Practical Application of PLC: Ammonia Leak Detection and Alarm System Design

Ammonia (NH₃) is a common industrial chemical widely used in fertilizer manufacturing, refrigeration systems, and other fields. Due to its strong corrosiveness and toxicity, leaks can pose a serious threat to the environment and personnel safety. Therefore, timely detection of leaks and taking measures is particularly important in areas where ammonia leaks may occur. This article will introduce how to implement an ammonia leak alarm control system using Siemens PLC, helping everyone understand the application of PLC control systems in actual industrial environments.

This project will use the Siemens S7-1200 series PLC for development, with main functions including:

1. Ammonia leak detection: Real-time monitoring of ammonia concentration through gas sensors.
2. Alarm system: When the ammonia concentration exceeds a set threshold, the PLC controls the alarm device to issue sound or visual alarms.
3. Emergency control: Depending on actual needs, the PLC can start fans or other devices to help clear ammonia leaks.

1. Use ammonia concentration sensors to collect gas data.
2. Set concentration thresholds to trigger alarms when ammonia concentration exceeds this threshold.
3. Use HMI (Human-Machine Interface) to display real-time data and allow operators to set alarm thresholds.
4. The system should be able to start emergency devices like fans when a leak occurs.

• Siemens S7-1200 PLC
• Ammonia concentration sensor (e.g., MiCS-5524)
• Alarm devices (buzzer, LED lights)
• Fan control module (e.g., relay)
• HMI (e.g., KTP700)

Step 1: Input Collection

First, we need to receive the signal from the ammonia concentration sensor through the PLC’s input module. Assuming the sensor output is analog, we will use the PLC’s analog input module (e.g., SM1231) to read the ammonia concentration.

Example Code:

// Assuming input I1 is the ammonia concentration sensor signal
// The output range of the sensor: 0 to 1000, corresponding to ammonia concentration 0% to 100%
ANALOG_INPUT(0, 1, 1000); // Read analog input value (0 to 1000)

Step 2: Set Alarm Threshold

Set the alarm threshold through the HMI interface. For example, we set the alarm to trigger when the ammonia concentration reaches 800. This threshold can be stored and managed using the PLC data block.

Example Code:

// Set alarm threshold
ALARM_THRESHOLD := 800; // Ammonia concentration threshold (unit: ppm)

Step 3: Compare Sensor Value with Threshold

In the PLC program, we will monitor the ammonia concentration value collected by the sensor in real-time and compare it with the set alarm threshold.

Example Code:

// Compare ammonia concentration with the threshold
IF ANALOG_INPUT_VALUE > ALARM_THRESHOLD THEN
    // Start alarm system
    ALARM_FLAG := TRUE;
ELSE
    ALARM_FLAG := FALSE;
END_IF

Step 4: Control Alarm Device

When the ammonia concentration exceeds the set threshold, the PLC will control the alarm device to issue a warning. We can use relays to drive the buzzer and LED lights.

Example Code:

// Control alarm device
IF ALARM_FLAG THEN
    OUTPUT_ALARM := TRUE; // Turn on buzzer and LED lights
ELSE
    OUTPUT_ALARM := FALSE; // Turn off buzzer and LED lights
END_IF

Step 5: Start Emergency Fan

When the ammonia concentration is too high, in addition to issuing an alarm, we can also start an emergency fan to reduce the impact of the leak. The PLC will start the fan by controlling relays or other actuators.

Example Code:

// Control fan
IF ALARM_FLAG THEN
    OUTPUT_FAN := TRUE; // Start fan
ELSE
    OUTPUT_FAN := FALSE; // Turn off fan
END_IF

The HMI interface is used to display ammonia concentration, alarm status, and allow operators to set alarm thresholds. Operators can adjust the alarm threshold for ammonia leaks and view real-time ammonia concentration through the HMI interface.

HMI Function Implementation:

1. Display real-time ammonia concentration.
2. Display alarm status (e.g., “Normal”, “Alarm”).
3. Allow operators to adjust alarm threshold.

1. Accuracy Issues with Analog Input: The accuracy of the ammonia sensor may be affected by environmental temperature, humidity, and other factors. In actual projects, we need to filter the input signal to avoid erroneous alarms caused by data fluctuations.

   Solution: Use filtering algorithms (e.g., averaging or weighted averaging) to smooth the collected signals.

2. Delay in Alarm Device Response: In some complex industrial environments, the alarm device may have a delayed response, leading to a failure to alert staff in a timely manner.

   Solution: Increase redundancy in the alarm mechanism, such as simultaneously activating the buzzer and flashing LED lights.

3. Fan Control Issues: The control of fan start and stop needs to consider the actual type of fan and electrical control requirements, especially in high-temperature environments.

   Solution: Choose appropriate relays and fan control modules to ensure the system operates normally under high load.

Through Siemens PLC, we can easily implement an ammonia leak alarm control system. This project not only demonstrates how to use PLC for data collection and processing but also involves interaction with sensors, alarm devices, and emergency control equipment. By studying this project, you will gain a better understanding of the practical applications of PLC in industrial control and master how to apply it in different types of automation control systems.