What do you know about the gas detection？

Ⅰ. Purpose of using gas detectors.

People use detectors to protect the health and life safety of personnel, and to safeguard property and fixed assets from damage. Also it's for comply with regional and national laws and regulations.

Ⅱ. The dangers of each gas are as follows.

 1. Fire or explosion hazards: such as methane, butane, propane, etc.

 2. Poisoning and harmful: such as carbon monoxide, hydrogen sulfide, sulfur dioxide and some volatile organic compounds and so on.

 3. Asphyxiation: lack of oxygen, oxygen is consumed or replaced by other gases.

Ⅲ. An introduction to some common nouns.

 1. Gas — A state of matter where the density and viscosity are extremely low (relative to liquids or solids), and can undergo significant expansion or compression in response to changes in pressure and temperature. It can diffuse with other gases and evenly occupy all spaces within any container. It's often interchangeable with "vapor".

 2. Atmosphere — The sum of all gases, vapors, dust, and smoke within a specific region.

 3. Ambient Air — The air surrounding the installation point of the sensing element.

 4. Flammable Gas, Combustible Gas — Gases that can be ignited and burn rapidly.

5. Toxic and Hazardous Gas — A gas may lead to death, injury, disability, or illness for people.

 6. Asphyxiating Gas — A substance that replaces oxygen and affects normal breathing.

Ⅳ. Common Causes of Failure of Fixed Detectors

Users had a lack of understanding of the detector's performance, or had improper selection of equipment, the user not following the specification requirements for installation, and inadequate maintenance, all of which can lead to failures. The following analysis mainly focuses on the reasons for failures in the use of combustible gas detectors by users, and at the same time, it proposes how to correctly use the combustible gas detector to minimize the occurrence of gas alarm failures.

1. Improper use by users.

Gas alarm users should be cautious when installing gas detectors in proximity to air conditioning and heating equipment. If during the use of these devices, cold or warm airflow directly blows over the combustible gas alarm, it may lead to changes in the alarm's resistivity and result in errors. Therefore, it is advisable to keep the combustible gas alarm away from air conditioning and heating equipment to prevent malfunctions caused by improper placement.

2. Irregularities in the construction process.

Irregularities in the construction process can cause the combustible gas detector to malfunction during use. If the combustible gas detector is not installed in proximity to equipment prone to leaking combustible gases, or if it is installed adjacent to an exhaust fan, the leaked combustible gases cannot diffuse sufficiently to the detector, preventing timely detection of the leakage hazard.

If a combustible gas detector is not grounded, it will not be able to eliminate electromagnetic interference, which will affect the voltage, and faulty detection data may appear. Therefore, the combustible gas detector should be reliably grounded during construction. Combustible gas alarms and terminals are set up in places that are prone to collision or water ingress, which may result in electrical line breaks or short circuits. Welding must use non-corrosive flux; otherwise, the joints may corrode or increase line resistance, affecting normal detection. Do not drop or throw the detector to the ground. Debugging should be carried out after construction to ensure that the combustible gas alarm is in a normal working condition.

3. Maintenance.

A combustible gas detector, which is used to detect the concentration of combustible gases, must be able to communicate with its environment for detection. Therefore, it is unavoidable that a variety of contaminating gases and dust from the environment will enter the detector. The damage caused to the detector by its working conditions is an objective fact, as the working environment of the combustible gas detector is relatively harsh. Many detectors are installed outdoors, and poor maintenance can lead to errors or non-detection in the combustible gas alarms.

Regular cleaning and maintenance of the combustible gas detector is an important task to prevent failures. The grounding should be tested regularly. If the grounding is not up to the standard requirements, or if it is not grounded at all, it will make the combustible gas detector susceptible to electromagnetic interference, resulting in failure.

V. Common causes of inaccurate display values

Issue 1: The gas detector cannot be calibrated.

The possible reasons may be: a bad sensor, a faulty circuit board, incorrect calibration gas, no power, or poor contact. Therefore, based on the cause, you can take the following actions: replace the sensor, replace the circuit board, use the correct calibration gas, turn on the power, or reconnect the wires.

Issue 2: The 4-20mA signal is incorrect.

