1. Process Control Instrumentation II
Module 4

2. Module 4
Final control element : I/P converter – pneumatic and electric actuators – valve positioner – control values – characteristics of control valves – inherent and installed characteristics – valve body – commercial valve bodies – control valve sizing – cavitation and flashing – selection criteria.

3. I/P Converter

4. ACTUATORS
If a valve is used to control a flow, some mechanism must physically open or close the valve.
For a temperature control some mechanism must turn the heater ON or OFF.
these are examples of ACTUATORS.
Between Signal conversion unit and Control Valve.

5. ACTUATORS TYPES ELECTRICAL ACTUATORS Solenoid Motors
PNEUMATIC ACTUATORS
Spring and Diaphragm Actuators
Piston Actuators
Rotary Valve Actuators
ELECTRO-PNEUMATIC ACTUATORS
HYDRAULIC ACTUATORS

6. ELECTRICAL ACTUATORS SOLENOID
ON- OFF Control

7. ELECTRICAL ACTUATORS MOTORS
Motor as Direct Actuators
Motor along with Gear boxes
Common Varieties
DC Motors
AC Motors
Stepping Motors

8. PNEUMATIC ACTUATORS SPRING & DIAPHRAGM ACTUATORS

9. PNEUMATIC ACTUATORS SPRING & DIAPHRAGM ACTUATORS

10. PNEUMATIC ACTUATORS SPRING & DIAPHRAGM ACTUATORS
DIRECT ACTING
REVERSE ACTING

11. PNEUMATIC ACTUATORS SPRING & DIAPHRAGM ACTUATORS
FAIL SAFE OPERATIONS
FAIL OPEN
(Air to Close)
FAIL CLOSED
(Air to Open)

12. PNEUMATIC ACTUATORS SPRINGLESS DIAPHRAGM ACTUATORS

13. PNEUMATIC ACTUATORS PISTON ACTUATORS

14. PNEUMATIC ACTUATORS PISTON ACTUATORS

15. ELECTRO-PNEUMATIC ACTUATORS

16. HYDRAULIC ACTUATORS
PH = F1/A1 = Fw/A2
Fw = F1A2/A1

17. HYDRAULIC ACTUATORS

18. ACTUATORS - COMPARISON
Actuator types
Advantages
Disadvantages
Electrical(servomotor or stepping motor)
Direct interface with computer system. Simple design.
Low thrust.
Slow speed.
No mechanical fail safe hazardous.
Electromechanical(Motors combined with gear boxes)
High thrust
High stiffness coefficient
Flexible adaptation
Complex design
No mechanical fail safe
Large, heavy structure
Hazardous
Hydraulic and Electro hydraulic
Fast speed
Self lubrication
Fluid Viscosity Sensitive
Pneumatic and Electro-pneumatic
Low cost
Mechanical Fail safe
Simple design
Small package
Suitable for highly hazardous areas also
Good control with control device
Slow speed
Lack of stiffness
Instability
Moderate trust
Quality air requirement

19. POSITIONERS
It guarantees that the valve does move to the position where the controller wants to be.
Can correct many variations, including changes in packing friction due to dirt, corrosion, or lack of lubrication; variations in the dynamic forces of the process; sloppy linkages; or non-linearities in the valve actuator.

20. ELECTRONIC VALVE POSITIONERS (For Electric Actuators)
High Gain Proportional controller.
It is considered as a slave controller of a cascade loop in which the master controller is the primary controller itself.
The primary controller’s output signal is the set point for the slave controller (positioner)

21. ELECTRONIC VALVE POSITIONERS

22. VALVE POSITIONERS for PNEUMATIC Actuators
Refer Krishnaswamy

23. Pneumatic Motion Balance Positioner

24. Pneumatic Force Balance Positioner

25. Electro-pneumatic Force Balance Positioner

26. CONTROL VALVES
Control Flow Rate

27. CONTROL VALVE CHARACTERISTICS
The relationship between control valve capacity and valve stem travel is known as the Flow Characteristic of the Control Valve.
Trim design of the valve affects how the control valve capacity changes as the valve moves through its complete travel.
Because of the variation in trim design, many valves are not linear in nature. Valve trims are instead designed, or characterized, in order to meet the large variety of control application needs.
Many control loops have inherent non linearity's, which may be possible to compensate selecting the control valve trim.

