Waste Acid Treatment Control System: A Safe and Efficient Intelligent Management Solution for Industrial Waste Liquids

The waste acid treatment system is a crucial aspect of industrial environmental protection. By utilizing Siemens PLC for automation control, it enhances processing efficiency and safety while reducing environmental risks and operational costs.

1. Hardware Configuration

PLC and Expansion Module Selection

In response to the specific requirements of the waste acid treatment system, we selected the Siemens S7-1500 series PLC as the main control unit, specifically the CPU 1516-3 PN/DP model. This model possesses sufficient processing power and communication interfaces to meet the control needs of medium-sized waste acid treatment systems. Considering the highly corrosive environment during the waste acid treatment process, all on-site equipment is designed with anti-corrosion features.

The configuration of expansion modules is as follows:

• Digital Input Module (SM 521): 32 points, 24VDC

• Digital Output Module (SM 522): 16 points, 24VDC/0.5A

• Analog Input Module (SM 531): 8 points, 4-20mA, 16-bit resolution

• Analog Output Module (SM 532): 4 points, 4-20mA, 16-bit resolution

• Communication Module (CM 1542-5): PROFIBUS DP interface

I/O Point Allocation Table (Partial)

Digital Input (DI):

• %I0.0~%I0.7: Feedback on the start/stop status of each pump

• %I1.0~%I1.3: Liquid level switch signals

• %I1.4~%I1.7: Valve limit switches

• %I2.0~%I2.3: Emergency stop buttons

• %I2.4~%I2.7: System operating mode selection

Digital Output (DO):

• %Q0.0~%Q0.7: Control for starting/stopping each pump

• %Q1.0~%Q1.7: Control for solenoid valves

Analog Input (AI):

• %IW64: pH value measurement

• %IW66: Oxidation-reduction potential

• %IW68: Temperature measurement

• %IW70: Pressure measurement

• %IW72: Liquid level measurement

Analog Output (AO):

• %QW64: Neutralizing agent flow control

• %QW66: Heater power control

2. Control Program Design

Program Architecture Design

The waste acid treatment control system adopts a hierarchical program structure to ensure maintainability and scalability. The overall architecture is divided into the following layers:

1. OB Layer: Main cycle (OB1), cyclic interrupt (OB30), fault handling (OB80~OB86)

2. FB Layer: Functional modules, including pump control, valve control, PID regulation, etc.

3. FC Layer: Functional functions, including data conversion, parameter calculation, etc.

4. DB Layer: Data blocks, storing system parameters, operational data, etc.

The program execution flow is as follows:

• OB1 calls various system initialization function blocks

• OB30 (100ms cycle) executes process control and data acquisition

• OB80~OB86 handles various faults and exceptions

Function Block Design

Taking the neutralization control function block as an example, this FB is responsible for the automatic adjustment of the pH value of the waste acid:

FUNCTION_BLOCK "FB_NeutralizationControl"

{ S7_Optimized_Access := 'TRUE' }

VERSION: 0.1

VAR_INPUT

    pH_Value : REAL;            // Current pH value, from pH meter

    SetPoint : REAL;            // Target pH value

    EnableAuto : BOOL;          // Enable automatic control

    Manual_Value : REAL;        // Manual control value (0-100%)

END_VAR

VAR_OUTPUT

    NeutralizingAgent_Output : REAL;  // Neutralizing agent output (0-100%)

    Status : INT;               // Status code

END_VAR

VAR

    pH_PID : PID;               // PID controller instance

    LastOutput : REAL;          // Last output value

END_VAR

The function block implements pH control based on the PID algorithm, automatically adjusting the amount of neutralizing agent added according to the deviation between the measured value and the set point. Additionally, considering the characteristics of waste acid treatment, anti-overshoot optimization and deadband handling have been added to avoid severe fluctuations during the neutralization process.

Status Control Design

The status control of the waste acid treatment system adopts a state machine model, mainly including the following states:

1. System Stop State (STOP): All devices stop operating

2. System Initialization State (INIT): System startup initialization

3. Standby State (STANDBY): System ready and waiting for processing

4. Normal Operation State (RUN): System normally processes waste acid

5. Fault State (FAULT): System detects a fault

6. Maintenance State (MAINTENANCE): System is in maintenance mode

Status transitions are triggered by operator commands and system events, with clearly defined device operation rules and safety boundary conditions for each state, ensuring the system operates safely and reliably under all circumstances.

