, in turn, translated by the unit’s linkage into a linear motion, which drives
the valve stem and plug assembly for flow modulation. • In case of electric signal failure, these actuators can be specified to fail in the stem-out,
stem-in, or last position. • Commonly used motors for electric actuators include steppers and servos.
Contd..
Diaphragm actuators – Diaphragm actuators have compressed air applied to a flexible
membrane called the diaphragm. These types of actuators are single acting, in that air is only supplied
to one side of the diaphragm, and they can be either direct acting (spring-to-retract) or reverse acting (spring-to-extend).
Contd..
Flapper nozzle amplifier
• A pneumatic control system operates with air. • The signal is transmitted in form of variable air pressure (often in the
range 3-15 psi, i.e. 0.2 to 1.0 bar) that initiates the control action. • One of the basic building blocks of a pneumatic control system is the
flapper nozzle amplifier. • It converts very small displacement signal (in order of microns) to
variation of air pressure. • The basic construction of a flapper nozzle amplifier is shown below.
Contd..
Characteristics of a flapper nozzle amplifier
Limitations of Flapper Nozzle Amp.
• The major limitation of a flapper nozzle amplifier is its limited air handling capacity. The variation of air pressure obtained cannot be used for any useful application, unless the air handling capacity is increased. • Another problem of a flapper nozzle amplifier is its sensitivity
variation.
Air Relay
The principle of operation of an air relay can be explained using the schematic diagram shown here.
It can be seen from Fig. that the air relay is directly connected to the supply line (no orifice in between). The output pressure of the flapper nozzle amplifier (p2) is connected to the lower chamber of the air relay with a diaphragm on its top.
Contd..
The variation of the pressure p2 causes the movement (y) of the diaphragm. There is a double-seated valve fixed on the top of the diaphragm. When the nozzle pressure p2 increases due to decrees in xi, the diaphragm moves up, blocking the air vent line and forming a nozzle between the output pressure line and the supply air pressure line. So more air goes to the output line and the air pressure increases. When p2 decreases, the diaphragm moves downward, thus blocking the air supply line and connecting the output port to the vent. The air pressure will decrease.
Limitations of Air Relay
Problem with of an air relay is its sensitivity variation.
Flapper Nozzle Amplifier with Feedback
Flapper nozzle amplifiers are never used in open loop; it is always used in closed loop. The scheme shown below is a torque balance arrangement.
Contd..
Anticlockwise moment: and
Clockwise moment:
Where AB1 and AB2 are the areas of the two bellows, a and b are the corresponding lengths of the link segments.
Thereby at balance:
solenoid
• Solenoid is an electromagnet which can be used as an actuator. • Electrically operated actuators.• Solenoid valves are used in hydraulic and pneumatic systems.
It moves a rod by electromagnetic energy
Contd..
Applications
Contd..
Contd..
Applications (Combined)
Contd..
Advantages of Pneumatic Actuators
• Weight• Cylinders much lighter than motors
• Simple• Much easier to mount than motors• Much simpler and more durable than other for linear motion
• Fast on/off type tasks • Big forces with elasticity• No leak problems
Disadvantages of Pneumatic Actuators
• All the components are quite expensive• A properly designed system is more complex than an equivalent
electromechanical system (electric motors, power screws and other linear actuators). • All these components take up quite a bit of valuable space (For
example within a robot). • No weight advantage if only one cylinder used (still need compressor,
reservoir, pressure sensors, regulator)
Operators
General manual
Push button
Pull button
Push/pull button
Lever
Pedal
Treadle
Manual
Rotary knob
OperatorsMechanical
Plunger
Spring normally as a return
Roller
Uni-direction or one way trip
Pressure
Pilot pressure
Differential pressure
Detent in 3 positions
Operators
Solenoid direct
Solenoid pilot
Solenoid pilotwith manual overrideand integral pilot supply
Solenoid pilotwith manual override and external pilot supply
Electrical
When no integral or external pilot supply is shown it is assumed to be integral
Pneumatic Rotary Actuators
Pneumatic Valve Positioner • Pneumatic valve positioner is another important component used in
process control.• The control valve should be moved up or down, depending on the air
pressure signal (3-15 psi). • The valve postioner can be of two types, (a) direct acting type and (b)
feedback type.• The direct acting type valve positioner is shown below.
