Process Dynamic and Control I CHEN 341 What is … Control...2 Dr. Mohamed Helmy CHEN 341 Process...

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Dr. Mohamed Helmy CHEN 341

Process Dynamic and Control I CHEN 341

What is process control?

1. Process: number of successive steps (sequence of operations involving a change in a

substance being treated)

Series of steps or actions (chemical or mechanical) taken in order to achieve a change or a

particular end.

Examples of processes include:

Change of energy state, such as from hot to cold, or liquid to gas, distillation of water

Change of composition, as occurs in a chemical reaction or in mixing

Change of dimension, as in breaking coal

Change in level, as in reservoir or tank filling

Change in media velocity, as in a reservoir feed pipe

Holding a pressure level, as in a water distribution network

2. Control: To maintain desired conditions in a physical system by adjusting selected variables

in the system

Process control is a branch of automatic control that is used to maintain process conditions at

their desired values by manipulating certain process variables to adjust the variables of

interest.

Note: Process automation involves using computer technology and software engineering to

help e.g. power plants and factories in industries as diverse as paper, mining and cement to

operate more efficiently and safely

Compare between process dynamic and process control?

Process dynamics is concerned with the study of transient and steady-state behaviour of

processes. (Study the change of process variable with time until it reaches the new steady state

value)

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Process control is concerned with the "control" or "manipulation" of process behaviour so that

the process operates close to the desired operating point even in the presence of disturbances.

Examples of simple control systems:

1- A human being riding a bicycle

Fig. 1, A man riding a bicycle and trying to advance on an imaginary straight line

The rider looks ahead and receives information about his current position (Controlled

Variable) and also about the imaginary dotted line (Set Point). So obviously the eye acts as

the Sensor, that sends the information to the human brain (the controller), and after processing

the data, the decision or control action is achieved by steering the bicycle by the rider’s hand

(Final Control Element).

2. Float type Liquid Level Controller

A rise in liquid level causes the hollow Ball–Float to lift, and the attached ‘plug’ of a valve

mechanism comes closer to its ‘seat’ (built on the end of the liquid inlet) to reduce the

liquid inflow. In the system, the set point is the location of the hinge on the tank wall which is

usually fixed because in major number of applications it is required that the tank should be

filled to the fullest before use.

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Fig. 2, The float type level controller

3. The temperature control system of a mixer pre-heater

Fig. 3, The temperature control system of a mixer pre-heater

A Thermocouple Sensor, TC, measures the exit stream temperature, TC. The TC output is

electronically conditioned in the Temperature Transmitter, TT, to 4–20 mA DC current signal

and transmitted as the measured variable, TM, to the Temperature Indicator Controller, TIC.

Such a Controller, beside its control capability has the facility of displaying the Measured

Variable, TM, and Set value, TR. Error is found as, e = TR – TM, and the Controller Output, CO, is

produced as a function of error. The low energy output from the Controller is amplified

through the FCE (which may be a Thyristor type power regulator) that changes the power

mains current to the resistance heater in the tank.

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4. The dynamic level system

Fig. 4, The dynamic level system and its control

A tank acts as a constant head reservoir for supplying liquid to a process. The process

has variable demands. A Constant head in the tank is required so that exit flow rate is only

dependent on the resistance in the outflow line. The resistance may be the Control Valve

(FCE) of the control system of the downstream process. The liquid level of the tank is

measured by a level sensor-transmitter, and sent to the level Controller. The Controller

Output manipulates inflow through the Control Valve. The open loop process, and the

process with its control system has been shown in Figure ( ).

5. The reflux ratio control of a distillation column

Fig. 5, The reflux ratio control of a distillation column

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In a distillation column, the reflux ratio is one of most important parameters to be

maintained to keep up the top product or distillate composition to the desired value; and in

many instances top product is the desired product of this operation. Reflux ratio, as the

name suggests, is the ratio of molar or mass flow rates of two streams R and D.

