Industrial Process Control

SKEE 4173

CHAPTER 1 Introduction To Process Control

• The importance of process control. • Control theory basics • Components of control loops and ISA symbology • Controller algorithms and tuning • Process control systems • Control objectives and benefits Industrial Process Control 2014 Introduction To Process Control

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• The importance of process control. ˃ Define process ˃ Define process control ˃ Describe the importance of process control in terms of variability, efficiency, and safety

Industrial Process Control 2014 Introduction To Process Control

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• The importance of process control. ˃ Define process ˃ Define process control methods changing or ˃ Describe the importance of of process control refining raw materials to in terms of variability, efficiency, and safety create end products


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• The importance of process control. ˃ Define process ˃ Define process control ˃ Describe the importance of process control methods that areand used to in terms of variability, efficiency, safety control process variables when manufacturing a product


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• The importance of process control. ˃ Define process ˃ Define process control ˃ Describe the importance of process control in terms of variability, efficiency, and safety


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Manufacturers control the production process for three reasons: ① Reduce variability ② Increase efficiency ③ Ensure safety


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① Reduce variability (Process Variability)


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②Increase Efficiency Some processes need to be maintained at a specific point to maximize efficiency.

Combined Heat & Power Device. Producing efficient output with only 10% loss Power


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Heat

③Ensure safety Precise process control is required to ensure safety For example, maintaining proper boiler pressure is crucial in preventing boiler implosions that can clearly threaten the safety of workers


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You built it as designed and ….

You lost $100,000 per week; no one would buy your product. …. Then, the pump was damaged $$$….

Vapor product

Then, an explosion killed 2 workers

Feed Also, it polluted Pasir Gudang Methane Ethane (LK) Propane Butane Pentane

Process fluid

Steam

Liquid product

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We need to be able to “drive” the process T6

Feed Methane Ethane (LK) Propane Butane Pentane

T1

T2

F1

T4

F2

Process fluid

P1

Vapor product We need an active element that uses measurements to influence the process

T5

T3

L1

F3

Steam

A1 L. Key

Liquid product

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We need to be able to “drive” the process T6

Feed Methane Ethane (LK) Propane Butane Pentane

T1

T2

F1

T4

F2

Process fluid

P1

We need an active element that uses measurements to influence the process. Vapor This can be automated product

T5

T3

L1

F3

Steam

A1 L. Key

Liquid product

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• The importance of process control. • Control Theory Basics • Components of control loops and ISA symbology • Controller algorithms and tuning • Process control systems • Control objectives and benefits Industrial Process Control 2014 Introduction To Process Control

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• Control Theory Basics The control loop Process Control Terms


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• Control Theory Basics The control loop Process Control Terms

Required Tasks 1. Measurement 2. Comparison 3. Adjustment


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control valve (LV) are used to control the liquid level in a process tank. The purpose of this control system is to maintain the liquid level at some prescribed height (H) above the bottom of the tank. It is assumed that the rate of flow into the tank is random. The level transmitter is a device that measures the fluid level in the tank and converts it into a useful measurement signal, which is sent to a level controller. The level controller evaluates the measurement, compares it with a desired set point (SP), and produces a series of corrective actions that are sent to the control valve. 1. What do you The valve controls the flow of fluid in the outlet pipe to maintain a level in the tank. want to measure?

The control loop

2. What do you compare your measurement with?

Liquid Level Transmitter

3. What do you adjust to obtain the required measurement?

LT 100

H LV 100

LC 100

Level Controller

Liquid Control Valve Industrial Process Control 2014 Figure 1-2. Process level Control control: Example Introduction To Process

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ates the measurement, compares it with a desired set point (SP), and produces a series of corrective actions that are sent to the control valve. The valve controls the flow of fluid in the outlet pipe to maintain a level in the tank.

