Module 2_PC.pptx

1. Actuators

2. Contents
• Introduction to various actuators such as flow control valves,
Hydraulic and Pneumatic, Servo motors, symbols and characteristics

3. Process control loop

4. Flow Control Valves
• A valve is a device that regulates, directs or controls the flow of a fluid by opening, closing, or
partially obstructing various passageways.
• A valve is defined as any device by which the flow of fluid may be started, stopped or regulated
by a movable part that opens or obstructs passage.
• Valve Functions
1. Stopping and starting fluid flow.
2. Varying (throttling) the amount of fluid flow.
3. Controlling the direction of fluid flow.
4. Regulating downstream system or process pressure.
5. Relieving component or piping over pressure.

5. Major Parts of valve

7. Flow Characteristics
• It describes how the flow rate changes with the movement or lift of the stem. The
shape of the plug primarily decides the flow characteristics.
(i) Inherent or ideal characteristics
(ii) Effective or installed characteristics
• An inherent characteristic is the ideal flow characteristics of a control valve and is
decided by the shape and size of the plug.
• On the other hand, when the valve is connected to a pipeline, its overall
performance is decided by its effective characteristic.

8. Control valve characteristics

9. Types of Control Valves based on Application
• Flow control- Ball valve, Gate valve, Globe valve, Needle valve,
Pinch/Diaphragm valve, Butterfly valve
• Pressure control- Relief valve, Safety valve
• Directional control-Check valve

10. Ball valve
• A Ball valve is a quarter-turn rotary motion valve that uses a ball-
shaped disk to stop or start the flow. Most ball valves are of the quick-
acting type, which requires a 90° turn of the valve handle to operate
the valve. The ball valve is Smaller and lighter than a gate valve of
same size and rating.

11. Gate valve
• Gate valve is the most common type of valve in any process plant. It is a linear motion
valve used to start or stop fluid flow. In service, these valves are either in fully open or fully
closed position. Gate valves are used in almost all fluid services such as air, fuel gas, feed
water, steam, lube oil, hydrocarbon, and all most any services. Gate valve provides good
shutoff.

12. Globe valve
• Globe valve is used to stop, start, and regulate the fluid flow. Globe Valves
are used in the systems where flow control is required and leak tightness is
also necessary. Globe valve provides better shut off as compared to gate
valve and it is costlier than gate valve.

13. Needle valve
• Needle valves are similar to a globe valve in design with the biggest difference is
the sharp needle like a disk. Needle valves are designed to give very accurate
control of flow in small diameter piping systems. They get their name from
their sharp-pointed conical disc and matching seat.

14. Pinch/Diaphragm valve
• The pinch valve is also known as clamp valve. It is a linear motion valve. Used to
start, regulate, and stop fluid flow. It uses a rubber tube, also known as a pinch
tube and a pinch mechanism to control the fluid. Pinch Valve is ideally suited for
the handling of slurries, liquids with large amounts of suspended solids, and
systems that convey solid material pneumatically.

15. Butterfly valve
• A Butterfly valve is a quarter-turn rotary motion valve, that is used to stop,
regulate, and start the flow. Butterfly valve has a short circular body. Butterfly
Valve is suitable for large valve applications due to Compact, lightweight
design that requires considerably less space, as compared to other valves.

17. Relief valve/Safety valve
• A pressure Relief valve or pressure safety valve are used to protect
equipment or piping system during an overpressure event or in the event of
vacuum. This valve releases the pressure or vacuum at pre-defined set
pressure.

18. Check valve
• The check valve prevents backflow in the piping system. The pressure of the
fluid passing through a pipeline opens the valve, while any reversal of flow
will close the valve.

19. Classification of control valves
Control valves are available in different types and shapes. They can be
classified in different ways; based on:
(a) application
(b) action
(c) number of plugs, and
(d) flow characteristics.

20. Types of Control Valves based on Action
Control valves operated through pneumatic actuators can be either
(i) air to open.
• They are designed such that if the air supply fails, the control valve
will be either fully open, or fully closed, depending upon the safety
requirement of the process.
• For example, if the valve is used to control level of liquid in a tank,
the valve should be shut off completely in case of air failure(A).

21. (ii) air to close.
• On the other hand, if the valve is handling cooling water to a reactor,
the flow should be maximum in case of emergency (B).

22. Types of Control Valves based on Number of plugs
• Control valves can also be characterized in terms of the number of plugs present, as
(i) single-seated valve
(ii) double-seated valve
• The advantage of single-seated valve is that, it can be fully closed and flow variation
from 0 to 100% can be achieved. But looking at its construction, due to the pressure
drop across the orifice a large upward force is present in the orifice area, and as a
result, the force required to move the valve against this upward thrust is also large.

23. • Thus this type of valves is more suitable for small flow rates. On the other hand, there are
two plugs in a double-seated valve; flow moves upward in one orifice area, and downward
in the other orifice.
• The resultant upward or downward thrust is almost zero. As a result, the force required to
move a double-seated valve is comparatively much less.