The Total Condenser condenses all the vapors coming from the top plate of the column. The

condensed liquid is diverted into two streams, R and D. R is fed back to the top plate, and D is

the product stream. A composition sensor-transmitter on the product line measures the

product composition and sends the information to the composition Controller, which

manipulates the reflux ratio through the three-way control valve. Figure ( ) depicts the

control system [Three way Control Valves are a variety of control valves and acts as special

final control elements. They come in two basic configurations:

(1) Diverting type has one inlet and two outlet ports. Liquid coming through inlet goes out

through the outlet ports subdivided into two streams at a definite volume ratio. This ratio is

dependent on the valve stem position.

(2) Mixing type has one outlet and two inlet ports.

Liquids coming through inlet ports are mixed in a definite volume ratio inside the valve

body, and go out through the outlet port. Again this ratio is dependent on the valve stem

position.]. The same control action may be achieved by using two sets of two-way

Control Valves.

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6. Multi-loop control system of a flashing stage

Fig. 6, Multi-loop control system of a flashing stage

What are the main components of a control system?

Any control system consists of main four components:

(a) process (b) measuring element (sensor)

(c) controller (d) final control element (FCE)

Why process control? What are the objectives of process control?

A control system is required to perform either one or both task:

(1) Safety (2) Environmental protection

(3) Equipment protection (4) Smooth operation

(5) Product quality (6) Profit

(7) Monitoring and diagnosis (8) Efficient use of raw materials and energy

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Tasks of a control engineer

Identifying the controlled system

Defining the controlled variable

Defining the measuring point

Determining the disturbance variables

Selecting of the final control element

Examining whether a closed-loop control system offers appreciable advantages

Selecting a suitable controller

Assembling the control devices taking into consideration guidelines

Commissioning, parameterisation, optimization

How is control documented?

There are two methods for control system documentation:

A. physical control system

Consists of a simple flow sheet showing the process and the location of the measuring element,

the controller, and the final control element

Fig. 7, Physical control system for a heating process in a stirred tank reactor

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B. block diagram

It indicates the flow of information around the control system, the relationship between signals,

and the function of each part of the system (Fig. 8).

Each component of the system is represented by a block, with little regard for the actual physical

characteristics of the represented component (e.g., the tank or controller).

Block diagram contains information like:

a) Material, Energy and Signal Flows within the system.

b) The Mathematical Processing done for conversion of a signal to another (as happens when

a temperature sensor-transmitter converts the process temperature to an electrical current).

c) Branching and algebraic addition of the signal streams.

A Block Diagram generally consists of four basic elements:

a) Lines represent a signal, material or energy stream path and the associated Arrow-

Head expresses the direction of flow.

b) The summing points represent the algebraic summation of signals coming through the

incoming arrows with the +ve or –ve signs as their algebraic attributes. The resulting

signal flows out through the outgoing arrow.

c) A branch point is the location on a line at which the signal branches out and goes parallel

to their individual destination.

d) A block represents the mathematical operation done on an incoming signal to convert that

to the next one. Thus, a block is where the input signal to the block terminates and the

output signal from the block is generated. In a block diagram the blocks generally

represent the hardware used in a control system.

Any block diagram can be handled or manipulated algebraically. Figure 9 shows the essential

rules of block diagram algebra.

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Fig. 8, generalized process control system (closed loop feedback).

Fig. 9, Rules of block diagram algebra

Where is control done?

Sensors, local indicators, and valves (FCE) are located in the process.

Displays of variables, calculations, and commands to valves are in the centralized control

center.

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Fig. 10, location of different parts of a control system (older style control panel)

Fig. 11, location of different parts of a control system (modern computer based control panel)

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DefinitionsandTerminologies

1. Input Variable

This variable shows the effect of the surroundings on the process. It normally refers to those

factors that influence the process. An example of this would be the flow rate of the steam

through a heat exchanger that would change the amount of energy put into the process. There

are effects of the surrounding that are controllable and some that are not. These are broken

down into two types of inputs.

a. Manipulated variable (control output): variable in the surroundings can be controlled

by an operator or the control system in place.

b. Disturbances: inputs that cannot be controlled by an operator or control system. There

exist both measurable and immeasurable disturbances.

2. Output variable (process variable)

Also known as the control variable (response to input) these are the variables that are process

outputs that affect the surroundings.