Liquid Level Transmitter LT 100

H

The control loop

LV 100

LC 100

Level Controller

Liquid

Process control block diagram

Control Valve Figure 1-2. Process level control: Example

Thus, a process control system consists of four essential elements: process, measurement, evaluation, and control. A block diagram of these elements is shown in Figure 1-3. The diagram also shows the disturbances that enter or affect the process. If there were no upsets to a process, there would be no need for the control system. Figure 1-3 also shows the input and output of the process and the set point used for control.


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H

The control loop

LV 100

LC 100

Level Controller

Liquid





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H

The control loop

LV 100

LC 100

Level Controller

Liquid





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Process Control Terms  Process variable, Measured/Controlled variable and Manipulated variable  Setpoint  Error  Offset  Load disturbance  Control algorithm  Manual and automatic control  Closed and open control loops Industrial Process Control 2014 Introduction To Process Control

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position, or blade pitch. The controlled variables are those conditions such as temperature, level, position, pressure, pH, density, moisture con tent, weight, and speed that must be maintained at some desired value For each controlled variable there is an associated manipulated variable. The control system must adjust the manipulated variables so the desired value or set point of the controlled variable is maintained despite any disturbances. Process Control Terms

 Process variable, Measured/Controlled variable and Manipulated variable  Setpoint Disturbances  Error  Offset Manipulated Controlled  Load disturbance Process Variables Variables  Control algorithm  Manual and automatic control Figure 1-1.Process variables Closedcontrol and open control loops

Industrial Process Control 2014 or affect the process 23 Disturbances enter and tend to drive the controlled Introduction To Process Control variables away from their desired value or set point condition. Typical di

control valve (LV) are used to control the liquid level in a process tank. The purpose of this control system is to maintain the liquid level at some prescribed height (H) above the bottom of the tank. It is assumed that the rate of flow into the tank is random. The level transmitter is a device that measures the fluid level in the tank and converts it into a useful measurement signal, which is sent to a level controller. The level controller evaluates the measurement, compares it with a desired set point (SP), and produces a series of corrective actions that are sent to the control valve. The valve controls the flow of fluid in the outlet pipe to maintain a level in the tank.

Process variable, Measured/Controlled variable and Manipulated variable

Liquid

Process Variable?

Level Transmitter LT 100

H LV 100

Measured/ Controlled Variable? LC 100

Level Controller

Manipulated Variable? Liquid Control Valve

Industrial Process Control 2014 FigureIntroduction 1-2. Process level control: To Process ControlExample

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Common Process Variables:  Pressure  Flow  Level  Temperature  Density  Ph (acidity or alkalinity)  Mass  Conductivity


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Process Control Terms  Process variable, Measured/Controlled variable and Manipulated variable  Setpoint  Error  Offset  Load disturbance  Control algorithm  Manual and automatic control  Closed and open control loops Industrial Process Control 2014 Introduction To Process Control

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Process Control Terms  Process variable, Measured/Controlled variable and Manipulated variable  Setpoint/ reference point  Error  Offset  Load disturbance  Control algorithm  Manual and automatic control  Closed and open control loops Industrial Process Control 2014 Introduction To Process Control

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Open loop system Closed loop system


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Process Control Terms Offset is a sustained

 Process variable, Measured/Controlled variable deviation of the process and Manipulated variable variable from the setpoint.  Setpoint  Error  Offset  Load disturbance Example  Control algorithm In the temperature control loop example, if the control  Manual and automatic control system held the process fluid  Closed and open control loops°C consistently, even at 100.5 Industrial Process Control 2014 Introduction To Process Control

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though the setpoint is 100 °C, then an offset of 0.5 °C exists.