24. ACTUATOR
• An actuator is a component of a machine that is responsible for moving and controlling
a mechanism or system, for example by opening a valve. In simple terms, it is a
"mover".
• An actuator requires a control signal and a source of energy. The control signal is
relatively low energy and may be electric voltage or current, pneumatic or hydraulic
pressure, or even human power. Its main energy source may be an electric
current, hydraulic fluid pressure, or pneumatic pressure.
• When it receives a control signal, an actuator responds by converting the signal's
energy into mechanical motion.

25. Pneumatic Actuators
• Pneumatic actuators enable considerable forces to be produced from
relatively small pressure changes. A pneumatic actuator converts energy
formed by vacuum or compressed air at high pressure into either linear or
rotary motion.
• Moreover, pneumatic actuators are safer, cheaper, and often more reliable
and powerful than other actuators. These forces are often used with valves
to move diaphragms to affect the flow of air through the valve.

27. Advantages of Pneumatic actuators –
• The biggest advantage of the pneumatic actuators is their failsafe action. By design of the
compressed spring, the engineer can determine if the valve will fail closed or open,
depending on the safety of the process.
• Provide high force and speed, which are easily adjustable and are independent of each other
• Have a delayed response which makes them ideal for being resilient against small upsets in
pressure changes of the source.
• Most economical when the scale of deployment matches the capacity of the
compressor.
• Provide inherent safety and are ideal for hazardous and explosive environment.
• Low component cost and smaller footprint.

28. Limitations of Pneumatic Actuators
• Maintenance and operating costs can be high, especially if a serious effort
has not been made to quantify and minimize the costs.
Maintenance costs include replacement cylinder costs and plugging air-
line leakages whereas the operating costs include the cost of compressed
air, i.e. electricity for the compressor.

29. Hydraulic actuators
• A hydraulic actuator consists of cylinder or fluid motor that uses hydraulic power to
facilitate mechanical operation. The mechanical motion gives an output in terms of linear
or rotatory motion. As liquids are nearly impossible to compress, a hydraulic actuator
can exert a large force. The drawback of this approach is its limited acceleration.
• The hydraulic cylinder consists of a hollow cylindrical tube along which a piston can slide.
The term single acting is used when the fluid pressure is applied to just one side of the
piston. The piston can move in only one direction, a spring being frequently used to give
the piston a return stroke. The term double acting is used when pressure is applied on
each side of the piston; any difference in pressure between the two sides of the piston
moves the piston to one side or the other.

31. Advantages of hydraulic actuation system
• Variable speed and direction
• Power-to-weight ratio
• Stall condition and overload protection(pressure relief valves)
The hydraulic fluid must essentially be non-compressible to be able to
transmit power instantly from one part of the system to another. The most
common liquid used in hydraulic systems is petroleum oil.

32. Electrical type actuators
• Solenoids
• Servomotor

33. SOLENOID ACTUATORS
• When current flows through the coil, a magnetic field forms around the coil. The magnetic
field attracts the 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 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.

35. Servomotor actuator
• A servomotor is a rotary actuator that allows for precise control of angular
position, velocity and acceleration. It consists of a suitable motor coupled to a
sensor for position feedback. It also requires a relatively sophisticated
controller, often a dedicated module designed specifically for use with
servomotors.
• Servomotors are used in applications such as robotics, CNC machinery or
automated manufacturing.

38. • Suppose, we have given a signal to rotate servomotor by an angle of 45° and then stop
and wait for further instruction.
• The shaft of the DC motor is coupled with another shaft called output shaft, with help of
gear assembly. This gear assembly is used to step down the high rpm of the motor's shaft
to low rpm at output shaft of the servo system.
• The voltage adjusting knob of a potentiometer is so arranged with the output shaft by
means of another gear assembly, that during rotation of the shaft, the knob also rotates
and creates an varying electrical potential. This signal i.e. electrical potential is increased
with angular movement of potentiometer knob along with the system shaft from 0° to
45°. This electrical potential or voltage is taken to the error detector feedback amplifier
along with the input reference commends i.e. input signal voltage.

39. • As the angle of rotation of the shaft increases from 0° to 45°
the voltage from potentiometer increases. At 45° this voltage reaches to a value which
is equal to the given input command voltage to the system. As at this position of the
shaft, there is no difference between the signal voltage coming from
the potentiometer and reference input voltage(command signal) to the system, the
output voltage of the amplifier becomes zero.
• From Figure, it can be observed that the output electrical voltage signal of the
amplifier, acts as input voltage of the DC motor. Hence the motor will stop rotating
after the shaft rotates by 45°. The motor will be at this rest position until another
command is given to the system for further movement of the shaft in desired direction.

41. Advantages of Electric actuators-
• Provide precise control and positioning in comparison to pneumatic actuators.
• Response time is essentially instantaneous.
• High degree of stability.
• Help adapt machines to flexible processes.
• Low operating cost. Controllers and drivers low voltage circuitry consume power to a far
lesser degree.

42. Disadvantages of Electric actuators-
• The primary disadvantage of an electric actuator is that, should a power failure occur,
the valve remains in the last position and the fail-safe position cannot be obtained easily
unless there is a convenient source of stored electrical energy.
• Higher cost than pneumatic actuators. High component costs often deter the use of
electric actuators because savings in operating costs compared to pneumatics are often
not adequately considered or are outright ignored.
• The actuator needs to be in an environment that is rendered safe. Generally not
recommended for flammable atmospheres.