As we consider a control problem. We are able to look at two major control structures.

a. Single input-Single Output (SISO): For one control (output) variable there exist one

manipulate (input) variable that is used to affect the process.

b. Multiple inputs-multiple outputs (MIMO): There are several control (output) variables

that are affected by several manipulated (input) variables used in a given process.

3. Set point SP (reference value): The set point is where you would like a controlled

process variable to be (the desired value of the controlled variable).

4. Error (system deviation): In process controls, error is defined as: Error = set point –

measured value of the process variable.

5. Cascade: A control system with 2 or more controllers, a "Master" and "Slave" loop. The

output of the "Master" controller is the set point for the "Slave" controller.

6. Control system constrains: Any process has certain operating constraints, which are

classified as hard or soft. An example of a hard constraint is a minimum or maximum

flow rate a valve operates between the extremes of fully closed or fully open. An example

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of a soft constraint is a product composition it may be desirable to specify a composition

between certain values to sell a product, but it is possible to violate this specification

without posing a safety or environmental hazard.

Remember: What are the objectives of process control?

A control system is required to perform either one or both task:

(1) Safety (2) Environmental protection

(3) Equipment protection (4) Smooth operation

(5) Product quality (6) Profit

(7) Monitoring and diagnosis

Let us discus these objectives on the following process:

Fig. 12, Hydrocarbon separation

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1. Safety

Fig. 13, control system as a safety tool

2. Environmental protection

Fig. 14, control system as a means of environmental protection

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When you see flaring, please be assured that flares play a key role in keeping refineries and

chemical plants running safely.

What are flares?

Flares are important safety devices used in refineries and petrochemical facilities. They safely

burn excess hydrocarbon gases which cannot be recovered or recycled. Excess hydrocarbon

gases are burnt in the flare systems in an environmentally-sound manner, as an alternative to

releasing the vapor directly into the atmosphere.

During flaring, excess gases are combined with steam and/or air, and burnt off in the flare system

to produce water vapor and carbon dioxide. The process of burning these excess gases is similar

to the burning of liquefied petroleum gases (LPG), which some of us use as fuel for home

cooking.

There are generally two types of flares

Elevated flares, where the flare tip is between 20 to 150 meters above ground.

Ground flares, where the flare tip is about two to three meters above ground, which is

fenced off with a high heat-shield fencing, which also acts as a safety zone

What can sometimes be seen or heard in instances of flaring?

A flame or a glow:

Members of the public may become concerned when they see flaring and mistake them

for fires.

The elevated flare presents itself as a flame.

The ground flare is not visible in the day. At night, it may cast an orange glow in the

night sky, depending on cloud cover.

Smoke:

Sometimes, a white cloud may be observed around the flare. It is actually steam that is

injected into the flare system to support clean combustion.

While we strive to achieve smokeless flaring, it may not always be possible during

operational situations.

On some occasions, there may be black smoke from the flare.

The smoke, which is mainly made up of carbon particles, occurs when there is

insufficient amount of air to support a complete combustion. This may happen when

there is a sudden release of excess gases to the system with a delay in response before

sufficient steam can be supplied to the burning process.

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Steam is usually added to the gases to increase turbulence in the gas flow. This increases

air intake that helps to achieve complete combustion and smokeless flaring.

Noise

Flaring may produce a rumbling noise. It may sound like thunder.

This is due to the turbulent mixing of gases, air and steam. The sound is similar to what

you hear when you fan a campfire, and oxygen is mixed with the flame.

Fig. 15, Flare stack

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3. Equipment protection

Fig. 16, control system as a method of equipment protection

4. Smooth operation production rate

Fig. 17, control system as a method to maintain smooth operation

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5. Product quality

Fig. 18, control system as a method to maintain product quality

6. High profit

Fig. 19, control system as a method to obtain high profit

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7. Monitoring and diagnosis

Fig. 20, control system as a method of monitoring and diagnosis

Attributes of control system

There are several common attributes of control systems:

1. Disturbance rejection: The ability to maintain the process variable at its desired value in spite

of disturbances that might be experienced

2. Set point tracking: The ability to move the process variable from one setting to a new desired

setting

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Types of control loops

a) Feedback control loop:

In a feedback control system, information about performance is measured and that

information is used to correct how the system performs.

