View
218
Download
0
Category
Preview:
Citation preview
7/27/2019 Chemical Process Variables-1
1/47
UNIT1
Introduction to ProcessControl
1PCT301T-2013
7/27/2019 Chemical Process Variables-1
2/47
OBJECTIVES:
Understand the terminologies associated with processcontrol.
How the four components / elements of control system
are linked. Know and classify process variables.
Control objectives are identified
Know Incentives of process control.
Use the Control Algorithms to control the measuredvariables.
Use of process control instrumentation to regulate theprocess.
UNIT1
2PCT301T-2013
7/27/2019 Chemical Process Variables-1
3/47
Unit 1:
Process control is the regulation / manipulation of variables
influencing the conduct of a process in such a way as to
obtain a product of desired quality and quantity in an
efficient and economic manner.
Most basic process control processes consist of a control
loop as shown in figure 1 below, having four main
components of a control system.
Process Control Terminology
UNIT1
3PCT301T-2013
7/27/2019 Chemical Process Variables-1
4/47
CONTROL SYSTEM
PROCESS
MEASUREMENT
CONTROLLER
Set point
Disturbance variables
Controlled
variable
Raw materials
FINAL CONTROLELEMENT
UNIT1
4PCT301T-2013
7/27/2019 Chemical Process Variables-1
5/47
Process - refers to the collection of unit operations or anyphysical and / chemical conversion of raw materials to useful
product
Measurement - is a device which calculates the action based
on the measured value against a preset or desired value (set
point).
A controller is a device used to control the process. It
compares a signal from the measured value (PV) with the set
point (SP) and produces an error signal which is used to
adjust the final control element.
Error = MVSP
Final control element- An output signal resulting from the
controller calculation, which is used to manipulate the
process action through some form of actuator.
UNIT1
5PCT301T-2013
7/27/2019 Chemical Process Variables-1
6/47
Control system can be defined as a combination of
components that act together to perform a certain objectives.
A system interact with its environment through signals as
shown in the diagram below.
Generally, signals are function of time as indicated in the
diagram. e.g u(t) and y(t).
6
U(t) y(t)
System
UNIT1
PCT301T-2013
7/27/2019 Chemical Process Variables-1
7/47
Two types of signals
Input signal, u(t) affect the system behaviour in some way
Output signal, y(t) give information about the system
behaviour
Set Point: A reference value representing the desired value ofthe process variable being controlled.
E.g: If the level of a liquid in a tank must be maintained within 5
ft of 50 ft, what is the liquids setpoint? = 50 ftSet-point change - implementing a change in the operating
conditions. The set-point signal is changed and the
manipulated variable is adjusted appropriately to achieve the
new operating conditions.7
UNIT1
PCT301T-2013
7/27/2019 Chemical Process Variables-1
8/47
Disturbance change -the process transient behavior when a
disturbance enters, also called regulatory control or load
change. A control system should be able to return each
controlled variable back to its set-point.
Process load is the level of material, force, torque, energy,
power, or other variable applied to or externally by another
instrument.
Process upset is any type of disturbance that causes the
control variable to depart from the set point.
Process demand is the requirement made by the control
system or the operator on the process.
Load response is the way in which the process behaves after
a change in the process load introduces an error in to the
system.
8
UNIT1
PCT301T-2013
More Terminologies
7/27/2019 Chemical Process Variables-1
9/47
Steady state is one that does not change with time. Every
time we take snapshot, all the variables have the same values
as in the first snapshot.
Transient state is one that changes with time. Every time we
take a snapshot, many of the variables have different values
than in the first snapshot.
Capacity is a measure of capacity to restore volume, mass,
heat, information, or any form of energy or matter.
Dead time is the time interval between the initiation of the
output change or stimulus and start of resulting response.
Control system lag the time for the process control loop to
make necessary adjustments to the final control element.
Process lag is the time delay caused by the process to
eliminate errors after the manipulated variables are adjusted.
9
UNIT1
PCT301T-2013
7/27/2019 Chemical Process Variables-1
10/47
UNIT1
Classification of variables in a chemical process.
The variables (Flow rates, Temperature, Pressure, Concentration,
etc) are conditions which can change a process in some way.They are divided in to two groups.
a) INPUT variables: which donates the effect of thesurroundings on the chemical process.
b) OUTPUT variables: which donates the effect of the process onthe surrounding.
The INPUT variables can be further classified into the followingcategories:
Manipulated ( or adjustable ) variables, if their values canbe adjusted freely by the human operator or a controlmechanism. Typically flow rates of streams entering orleaving a process that we can change in order to controlthe plant.
