Engineering Application of Combustible and Toxic Gas Detection and Alarm

Petrochemical equipment is flammable, explosive, toxic and harmful. It is very important for places involving the leakage of flammable and toxic and harmful gases to set up detection and alarm devices in accordance with national standards. One of the major safety hazard determination items proposed in the "Standards for the Determination of Major Production Safety Accidents in Hazardous Chemical Production and Operation Units (Trial)".

According to the requirements of the current national standard GB/T 50493-2019 "Design Standard for Detection and Alarm of Combustible and Toxic Gases in Petrochemical Industry", combined with other relevant documents, standards and engineering practices, this article explains the engineering of combustible and toxic gas detection and alarm in petrochemical industry application.

1. Applicability of GB/T 50493-2019

GB/T 50493-2019 is suitable for the design of combustible gas and toxic gas detection and alarm systems (GDS) in new and expansion projects in petrochemical industries. The GDS design of petrochemical reconstruction projects, chemical installations, factories or installations that use coal as raw material to produce fuels and chemical products, and factory analytical laboratory areas can be used as a reference.

The definitions of new construction, expansion and reconstruction can be referred to the "Measures for the Safety Supervision and Administration of Hazardous Chemical Construction Projects" promulgated by Order No. 45 of the State Administration of Work Safety on January 30, 2012 (Amended No. 79 on May 27, 2015).

Although GB/T 50493-2019 is a recommended standard, since it is cited by many documents such as Emergency [2019] No. 78 "Guidelines for the Investigation and Management of Safety Risks in Hazardous Chemical Enterprises", it is recommended to meet the scope of application of GB/T 50493-2019 The project strictly implements the requirements of GB/T 50493-2019 in accordance with the degree and specific terms given in the standard.

2. Combustible gas range

GB/T 50493-2019 and GB 50160-2008 (2018 Edition) "Petrochemical Enterprise Design Fire Protection Standards" focus on the range of flammable gases that require detectors to be set up in accordance with the standard requirements: Class A gases or Class A and B flammable liquid gases Combustible gas or vapor formed after chemical reaction.

The fire hazard classification of flammable gases is classified according to Table 1.

3. Toxic gas range

In actual projects, the applicable standards and specifications can be determined according to the project conditions and characteristics, and should comply with the requirements of laws and regulations. It is recommended to use several of the following ranges as the basis for determining the range of toxic gases:

1) 14 toxic gases and vapors listed in Appendix B of GB/T 50493-2019;

2) The gases and vapors listed in the "Catalogue of Highly Toxic Substances" (Wei Fa Jian Fa [2003] No. 142), a total of 30 media, see GB 50493-2009 Clause 2.0.2;

3) The toxic gases or vapors in the 358 media listed in Table 1 of GBZ 2.1-2019 are classified as acute toxic gases or vapors of Category 1 and Category 2 according to GB30000.18-2013;

4) The toxic gases or vapors in the "Catalogue of Hazardous Chemicals (2015 Edition)" (State Administration of Work Safety Announcement No. 5 of 2015) are classified as acute toxic hazard categories 1 and 2 in accordance with GB30000.18-2013. Toxic gases or vapors;

5) 56 toxic gases/vapors listed in "Table A.1" in Appendix A of GBZ/T 223-2009 "Specifications for Setting Up Toxic Gas Detection and Alarm Devices in Workplaces";

6) In the "List of Hazardous Chemicals under Key Supervision" issued by the Work Safety Supervisor No. 3 [2011] No. 95 and the "Second Batch of Key Supervised Hazardous Chemicals List" issued by the Work Safety Supervisor No. 3 [2013] No. 12, according to the key supervision Where hazardous chemical safety measures and accident emergency response principles require the installation of toxic gas leakage detection alarms;

7) Other current and applicable requirements of relevant laws, regulations, standards and specifications.

For petrochemical projects, item 5 in the above listed range is optional, and all other items are mandatory and should be strictly implemented. Item 5, GBZ/T223-2009, was formulated in accordance with the "Law of the People's Republic of China on the Prevention and Control of Occupational Diseases" and proposed by the Occupational Health Standards Professional Committee of the Ministry of Health. Although it is a recommended standard, when conditions are met, the range of toxic gases listed in item 5 should be considered.

