狠狠撸

CLL 361
Instrumentation & Automation
Department of Chemical Engineering
Source: 狠狠撸s prepared by Dr. M.A. Shaik

Outline
? Introduction
? Importance of Process Control
? Control Theory Basics
? Control Algorithms and Tuning
? Components of Control Loops
? Instrumentation

Introduction
Process Engineers are responsible for Design and Operation of
Chemical Processes
Ex: Boiler - controllers for water level and steam pressure
For large and complex processes process automation becomes
more important from the view point of:
v Safety
v Environmental aspects
v Economic aspects
v Ensuring uninterrupted operation of the plant with
minimum human intervention

Manual Vs Automatic Control
Heat Exchanger

Process Control
Three Main Tasks:
Measurement
Comparison
Adjustment
- Control Objective
- Input and Output variables
- Constraints
- Operating Characteristics
- Safety, environmental and
economic aspects
- Control structure
Development of Control Scheme:

Tank Level Problem
Feedback Control Strategy 1:
The level is measured and the inlet
flow rate (valve position) is
manipulated
- Control Objective
- Input and Output variables
- Constraints
- Operating Characteristics
- Safety, environmental and
economic aspects
- Control structure
Development of Control Scheme:

Tank Level Problem
Feedback Control Strategy 2:
The level is measured and the
outlet flow rate (valve position) is
manipulated
Feed-forward Control Strategy:
Inlet flow rate is measured and
outlet flow rate is manipulated

Taking a Shower
Input Variables: Manipulated input
variables are HW and CW valve
positions, body position. Disturbance
inputs include a drop in water pressure
and changes to HW temp.
Control Objective: to become clean, to
become refreshed, to be comfortable
(correct temp. and water velocity as it
contacts the body)
Output Variables: Measured output
variables are temp and flow rate of
mixed stream as it contacts your body.
Constraints: Min and Max valve
positions on both streams. Min and
Max flow rates (hard constraints).
Mixed water temp (soft constraint).
Control Structure: Adjusting either
valve affects both temp and flow rate
Feedback / Feed-forward control

Elements of a Control Loop
Instrumentation

Elements of a Control Loop (Instrumentation)
Primary elements/sensors: first element in the control loop to measure the process
variables (flow, pressure, temp, level, composition, pH, humidity, viscosity etc.)
Transducer: A device that translates a mechanical signal into an electrical signal. E.g.
pressure transducer
Converter: A device that converts one type of signal into another type. E.g.: analog
to digital, current-to-pressure (4-20mA electric signal to 3-15 psig pneumatic signal)
Transmitter: A device that converts a reading from a sensor or a transducer into a
standard signal and transmits that signal to a controller or monitor (PT, LT, FT, TT,
AT etc.)
Controller: device that receives data from a sensor, compares it with set point and
takes corrective action by sending signals to the control element (PLC, DCS etc.)
Controlling element / final controlling element: part of the control system that
physically changes the manipulated variable (e.g. control valve)
Actuator: part of the control element that physically changes the final control
element when signaled to do so. (pneumatically, hydraulically, electrically)

Process Control with Flow Regulation

Temperature of Nylon-6 Polymerization

Temperature and rpm control
of PMMA Polymerization

Pressure Sensors
Bourdon-Tube pressure gauges (a) negative (b) positive pressures
Bourdon-Tube Helical Bourdon-Tube

Differential Pressure Transducer

Position & Motion Measuring Devices
(a) LVDT: Linear Variable
Differential Transformer with
movable core and three windings
(b) Secondary voltage vs
core displacement
Potentiometers convert displacement in a sensor to an electric signal

Level sensors
Sight glass
Simple float Angular float

Capacitive Level Measurement
Conductive Probe Capacitive Probe
For conductive & nonvolatile liquids (as
a spark can occur)
For nonconductive liquids with high
dielectric constant
Liquid increases capacitance between
plates

Flow measurement
Orifice Plate
Venturi Tube
Flow Nozzle Dall Tube

Temperature measurement
Bimetallic Strip

Thermocouples
(b) thermocouples connected to
form a Thermopile
Thermopile: no of thermocouples connected in series to increase sensitivity and
accuracy for low temp differences
(a) Thermocouple Circuit
(c) Pyrometer
Pyrometer: by sensing heat radiated from a hot a body through a lens on to a
thermopile (non contact device, e.g. furnace temp through a small hole in furnace wall)

