Practical PLC Programming Techniques for Reactor Temperature Control in Chemical Plants

In chemical production, reactor temperature control directly affects product quality and production safety. Today, we will discuss in detail how to build a precise and reliable temperature control system using Siemens PLC.

1. Hardware Configuration

1.1 PLC Selection and Module Configuration

Main Unit Selection: S7-1500 CPU 1515-2 PN (with dual Ethernet ports)
Analog Input Module: AI 4xU/I/RTD/TC ST (6ES7531-7KF00-0AB0)
Analog Output Module: AO 4xU/I ST (6ES7532-5HF00-0AB0)
Digital Input Module: DI 16x24VDC BA (6ES7521-1BH10-0AA0)

1.2 Key I/O Point Allocation

// Analog Inputs

%IW100: Reactor temperature measurement (PT100, 4-20mA)

%IW102: Jacket inlet temperature (PT100, 4-20mA)

%IW104: Jacket outlet temperature (PT100, 4-20mA)

%IW106: Pressure transmitter (0-10bar, 4-20mA)


// Analog Outputs

%QW100: Heating control valve opening (4-20mA)

%QW102: Cooling control valve opening (4-20mA)


// Digital Inputs

%I0.0: Start button

%I0.1: Stop button

%I0.2: Emergency stop button

%I0.3: Temperature exceeds upper limit

%I0.4: Temperature falls below lower limit


// Digital Outputs

%Q0.0: Running indicator light

%Q0.1: Fault indicator light

%Q0.2: Heating system enable

%Q0.3: Cooling system enable

2. Control Program Design

2.1 Core Temperature Control Function Block

FUNCTION_BLOCK FB_TemperatureControl

VAR_INPUT

    Enable : BOOL;         // Enable control

    SetPoint : REAL;       // Temperature setpoint [℃]

    ActualValue : REAL;    // Actual temperature value [℃]

    PID_Manual : BOOL;     // Manual/Automatic switch

    Manual_Output : REAL;  // Manual output value [%]

END_VAR


VAR_OUTPUT

    HeatingValve : REAL;   // Heating valve opening [%]

    CoolingValve : REAL;   // Cooling valve opening [%]

    ControlMode : INT;     // Current control mode

    Deviation : REAL;      // Temperature deviation [℃]

END_VAR


VAR

    PID_Controller : PID_Compact;

    OutputValue : REAL;

    DeadZone : REAL := 0.5;

END_VAR


// Calculate temperature deviation

Deviation := SetPoint - ActualValue;


// PID controller configuration and execution

PID_Controller(

    Setpoint := SetPoint,

    Input := ActualValue,

    Input_PER := FALSE,

    ManualEnable := PID_Manual,

    ManualValue := Manual_Output,

    Reset := NOT Enable,

    Output =&gt; OutputValue
);


// Determine heating or cooling mode based on output value

IF OutputValue &gt; DeadZone THEN

    HeatingValve := OutputValue;

    CoolingValve := 0.0;

    ControlMode := 1; // Heating mode

ELSIF OutputValue &lt; -DeadZone THEN

    HeatingValve := 0.0;

    CoolingValve := ABS(OutputValue);

    ControlMode := 2; // Cooling mode

ELSE

    HeatingValve := 0.0;

    CoolingValve := 0.0;

    ControlMode := 0; // Hold mode

END_IF;

2.2 Temperature Segmentation Control Data Block

DATA_BLOCK DB_TempProfile

STRUCT

    Segments : ARRAY[1..10] OF STRUCT

        Temperature : REAL;    // Target temperature [℃]

        RampRate : REAL;      // Heating rate [℃/min]

        HoldTime : TIME;      // Hold time

        NextStep : INT;       // Next segment number

    END_STRUCT;


    CurrentSegment : INT;     // Current segment number

    SegmentTimer : TON;       // Segment timer

    TotalSegments : INT;      // Total number of segments

    ProfileActive : BOOL;     // Process curve active

END_STRUCT;

