Interpretation of GB/T50493-2019 Design Code for Detection and Alarm of Flammable and Toxic Gases in Petrochemical Industries

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This article is based on Professor Pei Bing’an from Sinopec Luoyang/Guangzhou Engineering Co., Ltd., one of the main drafters of the GB/T50493-2019 standard, whose analysis of the revisions to the \"Design Code for Detection and Alarm of Flammable and Toxic Gases in Petrochemical Industries\" GB50493-2019. http://yunrun.com.cn/upload/201911/07/201911070112304620.png The standard GB/T50493-2019 \"Design Standards for Detection and Alarm of Flammable and Toxic Gases in Petrochemical Industries\" will come into effect on January 1, 2020; the previous standard \"Design Code for Detection and Alarm of Flammable and Toxic Gases in Petrochemical Industries\" (GB50493-2009) will be repealed at the same time. Main contents of the revision of GB/T50493-2019: 1. The standard title has been changed from “Design Code” to “Design Standard”” ； 2. The range of toxic gases has been expanded from those listed in the Catalogue of Highly Toxic Substances to common highly toxic gases ； 3. Added requirements for the design compatibility, independence, and reliability of the combustible gas and toxic gas detection and alarm system (abbreviated as GDS) ； 4. Added the requirement that the combustible gas and toxic gas detection and alarm system (abbreviated as GDS) should be installed separately from the fire and firefighting monitoring system ； 5. Open-circuit (laser, infrared) detectors and noise detectors have been added, further improving the requirements for the placement and arrangement of detectors ； 6. Five appendices of regulatory standards, including guidelines for selecting common gas detectors and GDS configuration diagrams, have been added ； 7. Some chapters and clauses of the standards were revised and improved, and the mandatory provisions were removed. ◆Original name: GB50493-2009 \"Code for Design of Detection and Alarm Systems for Flammable and Toxic Gases in Petrochemical Industries\". Current name: GB/T 50493-2019 \"Design Standards for Detection and Alarm Systems of Flammable and Toxic Gases in Petrochemical Industries\". Explanation of the provisions: According to the latest national standards, as a regulatory document, all its provisions are mandatory; whereas a recommended standard does not contain any mandatory provisions. ◆Original: GB50493-2009 3.0.6: Combustible gas detectors shall be products that have received certification for manufacturing as measuring instruments, certification for explosion-proof performance, and fire protection certification from **designated agencies or their authorized testing units. ◆Current standard: GB/T50493-2019 3.0.5: Combustible gas detectors must obtain a type approval certificate for measuring instruments from a **designated institution or its authorized testing body, an explosion-proof certification, and a type test report for fire protection products ； The alarm control units involved in fire linkage shall be special combustible gas alarm controllers that have obtained a type test report from the **Fire Electronic Product Quality Supervision and Inspection Center ； **Regulations require that toxic gas detectors obtain a type approval certificate for measuring instruments from a **designated authority or an authorized testing unit. Toxic gas detectors installed in areas prone to explosions must also obtain an explosion-proof certification from a **designated authority or its authorized testing agency. ◆Explanation of the provisions: On December 26, 2017, the Ren administration approved the new measurement law, which came into effect on December 28, 2017. Main changes: Cancel administrative permits for the manufacture and repair of measuring instruments (that is, abolish certification for the manufacture of measuring devices), and strictly enforce permits for type approval of such instruments. Combustible gas detectors no longer require CCCF certification, but when sold, they must come with a type test report for fire protection products (which is mandatory) and a fire protection product certification certificate (obtained on a voluntary basis by the company). ◆Original: GB50493-2009 3.0.9: Detection and alarm systems for flammable gases and toxic gases should be installed separately. ◆Current: GB/T50493-2019 3.0.8: Flammable gas and toxic gas detection and alarm systems shall be installed separately, independent of other systems. ◆Explanation of the provisions: The combustible gas and toxic gas detection and alarm systems should not be integrated into the production process control system; this is to ensure that these detection and alarm systems can continue to function properly even if there are failures or outages in the production process control system of the industrial facility. Document No. 3116 issued by the General Administration of Work Safety in 2014 also stipulates clearly that gas detection and alarm systems for flammable gases and toxic gases should be installed separately. Therefore, when revising this standard, the requirements for the installation of GDS were revised in accordance with the relevant provisions of GB50116 and Document No. 116 issued by the State Administration of Work Safety. ◆Original: GB50493-2009 3.0.11: For