Resistance Temperature Detectors (RTD)
RTDs measure the change in electrical resistance of a wire-wound resistor with temp.
Typically platinum resistance element is used with 100 W
RTD connections using (a) common supply and meter leads (b) directly
connected meter

Selection of Temperature Sensor

Humidity Measurement
Dew Point: is the temperature at which condensation of the water vapor in air or
gas will take place as it is cooled at constant pressure.
Dry-bulb temperature: is the temperature of a room or mixture of water vapor
and air (gas) as measured by a thermometer whose sensing element is dry.
Wet-bulb temperature: is the temperature of the air (gas) as sensed by a moist
element.
Psychrometric chart: is a somewhat complex combination of several simple
graphs showing the relation between dry-bulb temperatures, wet-bulb
temperatures, relative humidity, water vapor pressure, weight of water vapor per
pound of dry air.

Regulators and Safety Valves
Self-compensating pressure regulators (a) Spring loaded (b) weight loaded
(c) Automatic pressure
safety valve (PSV)

Regulators and Safety Valves
Pilot operated regulator

Level Regulators
(a) Automatic fluid level
controller
(b) Automatic emptying of
a storage tank when full
(c) Means of detecting full level
or empty level in a fluid reservoir

Flow control actuators: Globe Valve
(a) Cross section of
Globe valve with linear
flow control plug
(b) Different flow
patterns for various
plugs vs plug travel
Actuator determines the speed of travel and distance the valve shaft travels

Travel
(fraction)
Flow (%
of Max)
0.0 0.0
0.2 6.25
0.4 12.5
0.6 25.0
0.8 50.0
1.0 100.0

Flow control actuators: Butterfly Valve
(a) Cross section of a
butterfly valve
(b) Flow vs travel
characteristic
The relation between flow and lift is approximately equal % up to 50% open after
which it is linear. Butterfly valves offer high capacity at low cost.

Flow control actuators: Butterfly Valve

Flow control actuators: Gate Valve

Flow control actuators: different valves
(a) Diaphragm (b) One-piece ball valve (c) Rotary plug valve
Rotary plug valve can be used with
corrosive liquids for forward and
reverse flow.
Diaphragm valves & ball
valves are good for slurries &
liquids with suspended solids

Valve Fail Safe
(a) Fail Open (b) Fail Close

P&ID diagrams: Documentation & Symbols
? Plant Equipment and Vessels showing locations, capacity,
pressure, liquid level operating range, usage and so on.
? All interconnection lines distinguishing between the types of
interconnection, i.e. gas or electrical and operating range of
line
? All motors giving voltage and power and other relevant
information
? Instrumentation showing location of instrument, its major
function, process control loop number, and range
? Control valves giving type of control, type of valve action,
fail save features, and flow plus pressure information
? The ranges for all safety valves, pressure regulators,
temperatures, and operating ranges
? All sensing devices, recorders, and transmitters with control
loop numbers

Symbols for instrument line interconnection
Abbreviations for secondary flow lines

Documentation & Symbols
Examples of the letter and numbering codes
? (a) First letter T indicates that the instrument is in Temperature loop number 178. The
second letter Y denotes conversion, from current to pressure, circle indicates discrete
instrument located in field.
? (b) F indicates Flow, R is for Recorder, and C is for Controller , in loop 97. Hexagon
indicates it is an accessible computer function.
? (c) P indicates Pressure, R is for Recorder, and T is for Transmitter , in loop 89 located in a
secondary accessible location and is a PLC function.
? (d) L is for level, A is alarm, H is high, which is an alarm for high liquid level located in
loop 222 and is not accessible.

Standardized instrument symbols

Instrument identification letters

Examples of basic and actuator symbols

Examples of primary elements used in P and ID diagram

Examples of regulators & safety valve symbols used in P and ID diagram

Example of P& ID for a mixing station

Examples of Experimental Computer
Control of Processes
Liquid Level Control System using a PC (Srinivas & Chidambaram, 1996)
? The tank can hold water upto a height of
100 cm
? Water is pumped into the tank
continuously by a small pump.
? Water height is measured by a
differential pressure transducer (at
bottom)
? Pneumatic signal from DPT is
converted into current signal.