10PCT301T-2013
7/27/2019 Chemical Process Variables-1
11/47
Disturbance variable, if their values are not the result ofadjustment by an operator or a control system. Flow rates,
temperatures, or compositions of streams entering theprocess. Variables in the process that affects the controlledvariables but cannot be manipulated.
The OUTPUT variables can be further classified into the following
categories:
Measured (Controlled) variables, if the values are known bydirectly measuring them. Flow rates, compositions,temperatures, levels, and pressures in the process that we cancontrol, either by holding them as constant or making them
follow some desired values.
The desired value of a controlled variable is referred to as its setpoint.
UNIT1
11PCT301T-2013
7/27/2019 Chemical Process Variables-1
12/47
unmeasured variable, if they are not measured or
cannot be measured directly.
In the figure.1.3 below,
the inputs x1
and w1
are the disturbances,
while x2and w2 are manipulated inputs.
The output variables x and w can be measured easily and
they are considered measured or the controlledoutputs.
12
UNIT1
PCT301T-2013
7/27/2019 Chemical Process Variables-1
13/47
13
UNIT1
PCT301T-2013
7/27/2019 Chemical Process Variables-1
14/47
EXERCISE 1
Imagine yourself in the shower
As observed on the insert
Diagram.
Identify the following:
a) The process variable(s)
b) The set point
c) The comparison element
d) The control unit
e) The correction unitf) The process
g)The measuring device
h) Manipulated variables.
14
UNIT1
PCT301T-2013
7/27/2019 Chemical Process Variables-1
15/47
EXERCISE 2
Imagine you are in a cabin in front of a small fire on a cold
winter evening. You feel uncomfortably cold, so you throwanother log into the fire the temperatuture to be at 5 0C
of 50 0C. Identify the following:
a) The process variable(s)
b) The set point
c) Controlled variable(s)
d) Measured variable(s)
h) Manipulated variable(s).
PCT301T-2013 15
UNIT2
7/27/2019 Chemical Process Variables-1
16/47
Unit2: Reasons for Control (Control incentives)
16
UNIT2
PCT301T-2013
7/27/2019 Chemical Process Variables-1
17/47
Safety: The safe operation of a chemical process is aprimary requirements for the well-being ofplant personnel,the community at large, and the economic viability of the
company. Thus the T, P, C (specially poisonous or explosivecomponents) should be within allowable limits.
Product specification / quality: The final products from theplant must meet demanding quality specifications set by
purchasers. Process control contributes to good plantoperation by maintaining the operating conditions required forexcellent product quality.
Environmental Regulation: Regulation requires that T, C,
and flow rate of certain chemical species be under some limits.The process must have the capacity to convert potentially toxiccomponents to being materials. Control can contribute to theproper operation of the plant units, resulting in consistently tolow effluent concentrations.
UNIT2
17PCT301T-2013
7/27/2019 Chemical Process Variables-1
18/47
Operational constraint: Different types of equipments
used in a chemical plant have limitations as a results of
their particular design, and inherent to their operation.
E.g. a centrifugal pump can deliver only a certain flow rate as
determined by its impeller size and the available pressure
drop in a line. Control systems need to recognize andsatisfy all such operational constrains.
Economics: The operation of plant should be as
economical as possible to utilize the raw material, energy,capital and human labor. Thus, the operating conditions
must be controlled effectively to minimize operating costs
and maximize profit.
UNIT2
18PCT301T-2013
7/27/2019 Chemical Process Variables-1
19/47
UNIT3. CONTROL STRATEGIES
The objective of a control system is to keep the controlled
variables at their desired values (or set points). This is
achieved by manipulating the manipulated variables using a
control algorithm.
3.1. CONTROL ALGORITHM
OPEN- LOOP SYSTEM (CONTROL)
An openloop system is a system with only the input and the
output but no regulation / no control action at all.
PCT301T-2013 19
UNIT3
7/27/2019 Chemical Process Variables-1
20/47
Example of an openloop system
Block diagram representation
U
Y
D
PROCESSU(t) Y(t)
Figure: A simplified open loop control system
D
20
In an open-loop system, the control
mechanism act without the current
information about the status of the
process.
Signal path is represented by
arrows, which show the direction
of information flow.