Example:

Fig. 21, the feedback control of the liquid level in a boiler drum

Another example for feedback control is the control of an outlet temperature of a heat

exchanger:

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Fig.22, feedback control of the outlet temperature of a heat exchanger process fluid

Feedback control can be implemented with different strategies as shown in the example of

controlling liquid level in a tank

Fig. 23, feedback control strategy 1 of the level is measured and the inlet flow rate

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Fig. 24, feedback control strategy 2 of the level is measured and the inlet flow rate

(Note there are two inputs to the LT for LAH and LAL)

Feedback control of reactor temperature:

Fig. 25, Feedback control of reactor temperature

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The following example shows feedback control loops for different parameters in a distillation

column:

This system shows the following control loops:

1) Pressure at the top of the tower by

adjusting the cooling water flow rate for

controlling rate of vapor condensation. Q =

m Cp ∆T = V∆H

2) Level control for the liquid in the reflux

drum

3) Temperature control of the top plates 4) Inlet feed temperature

5) Liquid level control at the tower bottom

Note: the control loop for steam is a feedforward

Fig. 26, Distillation column with six single-loop control systems

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Top product composition control in a distillation column:

Fig. 27, Top product composition control in a distillation column

Fig. 28, Pressure control loop of an evaporation unit

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pH control

Fig. 29, pH control system

The general block diagram for this type of control loops (feedback) is shown in figure 6.

Fig.30, General block diagram for the feedback control system

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There are two types of feedback control loops:

a. Closed loop feedback control system: in this type the controller automatically acts to

return the controlled variable to its desired value.

b. Open loop feedback control system: the measurement signal disconnected from the

controller, and the controller output would have to be manually adjusted to change the

value of the controlled variable. The block diagram of this loop is shown in figure 27.

Fig. 31, block diagram of the open loop system

Negative and positive feedback control systems

There are two main types of feedback control systems: negative feedback and positive feedback.

Negative feedback Positive feedback

In a negative feedback control the set point and

output values are subtracted (error signal is

computed from the difference between the set

point and the measured signal)

As a rule negative feedback systems are more

stable than positive feedback systems

In a positive feedback control system the

set point and output values are added.

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Advantages of feedback control system

1. Corrective action occurs as soon as the controlled variable deviates from the set point,

regardless of the source and type of disturbance. (counts for all disturbances)

2. Feedback control requires minimal knowledge about the process to be controlled; it

particular, a mathematical model of the process is not required, although it can be very useful

for control system design. (Easy to control)

Disadvantages of feedback control system

1. No corrective action is taken until after a deviation in the controlled variable occurs. Thus,

perfect control, where the controlled variable does not deviate from the set point during

disturbance or set-point changes, is theoretically impossible. (Allow error propagation)

2. Feedback control does not provide predictive control action to compensate for the effects of

known or measurable disturbances. (source of error is not defined)

3. It may not be satisfactory for processes with large time constants and/or long time delays. If

large and frequent disturbances occur, the process may operate continuously in a transient

state and never attain the desired steady state.

4. In some situations, the controlled variable cannot be measured on-line, and, consequently,

feedback control is not feasible.

b) Feed forward control system

If a particular load disturbance occurs frequently in a control process, the quality of control can

often be improved by the addition of feed-forward control.

The basic concept of feedforward control is to measure important disturbance variables and take

corrective action before they upset the process. (Does not allow error propagation).

The following scheme is a simple presentation of a feedforward control:

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Fig. 32, simple presentation of a feedforward control

The following scheme shows the difference between feedback and feedforward control:

Fig. 33, The difference between feedback and feedforward control:

Physically the difference is shown in this example for the control in the outlet temperature of a

heat exchanger:

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Fig.34, Physical control loop shows a comparison between feedback and feedforward strategies

Feedback measures the controlled variable and then take the corrective action while feedforward

measures the disturbance and corrects for it before the controlled variable changes.

An example of the feed forward control system is shown in figure 31.

