1. Plant-wide Control : part1
CH EN 5253 – Process Design II
Plant-wide Control : part1

2. Books
Product and Process Design Principles: Synthesis, Analysis and Evaluation
by J. D Seader, Warren D. Seider and Daniel R. Lewin
Chapter 20 (4th Edition)
Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design
by Gavin Towler, and Ray Sinnott
Chapter 5 (2nd Edition)

3. Overview Plant Wide Control is Different Profit Optimizer
Active Control System Objectives
Meet production rate and product quality targets
Keep system in Safe Operating Range for equipment, catalysts and materials of construction
Minimize Energy Utilized
Minimize Process Variability
Meet environmental regulations
Air quality
Water discharge quality
Profit Optimizer
Provide inputs for Active Control System
Separate Safety Control System
separate sensors, valves, controllers, power system
Identify Unsafe conditions
Take Action to Safely Bring the Plant/System Down to a Safe Point of Operation or Shut Down

4. Instrumentation and Control Objectives

5. Manipulated and controlled variables
Those streams or conditions to control or to maintain at some desired level (known as the set point).
Flow rates
Levels
Pressures
Temperatures
Compositions etc.
Manipulated variables
An associated variable which can be changed to get desired value
of controlled variable as a result.
Flowing streams
Disturbances
Variables tend to drive the controlled variables away from their set points,
Change in inlet/outlet
Change in set point
In process control, flow rate of the stream is manipulated through the use of a control valve.

6. Manipulated and controlled variables
Heat generation
Heat loss
Fuel Supply
Temperature

7. Selection of Manipulated Variables
Guideline 1:
Select manipulated variables that significantly
affect the controlled variables
If R>4, Controlled Variable: level in reflux drum
Manipulated variable: reflux flow rate ( not distillate)
Guideline 2:
Select manipulated variables that rapidly affect the controlled variables
Avoid variables with large delayed impact on controlled variable  X

8. Selection of Manipulated Variables
Guideline 3:
Select manipulated variables that effect the controlled variables directly rather than indirectly
If Exothermic reaction, Controlled Variable: temperature
Inject coolant directly (do not use cooling jacket)
Guideline 4:
Avoid recycling disturbances
Impurities in process
Better to purge than recycle

9. Single most important component to maintain parameters at desired values

10. Feedback Control System

11. Control Valves Instrument Connection
I to P converter
(Electro-pneumatic converter)
Output air from positioner
4-20 mA to 3-15 psi
Electric control signal
Pneumatic control signal
Instrument Air Supply
Instrument Air Supply
Positioner
Positioning valve stem following signal
Feedback to the controller the actual valve 

12. Control Valve Schematic

13. Control Valve Maintenance
Bypass valve
Drain valves
Isolation valves

14. Control Valves: Fail Safe Strategy
Fail safe does not mean “ failure is impossible or improbable”
“Prevents or mitigates unsafe consequences of the system's failure”
Traffic lights are designed to blink red in all directions if their controller goes down ( often during rain and snow)
An elevator is designed with special brakes that are held back by the tension of the elevator cable. If the cable snaps, loss of tension causes brakes to be applied.
Dead man's switch : Lawnmowers and snow blowers. Hand closed lever/held down all times If it is released, it stops rotating.

15. Control Valves: Fail Safe Strategy
Reasons of failure in process plant
Process abnormality
Instrument air failure
Power shutdown
Emergency situations fire, storm etc.

16. Control Valves Example: Polymerization: reactor should be emptied
Gas Processing: Gas should be flared

17. Sensors

18. Type of sensors in Process Control
Transmitter
Current output directly proportional to a variation in measured variable 
Typical output of 4 to 20 mA
 While transmitters consume more power during operation
Less susceptible to electrical noise as cable lengths increase in the applications
Transducer
Voltage output directly proportional to a variation in measured variable. 
lower operating power and consumption requirements
Switch
Active in two conditions like switch
Either “On” or “Off” signal
Used for alarms for high or low values
Float Type

19. Why is “Switch” important?
Why is the value from transmitter/transducer not always used for alarm ( like switch)?
Why is transmitter output 4 to 20 mA ,not 0-16mA ?

20. Location of Sensor/Switch
Ultra-sound
pulses
Float type
Level Switch
Glass Liquid Level Gages
Level Transmitter
(differential pressure)

21. Controlled variables
Pressure
Level
Flow
Ratio
Temperature
Vaporizer

22. Pressure Control Loop Pressure Control Valve – fail closed
Venting of noncondensables
after a condenser
Pressure Control Valve – fail closed
Is fail closed best?
Pressure Transducer/Transmitter – PT
Piezoelectric, diaphragm displacement
Pressure Indicator Controller – PIC
PID
Alarm Low- PAL
Alarm high -PAH
Direct venting
Condenser
Condenser
Condenser
Varying HT area
( dependent on liquid level)
Controlling coolant flow

23. Level Control Loop Level Control Valve – fail open
Level Transducer /transmitter– LT
DP cell, Ultra-sound pulses
Level Indicator Controller – LIC
PID
Alarm High-LAH
Alarm Low-LAL
Pump may dead head
Interlock valve to never goes to zero