PCT301T-2013
UNIT3
7/27/2019 Chemical Process Variables-1
21/47
3.1.1. Feedforward Control (FFC) Algorithm
Feedforward control is an example of an open-loop
control system that converts one or more conditions that
can disturb the controlled variable outside of any
feedback loop, in to corrective action to minimize
deviation of the controlled variable / mechanism whichanticipate the effect of the disturbances to produce the
corrective action if:
The disturbance can be measured
We know how the disturbances affect the output
We know how the control signal affects the output
21PCT301T-2013
UNIT3
7/27/2019 Chemical Process Variables-1
22/47
Feedforward Control Algorithm/Strategy
22
Fig:A
SP
U(t)Y(t)PROCESS
Disturbance (d)
CONTROL
EQUATION
FF CONTROLLER
PCT301T-2013
UNIT3
7/27/2019 Chemical Process Variables-1
23/47
Block diagram representation of fig: A
23
Fig: B
control system
PROCESSU(t) Y(t)
Disturbance (d)
MEASUREMENT
CONTROLLERSP
A , B : Typical Feedforward control systems
PCT301T-2013
UNIT3
7/27/2019 Chemical Process Variables-1
24/47
Advantages and disadvantages of FF control loop
ADVANTAGES
i. Acts before the effect of a disturbance has been felt by
the system.
ii. Is good for slow systems
iii. It does not introduce instability in the closed loop
response
PCT301T-2013 24
UNIT3
7/27/2019 Chemical Process Variables-1
25/47
DISADVANTAGES
i. Requires identification of all possible disturbances
and their measurement.
ii. Cannot cope with unmeasured disturbance.
iii. Sensitive to process parameter variation.
PCT301T-2013 25
UNIT3
7/27/2019 Chemical Process Variables-1
26/47
Example
Consider the example of the stirred tank heater.The objective is to control the temperature. The
disturbance source is Ti.
26
T
C
I/
P
Feedforward control for a heated tankPCT301T-2013
UNIT
3
7/27/2019 Chemical Process Variables-1
27/47
CLOSED LOOP SYSTEM
A closed loop system is a system with a total
control action / regulation around the entire
process.
The total control action is called the feedback
control. The controlled variable is measured, and
the measurement is fed to the controller, thus the
controller receiving information about how acontrol action affects the output.
27PCT301T-2013
UNIT
3
3 1 2 db k C l ( C)
7/27/2019 Chemical Process Variables-1
28/47
3.1.2. Feedback Control Loop (FBC)
In Feedback control, the measurement is fed to
the controller. Thus the controller receivesinformation about how a control action affects theoutput. Usually the measured variable is thevariable we want to control
The basic elements of a feedback controller
The process variable (PV). I.e. the variable that is tobe maintained under control.
The set point (SP) which is the desired value of theprocess variable
28PCT301T-2013
UNIT
3
h ( ) hi h i f h diff
7/27/2019 Chemical Process Variables-1
29/47
The error (e), which is a measure of the differencebetween the PV and the SP.
The controller, whose control law and turning
drive the corrective action . The final control element ( typically a valve)
The manipulated variable (MV), the variable in theprocess to which the FCE is attached.
Two dividends of feedback control loop
3.1.2.1. Negative feedback, this refers to thedesirable situation where the corrective actiontaken by the controller forces the controlledvariable towards the set point.
29PCT301T-2013
UNIT
3
7/27/2019 Chemical Process Variables-1
30/47
Error = reference feedback signal
3.1.2.2 Positive feedback occurs when the
controller forces the controlled variables farther
away from the set point.
Error = reference + feedback signal
30PCT301T-2013
UNIT
3
7/27/2019 Chemical Process Variables-1
31/47
Advantages and disadvantages of FB control loop
ADVANTAGES
I. Does not require identification of all possibledisturbances and their measurements.
II. It is insensitive to modelling error.
III. It is insensitive to parameter changes.
DISADVANTAGES
I. It waits until the effect of disturbance is felt by the
system.II. It is unsatisfactory for slow system.
III. It may create instability in the closed loop response.
PCT301T-2013 31
UNIT
3
Example 1:
7/27/2019 Chemical Process Variables-1
32/47
Example 1:
Man in the shower.
Process variables
The disturbances = Water from toilet flushing
Manipulated variables = Temperature and the flow rate of thewater
Measured / controlled variables = Temperature of the
water
32PCT301T-2013
UNIT
3
7/27/2019 Chemical Process Variables-1
33/47
Block diagram representation
33
PROCESS
MEASUREMENT
CONTROLLER
Set point
Disturbance =flushing
Water
temperatureFw , T
FINAL CONTROL
ELEMENT
PCT301T-2013
UNIT
3
7/27/2019 Chemical Process Variables-1
34/47
EXAMPLE 2
Consider the example of the stirred tank heater. Theobjective is to control the temperature. The
disturbance source is Ti.