Fig. 35, Physical feed-forward control system to control the outlet stream temperature of a H.X

In this example the possible disturbances (change in inlet feed temperature, change in water

temperature, and change in water flow rate) are accounted for by the inlet control valve.

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Another examples:

Control of a reactor temperature:

Fig. 36, Control of a reactor temperature using feedback and feedforward strategy.

Level control in a tank

Fig. 37, Feedforward level control in a tank

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Fig. 38, Feedforward control of heat exchanger

Flow feedforward control for temperature control of a column:

In this example there is a feedforward control on the inlet feed and also for steam in the reboiler.

Fig. 39, Flow feedforward control for temperature control of a column.

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The general block diagram for feed-forward control system is shown in figure 9.

Fig. 40, Feed-forward block diagram

Feedforward control has several disadvantages:

1. The disturbance variables must be measured on-line. In many applications, this is not

feasible. (Difficult to control)

2. To make effective use of feedforward control, at least a crude process model should be

available. In particular, we need to know how the controlled variable responds to changes in

both the disturbance and manipulated variables. The quality of feedforward control depends

on the accuracy of the process model.

3. Ideal feedforward controllers that are theoretically capable of achieving perfect control may

not be physically realizable. Fortunately, practical approximations of these ideal controllers

often provide very effective control.

c) Cascade control

Cascade control is used to improve control system performance over single-loop control.

In this type of control system at least two control loops are used with two controllers.

One controller is the master controller which monitors the controlled variable.

The other controller is the secondary or slave controller which monitor the disturbances.

The set point of the master controller is the desired value of the controlled variable and the set

point of the secondary (slave) controller is the output signal from the master controller.

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In a cascade control arrangement, there are two (or more) controllers of which one controller’s

output drives the set point of another controller. For example: a level controller driving the set

point of a flow controller to keep the level at its set point. The flow controller, in turn, drives a

control valve to match the flow with the set point the level controller is requesting.

Examples for cascade control:

Fig. 41, Cascade control of a liquid level

The controller driving the set point (the level controller in the example above) is called the

primary, outer, or master controller. The controller receiving the set point (flow controller in the

example) is called the secondary, inner or slave controller.

Fig.42, Comparison of cascade with a feedback loop applied to the temperature control of

stripping section of a distillation column

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Cascade control for the CSTR temperature

Fig. 43, Control of heat exchanger outlet temperature using steam flow as secondary loop.

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Fig. 44, Cascade control of heat exchanger outlet temperature using steam flow as secondary

loop

Note: Be aware with the control symbols.

Information flow diagram of a cascade control structure:

This scheme shows the flow of information in the cascade system. There are two control loops,

the slave control loop and the master control loop.

Fig. 45, Information flow diagram of a cascade control structure

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What are the advantages of cascade control?

There are several advantages of cascade control, and most of them boil down to isolating a slow

control loop from nonlinearities in the final control element. In the level control example the

relatively slow level control loop is isolated from any control valve problems by having the fast

flow control loop deal with these problems.

Imagine that the control valve has a stiction problem (see blog on valve problems.) Without the

flow control loop, the level control loop (driving the sticky valve) will continuously oscillate in a

stick-slip cycle with a long (slow) period, which will quite likely affect the downstream process.

With the fast flow control loop in place, the sticky control valve will cause it to oscillate, but at a

much shorter (faster) period due to the inherent fast dynamic behavior of a well-tuned flow loop.

It is likely that the fast oscillations will be attenuated by the downstream process without having

much of an adverse effect.

Or imagine that the control valve has a nonlinear flow characteristic (see blog on valve

problems.) This requires that the control loop driving it be detuned to maintain stability

throughout the possible range of flow rates. If the level controller directly drives the valve, it

must be detuned to maintain stability – possibly resulting in very poor level control. In a cascade

control arrangement with a flow control loop driving the valve, the flow loop will be detuned to

maintain stability. This will result in relatively poor flow control, but because the flow loop is

dynamically so much faster than the level loop, the level control loop is hardly affected.

i.e. in a cascade control system we try to overcome the slow dynamic of the master loop by using

a faster dynamic secondary loop

When should cascade control be used?