4. Detector setting principles

In the areas of production facilities and storage and transportation facilities that produce or use flammable gases and toxic gases, detectors should be set up according to the following principles:

1) When the concentration of combustible gas in the leaked gas may reach the alarm setting value, a combustible gas detector should be installed;

2) When the concentration of toxic gases in the leaked gas may reach the alarm setting value, toxic gas detectors should be installed;

3) Single-component gas media that are both flammable and toxic gases should be equipped with toxic gas detectors;

4) For multi-component mixed gases where combustible gas and toxic gas exist at the same time, the combustible gas concentration and the toxic gas concentration may reach the alarm setting value at the same time during leakage. Combustible gas detectors and toxic gas detectors should be installed respectively.

5. Judgment of lighter and heavier than air

To determine whether the leaked gas medium is heavier than air, the ratio of the molecular weight of the leaked gas medium to the molecular weight of ambient air should be used as the benchmark, and the judgment should be based on the principles given in Table 3.

 Table 3 Principles for judging severity

6. Determination of detector installation location

1) Basic requirements

Determine the detector installation location based on comprehensive consideration of the following aspects:

a) Release source location

Dynamic seals for gas compressors and liquid pumps;

Liquid sampling port and gas sampling port;

Liquid (gas) drain (water) port and vent port;

Frequently disassembled flanges and frequently operated valve groups.

b) Operation inspection route (refer to clause 4.1.1 of the main text of GB/T 50493-2019)

c) Peripheral boundary area (refer to GB/T 50493-2019 text 4.1.5 and clause 6.1.2)

2) Specific installation location

Clauses 4.2 to 4.4 of the main body of GB/T 50493-2019 stipulate the installation position of the detector in the horizontal direction relative to the release source. Clause 6.1 of the main text stipulates the installation position of the detector in the vertical direction relative to the release source. Clause 6.1.2 is explained in the text. Supplementary instructions are given that in some scenarios it is necessary to reduce the horizontal and vertical distance of the detector from the release source. Other relevant requirements of other provisions of GB/T 50493-2019 should also be followed. An example of detector installation requirements is shown in Table 4.

Table 4 Examples of detector installation engineering applications

7. Oxygen detector

Changes in oxygen concentration in the air will affect the normal operation of some gas detectors (combustible gas detectors with oxygen detection, such as catalytic combustion type), affect the respiratory health of operators, change the explosion limit of combustible gases in the environment, and ignite combustible gases able. The environmental under-oxygen and over-oxygen alarm setting values stipulated in GB/T 50493-2019 (see Table 5) are to protect the normal health of operators. The setting of oxygen over-oxygen alarm value and under-oxygen alarm value can only set one level alarm. Each enterprise can set the secondary overoxygen alarm value and secondary underoxygen alarm value of oxygen according to the needs of production safety and occupational health work.

During the production process, the ambient oxygen concentration may change, and if there is a lack of oxygen or over-oxygen, and people enter the active place, oxygen detectors should be installed. When the relevant gas release source is a combustible gas or toxic gas release source, the oxygen detector can be arranged together with the relevant combustible gas detector and toxic gas detector.

Places where the ambient oxygen concentration may be deficient during the production process are mainly confined space operations. During the production process, the ambient oxygen concentration may be excessive, mainly in the factory's nitrogen and oxygen station and the working conditions of local oxygen points.

The online analysis instrument room located within Zone 2 of the explosion hazard area should be equipped with flammable gas and/or toxic gas detectors, as well as oxygen detectors.

The installation height of the ambient oxygen detector should be 1.5m to 2.0m from the floor or floor, mainly based on the height range of the operation and maintenance personnel.

8. Measuring range and alarm value

The measurement range and alarm value of combustible gas, toxic gas and ambient oxygen detectors should comply with the requirements of Table 5.

           Table 5 Detector measurement range and alarm value requirements

Note 1: For some toxic gases, such as acrylonitrile vapor, it is difficult to measure under the conditions of 0~300% OEL due to the limitations of instrument manufacturing technology. In this case, the measurement range of toxic gases can be 0~300% OEL. 30% IDLH.

Note 2: The specific numerical value of the ambient oxygen alarm concentration should be determined according to the environment where the project is located. For example, in plateau areas, at an altitude of 1610m, the normal oxygen content in the atmosphere is about 17.3% VOL, and its hypoxic and superoxic atmospheric environments The oxygen content should be adjusted appropriately.