Process Control Terms  Process variable, Measured/Controlled variable undesired change in one of and Manipulated variable the factors that can affect the  Setpoint process variable.  Error  Offset  Load disturbance  Control algorithm  Manual and automatic control  Closed and open control loops Industrial Process Control 2014 Introduction To Process Control

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Process Control Terms  Process variable, Measured/Controlled variable and Manipulated variable  Setpoint a mathematical expression of  Error a control function.  Offset  Load disturbance  Control algorithm  Manual and automatic control  Closed and open control loops Industrial Process Control 2014 Introduction To Process Control

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Manual and Automatic Control


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 Primary element/sensor  Transducer, Converter, Transmitter  Signal


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 Primary element/sensor ˃ Pressure sensing diaphragms, strain gauges, capacitance cells ˃ Resistance temperature detectors (RTDs) ˃ Thermocouples ˃ Orifice plates ˃ Pitot tubes ˃ Venturi tubes Industrial Process Control 2014 Introduction To Process Control

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 Transducers/ Transmitters/ Converter A transmitter is a device that converts a reading from a sensor or transducer into a standard signal and transmits that signal to a monitor or controller. ˃ Pressure transmitters ˃ Flow transmitters ˃ Temperature transmitters ˃ Level transmitters ˃ Analytic (O2 [oxygen], CO [carbon monoxide], and pH) transmitters Industrial Process Control 2014 Introduction To Process Control

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 Signals ˃ Pneumatic signal (3-15 psig) ˃ Analog signal (4-20 mA) ˃ Digital signal


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Local controller

 Controllers


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 Final Control Elements final control element is part of the control system that acts to physically change/ adjust the manipulated variable. Examples; valve, pump, solenoid


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The Instrumentation, Systems, and Automation Society (ISA) Developed a set of symbols for use in engineering drawings and designs of control loops. Piping and Instrumentation Drawing (P&ID). Industrial Process Control 2014 Introduction To Process Control

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The Instrumentation, Systems, and Automation Society (ISA) Piping and Instrumentation Drawing (P&ID). Diagram in the process industry which shows the piping of the process flow together with the installed equipment and instrumentation


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 Valves


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 Pump Directional arrows showing the flow direction


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 Piping and connection


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 Identification letters and tag numbers


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Device for efficient Heat transfer from one medium to another

Example (Heat exchanger).

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Appendix A

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Appendix A

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Appendix A

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Appendix A

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Appendix A A control valve which should go to the open position on control signal failure.

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Fail open or Fail close?

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Valve 3 : affect flow rate of A Chemical Engineering Example: both fresh and A chemical reactor with recycle recycled feed a. Can T4, the reactor feed temperature be controlled by feedback? without changing (Hint: determine which valves have a causal effect on T4.) the ratio b. Select the best valve to control T4 if more than one valves are possible. Valve 5,6 : No effect

Yes, strong

Valve 2 : affect the flow of heating oil to the feed heat exchanger

Valve 1 : Influence the flow rate through the heat exchanger

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Determine Determine sensors sensors & & final final elements elements important important variables variables can can be be controlled. controlled. Select controller pairings which measured variable should be controlled by adjusting which manipulated variable. Vapor

Heat Exchangers

Drum

Variables to be controlled are input flow rate, temperature, compostion and pressure.

Liquid Pump 57

Determine Determine sensors sensors & & final final elements elements important important variables variables can can be be controlled. controlled. Select controller pairings which measured variable should be controlled by adjusting which manipulated variable. Vapor

Heat Exchangers FC

Drum

Variables to be controlled are input flow rate, temperature, compostion and pressure.

TC

Liquid Pump 58

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In-class Assignment. Identify the process by explaining the symbols

 Example of P&ID


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Types of controllers:  Discrete controllers  Multistep controllers  Continuous controllers


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Discrete controllers


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Multistep controllers


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Continuous controllers


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Continuous Controller Modes  Proportional Control Mode (P)  Integral Control Mode (I)  Derivative Control Mode (D)

Composite Controller Modes  Proportional-Integral (PI)  Proportional-Derivative (PD)  Proportional-Integral-Derivative (PID) Industrial Process Control 2014 Introduction To Process Control

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Controllers are tuned in an effort to: » The system responds quickly to errors. » The system remains stable (PV does not oscillate around the SP).