Cascade control should always be used if you have a process with relatively slow dynamics (like

level, temperature, composition, humidity) and a liquid or gas flow, or some other relatively-fast

process, has to be manipulated to control the slow process. For example: changing cooling water

flow rate to control condenser pressure (vacuum), or changing steam flow rate to control heat

exchanger outlet temperature. In both cases, flow control loops should be used as inner loops in

cascade arrangements.

Does cascade control have any disadvantages?

Cascade control has three disadvantages.

a) It requires an additional measurement (usually flow rate) to work.

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b) There is an additional controller that has to be tuned.

c) The control strategy is more complex – for engineers and operators alike.

These disadvantages have to be weighed up against the benefits of the expected improvement in

control to decide if cascade control should be implemented.

When should cascade control not be used?

Cascade control is beneficial only if the dynamics of the inner loop are fast compared to those of

the outer loop. Cascade control should generally not be used if the inner loop is not at least three

times faster than the outer loop, because the improved performance may not justify the added

complexity.

In addition to the diminished benefits of cascade control when the inner loop is not significantly

faster than the outer loop, there is also a risk of interaction between the two loops that could

result in instability – especially if the inner loop is tuned very aggressively.

Typical example of cascade control loop along with a block diagram:

Fig. 46, cascade control of a heat exchanger outlet temperature

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Fig. 47, Block diagram of a cascade control system for the heat exchanger outlet temperature

Example on different strategies of cascade control:

For the heat exchanger shown in figure 10, the objective is to control the output temperature of

the tube side effluent stream.

The possible disturbances are:

The flow rate of steam into the exchanger

The temperature of steam

The steam pressure in the exchanger

Any one of those mentioned disturbances can be used as the secondary output in a cascade

control system. The choice of which to use depends on the disturbances that affect the effluent

temperature.

Figure 48, a shell and tube heat exchanger

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Fig. 49, cascade control of effluent temperature via steam flow control

Fig. 50, cascade control of effluent temperature via shell side pressure control

The general block diagram for cascade control system:

Fig.51, Traditional cascade block diagram

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Example on the feedback and cascade loops:

For a system consists of a kettle through which water, entering at temperature 𝑇𝑖, is heated to 𝑇𝑜

by the flow of hot oil through a jacket surrounding the kettle. The temperature of the water in the

kettle is measured and transmitted to the controller, which in turn adjusts the flow of hot oil

through the jacket.

Draw a physical feedback and cascade control system to control the temperature inside the kettle

and draw the corresponding block diagram for each case.

Solution:

a. Feedback control system

Fig. 52, feedback loop

Feedback block diagram

Fig. 53, block diagram of the feedback loop

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b. Cascade control system

Fig. 54, Cascade control loop

Cascade block diagram

Fig. 55, block diagram of the cascade control loop

d) Ratio control

An important control problem in the chemical industry is the combining of two or more streams

to provide a mixture having a desired ratio of components.

Ratio control is a special type of feedforward control that has had widespread application in the

process industries. The objective is to maintain the ratio of two process variables as a specified

value. The two variables are usually flow rates, a manipulated variable u, and a disturbance

variable d. Thus, the ratio R is controlled rather than the individual variables.

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𝑅 =𝑢

𝑑

Typical application examples of ratio control include:

e) Setting the relative amounts of components in blending operations

f) Maintaining a stoichiometric ratio of reactants to a reactor

g) Keeping a specified reflux ratio for a distillation column

h) Holding the fuel-air ratio to a furnace at the optimum value

i) Ratio control of feed flow to the column and steam to the re-boiler of a distillation

column.

j) Ratio control of added chemicals to waste water in water treatment and pollution

control applications.

Implementation of ratio control

There are two methods:

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Fig. 56, Ratio control, method I

Fig. 57, Ratio control, method II

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Examples of ratio control system

1. Maintaining a stoichiometric ratio of reactants to a reactor

Fig. 58, maintaining a stoichiometric ratio of reactants to a reactor

2. Controlling the ratio of combustion air to fuel flow

Fig. 59, controlling the ratio of combustion air to fuel flow

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Important note: the final control element is always on the manipulative variable stream.