34PCT301T-2013
UNIT
3
7/27/2019 Chemical Process Variables-1
35/47
SOLUTION
35
TC
T
T
SP
Q
PCT301T-2013
UNIT
3
7/27/2019 Chemical Process Variables-1
36/47
REVISIT THE BLENDING PROCESS: A CONBINATION OFFEEDBACK FEEDFORWARD CONTROL LOOPS
36
Pure A
W2
X2=1Mixture (A,B)W1
X1
W
X
AT
AC
I/P
xsp
AT AC I/P
PCT301T-2013
UNIT
3
7/27/2019 Chemical Process Variables-1
37/47
Class exercise
1. Riding a bicycle is an inherently unstablesystem.
a) Identify all process variables.
b) Could the process be considered as an open loop
or closed loop? Explain.
c) Construct the control mechanism of this process.
PCT301T-2013 37
UNIT
3
7/27/2019 Chemical Process Variables-1
38/47
CLASS EXERCISE
2. Your bathtub at home is a control system that keeps thewater level constant. A constant flow from the tap yields a
constant water level, because the flow rate through thedrain increases as the water level increases, and decreasesas the water level decreases. After equilibrium has beenreached, the level can be controlled by controlling the inputflow rate. A low input flow rate yields a lower level, while a
higher input flow rate yields a higher level.a) Which control strategy is used in this process?
b) Sketch a control system that uses this principle toprecisely control the fluid level in a tank. Show the intakeand the drainage valves, the tank, any sensors and
transmitters, and the interconnection of all components.c) Draw a block diagram of the system, identifying the input
and output signals of each block.
38PCT301T-2013
UNIT
3
7/27/2019 Chemical Process Variables-1
39/47
Unit 4: Design element of a control system
In the design of a process control system, one should be able
to have answers to the WHAT,WHYand the HOWmethods
of achieving the process needs.
Series of steps to follow during the design of a control system
to follow the decision making process.
4.1. Define the control objectives
Question: what are the operational objectives that a control
system is called upon to achieve?
39PCT301T-2013
UNIT
4
7/27/2019 Chemical Process Variables-1
40/47
The answer to this question will be determining the control
objective .
Answer: For the blending process in the example, thecontrol objective is to Keepxat a desired value (or set
point)xsp despite variations inx1(t).
4.2. Select measured variables.
Question : What variables should be measured to monitor
the operational performance of the process.
Answer: the first attempt will be to install the measuring
devise that will monitor the values ofxand wdirectly.
PCT301T-2013 40
7/27/2019 Chemical Process Variables-1
41/47
4.3. Select manipulated variables.
Question : What variables are to be manipulated to achieve
the control objective ?
Answer: the first attempt will be to install the measuringdevise that will monitor the values ofxand wdirectly.
4.4. Design the controller.
In every control configuration, the controller is the active
element that receives the information from themeasurements and takes appropriate control action toadjust the values of the manipulated variables. E.g FF,FB.
41PCT301T-2013
UNIT
4
7/27/2019 Chemical Process Variables-1
42/47
Question:
How is the information, taken from the measurement used toadjust values of the manipulated variables?
DESIGN QUESTION
Suppose that the inlet concentration x1 changes with
time. How can we ensure that x remains at or near
the set point ?
As a specific example, if and ,
thanx>xSP.
1 1x x 2 2w w
42
UNIT
4
PCT301T-2013
7/27/2019 Chemical Process Variables-1
43/47
4.4.1. Some Possible Control Strategies:
Method 1. Measure x and adjust w2.
ifxis too high, we should reduce w2;
43
UNIT
4
PCT301T-2013
7/27/2019 Chemical Process Variables-1
44/47
Method 2. Measure x1 and adjust w2.
Thus, if x1 is greater than , we would decrease w2 so that
44
2 2;w w
UNIT
4
PCT301T-2013
7/27/2019 Chemical Process Variables-1
45/47
Method 3.Measure x1 and x, adjust w2.
This approach is a combination of Methods 1 and 2.
45
Pure A
W2
X2=1Mixture (A,B)
W1
X1
W
X
AT
AC
I/P
xsp
AT AC I/P
UNIT
4
PCT301T-2013
7/27/2019 Chemical Process Variables-1
46/47
46
Method 4. Use a larger tank.
If a larger tank is used, fluctuations inx1 will tend to be
damped out due to the larger capacitance of the tank
contents.
However, a larger tank means an increased capital cost.
UNIT
4
PCT301T-2013
7/27/2019 Chemical Process Variables-1
47/47
EXERCISE 2
On the process, construct the
possible control strategies
that will help to achieve the
desired products.
UNIT4
Recommended