combustible gas and toxic gas detection and alarm systems that are fixedly installed on-site in process units and storage and transportation facilities, it is advisable to use uninterruptible power supply (UPS) for power supply. For gas stations, CNG stations, and dispersed or standalone facilities handling toxic, flammable, and explosive substances, their combustible gas and toxic gas detection and alarm systems may be powered by a regular power supply. ◆Current standard: GB/T50493-2019 3.0.9: The power consumption requirements for gas detectors, alarm control units, on-site alarms, etc., in combustible gas and toxic gas detection and alarm systems should be treated as those of particularly important loads within category 1 electrical loads; it is advisable to use UPS power supply devices for power supply. ◆Explanation of the provisions: For decentralized or standalone facilities that handle toxic and flammable/explosive substances, such as gas stations and CNG filling stations, panel-mounted or wall-mounted units are generally used, and their power consumption is low; therefore, it is stipulated that the detection and alarm systems can also be powered by ordinary electrical sources. ◆Original: GB50493-2009 3.0.4: Alarm signals shall be sent to on-site alarms and indicating alarm devices in a manned control room or on-site operating room, with both audible and visual alarms activated. ◆Current standard: GB/T50493-2019 3.0.3: Detection and alarm signals for flammable gases and toxic gases shall be sent to manned on-site control rooms, central control rooms, etc., for display and alarm ； The secondary alarm signals for combustible gases, as well as the fault signals from the alarm control units of the combustible gas and toxic gas detection systems, should be sent to the fire control room. ◆Explanation of provisions: The fire control room must also monitor the secondary alarm signals for combustible gases, as well as any fault signals from the alarm control units of the combustible gas and toxic gas detection and alarm systems. ◆Original: GB50493-2009 3.0.8: Detectors for flammable or toxic gas areas shall be of the fixed type. ◆Current standard: GB/T50493-2019 3.0.6: In areas where combustible gas and toxic gas detectors are required, fixed detectors should be used ； In locations where it is necessary to temporarily detect flammable and toxic gases, portable gas detectors should be provided. ◆Explanation of the provisions: In facilities where it is not possible to install fixed detectors for combustible or toxic gases, mobile detectors for such gases can usually be installed to meet the safety requirements in terms of production and maintenance. For example: Excessively high environmental humidity ； The ambient temperature is too low ； Or areas that are considered non-explosive or non-toxic under normal conditions, but may become explosive or toxic hazard areas during production and maintenance. Due to the limitations of the properties of the products being tested, mobile detectors for flammable or toxic gases can usually be installed to meet the safety requirements of production and maintenance. ◆Original: GB50493-2009 2.0.2: Toxic gases: Refer to gases that can cause acute or chronic health hazards to workers through physical contact during their occupational activities. In these specifications, the range of toxic gases refers to the toxic vapors or gases listed in the \"Catalogue of Highly Toxic Substances\" (Document No. 142 issued by the Health and Supervision Department). Common ones include: nitrogen dioxide, hydrogen sulfide, benzene, hydrogen cyanide, ammonia, chlorine, carbon monoxide, acrylonitrile, vinyl chloride, phosgene (carbonyl chloride), etc. ◆Current standard: GB/T50493-2019 2.0.2: Toxic gases refer to toxic gases or vapors that can cause death or permanent injury to workers during their occupational activities through skin contact or inhalation. ◆Explanation of the provision: Any gas that can cause death or permanent injury through skin contact or inhalation is referred to as a toxic gas. All must comply with these specifications. ◆Original: GB50493-2009 3.0.5: The layout of on-site alarms within the equipment area should be determined comprehensively based on factors such as the area of the equipment area, the arrangement of equipment and buildings, the physicochemical properties of the emission sources, and the characteristics of air flow in the area. For on-site alarms, speakers or alarm lights can be used. ◆Current: GB/T50493-2019 3.0.4: An audible and visual alarm for combustible gases and toxic gases shall be installed in the operation area of the control room ； Field area alarms should be installed based on the area occupied by the equipment, the layout of the devices and structures, the physical and chemical properties of the emission sources, as well as the characteristics of air flow in the area. Field area alarms must have both audible and visual alarm functions. ◆Explanation of provisions: When the number of combustible and/or toxic gas detectors at a site is less than 10, the ambient noise level at the site is below 85 dBA, and the detectors are equipped with integrated audible and visual alarms, it is permissible not to install any area alarms on site, provided that this does not compromise the overall alarm effectiveness. When