Assignment:

A ratio control scheme is to be used to maintain a stoichiometric ratio of H2 and N2 as the feed to

an ammonia synthesis reactor. Draw a schematic diagram for the ratio control scheme.

e) Override control

Override control (or “selective control” as it is sometimes called) is a form of multivariable

control in which a manipulated variable can be set at any point in time by one of a number of

different controlled variables.

If there are more controlled variables than manipulated variables in a process to be controlled, a

common solution to this problem is to use a selector to choose the appropriate process

variable from among a number of available measurements.

Selectors can be based on multiple measurement points, multiple FCE or multiple

controllers. Selectors are used to improve the control system performance as well as to protect

equipment from unsafe operating conditions.

In an Override Control the selected control action depends on whether the process variable has

reached any of the two pre-chosen limits called hi- or low-limits.

Consider the process in Figure 14(a), where a hot saturated liquid pours in surge tank (capacity

element to reduce flow fluctuation) and from there is pumped under flow control to some

downstream process. Under normal operation, the controller tries to keep the level at H1. If, for

some reason, the level drops down to H2, the liquid will not have enough net +ve

suction head (NPSH) and the pump may get vapor bound which is a highly undesirable

condition in pump operation. Therefore, it is necessary to introduce a control scheme to avoid

this situation. This new scheme has been shown in Figure 14 (b).

The level in the tank is now measured and controlled. The FCE in the flow loop is a variable

speed pump that pumps more liquid for an increase of controller output. Hence, the flow

controller (FC) must have a reverse action, while the level controller (LC) is a direct acting

controller. The output of each controller goes to Low Selector (LS). The low selector’s output

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goes to the pump. Under normal operating condition with level at H1 (which is above the

set point), the LC would try to speed-up the pump by increasing its output to saturation (say

to 20 mA), while the output from FC might be 10–12 mA. The low selector (LS) will allow

only the FC output to manipulate pump speed. This is the desired operating condition.

Another kind of contingency may appear if the in-flow rate of the hot liquid decreases

so that tank level continues to drop down. As the level drops below set point, the LC will

decrease its output to slow down the pump speed. If LC output goes below that of FC, the LS

will again allow only the LC output to manipulate pump speed thus avoiding any

undesirable low level value inside the tank. It can be said that the LC overrides the FC.

Fig. 60, Tank level control. (a) Conventional. (b) Control with override

Example: Override control to protect a boiler system

Fig. 61, Override control to protect a boiler system

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Another category of control strategies involves the use of signal relays or function blocks with

the ability to switch between different signal values, or re-direct signals to new pathways. Such

functions are useful when we need a control system to choose between multiple signals of

differing value in order to make the best control decisions.

The “building blocks” of such control strategies are special relays (or function blocks in a digital

control system) shown here:

Fig. 62, Signal relays or Function blocks

High-select functions output whichever input signal has the greatest value. Low-select functions

do just the opposite: output whichever input signal has the least value. “Greater-than” and “Less

than” symbols mark these two selector functions, respectively, and each type may be equipped to

receive more than two input signals.

Sometimes you will see these relays represented in P&IDs simply by an inequality sign in the

middle of the large bubble, rather than off to the side in a square. You should bear in mind that

the location of the input lines has no relationship at all to the direction of the inequality symbol –

e.g., it is not as though a high-select relay looks for the input on the left side to be greater than

the input on the right. Note the examples shown below, complete with sample signal values:

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High-limit and low-limit functions are similar to high- and low-select functions, but they only

receive one input each, and the limit value is a parameter programmed into the function rather

than received from another source. The purpose of these functions is to place a set limit on how

high or how low a signal value is allowed to go before being passed on to another portion of the

control system. If the signal value lies within the limit imposed by the function, the input signal

value is simply passed on to the output with no modification.

f) Split range control

This type of control is used, where there are several manipulated variables, but a single output

variable. The coordination among different manipulated variables is carried out by using Split

Range Control.