Heat Exchanger Control using a PC (Singh et al., 1996)

Heat Exchanger Control using a PC (Singh et al., 1996)
? The process consists of 1-2 shell & tube heat exchanger
? The carbon steel shell is 11.5 cm in diameter & 74 cm long
? Copper tubes are 1.27 cm in diameter and 122 cm long
? Hot fluid (steam) on shell side & cold process stream (water)
on tube side
? Steam line consists of a steam regulator, a control valve, an
absolute pressure transmitter, a pressure guage, a temperature
transmitter and a steam trap.
? Water line consists of a control valve, a rotameter, an orifice
meter along with a DPT at the inlet of heat exchanger.
? Cascade control to minimize the fluctuations in steam line
pressure.

A to D & D to A converters
Signals
Pneumatic
Electric
Digital
Analog

Smart Sensor Block Diagram
? Smart sensor: Integration of sensor with ADC, processor, and DAC for
actuator control.
? Smart sensor contains all necessary circuits to interface to the sensor, amplify
the signal, apply PID control, ADC to convert signal into digital format for
internal processor, DAC to convert signal back into analog format for actuator
control.

Digital to Analog Conversion
(a) 1 kHz sine wave form reproduced
from a DAC
(b) Commercial 8-bit DAC
? Two methods of converting digital to analog signals:
(i) DAC converter (ii) Pulse Width Modulation (PWM)
? Digital signal is converted to a voltage
every 0.042 ms giving step wave form.
? Finer time grid will give better approx.
? 4 bit binary coding

Pulse Width Modulation (PWM signal)
PWM signal to give a 1 kHz sine wave using positive and negative supplies
? PWM changes the duration for which voltage is applied going
from narrow to wide and back to narrow.
? If the voltage pulses are averaged it gives a half-sine wave.
? The other half is generated using negative supply

Different Control Modes
? On/Off action: Simplest form of control
E.g. Room heating system: reference temp is 75o F
Simple On/Off action
(considering only inertia)
? When heating, the temp in the center of
room has already reached 77o F before the
temp at sensor reaches 75o F
? As the room cools, the temp in the room will
drop to 73o F before the temp at sensor
reaches 75o F
? Temp varies between 72o F to 78o F due to
inertia of system
Differential or Delayed On/Off action
(considering dead time & inertia)
? The switching points for thermostat are
delayed by ± 3o F
? Temp varies between 70o F to 80o F

Different Control Modes
? Proportional, Integral & Derivate action
General guidelines:
1. Pressure control requires P&I.
D is normally not required
2. Level control uses P and
sometimes I. D is normally not
required
3. Flow control uses P&I. D is
normally not required
4. Temp control uses PID with
integral set for a long time
period.

Programmable Logic Controllers (PLC)
? PLC can be configured to receive a small number of inputs (both
analog and digital) & control a small number of outputs
? PLC can be expanded with plug-in modules to receive a large
number of signals and control a large number of actuators
Block diagram of a control loop:

Programmable Logic Controllers (PLC)
? I/O Scan Mode: the output control signal is updated based on the
information received from previous I/O scan cycle
? Execution Mode: In this mode the processor evaluates input data
stored in memory against the data programmed into the CPU.
The programs are set up using ladder network (series of
steps/instructions to be executed sequentially)
? Ladder diagram: symbolic and schematic way to represent the
interconnections between the elements of a PLC.
E.g. Liquid heating system

Symbols used in Ladder Diagrams
Push to open
Push to close
NO: Normally Open NC: Normally Close
Position

Ladder diagram for liquid heating system
? The first three rungs S1,S2, and S3 are used for control of indicator lights and
the power to the rest of the circuit.
? Closure of switch 1PB will energize control relay 1CR, which in turn closes
1CR NO (red light ON); opens 1CR NC (green light OFF); heater is turned ON
? In rung S5 when temp reaches desired level it will energize 2CR which in turn
sets alarm ON in rung S6
? In rung S1 2CR will turn OFF and de-energizes 1CR, turns lights red to green
and turns off heater.
? Alarm will remain enabled
until turned Off by switch 3PB
? Switch 3PB ON de-energizes
2CR then alarm is OFF
? 1CR can also be turned off by
switch 2PB
Power on Relay
Red light control
Green light control
Heater control
Temp sense &
Control Relay
Alarm Control

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