the ambient noise level at the site exceeds 85 dBA and the integrated sound and light alarms built into the detector are not sufficient to trigger an alert, area alarms need to be installed at the main entrances and exits of the production site as well as in areas with high noise levels, in order to warn the workers on site. ◆Original: GB50493-2009 3.0.10: The installation of portable combustible gas and/or toxic gas detectors shall be determined comprehensively based on the site conditions of the production facility, the flammability and explosiveness characteristics as well as toxicity of the industrial gases, and the number of operators. ◆Current standard: GB/T50493-2019 3.0.7: On-site workers entering explosive gas environments and/or toxic gas environments shall be equipped with portable combustible gas and/or toxic gas detectors. When an environment contains both explosive gases and toxic gases, portable combustible gas and toxic gas detectors can be of the multi-sensor type. ◆Original: GB50493-2009: Gases with a density greater than 0.97 kg/m3 (under standard conditions) are considered heavier than air ； Gases with a density of less than 0.97 kg/m3 (under standard conditions) are considered lighter than air. ◆Current standard: GB/T50493-2019 4.1.2: To determine whether the leaking gas is heavier than air, the ratio of the molecular weight of the leaking gas to that of ambient air should be used as a criterion; if this ratio is ≥1.2, then the leaking gas is heavier than air ； 1.0 ≤ Ratio < 1.2, the leaking medium is slightly heavier than air ； 0.8 < ratio < 1.0; the leaking medium is slightly lighter than air ； If the ratio is ≤0.8, the leaking medium is lighter than air. ◆Explanation of the provisions: Since temperature and altitude have a significant impact on the density of gases, in order to determine whether the gas leaking into the atmosphere is heavier than air, this standard uses the relative ratio of the molecular weight of the leaking gas to that of the local air as a criterion. ◆Original: GB50493-2009 4.4.2: In the online analysis instrument rooms located within Zone 2 of the explosive hazard area, combustible gas detectors shall be installed. ◆Current standard: GB/T50493-2019 4.4.2: In online analysis instrument rooms located in Zone 2 of explosive hazard areas, combustible gas and/or toxic gas detectors shall be installed, along with oxygen detectors as well. ◆Original: GB50493-2009 6.1.1: Gases with a relative density greater than 0.97 kg/m³ (under standard conditions) are considered heavier than air ； A substance with a relative air density less than that of air (under standard conditions) is considered lighter than air. When detecting flammable gases and/or toxic gases that are heavier than air, the recommended installation height for the detectors should be 0.3 m to 0.6 m above the floor level (or floor surface). If it is too low, rainwater dripping and splashing can cause damage to the detector ； If it is too high, it exceeds the altitude at which gases heavier than air can readily accumulate. ◆Currently: GB/T 50493-2019, Clause 6.1.2: When detecting combustible or toxic gases heavier than air, the detector should be installed at a height of 0.3 m to 0.6 m above the floor (or ground) ； When detecting flammable gases or toxic gases that are lighter than air, the installation height of the detector should be within 2 meters above the source of release. When detecting flammable gases or toxic gases that are slightly heavier than air, the installation height of the detector should be between 0.5 m and 1 m below the source of emission ； When detecting flammable or toxic gases that are slightly lighter than air, the detector should be installed at a height of 0.5 m to 1 m above the release source (newly added). ◆Explanation of the provisions: When detecting flammable gases or toxic gases that are lighter than air, the installation height of the detector should be within 2 meters above the source of emission; according to the 09 edition of the standards, it should be between 0.5 and 2 meters above the source of emission. 6.1.3 The installation height of the ambient oxygen detector should be between 1.5 m and 2.0 m above the floor or floor slab. (New) 6.1.4 Linear combustible gas detectors should be installed in large, open areas, and the length of their detection area should not exceed 100 m (new). ◆Original: GB50493-2009 3.0.5: When a limited number of combustible or toxic gas detectors need to be installed on-site, and without compromising the effectiveness of the local alarm system, the local alarms can be integrated with the sensors of these combustible and toxic gas detectors. When a large number of combustible or toxic gas detectors need to be installed on site, the on-site alarms should be installed separately from these detectors. Based on the site conditions, requirements for audible and visual warnings should be specified, with alarms being set up in designated zones. To alert on-site staff, audible alarms with a volume of 105 dBA are often used; in areas with high noise levels as well as at the main entrances and exits of the production area, rotary alarm lights are also typically installed. ◆Currently: GB/T 50493-2019 5.3.2: The alarm signal sound level of area alarms shall be higher than 110 dBA (the 2009 standard specified 105 