3. Safety and Redundancy Design

3.1 Temperature Safety Interlock System

FUNCTION_BLOCK FB_TempSafetyInterlock

VAR_INPUT

    Temperature : REAL;        // Actual temperature

    HighLimit : REAL;         // High temperature limit

    LowLimit : REAL;          // Low temperature limit

    HighHighLimit : REAL;     // Extremely high temperature limit

    EmergencyStop : BOOL;     // Emergency stop signal

END_VAR


VAR_OUTPUT

    SafetyOK : BOOL;          // Safety status

    AlarmHigh : BOOL;         // High temperature alarm

    AlarmLow : BOOL;          // Low temperature alarm

    AlarmHighHigh : BOOL;     // Extremely high temperature alarm

    ShutdownRequired : BOOL;  // Shutdown required

END_VAR


// Temperature range check

AlarmHigh := Temperature &gt; HighLimit;

AlarmLow := Temperature &lt; LowLimit;

AlarmHighHigh := Temperature &gt; HighHighLimit;


// Safety interlock logic

ShutdownRequired := AlarmHighHigh OR EmergencyStop;

SafetyOK := NOT ShutdownRequired AND 

            NOT (AlarmHigh AND AlarmLow);


// Interlock actions

IF ShutdownRequired THEN

    // Execute emergency shutdown procedure

    // Close all heating/cooling outputs

    // Send alarm notification

END_IF;

3.2 Redundant Temperature Measurement Design

Use dual PT100 sensor configuration
Sensor signal comparison and fault detection
Automatic switch to backup sensor
Deviation limit alarm function

4. Human-Machine Interface Design

4.1 Main Operation Screen Layout

Process Display Area

Real-time temperature trend graph
Current temperature/setpoint display
Valve opening indicator bar
Control mode status

Operation Control Area

Temperature setpoint input box
Manual/Automatic switch button
Start/Stop button
Process curve selection

Alarm Information Area

Real-time alarm list
Historical alarm records
Alarm confirmation button

4.2 Parameter Setting Screen

// HMI parameter structure definition

TYPE ST_PIDParameters :

STRUCT

    Kp : REAL;           // Proportional coefficient

    Ti : TIME;           // Integral time

    Td : TIME;           // Derivative time

    DeadBand : REAL;     // Deadband range

    MaxOutput : REAL;    // Maximum output

    MinOutput : REAL;    // Minimum output

END_STRUCT

END_TYPE

5. Fault Diagnosis and Troubleshooting

5.1 Fault Diagnosis Function Block

FUNCTION_BLOCK FB_DiagnosticSystem

VAR_INPUT

    SensorStatus : ARRAY[1..4] OF BOOL;  // Sensor status

    ValveStatus : ARRAY[1..2] OF BOOL;   // Valve status

    CommStatus : BOOL;                   // Communication status

END_VAR


VAR_OUTPUT

    FaultCode : WORD;                    // Fault code

    FaultDescription : STRING[80];       // Fault description

    MaintenanceRequired : BOOL;          // Maintenance required

END_VAR


// Fault diagnosis logic

CASE FaultCode OF

    16#0001: 

        FaultDescription := 'Temperature sensor 1 fault';

        MaintenanceRequired := TRUE;

    16#0002: 

        FaultDescription := 'Heating valve actuator fault';

        MaintenanceRequired := TRUE;

    16#0004: 

        FaultDescription := 'Cooling system circulation pump fault';

        MaintenanceRequired := TRUE;

    16#0008: 

        FaultDescription := 'PLC-HMI communication interruption';

        MaintenanceRequired := FALSE;

END_CASE;

5.2 Common Fault Quick Handling Table

| Fault Phenomenon | Possible Cause | Handling Method |

|———|———|———|

| Temperature instability | Improper PID parameters | Retune PID parameters |

| Slow heating | Insufficient heating power | Check heater and power supply |

| Valve not actuating | Actuator fault | Check air source and positioner |

| Abnormal readings | Sensor wiring issue | Check wiring and shielding |

6. Debugging Methods and Experience Summary

6.1 Step-by-Step Debugging Process

Hardware Check

Check the correctness of all wiring
Test analog input and output
Verify digital signal

Unit Testing

Individually test temperature reading
Individually test valve control
Test alarm system

System Integration Testing

Manual mode testing
Automatic control testing
Abnormal condition simulation

6.2 PID Parameter Tuning Techniques

Initial parameter settings: Kp=1.0, Ti=300s, Td=0s
Use the critical ratio method for tuning
Note the differences in heating and cooling characteristics
It is recommended to set a dead zone to prevent frequent switching

Reactor temperature control is a typical process control application. Mastering PLC programming techniques and control strategies can help build a safe and reliable control system. What temperature control challenges have you encountered in your actual projects? Feel free to leave a message for discussion!