There are usually three occupational exposure limits (OEL) for toxic gases. From low to high, they are: maximum allowable concentration (MAC), time-weighted average allowable concentration (PC-TWA) and short-term exposure allowable concentration (PC-STEL). ). When determining the measurement range of toxic gas detectors, give priority to the OEL value with a lower value, that is, the selection order is: MAC, PC-TWA and PC-STEL. Only when a suitable toxicity detector product cannot be selected according to the measuring range of 0 to 300% OEL, the measurement range is allowed to be relaxed to 0 to 30% IDLH.

9. Setting of on-site alarm

There are three types of on-site alarms: on-site area alarms; toxic gas detector integrated sound and light alarm; combustible gas detector integrated sound and light alarm. The setting principles are shown in Table 6.

Table 6 Principles for setting up on-site alarms

Combustible gas and toxic gas detection and alarm systems should be divided into alarm zones according to the devices or units of production facilities and storage and transportation facilities, and each alarm zone should be equipped with on-site regional alarms. The number of alarms in the on-site area should be such that the alarm can be perceived by on-site personnel anywhere in the area.

On-site area alarms can be set reasonably according to the situation, and can be classified appropriately when necessary, for example: area one is flammable, area one is toxic; area two is flammable, area two is toxic;...

Depending on the circumstances, toxic and flammable site area alarms may also be combined in smaller quantities or in some other cases.

On-site area alarms should be set up in the following three occasions and locations:

1) Set up on-site area alarms at the main entrances and exits of the production site;

2) Set up on-site area alarms in high-noise areas [noise exceeding 85dB(A)];

3) If someone enters relatively closed places such as compressor workshops, pump rooms, drum (material) warehouses, analysis cabins, and analytical laboratories where flammable or toxic gases may accumulate for inspection operations, set them at eye-catching locations such as their entrances and exits. Sound and light alarm.

In addition to the above three occasions and locations where on-site area alarms need to be set, whether further on-site area alarms need to be set up in a certain area should be determined based on the actual situation on site. When the following three conditions are met at the same time, there is no need to set up on-site area alarms in the area. Area siren:

1) The number of detectors in the alarm zone is less than 10;

2) On-site noise is lower than 85dBA;

3) The on-site detector is equipped with integrated sound and light alarm.

As long as one of the above three conditions is not met, further on-site regional alarms need to be set up inside the area.

10. System requirements and configuration diagram

The flammable gas and toxic gas detection and alarm system (GDS) should be set up independently from other systems and should be set up separately from the fire and fire monitoring system. This is an important revision in the revision and upgrade of GB 50493-2009 to the 2019 version of the standard. The requirements of the 2009 version of the standard are: GDS should be set up independently, and it is clear that the I/O card is allowed to be independent and the shared controller method uses a shared system of GDS and DCS. ; Allow GDS to be combined with the fire detection and alarm system, and it is proposed in the explanation that GDS for large joint installations, regional control centers and the central control room of the whole plant can be given priority to be combined with the fire detection and alarm system. GB/T 50493-2019 clearly requires that GDS should be independent, including independence from DCS and other systems, as well as independence from fire monitoring systems.

The GDS configuration diagram is shown in Figure 1.

The configuration diagram shown in Figure 1 includes three parts: the standard GDS system, the fire linkage GDS system and the relevant parts of the SIS system, which are explained as follows:

As part of the standard GDS system, the alarm control unit should use independently configured microprocessor-based electronic products and should meet the requirements of clause 5.4.1 in GB/T 50493-2019.

For the fire linkage GDS system, the alarm control unit participating in the fire linkage should use a special combustible gas alarm controller that is manufactured according to the special combustible gas alarm controller product standards and has obtained a test report; a special combustible gas alarm controller refers to one that complies with current national standards. Products that meet the quality requirements of "Combustible Gas Alarm Controller" GB 16808-2008 and have a fire product type inspection report. When the combustible gas detector participates in fire linkage, the detector signal should first be sent to a special combustible gas alarm controller that is manufactured according to the special combustible gas alarm controller product standards and has obtained a test report. The alarm signal should be output by the special combustible gas alarm controller to Fire alarm controller in the fire control room. There should be a clear distinction between combustible gas alarm signals and fire alarm signals in the fire alarm control system.