dBA); however, the overall sound pressure level at a distance of 1 meter from the alarm shall not exceed 120 dBA. ◆Explanation of provisions: The selection of on-site alarms has been revised from the original 105 dBA sound level to 110 dBA. ◆Original: GB50493-2009 3.0.2: The first-level alarm for the detection of flammable and toxic gases is a conventional gas leakage warning alarm, intended to prompt operators to go to the site for inspection. When the concentrations of combustible and toxic gases reach the secondary alarm levels, operators are prompted to take emergency measures. When coordinated protection is required, the output contact signal of the secondary alarm can be used. In the event of a leak of flammable gases and toxic gases at the site, in order to protect the health of the workers on site and enable operators to handle the situation promptly, the handling of the detection and alarm signals for both toxic gases and flammable gases should follow the principle that secondary alarms take precedence over primary alarms ； When they belong to the same alarm level, the principle of giving priority to the alarm level of toxic gases applies. ◆Current: 3.0.2 Detection and alarm systems for flammable gases and toxic gases shall employ a two-stage alarm system. When toxic gases and flammable gases of the same level trigger alarms simultaneously, the alarm level for the toxic gases shall take precedence. ◆Explanation of provisions: ① Level 1 alarm serves as a warning of gas leakage, prompting operators to promptly go to the site for inspection and handling ； ②The secondary alarm is an emergency alarm for gas leaks, indicating to operators that they must take immediate corrective actions ； ③When interlocked protection is required in case of a gas leak, a two-stage alarm contact signal should be used as the condition for such interlocked protection ； ④The alarm built into the field detectors receives first- and second-level alarm signals output by the detectors; the area alarms on-site receive second-level alarm signals output by the GDS system. ◆Current standard: GB/T 50493-2019 3.0.1: In areas of production or use facilities for flammable gases and toxic gases, as well as storage and transportation facilities, gas detectors for flammable gases should be installed when the concentration of such gases in any leaks may reach the alarm set value ； Toxic gas detectors should be installed when the concentration of toxic gases in the leaking gas may reach the alarm set value ； For single-component gas media that are both flammable and toxic, only toxic gas detectors are installed ； In multi-component gas mixtures where flammable gases and toxic gases are present simultaneously, it is possible for both the concentration of the flammable gas and that of the toxic gas to reach their respective alarm setpoints upon a leak. Therefore, separate detectors for flammable gases and toxic gases should be installed. ◆Explanation of provisions: For mixtures containing multiple toxic gas components, or when there are significant differences in the composition of leaked gases under different operating conditions, it is necessary to install separate toxic gas detectors to ensure operational safety, provided that the concentration of each toxic gas component has the potential to reach its respective alarm setpoint. ◆Original: GB50493-2009 4.1.2: Detection points shall be installed at the following main release sources where flammable or toxic gases may leak: 1. Seals of gas compressors and liquid pumps ； 2. Liquid sampling port and gas sampling port ； 3. Liquid discharge (water) port and vent port ； 4. Flanges and valve assemblies for equipment and pipelines ； ◆Current: GB/T50493-2019 4.1.3: Detection points shall be installed around the sources of flammable gases and/or toxic gases: 1. Dynamic seals of gas compressors and liquid pumps ； 2. Liquid sampling port and gas sampling port ； 3. Liquid/gas discharge (water) port and vent port ； 4. Flanges that are frequently disassembled and valve assemblies that are often operated. ◆Explanation of the provisions: In accordance with the Code for Design of Electrical Installations in Explosive Atmospheres GB50058, release sources shall be classified into continuous release sources, first-class release sources, and second-class release sources based on the frequency of substance release and the duration of such release. Primary release source: Periodic or accidental release during normal operation ； Secondary release source: It does not release under normal conditions; if it does, it is only on an occasional basis for a short period of time. The main targets detected by combustible gas and toxic gas detectors are devices or locations that belong to category 2 release sources. ◆Current standard: GBT 50493-2019 4.3.1 Detectors shall be installed within the fire dike of liquid storage tanks that produce flammable gases, such as liquefied hydrocarbons, Class B methanol, and Class A ethyl alcohol liquids. The horizontal distance between a combustible gas detector and any source of release within its coverage area should not exceed 10 m, while the horizontal distance between a toxic gas detector and any source of release within its coverage area should not exceed 4 m. ◆Note: According to 7.2.1.1 of AQ3036-2010 \"Regulations on the Installation of