Background of the plan:

With the increasingly powerful development of science and technology in our country, the development and construction of highways and tourist areas are growing day by day, and the proportion of tunnels in construction is also increasing. Tunnel excavation work has also received high attention from project construction enterprises and the country. Tunnels are confined spaces with poor ventilation. During excavation and construction, it is easy to produce flammable and toxic gases, which can remain in the tunnel and pose a threat to the health and safety of tunnel workers. They may also cause significant disasters and losses to tunnel engineering. Therefore, the prevention and control of harmful gases in tunnels is the focus of tunnel construction, and 24-hour online detection of harmful gases in tunnels is also a protective measure for tunnel construction.

Solution:

During tunnel excavation, the most easily generated toxic and harmful gases are mainly CO, carbon monoxide, CH4, methane, H2S, hydrogen sulfide, O2 oxygen, SO2, sulfur dioxide, CO2, and carbon dioxide. In addition, to prevent oxygen deficiency and suffocation of workers, the concentration of O2 oxygen must also be detected.

Monitoring locations: dangerous areas where gas leaks are likely to occur, such as tunnel entrances, middle of tunnels, tunnel bifurcations, tunnel excavations, and near explosions.

Monitoring gases and other hazardous substances: CO, CH4, H2S, O2, SO2, CO2, dust particles (PM2.5+PM10)

Installation requirements: If strictly following the new national standard requirements, a gas alarm should be installed on an area of 25 square meters. Users can also choose according to the actual situation on site.

The installation precautions are as follows (to be considered before installation and construction):

1: When installing a gas detector in a tunnel, the first thing to consider is whether there is a signal and a network inside the tunnel. If not, a walkie talkie must be equipped to facilitate communication, debugging, and docking, otherwise it will be troublesome to travel back and forth.
2: The wiring area requires a waterproof junction box (because tunnels are definitely damp). Wiring requires a sleeve, and considering cost issues, PV pipes can be used for closer distances, while corrugated pipes can be used for farther distances. If it is a mandatory explosion-proof area, explosion-proof galvanized pipes, explosion-proof joints, and explosion-proof junction boxes must be used.
3: If the tunnel power supply is 220V, then a 220V to 24V power adapter needs to be considered, as the gas detector requires 24V power supply
4: If it is convenient to obtain electricity, wireless LORA transmission scheme can be used in the middle section of the tunnel to avoid wiring and save labor and wire costs

Installation height: The installation height of the detector from the ground should be greater than 30 centimeters to prevent water splashing in. In general, for detecting harmful gases with a specific gravity lower than that of air, the probe should be installed above the house or equipment; For harmful gases with a specific gravity greater than that of air, the probe should be installed close to the ground. The probe of the detector should be installed facing downwards.

data transmission

Several gas detection alarm devices can be installed in each monitoring area according to the actual situation on site. All gas detection devices must have real-time data display and sound and light alarm functions. The relevant gas concentration values detected by the detectors at each point can be transmitted and communicated to the gas alarm controller in the guard room or control room through the following two methods. When any monitored gas concentration reaches the alarm value, the nearby fan will be automatically started through a relay for air circulation treatment. At the same time, the corresponding channel of the gas alarm controller in the transmission room will also give an alarm prompt.

1: Wired transmission

There are two main ways of wired transmission: 4-20mA and RS485

4-20mA is called analog signal output, also known as linear output, and the wiring method is a split line system.

2: Wireless transmission

There are two common methods of wireless transmission: LORA and 4G transmission

The Lora wireless module has been widely used in the industrial field, and its main advantage is that it is much farther than other series of wireless modules under the same power conditions. The transmission distance is 5 kilometers in an open environment with obstacles. If there are obstacles in the middle, the transmission distance will be attenuated, depending on the site situation. In addition, Lora belongs to low power, has good anti-interference ability, and high sensitivity.

4G wireless transmission is a direct connection mode for data exchange with servers. Gas concentration data is sent to the cloud platform through 4G signals, and finally transmitted to the user's computer or mobile phone for viewing. Mobile phone or SMS alarm prompts can also be set. It is important to ensure that the tunnel network is smooth and stable under the premise of using 4G modules. The advantage is that there is no distance limit, and transmission can be carried out wherever there is a signal

Scheme composition:

Gas detection alarm, gas alarm controller, shielded cable, wiring construction consumables, client gas online monitoring software platform, user computer, PLC and DCS system, etc

Norms or industry standards for implementing the plan:

Design Code for Electrical Installations in Explosive and Fire Hazardous Areas GB 50058-92

Code for Construction and Acceptance of Industrial Automation Instrumentation Engineering GBJ 93-869

Technical Requirements and Test Methods for Combustible Gas Alarm Controllers GB16808-2008

General Technical Conditions for Gas Detection and Alarm Devices in Work Environment GB12358-2006

Electrical Equipment for Explosive Gas Environments Part 1: General Requirements GB3836.1-2010

Electrical Equipment for Explosive Gas Environments Part 2: Explosion proof Type "d" GB3836.2-2010

Technical Regulations for Environmental Testing of Highway Tunnels DB36/T 1709-2022