Do storage tanks require level measurement instruments based on two different principles?

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The domestic standards for measuring liquid levels in storage tanks are primarily based on **national standards (GB), industry standards (such as those in the petrochemical and chemical sectors), as well as relevant safety regulations issued by the Ministry of Emergency Management. The following is a brief overview of the specific regulations and requirements: **Standards and industry standards (1) SH/T 3007-2014 \"Code for Design of Storage Areas in Petrochemical Storage and Transportation Systems\" – Article 6.4 specifies that for storage tanks holding flammable, explosive, or toxic substances, level measurement must be carried out using instruments based on two different principles (such as radar level gauges + servo level gauges). These two types of instruments must be installed separately, serving as backups for one another.    The high, very high, low, and very low level alarm signals should be transmitted to the control system in a redundant manner, and should be interlocked with the emergency shut-off valve. (2) SH/T 3184-2017 Code for Design of Automation Systems in Petrochemical Tank Farms 4.2.3 Pressure Storage Tanks for Measurement Purposes 4.2.3.1 Two sets of liquid level measurement instruments equipped with on-tank indicators shall be installed at the top of the tank; one of these sets is used to set high and low liquid level alarms in the control system. 4.2.3.1 The two sets of liquid level continuous measurement instruments can be instruments based on different measurement principles; when an instrument of a certain principle is not suitable for the properties of the medium, two identical instruments can also be used. (3) GB/T 50493-2019 \"Design Code for Detection and Alarm of Flammable and Toxic Gases in Petrochemical Industries\": For storage tanks involving major hazard sources, the level measurement system must be interconnected with the flammable/toxic gas detection system; an interlock action should be triggered in case of abnormal liquid levels (such as emergency shutdown of critical systems).    Although a dual instrument setup is not directly required, it is suggested that \"critical parameters should be made more reliable through redundant configuration.\" (4) GB 50160-2008 \"Code for Fire Protection Design of Petrochemical Enterprises\" requires that large storage tanks (such as those for crude oil and liquefied hydrocarbons) be equipped with an independent Safety Instrumented System (SIS). The level measurement function, as part of the Safety Instrumented Function (SIF), must meet the requirements regarding SIL levels (Safety Integrity Levels).    SIL2/SIL3 systems typically require redundant measurements (two instruments based on different principles). (5) GB 17681-2024 Technical Specifications for Safety Monitoring of Major Hazard Sources of Hazardous Chemicals 6.3.2 Monitoring Requirements for Atmospheric and Low-Pressure Tank Areas 6.3.2.1 Tanks shall be equipped with at least 2 sets of continuous level detection instruments, or 1 set of continuous level detection instrument and 2 level switches. 6.3.3 Monitoring requirements for pressure storage tank areas 6.3.3.1 Pressure storage tanks shall be equipped with at least 2 sets of continuous level detection instruments and 1 high-high level switch, or with 3 sets of continuous level detection instruments. 6.3.4 Monitoring requirements for the fully frozen storage tank area 6.3.4.1 The installation of liquid level monitoring instruments in the storage tanks and the control of liquid levels shall comply with the following provisions. a) At least 3 sets of level detection instruments should be installed, of which at least 2 sets must be continuous level detection instruments, used for level measurement as well as for generating alarms for low and high levels, high-high level alarms and interlocks, and low-low level alarms and interlocks. According to the regulations issued by the Ministry of Emergency Management: (1) Article 32 of the Interim Provisions on the Supervision and Management of Major Hazard Sources of Hazardous Chemicals (Order No. 10 of the Ministry of Emergency Management) requires that storage tanks for major hazard sources of hazardous chemicals must be equipped with continuous level monitoring systems, as well as functions for detecting high and low liquid levels.    For storage tanks classified as major hazard sources of level 1 and level 2, redundant measurement is recommended, and the alarm signals should be transmitted independently to the monitoring platform. (2) \"Guiding Opinions on Strengthening the Management of Safety Instrumented Systems in the Chemical Industry\" (Safety Supervision General Administration Document No. San [2014] 116): For installations involving \"highly regulated hazardous chemicals, highly regulated hazardous chemical processes, and major hazard sources\" (i.e., \"two key areas and one major hazard\"), level measurement instruments shall be incorporated into the Safety Instrumented System (SIS) and must meet the following requirements: redundant configuration (such as 1oo2 or 2oo3 voting mechanisms) ；    Combination of instruments based on different principles (to avoid common cause failure). Other industry standards: (1) HG/T 20507-2014 \"Specifications for the Selection and Design of Automation Instruments\" – Article 5.3 states that for level measurement, which is critical for safety interlocks or economic calculations, it is advisable to use instruments based on two different measurement principles, and the measurement systems should be independent of each other (for example, with separate power supply and signal transmission).    For media that crystallize easily and are highly corrosive, it is explicitly required to use a combination of non-contact (such as radar) and contact (such as magnetostrictive) instruments. (2) HG/T 30033-2017 Safety Technical Specifications for Hydrogen Fluoride Production 5.2 Process for producing hydrogen fluoride using fluorite and sulfuric acid 5.2.6 Packaging and storage procedures 5.2.6.3 Safety operation technical measures 5.2.6.3.9 Each storage tank shall be equipped with two measurement methods (level and weight). (3) NB/T 47003.1-2022 \"Atmospheric Pressure Storage Tanks – Part 1: Steel Storage Tanks\" recommends that for large atmospheric pressure storage tanks (such as crude oil storage tanks), a dual measurement system consisting of a radar level gauge plus a manual sight glass, or a radar gauge combined with a servo-level gauge, be used to ensure data reliability. Guidelines for Identifying Safety Risks and Hazards in Liquid Chlorine (Chlorine Gas) Production Enterprises (Trial). The installation of liquid chlorine storage tanks must meet the following requirements: (5) Level measurement should be carried out both locally and remotely, using level gauges based on different measurement principles; remote level gauges should be of the leak-proof type that does not come into contact with the medium. Local level indication shall be used; glass plate level gauges are not permitted. Typical compliance solution 1: High-risk storage tanks (such as liquefied hydrocarbons, benzene series) – radar level gauge (non-contact) + servo level gauge (contact type) + high/low level switches.     The signals are connected to the DCS and SIS systems respectively to achieve independent control.  2. Atmospheric crude oil storage tanks: guided wave radar + magnetostrictive level gauge, supplemented by regular calibration using manual gauging ports.  3. Acid and alkali storage tanks: Ultrasonic level gauge + pressure transmitter (level is determined via density conversion) to prevent damage caused by medium corrosion. Implementation notes – Common failure prevention: The installation locations, power supply, and signal transmission paths of the two instruments must be physically separated.  Regular calibration: In accordance with the requirements of the \"JJG 971-2019 Calibration Regulations for Level Gauges\", calibration should be carried out at least once a year.  Document records: HAZOP analysis reports, LOPA protection layer analysis, and SIL verification documents must be archived for reference. Domestic regulations require that high-risk storage tanks (especially in those involving \"two key aspects and one major risk\") be equipped with level gauges based on two different principles, which are also to be integrated into the safety interlock system ； A redundant configuration is recommended for ordinary storage tanks. The actual design must take into account specific standards (such as SH/T 3007 and GB 50160) as well as the safety regulations set by the Ministry of Emergency Management, with the final design being determined through HAZOP and SIL analyses. When there are no special requirements, it is not necessary to use two level gauges based on different principles for measuring the liquid level in a storage tank; either level gauges of different principles or those of the same principle can be used.