The figure attached below shows an example of a typical split range control scheme. The

steam discharges from several boilers are combine at a steam header. Overall steam

pressure at the header is to be maintained constant through a pressure control loop. The

command from the pressure controller is used for controlling simultaneously the steam flow rates

from the boilers in parallel. Clearly, there is a single output variable (steam header pressure)

while there are a number of manipulating variables (discharge from different boilers).

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Fig.63, Steam header with split range control

Another example of split control (pressure control)

Fig. 64, Split range control scheme

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Types of controllers

1. On/off control

In this type the output from the controller is either “full on” or “full off” and the final control

element (control valve or whatever) is full open or full closed; there are no intermediate

values of the output. Example of this type is the electrical heater, an iron, etc..

2. Proportional control

The controller changes the manipulated variable by an amount proportional to the error.

3. Proportional integral control

The controller is instructed to change the manipulated variable by an amount proportional to the

error and an additional amount proportional to the time integral of the error.

4. Proportional derivative control


error and an additional amount proportional to the time derivative of the error.

5. Proportional integral derivative control


error and an additional amount proportional to the time integral and time derivative of the error.

These types of controllers will be discussed in detail in the next chapters.

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Control symbols and signals

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Summary of instrument type & location Summary of

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Line symbols (signals)

Piping and connection symbols

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Common Instrumentation Symbols for Control Loop Presentation:

Information flow inside a process control system:

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Notes about control valves

Butterfly valve Solenoid valve

Pneumatic valve (air to close)

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Pneumatic valve (air to open)

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Exercise:

Explain the meaning of the following signals and lines

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DesignMethodologyforProcessControl

1) Understand the process:

Before attempting to control a process it is necessary to understand how the process works and

what it does.

2) Identify the operating parameters:

Once the process is well understood, operating parameters such as temperatures, pressures, flow

rates, and other variables specific to the process must be identified for its control.

3) Identify the hazardous conditions:

In order to maintain a safe and hazard-free facility, variables that may cause safety concerns

must be identified and may require additional control.

4) Identify the measurable:

It is important to identify the measurable that correspond with the operating parameters in order

to control the process.

Measurables for process systems include:

Temperature Pressure Flow rate

pH Humidity Level

Concentration Viscosity Conductivity

Turbidity Redox/potential Electrical behavior

Flammability

5) Identify the points of measurement:

Once the measurables are identified, it is important locate where they will be measured so that

the system can be accurately controlled.

6) Select measurement methods:

Selecting the proper type of measurement device specific to the process will ensure that the most

accurate, stable, and cost-effective method is chosen. There are several different signal types that

can detect different things.

These signal types include:

Electric Pneumatic Light

Radiowaves Infrared (IR) Nuclear

Electromagnetic Hydraulic

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7) Select control method:

In order to control the operating parameters, the proper control method is vital to control the

process effectively. On/off is one control method and the other is continuous control. Continuous

control involves Proportional (P), Integral (I), and Derivative (D) methods or some combination

of those three.

8) Select control system:

Choosing between a local or distributed control systems that fit well with the process effects both

the cost and efficacy of the overall control.

9) Set control limits:

Understanding the operating parameters allows the ability to define the limits of the measurable

parameters in the control system.

10) Define control logic:

Choosing between feed-forward, feed-backward, cascade, ratio, or other control logic is a

necessary decision based on the specific design and safety parameters of the system.

11) Create a redundancy system:

Even the best control system will have failure points; therefore it is important to design a

redundancy system to avoid catastrophic failures by having back-up controls in place.

12) Define a fail-safe:

Fail-safes allow a system to return to a safe state after a breakdown of the control. This fail-safe

allows the process to avoid hazardous conditions that may otherwise occur.

13) Set lead/lag criteria:

Depending on the control logic used in the process, there may be lag times associated with the

measurement of the operating parameters. Setting lead/lag times compensates for this effect and

allow for accurate control.

14) Investigate effects of changes before/after:

By investigating changes made by implementing the control system, unforeseen problems can be

identified and corrected before they create hazardous conditions in the facility.

15) Integrate and test with other systems:

The proper integration of a new control system with existing process systems avoids conflicts

between multiple systems.

Process Dynamic and Control I CHEN 341 What is … Control...2 Dr. Mohamed Helmy CHEN 341 Process...

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