Gas warning system
A gas warning system is a warning device for protecting people and property from dangerous gases and gas-air mixtures.
Function and structure
The gas warning system consists of a gas detector, gas warning center and alarm devices, which are placed, configured and adjusted according to the requirements of a gas warning system. The gas warning center processes the signals from the gas detectors. The gas warning center controls external alarm devices via potential-free contacts or analog and digital signals or triggers countermeasures independent of humans (e.g. triggering ventilation or switching off electrical equipment).
In contrast to portable gas warning devices , a gas warning system is permanently installed and permanently monitors dangerous gas concentrations. Monitoring tasks can thus be carried out without endangering or even harming people. With the help of gas warning systems, flammable, toxic gases and a lack of oxygen can be monitored separately or in combination. In most cases, these gases are monitored in the measuring areas where gases form dangerous concentrations. In the case of flammable gases, monitoring is carried out in the area of the lower explosion limit (LEL) , in the case of toxic gases in the ppm range and in the case of a lack of oxygen in the volume percentage range.
The gas warning system is also referred to as a gas warning device. This designation can be found e.g. B. in the leaflets of the former trade association for chemistry, today trade association raw materials and chemical industry . However, the term gas warning system is widely used in everyday language. For this reason the gas warning system is used here. In colloquial terms, adjustment is usually referred to as calibration . The following text therefore also speaks of calibration or calibration.
Possible uses
The use of gas warning systems is not limited to specific applications or branches of industry , but takes place across all branches wherever dangerous gases occur and create potential hazards. On a larger scale, it is used in the chemical and petrochemical industry, the exploration of oil and gas, but also by energy and gas suppliers. Gas warning systems are also used in the automotive industry, the printing industry, in hazardous materials storage, in the transport sector, in wastewater management and in many areas of the processing industry. Gas warning systems can also be found in underground car parks , laboratories or even in swimming pools and ice rinks. There are still two basic types of application. Domestic, residential, and commercial industrial sectors. No approval by a ZÜS is necessary for gas warning systems in the private sector. It is different in the industrial or commercial sectors. In addition to industry and trade, this also includes landlords or operators of underground garages (municipalities and municipalities). Agricultural businesses are also part of the commercial sector. In these commercial areas, the installation of EX protection gas warning systems and protection systems is subject to EU Directive 2014/34. This was implemented in German law with the 11th ProdSV and has been binding since April 20, 2016. Old certificates and approvals according to RL 94/9 / EG, which were issued before April 20, 2016, remain valid. All other standards or laws contradicting this regulation are invalid on the day the 11th ProdSV comes into force. Gas warning systems for EX protection are systems that are subject to inspection in Germany by the BetrSichV. Protection of old systems remains.
Gas detectors
The gas concentrations are recorded by gas detectors and forwarded to the gas warning center. Various measuring principles are used for this. The selection of the measuring principle and thus the corresponding gas warning detector is based on the type of gas to be monitored, the expected concentration and the ambient conditions at the place of use. The gas detector determines the gas concentration and converts this information into electrical signals (voltage signals, current signals). These signals are transmitted to the gas warning center via cables. For the use of a gas detector in a gas warning system, the selection of the correct measuring principle is of fundamental importance.
Measurement principles of gas detectors
Catalytic gas detectors (catalytic detectors)
- Application: Monitoring of flammable gases and vapors
- Measuring range: 0–100% LEL or in special versions 0–10% LEL
- Selectivity: none; all flammable gases and vapors are recorded simultaneously. The signal level is different.
The measuring principle of catalytic combustion is implemented with the help of a Wheatstone measuring bridge . One branch of the bridge is located in the evaluation unit and the other branch of the bridge consists of sensors. These sensors, also known as pellistors, are wound from fine platinum wire and embedded in a carrier material. One sensor is catalytically active. Catalytic combustion takes place on its surface when it comes into contact with a combustible gas. When the gas burns on the surface of the sensor, an amount of heat is generated that increases the temperature of the sensor and thus changes the resistance of the platinum wire . The "Wheatstone Bridge" is detuned by the change in resistance. A voltage can be measured between the bridge arms, which is proportional to the gas concentration. The second sensor is catalytically inactive. No combustion takes place here. The task of this sensor is to largely eliminate disturbance variables such as temperature, air pressure and humidity.
|
Advantages:
|
Disadvantage:
|
Semiconductor gas detectors
- Application: Monitoring of flammable and toxic gases and vapors
- Measuring range: 0–50% LEL when monitoring flammable gases and vapors. For toxic gases, the typical monitoring range is 0–1000 ppm.
- Selectivity: none; all oxidizable gases and vapors are recorded simultaneously. The signal level is very different.
As a semiconductor material, for. B. tin dioxide is used. When oxidizable gases are adsorbed on the sensor surface, the internal resistance of the sensor is reduced. This change in resistance is evaluated. The heating of the sensor increases the reaction speed of the gases on its surface.
|
Advantages.
|
Disadvantage:
|
Infrared gas detectors
- Application: Monitoring of flammable gases and vapors
- Measuring range: 0–100% LEL
- Selectivity: dependent on substance group; Sensitivity is limited to certain types of gas, cross-sensitivities only exist within these groups.