On-site Safety Monitoring Equipment in Tank Areas for Major Hazardous Sources of Hazardous Chemicals\", in areas where flammable gases or liquids are stored, a flammable gas alarm should be installed every 20–30 meters within the fire dike. Moreover, the distance between the monitoring alarm and locations such as the drain outlets, connections, and valves of the storage tanks, where materials may leak out, should not exceed 15 meters. ◆Current: GBT 50493-2019 3.0.10 When determining the occupational exposure limits for toxic gases, the highest permissible concentration, the time-weighted average permissible concentration, and the short-term exposure permissible concentration should be considered in that order of priority. ◆Explanation of the provisions: The three occupational exposure limits (OELs) for toxic gases, in order from lowest to highest, are: the Maximum Allowable Concentration (MAC), the Time-Weighted Average Allowable Concentration (PC-TWA – 8 hours per day, 5 days per week), and the Short-Term Exposure Limit (PC-STEL – 15 minutes). Based on the current manufacturing standards for toxic gas detectors both domestically and internationally, if no suitable detectors are available on the MAC market, and there are multiple toxic gases with different occupational exposure limits, these detectors should be selected in the order of MAC, PC-TWA, and PC-STEL, all while ensuring the health and safety of operators ； For toxic gases for which no OEL value is provided, the Immediately Dangerous to Life and Health concentration IDLH can be used as a reference. ◆Currently, according to GBT 50493-2019 5.4.3, when combustible gas detectors are used in fire protection systems, the detector signals must first be sent to a dedicated combustible gas alarm controller that has obtained a type inspection report from a **Fire Electronic Product Quality Supervision and Inspection Center. The alarm signals should then be transmitted by this dedicated combustible gas alarm controller to the fire alarm controller in the fire control room. The combustible gas alarm signals and fire alarm signals should be clearly distinguished in a fire alarm control system. In accordance with the relevant provisions and requirements of GB50493, GB50116, and Document No. 3116 issued by the General Administration of Work Safety, the combustible gas and toxic gas detection and alarm system (GDS) shall be designed in accordance with the following principles: 1. The GDS system shall consist of combustible gas or toxic gas detectors, on-site area alarms, and an indoor alarm control unit. Gas detectors installed on-site should be equipped with integrated audible and visual alarms; combustible gas detectors can also be fitted with integrated audible and visual alarms. 2. The alarm control unit shall be an independently installed microprocessor-based electronic device (including separately installed PLCs, dedicated gas alarm controllers, DCS controllers, etc.). 3. When the alarm control unit issues a secondary alarm signal, it shall activate the area alarms installed in the corresponding alarm zones on site. 4. The secondary alarm signals for combustible gases and the fault signals from the alarm control unit should be sent to the fire control room for graphical display and alarm activation. A separate monitor can be set up. 5. When combustible gas detectors are used in fire protection interconnection systems, the detector signals should first be sent to a dedicated combustible gas alarm controller that has obtained a type inspection report from the **Fire Electronic Product Quality Supervision and Inspection Center. The fire protection interconnection signals are then sent by this alarm controller to the fire alarm controller in the fire control room, which is responsible for carrying out the fire protection interconnection functions. The signal from a combustible gas detector cannot be connected directly to the input circuit of a fire alarm controller. 6. When the detection signals of flammable gases or toxic gases are used as inputs for a Safety Instrumented System (SIS), the detectors shall be installed independently. The configuration of these detectors shall be determined based on the SIL rating of the relevant circuit, and it must comply with the provisions specified in the \"Code for Design of Safety Instrumented Systems in Petrochemical Industries\" GB/T 50770. ◆Note: When the detector signal is used for warning alarms, the GDS alarm control unit can utilize a separately designed microprocessor-based electronic device; there is no need for SIL certification nor for a type test report for fire protection products ； When the detector signals are used for fire protection interconnection, the GDS alarm control unit shall be a dedicated combustible gas alarm controller that has obtained a type inspection report from the Fire Electronic Product Quality Supervision and Inspection Center ； When detector signals are used for safety interlocks, in accordance with the relevant provisions of the \"Design Code for Safety Instrumented Systems in Petrochemical Industries\" GB/T 50770, interlock signals with a safety level of SIL1 and below can be connected to the GDS, while interlock signals with a safety level of SIL2 and above should be connected to the SIS. 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