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Gain Gain is defined simply as the change in output divided by the change in input. Example: Change in Input to Controller - 10% Change in Controller Output - 20% Gain = 20% / 10% = 2 Industrial Process Control 2014 Introduction To Process Control

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Proportional Mode Integral Mode Derivative Mode


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 Single Control Loops  Multivariable/Advanced Control Loops


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Feedback Control


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Feedback Control  Feedback loop measures a process variable and sends the measurement to a controller for comparison to set point  Control action is taken to return process variable to set point  Figure shows a transmitter that measure fluid temperature, opens and closes hot steam valve when necessary to adjust the fluid’s temperature

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Pressure Control  Vary in speed hence able to respond to load changes and control action quickly  Speed required in pressure control dictated by volume of process fluid  High volume systems (natural gas storage) tend to change more slowly than low-volume system


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Flow Control  Fast loops:response quickly to changes: fast sampling & response time  Flow transmitter produce rapid noise  sensitive devices  Temperature of process fluid affects its density temperature measurements are often taken with flow measurement and compensation for temperature is accounted for in the flow calculation  Typically  flow sensor, transmitter, controller, valve/ pump are used in flow control loop


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Level Control  Speed of changes depend on size and shape of process (Large vessel takes longer time to fill), flow rate of input & outflow pipes  Final control element  valve on the input and/or outflow connections to tank  To avoid overflow, redundant level control systems are employed


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Temperature Control  Temperature loop is relatively slow  time required to change the temperature of a process fluid  To increase speed of temperature loop response  feedforward control strategies are used  RTDs and thermocouple are typical temperature sensors used  Final control element  fuel valve to a burner or a valve to some kind of heat exchanger


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Find a Piping & Instrumentation Drawing (P&ID) Diagram of a single control loops process (of your choice) and explain the process of the system briefly using the P&ID Diagram. 79


 Single Control Loops  Multivariable/Advanced Control Loops  Control loops in which a primary controller controls one process variable by sending signals to a controller of a different loop that impacts the process variable of the primary (main) loop  Primary controller will manipulate the setpoint of the secondary controller to maintain the setpoint temperature of the primary process variable  Normally, the secondary loop will be tuned first before the primary loop  adjustment to secondary loop will impact the primary Industrial Process Control 2014 loop 80 Introduction To Process Control

Feedforward Control  Control system that anticipates load disturbances and controls them before giving any impact to the process variable  Mathematical understanding of how the manipulated variables will impact the process variable  Example : Flow transmitter opens or closes a hot steam valve based on how much cold fluid passes through the flow sensor  Error is prevented before occurring, rather than corrected  Generally used where controlled variable has the potential of being a major load disturbance on the process variable ultimately being controlled Industrial Process Control 2014 Introduction To Process Control

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Feedforward Plus Feedback


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Feedforward Plus Feedback  To overcome the difficulty of anticipating every possible load disturbances in a feedforward systems, combination with feedback system is used  Total input from feedforward and feedback loop is combined  unified signal to the final control element  Flow transmitter and temperature transmitter provide information for controlling a hot steam valve

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Methods of control currently being used in process industries.

Cascade Control  Control system in which a secondary (slave) control loop is set up to control variable that is a major source of load disturbance for another primary (master) control loop  The controller of the primary loop determines the setpoint of the summing controller in the second loop


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Batch Control  Those processes that are taken from start to finish in batches  Example, mixing ingredients for a juice drink    

Limited amount of one flavour is mixed at one time Not practical to have a continuous process running Getting correct proportion of ingredients into batch At various stages of batch processes  Level, flow, pressure, temperature, mass measurement

 Disadvantage :  Process must be frequently restarted  Start-up problem  all measurements are below set point  As recipe changes, control instruments need recalibration


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Ratio Control  A process which requires a ratio of certain amount to be mixed. Example  diluting acid in water  A control system can be developed to control the ratio of acid to water though the water supply may not be controlled  Used in many applications and involves a controller that receives input from a flow measurement device on the unregulated (wild) flow  Controller performs a ratio calculation and signals the appropriate setpoint to another controller that sets the flow of the second fluid  proper proportion of the second fluid can be added  Used  continuous process is going on and an additive is being put into the flow