The infrared measuring method is a gas measuring method for all heteroatomic gases. These gases, which consist of different types of atoms, absorb infrared light in different bands. This infrared radiation crosses a measuring section with sample gas. The sample gas weakens the radiation energy through absorption. This is not directly proportional to the gas concentration. A wavelength with typical measuring behavior for hydrocarbons is used for the measurement. A second wavelength, without absorption behavior for hydrocarbons , is used as a reference. The gas concentration is calculated by a microprocessor.
|
Advantages:
|
Disadvantage:
|
Electrochemical gas detectors
- Application: Monitoring of toxic gases and vapors as well as oxygen deficiency or excess
- Measuring range: lower ppm range ( parts per million )
- Selectivity: high; Sensitivity is limited to certain types of gas, but there are cross-sensitivities.
Electrochemical detectors work with electrochemical cells ( EC cells ) as sensors. EC cells are similar to a galvanic element with 2 or 3 electrodes made of different metals and an electrolyte solution. The change in the ion flow in the electrolyte is recorded when gas enters the interior of the EC cell through a gas-permeable membrane . This is directly proportional to the gas concentration. The gas concentration is calculated by a microprocessor.
|
Advantages:
|
Disadvantage:
|
Versions of gas detectors
Passive gas detectors
Passive gas detectors can only be operated with a specific gas warning center. As a rule, this is a gas detection center from the same manufacturer. A passive gas detector cannot operate without this gas warning center. Operation with a third-party gas warning center is not possible in most cases.
Active gas detectors
Active gas detectors are also known as gas transmitters. These detectors already emit a standard signal. In most cases it will be a standard 4-20 mA signal. Gas transmitters are closed systems and can be used separately. Operation with a third-party gas warning center is possible without any problems if this gas warning center has a 4–20 mA input module.
Gas warning centers
The signals from the gas detectors are received and processed in the gas warning center. The gas warning center can transmit signals directly to alarm devices via potential-free contacts. Analog or digital signals can also be passed on to external systems. (e.g. fan control). With the help of defined interfaces, signals can be passed on to higher-level systems (e.g. process control system).
Versions of gas warning systems
Compact gas warning centers
This type of center is used in smaller gas warning systems. There are control centers for the operation of one gas detector, but also for the operation of 8 gas detectors. These systems are preferred when no control cabinet is available and signal processing has to be implemented on site. As a rule, all of the required components are housed in a compact housing made of plastic or metal. Compact gas warning systems are very user-friendly and offer a good price-performance ratio. However, it is disadvantageous that the system cannot be expanded or retrofitted at all, or only to a limited extent.
Gas warning centers in modular design with duct inserts
This technology is the classic form of gas warning centers. One or more gas detectors are operated from one channel insert each. All components required for operation are integrated in the sewer card. The plug-in units work independently of one another and are housed in a subrack. It is often possible to link the functions of the individual cards. This system is very well suited for installation in an existing control cabinet . These systems are economical for the operation of max. 15 or 20 gas detectors. Due to the modular structure with separate channel cards, the system is only flexible to a limited extent. This is disadvantageous in programming, maintenance and control cabinet construction.
Digital gas warning centers
Digital gas warning centers are well suited for use in larger gas warning systems. Several hundred gas detectors can be processed in the maximum version. The structure is flexible. In contrast to modular systems, channel cards do not have to be set according to the number of gas detectors. Only the modules required for operation need to be integrated into the system. These modules are networked with each other and are operated by a central processor unit. This flexible structure enables a good price-performance ratio for larger gas warning systems. The flexible structure is also an advantage for programming, maintenance and control cabinet construction. Extensions and changes in the configuration are often possible without any problems and with relatively little effort. Digital gas warning centers are not suitable for use in smaller gas warning systems, as the control center must have a basic structure to operate. It makes economic sense to use around 15 to 20 gas warning detectors.
Alarm means
In most cases, alarm devices are part of gas warning systems. In the event of an alarm, these alarm devices are activated and warn personnel of a risk from dangerous gas concentrations. This alarm can take place optically, acoustically or in combination of these two signals.
Withdrawal systems
For special applications, extraction systems can also be part of a gas warning system. If the conditions at the measuring location are unsuitable for a gas detector, an extraction system can be used to extract the gas and, associated therewith, to process the measuring gas accordingly. For example, this can apply to measuring locations with high temperatures. In addition to suction, the sample gas is then also cooled in the extraction system. The cooling must then take place up to the temperature level at which the gas detector can work.
Criteria for planning and using gas warning systems
- Selection of gas detectors
- Selection of calibration (type of gas to be monitored)
- Number of gas detectors
- Positioning the gas detectors
- Definition of the alarm thresholds
- Alarm evaluation and forwarding
- Triggering of subordinate measures
- Create a safety concept, risk assessment
- Prepare maintenance and inspection plan for compliance with legal requirements and add to the documentation of the gas warning system.
- EX protection systems fall under the Explosion Protection Product Regulation, 11 ProdSV (EU 2014/34)
- Depending on the type of protection system, an approved monitoring body must carry out an initial acceptance. (TÜV, DEKRA, or other approved monitoring body)
- CE declaration of conformity for minimum standards must be requested or drawn up. The evaluation procedure is based on the 11th ProdSV, Annex III to IX.