28. CONTROL VALVE CHARACTERISTICS
Inherent Characteristics
Installed/ Effective Characteristics

29. CONTROL VALVE CHARACTERISTICS Inherent Characteristics (Ideal)
These curves are based on constant pressure drop across the valve (pressure difference is determined by the valve alone).
based on the design conditions, parameters and experimental results.
Different from actual because of the variations in parameters and limitations in manufacturing process.

30. CONTROL VALVE CHARACTERISTICS Inherent Characteristics (Ideal)
ASSUMPTIONS:
The actuator is linear.
The pressure difference across the valve is constant.
The process fluid is not flashing, cavitating or approaching sonic velocity (Choked flow).

31. Inherent Characteristics….

32. 1. Quick Opening CV Used for Full On/ Full OFF applications.
Relatively small motion of valve stem results in maximum possible flow rate through the valve.
Eg: 30% travel of stem may results in 90% of full flow
Also called ‘DECREASING SENSITIVITY TYPE VALVE’
The valve sensitivity (∆Q/∆S) at any flow decreases with increasing flow.

33. 2. Linear Valve Flow rate varies linearly with stem position.
Q/Qmax = S/ Smax
Sensitivity Constant.
Eg: if valve open 25% the flow through the valve is 25% of full flow.
Process tank level control, De-aerated level control etc.

34. 3. Equal Percentage Valve
A given percentage change in stem position produces an equivalent change in flow.
These valves does not shut off flow completely.
RANGEABILITY, R = Qmax /Qmin
‘INCREASING SENSITIVITY TYPE’
The valve sensitivity at any given flow rate is a constant percentage of the given flow rate, thus equal percentage.

35. 3. Equal Percentage Valve…
Eg: consider a valve having a maximum flow rate of 60 tonnes/hr.
Case 1
when valve delivering 10 tonnes/hr, when the valve is permitted to open 10% more, then the incremental flow is 1 tons/hr.
Case 2
when valve delivering 40 tonnes/hr, when the valve is permitted to open 10% more, then the incremental flow rate is 4 tonnes/hr.

37. CONTROL VALVE CHARACTERISTICS Installed Characteristics (Real)
When valve is installed as part of a process plant, its flow characteristics are no longer independent of the rest of the system.
Fluid flow through the valve is subject to frictional resistances in series with that of the valve.
Distortion Coefficient,

38. CONTROL VALVE CHARACTERISTICS Installed Characteristics (Real)
Deviation in inherent valve characteristics.
Actuators without positioner will introduce nonlinearities.
Pump curves will also introduce nonlinearities.

39. Valve Body & Commercial Valve Bodies
Refer Krishnaswamy

40. CONTROL VALVE SIZING Proper sizing of control valve is important.
If control valve is oversize,
The valve must operate at low lift and the minimum controllable flow is too large.
Lower part of the flow-lift characteristic is most likely to be non uniform in shape.
If control valve is undersize,
the maximum flow desired for operation of a process may not be provided.

41. Control valve sizing FACTORS AFFECTING SIZING
Pressure Drop across the control valve
Flow Rate through the control valve
Specific Gravity
Other factors such as type of fluid, gas or liquid, critical flow conditions for gases and vapours and viscosity of liquids influence valve size

42. FLOW COEFFICIENT, CV
The flow coefficient indicates the amount of flow the control valve can handle under a given pressure drop across the control valve.
‘The number of US Gallons of cold water at 60◦ F flowing through a fully open control valve for 1 minute at a pressure difference of 1 psi.’
US Gallon = ltr
Imperial Gallon = 4.53 ltr

43. FLOW COEFFICIENT, KV CV = 1.17 KV KV = 0.86 CV
Whenever the flow coefficient is mentioned in metric units.
The flow rate of water in m3/hr at about 300C flowing through the fully opened control valve at a pressure drop of 1 kg/cm2 across the valve.
CV = 1.17 KV
KV = 0.86 CV