The reasons may be: a problem with the circuit board, an issue with the instrument, loose or broken wiring, or incorrect wiring. Therefore, according to the cause, you can take the following actions: replace the circuit board, read the instrument manual, connect the wires, and correct the wiring.

Issue 3：No relay switching contact output.

The reasons may be: the circuit board is faulty; the relay is bad; the wiring is loose or broken; the wiring is not correct. Therefore, you can also look for countermeasures according to the causes: replace the circuit board if faulty, replace the relay if bad, ensure the wiring is securely connected, and correct any incorrect wiring.

VI. Installation Location

The locations in the plant that need to be protected are around gas boilers, compressors, pressurised storage tanks, cylinders or pipework. Potential leakage locations include valves, pressure gauges, flanges, T-joints, fill or drain joints, etc. These are the locations where we would consider installing them, and the following possibilities should be considered when determining the specific gas detector placement.

1. For detecting gases lighter than air (e.g., methane and ammonia), the fixed gas detector should be installed at a higher position, and a conical collector should be used.

2. When detecting gases heavier than air (e.g., butane and sulphur dioxide), the detector should be installed at a lower position.

3. Consider the possible behavior of escaping gases under natural and pressurised airflow, and install the detector in a ventilation duct if appropriate.

4. When determining the location of the detector, consider the possible damage caused by natural events (e.g., rain or flooding). For detectors installed outdoors, use weatherproofing measures.

5. If the detector is installed in a hot climate and in direct sunlight, use a detector sunshade.

6. When considering process conditions, note that gases like butane and ammonia are usually heavier than air. However, if released from a hot or pressurised production line, these gases may rise instead of falling.

7. Detectors should be placed slightly away from high-pressure components to prevent aerosol formation. Otherwise, leaking gases are likely to pass through the detector at high velocity without being detected.

8. The ease of function testing and maintenance should be taken into account.

9. The detector should be mounted vertically, with the sensing element facing downwards. This effectively prevents dust or moisture from collecting in front of the detector and allows the gas to enter the detector smoothly.

10. When installing open-circuit infrared devices, ensure that the infrared beam is not obscured or blocked for extended periods. Short-term blocking by vehicles, site staff, birds, etc., is acceptable.

11. Ensure that the open-circuit device is mounted on a stable structure that is not susceptible to vibration.

VII. Advantages and disadvantages of bus wiring system and branch wiring system

The bus wiring system is also known as RS485, while the branch wiring system is also referred to as the 4-20mA model. These two wiring methods each have their corresponding alarm hosts.

Generally speaking, most gas detectors utilizing the bus wiring system employ a 4-core shielded cable, consisting of 2 power lines and 2 signal lines, with a relatively long transmission distance of approximately 1-2Km. On the other hand, gas detectors using the branch wiring system utilize a three-core cable, including 2 power lines and 1 signal line, with the negative power line shared with the signal line. These detectors have a shorter transmission distance, typically within 1Km or less.

Advantages and disadvantages of the bus wiring system and the branch wiring system:

Bus Wiring System Advantages:

Uniform signals ensure a low probability of malfunction. The bus wiring system eliminates any inconvenience associated with data transmission, as it carries data in a consistent format on the data line, thereby enhancing data reliability. Additionally, it boasts simple wiring and a reduced workload. A key advantage of the bus system lies in its minimal wiring requirements, simplicity, and cost-effectiveness. With a four-bus configuration consisting of two signal lines and two power lines, wiring is straightforward and convenient.

Disadvantages of the Bus Wiring System:

Signal delay can occur. Data transmission is sequential, which becomes particularly evident when there are numerous probes. Power supply issues may arise as well. All probes are powered centrally through the host. When the number of probes increases, the host's power supply capacity may become insufficient, necessitating local power supply solutions.

Advantages of the branch wiring system:

Good data synchronization and no limitation of power supply. Compared with the bus wiring system, in the branch wiring system, each gas detector communicates separately with the controller, allowing timely transmission of site situations to the control unit. This enables monitors to make prompt and effective decisions, while peripheral control equipment can respond promptly and effectively to prevent dangerous accidents.

Disadvantages of the branch wiring system:

Complex wiring and significant signal interference are issues. The large amount of wiring results in increased workload, complicated installation, and high material costs.