Direct acting type valve positioner
Contd..• Here the control pressure creates a downward pressure on the
diaphragm against the spring, and the stem connected to the diaphragm moves up or down depending on the control pressure p. At equilibrium the displacement of the stem can be expressed as:
pA=Kx ---------------(1)where A is the area of the diaphragm and K is the spring
constant. • But the major shortcoming of this type of positioner is the nonlinear
characteristics. • Though ideally, the stem displacement is proportional to the control
pressure (from (1)), the effective area of the diaphragm changes as it deflates.
Contd..• The spring characteristics is also not totally linear. Moreover, in (1) we
have neglected the upward thrust force exerted by the fluid. • The change in thrust force also causes the change in performance of
the positioner. • Besides the force exerted on the control valve is also not sufficient for
handling valves for controlling large flow. • As a result, the use of direct acting type valve positioner is limited to
low pressure and small diameter pipelines.
Feedback type valve positioner
Contd..• The feedback type valve positioner has a pilot cylinder with which the
diaphragm is attached. • The piston of this pilot cylinder opens or closes the air supply and vent ports to
the main cylinder whose piston is connected to the stem of the control valve (not shown). • There is a mechanical link connected to the stem that adjusts the fixed end of
the spring connected to the diaphragm. This link provides the feedback to the postioner. • As the control pressure increases, the diaphragm moves down, so is the piston
of the pilot cylinder. This causes the lower chamber of the main cylinder to be connected to the 20 psi line and the upper chamber to the vent line. • Compressed air enters the bottom of the main cylinder and the piston moves
up.
Contd..• As the piston moves up, the feedback link compresses the spring
further and this causes the diaphragm to move back to its original position. • The air supply and the vent ports are now closed and the piston of the
main cylinder remains at its previous position. The relationship between the control pressure and movement of the stem in this case is more or less linear. • Moreover due to presence of power cylinder, the scheme is more
suitable to position large control valves.
Hydraulic actuators: cylindersCylinder types:
Single acting: work can be done only in one direction
Piston
Double acting piston:
Piston rod on both sides
Plunger
Work is done in both directions
Telescopic Telescopic Fast moving
Tandem
Fast moving
Hydraulic cylinders
Properties:
The cylinders have to be good quality steel with close tolerances.
There have to be good sealing both at the piston rod and at the cylinder.
With time dirt may come in and damage the surfaces. This has to be possibly reduced.
In this case, the leakage will increase all the time.
Hydraulic cylindersCalculation of cylinders
1maxmaxFFFF FLc FLC FFF
maxmax
22110 pApAFC
1122 pApAFCB
10 A
Qv
2A
QvB
friction forces inertial forcesmaximum loadslow motion, can be often neglected
Backward:
Outward:
Q
A2A1
p1
p2vB
v0
Hydraulic cylindersCalculation of cylinders
Hydraulic cylinders should be possibly operated in the 3rd region for smooth operation.
If the cylinder is new, the leakage losses are negligibly small so that:
ηc = ηmech
Ff1
v
2 3
1. Stick-slip2. Transition3. Normal behaviour
22112211 ApAp
F
QpQp
Fv LLc
92,085,0max
c
ηc
Δp
outwardsinwards at higher pressures
Hydraulic cylindersChecking for buckling
n: safety factor: 1-3,5
lk: buckling length
I1: moment of inertia of the piston rod
E: elasticity modulus of the rod material
1
2
max
1IE
lnF
kL
64
4d
Maximum permissible force:
Hydraulic cylindersCushioning of cylinders:
A hard impact of the piston at the end surfaces has to be inhibited – kinetic energy has to be absorbed.
This is done by increasing the hydraulic resistance at the end of the stroke.
Rotary hydraulic actuators
Swivel vane rotary actuator:
Limited angle in both directions
Maximum angle always smaller than 360°
The same torque in both directions
Piston rotary actuator:
With rack and gear coupling
Here maximum angle may be larger than 360°
Párhuzamdugattyús
lengőhajtás
Limited angle rotary actuator
Limited angle rotary actuator
Parallel piston rotary
actuator
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