In the relevant parts of the SIS system, when the flammable gas or toxic gas detection signal is used as the input of the safety instrument system, the detector should be set independently, the detector output signal should be sent to the corresponding safety instrument system, and the hardware configuration of the detector should comply with the current national standard GB /T 50770-2013 "Petrochemical Safety Instrument System Design Specifications" relevant provisions. Conventional flammable gas and toxic gas detection and alarm systems, including standard GDS systems and fire-fighting linkage GDS systems, do not have safety instrument functional requirements and do not need to comply with functional safety standards.

11. Control indoor sound and light alarms

Sound and light alarms for flammable and toxic gases should be set up in the control room operating area to facilitate timely detection and action by operators in the control room. The sound pressure level should be no less than 75dBA 1m in front of the equipment. The sound and light alarms should be activated. The signal should use the second-level alarm set value signal, and the operating station loudspeaker should not be used to replace the sound and light alarm in the control room.

12. Detector selection

The output of the detector can be 4mA ~ 20mA DC signal, digital signal, contact signal. It is not recommended to use bus signal. The selection of combustible gas and toxic gas detectors should be based on the technical performance of the detector, the physical and chemical properties of the gas being measured, the component types and detection accuracy requirements of the medium being measured, the compatibility of the detector material with the on-site environment, and the production environment. Characteristics, etc. are determined. For specific requirements, please see the requirements in Chapter 5 of GB/T 50493-2019 and other relevant provisions, including the requirements given in the description of the provisions.

13. Portable detection alarm instrument

In engineering practice, it is recommended to pay attention to the use of portable detection alarms.

Portable flammable and toxic gas detectors refer to gas detection and alarm devices that can be carried with you and complete detection and alarm tasks during the carrying process. They are used for medium leakage detection at production sites or storage and transportation sites, confirmation of on-site leakage media, and safety monitoring of the on-site environment. .

According to the requirements of GB/T 50493-2019, fixed detectors should be used in places where combustible gas and toxic gas detectors are required; mobile gas detectors should be used in places where flammable and toxic gases need to be temporarily detected. On-site workers entering explosive gas environments or toxic gas environments should be equipped with portable flammable gas and/or toxic gas detectors. When the entering environment contains both explosive gases and toxic gases, portable flammable gas and toxic gas detectors can use multi-sensor types.

According to GBZ/T 223-2009, fixed toxic gas detection alarm points are not required to be set up in special places where the concentration of toxic gases in the air is known to frequently or continuously exceed the alarm setting value. If they need to enter the workplace for work, relevant personnel should be equipped with portable toxic gas detection alarms and effective personal protective equipment. Generally speaking, the place where a toxic gas detection alarm should be installed should be a fixed type. When it is not necessary or does not have the conditions to install a fixed type, a mobile or portable detection alarm should be installed. In addition, portable detection and alarm devices should also be used for safety inspections and accident inspections.

14. Certificates and inspection reports

Relevant evidence includes the following documents:

1) "Announcement of the State Administration for Market Regulation on the Implementation Requirements for Converting Production Licenses to Compulsory Product Certification Management for Explosion-Proof Electrical and Other Products" [2019 No. 34]

2) "Announcement of the State Administration for Market Regulation on Optimizing the Catalog of Compulsory Product Certification" [2020 No. 18]

3) "Announcement of the State Administration for Market Regulation on Adjusting the Catalog of Measuring Instruments Subject to Compulsory Management" [2020 No. 42]

4) "Announcement of the State Administration for Market Regulation and the National Certification and Accreditation Administration on Reform and Adjustment of the Compulsory Product Certification Catalog and Implementation Methods" [2018 No. 11]

5) GB/T 50493-2019 Clause 3.0.5 (text and description of clauses)

6) Technical resolution on the definition of CCC certification catalog for tablet computers and other explosion-proof electrical products (TC28-2021-01)

Based on the above documents, the certificate and inspection report requirements are as follows.

Combustible gas detectors should obtain an explosion-proof certificate. They do not need a mandatory explosion-proof certification CCC certificate (based on 1, 2 and 6). They should obtain a fire protection product inspection report (based on 5). It is not mandatory to obtain a fire protection product certification certificate. You can voluntarily obtain a fire protection product certification certificate. Product certification certificate (based on 4). There is no requirement for a measuring instrument type approval certificate for combustible gas detectors other than methane. Methane detectors should obtain a measuring instrument type approval certificate (based on 3).