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Methods of control currently being used in process industries. Selective Control More important of two variables will be maintained Selective control is most often used when equipment must be protected or safety maintained. For example, in a boiler control system, if fuel flow outpaces air flow, then uncombusted fuel can build up in the boiler and cause an explosion. Selective control is used to allow for an air-rich mixture, but never a fuel-rich mixture. Industrial Process Control 2014 Introduction To Process Control

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Fuzzy Control  The controller uses fuzzy logic to make decisions about adjusting the process  A form of computer logic whether something is included/not based on grading scale  Kind of artificial intelligence that will account for numerous variable, formulate a theory of how to make improvement, adjust the process and learn from the results  Fuzzy control is a relatively new technology  removes operators the ability but none of the responsibility to control a process Industrial Process Control 2014 Introduction To Process Control

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1. 2. 3. 4. 5. 6. 7.

Safety Environment protection Equipment protection Smooth operation production rate Product quality High profit Monitoring & diagnosis

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• •

A process which is generally used to separate a mixture of two or more liquids and vapor based on their boiling points by passing it to a flash drums at a fixed temperature and pressure. The mixture is allowed to reach equilibrium, and vapor exits at the top and liquid exits the bottom of the drum

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The process separates 2 components (A & B) based on their different vapor pressures.

Stream flows through a valve to a vessel at a lower pressure.

Liquid feed stream, consisting of components A and B, is heated using two heat exchangers.

A

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B

Due to higher T and lower P, stream forms two phases, vapor and liquid.

Control Pressure

1. 2. 3. 4. 5. 6. 7.

Safety Environment protection High pressure drum is dangerous protection Equipment Smooth operation production rate Product quality High profit Monitoring & diagnosis

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Control Pressure

1. 2. 3. 4. 5. 6. 7.

Safety Environment protection High pressure drum is dangerous protection Equipment Smooth operation production rate Product quality High profit Monitoring & diagnosis

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To flare for combustion

2. Environment protection Never release harmful gasses to atmosphere

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Flow of liquid is controlled

3. Equipment protection

LC

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4. Smooth operation production rate The production rate need to be kept smooth

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5. Product quality Level Key is achieved by adjusting the heating

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6. High profit

Use the least costly heating Industrial Process Control 2014 Introduction To Process Control

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7. Monitoring & diagnosis Calculate & plot key parameters

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1. 2. 3. 4. 5. 6. 7.

Safety Environment protection Equipment protection Smooth operation production rate Product quality High profit Monitoring & diagnosis All seven must be achieved Otherwise the process can be unprofitable, or worse unsafe


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Manufacturers control the production process for three reasons: ① Reduce variability ② Increase efficiency ③ Ensure safety

Remember this? Industrial Process Control 2014 Introduction To Process Control

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When we control a process, variability of the process key variables are reduced  seven objectives achieved

When we control a process, variability of the process key variables are reduced  seven objectives achieved Reduced key variables variation economically beneficial

What statistics can we calculate from this data?

Variations of key plant variable frequency distribution

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Increase variation  broader distribution of frequency

How do we relate variability to process peformance?

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 Fired heater (furnace) with a chemical reaction occurring as fluid flows through the pipe

Small variations temperature can be operated very close to maximum constraint

Temperature of the reactor must be held minimum (no reaction) and maximum (metal damage)

When economic favors increased conversion feed profit increases as temperature increases From data  temperature varies about the desired value due to disturbance  fuel composition and pressureHence, the effectiveness of control in max profit depends on reducingthe temperature variations

Base case reactor temperature  plant performance with poor control

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Uses tighter distribution improved control

Calculating average performance of the process yield the following equation: M

Pave   F j Pj j 1

Pave=average process performance

Fj=fraction of data in interval j=Nj/NT Nj=Number of data points in interval j NT=total number of data points

Pj=performance measured at the midpoint of interval j 109

M=number of intervals in the frequency distribution

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Comparing Operation A and B

T; Temperature

Pj;Performance measured at the midpoint of interval

Fj; Fraction of data in interval Improved performance  reduce variance & increase average temperature

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QUESTION??

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Industrial Process Control

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