The most common method applied for continuous detection of hazardous gas leaks is to place a number of sensors where leaks are most likely to occur. These sensors are then connected via cable to a multi-channel controller located at a safe distance from the area and in a gas-free area, which has a display with alarm devices, event recording devices, etc. People often refer to this system as a fixed-point system. This is true that the system is indeed permanently fixed in an area (e.g. offshore rigs, oil refineries, laboratory cold storage, etc.).
The complexity of all gas detection systems depends on how they use data. Data logging allows information to be used to identify problem areas and help implement security measures. If the system is only intended to be used for warnings, the system output can be simple and there is no need for data storage. Therefore, when choosing a system, it is important to know what the information is used for so that the appropriate system components can be selected. In toxic gas monitoring, multi-point systems have quickly demonstrated their potential to address a wide range of workplace exposures, and their invaluable value in identifying problems to keep workers and management agencies informed of workplace pollutant concentrations.
In the design of multipoint systems, considerable consideration must be given to the various components and their interconnections. For example, when using a combustion detection sensor, the cable connection to the sensor should be three-core, each core has a cross-sectional area of 1 square millimeter, which not only transmits the output signal, but also transmits the power to the bridge circuit, where the bridge circuit is located on the sensor and acts to reduce the signal voltage drop along the cable. In toxic (some flammable) gas monitoring systems, air is often sampled away from the components and then pumped into the sensor (through some composite materials and narrow inner diameter ducts). Attention should be paid to the selection of appropriately sized pumps and pipes, the selection of a range of sampling components for sequentially sampling from each pipe, and the selection of filters to stop particulates or cut off the sample airflow with water.
The size of the inner diameter of the pipe is critical because the inner diameter must be large enough to achieve a fast response time with a standard-sized pump, but not too large to allow excess air to enter and cause sample dilution. Each sampling point must be connected to a separate pipe, and if there are several sampling points connected to a central sensor, it must be cleaned with clean air between samples. The controllers used in fixed systems can be located in the center of the plant or distributed throughout the plant, depending on the application requirements. The controller has a control panel with a single channel (i.e., a control card per sensor) or a multi-channel configuration, which is effective when power, space, or cost must be limited.
The gas concentration is indicated on each sensor, and under normal circumstances, an internal relay is used to control functions such as alarm, fault and shutdown. The number of effective alarm levels varies depending on the controller, but generally 3 levels can be set, depending on the statutory requirements or work specifications of the industry. Other useful features such as alarm suppression and reset, over-measuring range indication, and analog 4-20mA outputs are also included. Often, there is also a digital output function to connect the controller to the DCS/BMS. It is important to remember that the main purpose of a gas detection system is to detect the accumulated gas concentration when it reaches a dangerous level and to perform a mitigation operation to prevent the danger from occurring. If the gas concentration continues to approach the danger level, the execution program is turned off and the alarm is activated. It is not enough to record events or measure the levels of gases people are exposed to.
Cables and junction boxes
In a typical industrial gas detection system (as just described), the sensors are located at a number of key points around the plant, and the distance between the sensors and the controller is different. When installing an electrical connection to the controller, it is important to remember that the electrical circuit resistance of each sensor varies depending on the length of the cable. When performing calibration procedures for detectors of constant voltage types, a person is required both at the sensor site and at the controller. When calibrating a detector of a constant current type or using a local transmitter, the calibration of field equipment can be performed separately from the calibration of the control. The way to protect the sensor cable from external damage is to pass it through
A metal pipe can also be used with appropriate mechanical protection cables. Each cable termination should be equipped with a protective gasket and the sensor should be mounted on a junction box to assist in the completion of simple, low-resistance "clean" terminations. It is also important to ensure that all gasket sizes and threads match the outer diameter of the junction box and cables used. The correct gasket should be used to ensure that the connection between the detector and the junction box is climate-free. It is also important to remember that sensor manufacturers usually indicate the maximum loop resistance (not the line resistance) of their sensor connection when providing information to calculate the diameter of the cable core for installation.
Positioning of the sensor
The two most common, but probably the most difficult questions to answer about gas detection systems, are "How many detectors do I need?" And "Where should I put them?" "Unlike other safety-related types of detectors, such as smoke detectors, the location and number of detectors required for different applications are not clearly defined.
Standards such as EN50073 guidelines provide considerable guidance on the selection, installation, use, and maintenance of flammable gas or oxygen detection and measurement equipment. Similar guidelines, such as the National Electrical Code (NEC) or international regulations in the Canadian Electrical Standard (CEC), may be used where applicable. In addition, specific regulatory bodies have published specifications that specify minimum gas detection requirements for specific applications. These references are very useful, but they are either too generic and therefore too broad in terms of detail, or they are only for specific applications and therefore irrelevant to most applications.
Detector replacement should be determined according to the recommendations of experts with specialized knowledge of gas diffusion, experts with relevant operating device systems and equipment, safety and engineering technicians. Agreements on detector positioning should also be documented. The detector should be installed where gas is most likely to be present. The most protected areas in the plant should be around gas water heaters, compressors, pressurized storage tanks, cylinders or pipes. The areas most likely to leak are valves, gauges, mounting flanges, T-connections, oil fill pipes, or drain pipes, etc.
When determining the detector location, there are a few simple and very obvious references that may help you:
• When detecting gases are lighter than air (e.g. methane and ammonia), the detector should be mounted at a higher level, preferably with a collection cone.
• When the detection gas is heavier than air (e.g. butane and sulfur dioxide), the detector should be installed at a lower level.
• Consider how the leaking gas will behave when encountering a natural or pressurized airflow. If appropriate, install the detector in the ventilation pipe.
• When locating detectors, consider possible damage caused by natural events such as rain or flooding. For detectors installed outdoors, it is best to use weather impact protection components.
• If the detector is installed in a hotter environment and in direct sunlight, use a detector parasol.
• Consider operating conditions. For example, butane and ammonia are usually heavier than air, but if they leak from the line under high temperature and/or negative pressure, these gases will rise rather than fall.
• The detector should be mounted some distance behind the high-pressure component to allow gas clouds to form. Otherwise, all leaking gases pass in the form of high-speed jets, so that they are impossible to detect.
• Consider easy entry procedures for functional testing and repair.
• The detector should be installed in the designated location and pointing downward. This ensures that dust and water do not accumulate on the sensor's face, which can prevent gases from entering the detector.
• When locating an open-circuit IR device, it is important to ensure that there is no permanent occlusion or obstruction on the IR beam. Short-term obstacles such as transportation, construction site workers and birds can be adjusted.
• Ensure that the building where the open circuit equipment is located is strong and not prone to vibration.