3. Fault Diagnosis and Troubleshooting

Common Fault Analysis

Common faults in the waste acid treatment system and their possible causes:

1. Abnormal pH Fluctuations

• Neutralizing agent addition system failure

• pH sensor calibration deviation

• Insufficient mixing

• Feed composition sudden change

Liquid Level Control Abnormalities

• Liquid level sensor failure

• Inlet and outlet valve failure

• Unreasonable control parameters

Pump Operation Failures

• Motor overload

• Pump impeller corrosion damage

• Pipeline blockage

• Relay contact failure

Use of Diagnostic Tools

The Siemens programming software TIA Portal provides powerful diagnostic tools for fault diagnosis in the waste acid treatment system:

1. Online Monitoring: Real-time monitoring of program execution status and data changes

2. Tracking Function: Records the trend of key variables over time

3. Diagnostic Buffer: View diagnostic information generated by the system

4. Forced Table: Force I/O states for testing

It is highly recommended to use the SIMATIC ProDiag function, which can automatically generate alarm information and diagnostic guidance based on predefined monitoring conditions, greatly simplifying the fault location process.

Troubleshooting Methodology

For troubleshooting the waste acid treatment system, the following steps are recommended:

1. Safety Assessment: First, confirm whether the system is in a safe state, and perform an emergency shutdown if necessary

2. Phenomenon Recording: Carefully record the system status and alarm information at the time of the fault occurrence

3. Cause Analysis: Infer possible causes based on the fault phenomenon

4. Testing and Verification: Test potential fault points one by one using the control variable method

5. Repair and Resolution: Repair confirmed fault points

6. Verification Confirmation: Conduct comprehensive testing after repairs to confirm that the fault has been eliminated

4. Safety and Redundancy Design

Safety Circuit Design

The waste acid treatment system involves corrosive chemicals, making safety design crucial:

1. Hardware Safety Circuit: Uses hardwired safety circuits independent of the PLC to ensure safe shutdown in case of PLC system failure

2. Safety Level: Key safety components achieve SIL 2 safety level based on risk assessment results

3. Emergency Stop: Emergency stop buttons are set up in multiple locations in the control room and on-site, immediately cutting off power to all drive equipment when pressed

4. Interlock Protection: Implements multiple interlock protections, such as:

• Automatically stop feeding when liquid level is too high

• Automatically stop adding neutralizing agent when pH value is abnormal

• Automatically open the pressure relief valve when container pressure is abnormal

Redundant Control Strategy

To enhance system reliability, key components are designed with redundancy:

1. Sensor Redundancy: Key parameters (such as pH value) are measured using dual sensors, with a comparison algorithm to detect sensor failures

2. Controller Redundancy: Optional configuration of S7-1500R/H redundant controllers to ensure seamless switching of the control system

3. Communication Redundancy: Uses a ring network topology, automatically switching paths when communication lines are disconnected

4. Power Redundancy: The control system is equipped with redundant power supplies and UPS for backup

Fail-Safe Mechanism

The system design includes a comprehensive fail-safe mechanism:

1. Fault Detection: Regular self-checks and operational monitoring

2. Safe State: Defines the fail-safe state of each device to ensure the system enters a safe state in case of failure

3. Fault Logging: Automatically records fault information for subsequent analysis

4. Remote Alarm: Implements remote fault alarms via SMS, email, etc.

5. User Interface Design

Interface Layout Description

The HMI interface for waste acid treatment adopts an intuitive information organization method, mainly including:

1. Overview Screen: Displays the entire system process flow and key parameters

2. Process Unit Screen: Detailed parameters and controls for each processing unit

3. Trend Curve Screen: Historical trends of key parameters

4. Alarm Screen: Current and historical alarm information

5. Parameter Setting Screen: Configuration of process parameters

6. User Management Screen: User permission management

The interface design follows ergonomic principles, using color coding to indicate device status, with critical data displayed in large fonts to ensure operators can quickly access information.

Parameter Setting Description

The parameter setting interface is designed in layers, divided by permission levels:

1. Operator Level: Can only adjust routine operating parameters, such as pH set points

2. Engineer Level: Can adjust control parameters, such as PID parameters

3. Administrator Level: Can modify system configurations and permission settings

Key parameter settings include rationality checks and confirmation mechanisms to prevent erroneous operations that could lead to system anomalies.

Summary and Outlook

By implementing automation control for waste acid treatment using Siemens PLC, not only has processing efficiency and safety been improved, but data traceability has also been achieved. In the future, AI technology can be integrated to further optimize control strategies, and discussions are welcome!