Sulfur dioxide (SO2), hydrogen sulfide (H2S) and carbon monoxide (CO) toxic gas detectors should obtain a measuring instrument type approval certificate (basis 3). Toxic gas detectors used in explosive hazardous locations should obtain explosion-proof certificates and do not need the mandatory explosion-proof certification CCC certificate (based on 1, 2 and 6).

On-site area alarms used in explosion-risk locations should obtain the mandatory explosion-proof certification CCC certificate (according to 1, 2 and 6).

There are no mandatory calibration requirements for combustible gas and toxic gas detectors. Users can choose non-mandatory calibration or calibration methods to ensure that the values are accurate and meet the usage requirements (Based on 3).

Starting from June 11, 2018, combustible gas alarm products (including combustible gas detectors and GDS controllers) will no longer implement compulsory product certification management (CCCF) (basis 4).

When detectors and GDSs do not perform a safety instrumented function (SIF), they do not need to meet functional safety standards and there are no safety integrity level (SIL) certification requirements. When detectors and GDS perform safety instrumented functions (SIF), they should meet the requirements of functional safety standards, including the requirements of the GB/T 50770-2013 standard.

Methane detector; sulfur dioxide (SO2), hydrogen sulfide (H2S) and carbon monoxide (CO) toxic gas detectors. For the applicable type evaluation outline, please see the General Office of the State Administration for Market Regulation’s release of the "Catalogue of Measuring Instruments Subject to Compulsory Management and Applicable National Metrology Technical Specifications" 》Notice (City Supervision and Measurement [2020] No. 6).

15. Unit conversion

There are two ways to express the concentration of toxic gases in the ambient atmosphere: mass concentration (the mass of toxic gases per cubic meter of air, i.e. mg/m3) and volume concentration (the number of toxic gases contained in one million volumes of air). Volume number, that is, ppm (v/v) or μmol/mol. Usually, the gas concentration measured by most gas detection instruments is the volume concentration ppm (v/v), while the gas concentration used in national standards is the mass concentration unit. (mg/m3). mg/m3 complies with GB. 3100-1993 "International System of Units and Its Applications", ppm does not belong to the International System of Units.

Clauses A.2.6 of GB/T 50493-2019 Appendix B and GBZ 2.1-2019 Appendix A both give the conversion formulas for ppm (v/v) and mg/m3. The former gives the conversion formula under working conditions. , the latter gives the conversion formula at 20℃ and 101.3kPa. The conversion can also be calculated using the conversion coefficient. Appendix B of GB/T 18664-2002 "Selection, Use and Maintenance of Respiratory Protective Equipment" gives the conversion of 1ppm (v/v) to mg/ at 20°C and 1 atmospheric pressure mg/m3. coefficient of m3.

16. Power supply

The power supply loads of gas detectors, alarm control units, on-site sirens, etc. of combustible gas and toxic gas detection and alarm systems should be considered as particularly important loads among the primary electrical loads, and UPS power supply devices should be used for power supply.

GB/T 50493-2019 Clause 3.0.5 points out that dispersed or independent facilities for toxic, flammable and explosive goods, such as gas stations, gas filling stations, etc., generally adopt panel-mounted or wall-mounted types, and the power supply power is small. Therefore, it is stipulated that the detection and alarm system can also be powered by ordinary power supply.

17. Other engineering applications

Set up detectors or sound and light alarms as required in the following scenarios:

a) Set up sound and light alarms at the entrance and exit of the analysis cabin.

b) The online analysis instrument room located within Zone 2 of the explosion hazard area should be equipped with flammable gas and/or toxic gas detectors, as well as oxygen detectors.

c) Combustible gas and/or toxic gas detectors should be installed in places where combustible gases and toxic gases may enter the building, such as the air-conditioning fresh air vents in control rooms and cabinet rooms.

d) Combustible gas and/or toxic gas detectors should be installed in places such as process valve wells and pipe trenches where someone enters for inspection operations and may accumulate flammable or toxic gases heavier than air.

Summarize

In order to protect the personal safety and production safety of petrochemical enterprises, real-time monitoring of flammable and toxic gas leaks in the production process and storage and transportation facilities, and timely alarms to prevent personal injuries and fire and explosion accidents, how to comply with regulations, reasonably and Effective GDS engineering design and engineering application in production processes and storage and transportation facilities is crucial. According to the requirements of GB/T 50493-2019, combined with other relevant documents, standards and engineering practices, this article explains the petrochemical GDS engineering design principles and precautions as a reference for engineering applications.