44. FLOW COEFFICIENT, KV
Q = flow rate
SG = Specific Gravity

45. Guidelines for Sizing of Control Valves
The valve shall be sized for the actual flow condition and not for the ultimate design capacity of the system. Normal maximum flow rate is normally about 70% of the ultimate design capacity.
Most of the pressure drop of the system should be across the control valve. (about 70%)
When the pipe line is dimensioned with normal allowable velocities the control valve will be a few sizes smaller than the pipe line. Only in extreme cases where very high velocities have been used in the pipe line, the size of the control valve will be the same as that of the pipe line.

46. Guidelines for Sizing of Control Valves…
The selection must be done such that the calculated CV is attained at about 75 to 80% of the full wave travel.
Regardless of the application such as flow control or pressure control the valve sizing is done on the basis of flow coefficient CV.

47. CAVITATION
Related to Bernoulli’s theorem which describes the pressure profile as the fluid passes through a narrower passage, restriction, orifice or control valve.
As the fluid accelerates pressure head is converted into velocity head.
Fluid accelerates to maximum velocity, minimum pressure known as Vena Contracta.
Fluid then slows down as it again expands to full pipe area.
Static pressure recovers but a part is lost due to friction.

48. CAVITATION…
If the pressure head drops below the liquid vapour pressure at that temperature, then vapour bubbles will form downstream of the restriction.
As the static pressure recovers to a point greater than vapour pressure, the vapour bubbles collapse back into their liquid phase.
This collapse of bubbles produces high-energy implosions which is called ‘CAVITATION’.
These implosions generate noise, fluid shock cells, and gets that impinge upon the trim metal parts.
It generates a tremendous and concentrated impact force that destroys the metal as it fractures out tiny metal particles.

49. PRESSURE PROFILE

50. CAVITATION

51. CAVITATION…
Cavitation is usually coupled with vibration and a sound like rock fragments or gravel flowing through the valve.

52. Elimination of CAVITATION
No known material can withstand continuous cavitation without damage and eventual failure.
The length of time it will take is a function of the fluid, metal type and severity of the cavitation.
The greatest damage is caused by a dense pure liquid with high surface tension.
Density governs the mass of the micro jet stream, and surface tension governs the jet velocity.

53. Cavitation Elimination Methods
Revised Process Conditions
Revised Valve
Gas Injection
Revised Installation

54. Cavitation Elimination Methods

55. Cavitation Elimination Methods…

56. FLASHING
Cavitation occurs when P2˃PV , while flashing takes place when P2˂PV.
Liquid flashes into vapour and stays in the vapour phase.
The specific volume increases as liquid changes to vapour which in turn causes an increase in the fluid velocity.
If enough vapour is formed, the resulting high velocities can erode metals.
So the piping downstream of a valve needs to be much larger than the inlet-piping in order to keep the velocity keep the velocity of the stream low enough to prevent erosion.

57. SELECTION OF CONTROL VALVES
Need for a Control Valve
Collection of Process Data
Assigning Valve Pressure Drop
Control Valve Performance
Valve characteristics
Gain of control loop components like process, sensor, controller and valve gain.
Process Nonlinearity
Valve Rangeability
Control Valve Sequencing
Split-ranging or floating

58. SELECTION OF CONTROL VALVES…
Control Valve Sizing
Valve Actuator Selection
Whether electrical, pneumatic or hydraulic
Actuator Speed of Response
Actuator power or torque
Valve failure position (Fail safe operation)
Valve Positioner
When not to use positioner
To eliminate dead band
Split range operation

59. SELECTION OF CONTROL VALVES…
Process Application Considerations
High Pressure service
High differential pressure usage
Vacuum service
High temperature service
Low temperature service
Cavitation and erosion
Flashing and erosion
Viscous and slurry service
Leakage
Small flow values
Control valve noise
Piping and installation considerations
Climate and atmospheric corrosion

60. SELECTION OF CONTROL VALVES…
9. Control Valve Specification form
10. Test report and test specification