Which storage tanks require interlocks? Should these interlocks be connected to an independent Safety Instrumented System (SIS)?

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Storage tanks are important facilities in petrochemical enterprises, used to store liquid and gaseous raw materials, products, and intermediates. The amount of material stored in petrochemical storage tanks far exceeds the critical threshold for major hazard sources, resulting in high potential risks and a high-risk situation; even a minor error can lead to catastrophic accidents. According to statistics, the accident rate of storage tanks is much higher than that of high-temperature, high-pressure, continuous-reactor units in petrochemical enterprises. It is evident that storage tanks are a key focus of safety management in petrochemical enterprises. I. Three key issues regarding the interlock setup for chemical storage tanks: In domestic and international petroleum and chemical industries, accidents involving storage tanks are mostly caused by leaks of the stored substances, and these can easily lead to devastating incidents such as fires and explosions, with severe consequences and high costs. Next, we will explore the basic approach to accident prevention from the perspective of tank interlock settings. Which tanks exactly need to be equipped with interlocks? Should these interlocks be connected to an independent Safety Instrumented System (SIS)? Next, Little Swallow will guide you through China’s current laws, regulations, standards, and norms in order to find answers to the questions mentioned above. 1. “Notice of the General Administration of Work Safety on Further Strengthening the Safety Management of Chemical Storage Areas” (An Jian Zong Guan San [2014] No. 86): (i) Install high and low liquid level alarms for the storage tanks in accordance with regulatory requirements, and implement automatic interlock systems that shut off the feed valves to the tanks when the liquid level is too high, as well as systems that stop material transfer when the liquid level is too low. Emergency shut-off valves must be installed on key storage tanks such as those for large-scale liquefied gases and highly toxic chemicals. 2. Article 13(3) of the **Interim Provisions on the Supervision and Management of Major Hazard Sources of Hazardous Chemicals** (Order No. 40 of the State Administration of Work Safety) requires that emergency shutoff devices be installed for key facilities within major hazard sources, such as toxic gases, highly toxic liquids, and flammable gases ； For facilities handling toxic gases, emergency devices for dealing with leaks must be installed. Major hazard sources of Class 1 or Class 2 involving toxic gases, liquefied gases, or highly toxic liquids are equipped with an independent Safety Instrumented System (SIS). 3. \"Criteria for Identifying Major Safety Hazards in Chemical and Hazardous Chemicals Production and Operation Units (Trial)\\" (Safety Supervision General Administration Document No. San [2017] 121): V. Hazardous chemicals storage areas that constitute level 1 or level 2 major hazard sources do not have an emergency shutdown function in place ； The hazardous chemical storage areas that constitute Class I and Class II major hazard sources involving toxic gases, liquefied gases, and highly toxic liquids are not equipped with independent safety instrument systems. 4. Code for Fire Protection Design of Oil and Gas Engineering GB 50183-2004, 6.6.13: Gas condensate storage tanks and liquefied petroleum gas storage tanks shall be equipped with level gauges, thermometers, pressure gauges, safety valves, as well as high-level alarm devices or automatic interlock systems to cut off feed when the liquid level reaches a certain threshold. 5. Code for Design of Oil Reserve Tanks GB 50737-2011 6.2.3 The control valves on the oil inlet pipes of oil tanks shall be equipped with automatic interlock shutdown functions that activate when the liquid level is too high. 6.2.4 Oil tanks should be equipped with an automatic interlock to shut down the pumps when the liquid level is very low. 11.1.2 Each oil tank shall be equipped with continuous level measurement instruments as well as high-level and low-level switches, and shall comply with the following requirements: The continuous level gauge shall have the functions of issuing alarms for high level and low level, as well as triggering interlock to close the tank’s inlet valve at extremely high levels; the setting height for the low-level alarm (measured from the bottom of the tank) should not be less than 2 m. 3 The high level switch should have the function of interlocking to close the oil tank inlet valve when the level is too high. 4 The low-low level switch should have the function of interlocking to stop the oil pump and closing the pump outlet valve when a low-low level is detected; the setting height of this switch (from the bottom of the tank) should be no less than 1.85 m. 6. According to Article 15.1.2 of the \"Code for Design of Oil Depots\" GB 50074-2014, the following storage tanks shall be equipped with high liquid level alarms and interlocks; such alarms shall be capable of triggering the simultaneous closure of the control valves on the inlet pipelines of the tanks: – Storage tanks for Class A B and Class B liquids with an annual turnover rate of more than 6 times and a capacity of 10,000 m3 or more ； --Class A B and Class B liquid storage tanks with an annual turnover rate of 6 times or less and a capacity of more than 20,000 m3 ； --Storage tanks for liquids of toxicity grades I and II. 15.1.3 External floating roof tanks and internal floating roof storage tanks with a capacity of 50,000 m3 or more shall be equipped with a low-low liquid level alarm. The set height for the low-low liquid level alarm (measured from the bottom of the tank) should not be lower than the height at which the floating roof touches the bottom; the low-low liquid level alarm should also be capable of triggering the pump to stop simultaneously. 7. \"Code for Fire Protection Design of Petrochemical Enterprises\" GB 50160-2008 (2018 edition), 6.2.23: Storage tanks for flammable liquids should be equipped with automatic dehydrators, as well as level gauges and high-level alarms; where necessary, automatic interlock systems to shut off the feeding mechanism can also be installed. 6.3.11 Storage tanks for liquefied hydrocarbons shall be equipped with level gauges, thermometers, pressure gauges, safety valves, as well as high-level alarms and automatic interlock mechanisms to cut off the feed at excessively high levels. 8. \"Code for Safety Design of Spherical Storage Tanks for Liquefied Hydrocarbons\" SH 3136-2003, 5.3.2: Spherical storage tanks for liquefied hydrocarbons shall be equipped with high liquid level alarms and high-high liquid level interlocks. A low liquid level alarm should be installed if necessary. 5.3.3 For spherical liquefied petroleum gas storage tanks loaded and unloaded by tank trucks under intermittent operation, an automatic high liquid level interlock emergency shutdown feed cut-off device shall be installed. For single-component liquefied hydrocarbons or spherical storage tanks used in the continuous operation of refining and petrochemical production units, the interlock requirements shall be determined based on the needs of the upstream and downstream process flows. 9. \"Code for Design of Tank Areas in Petrochemical Storage and Transportation Systems\" SH/T 3007-2014: 5.4.3 Tanks storing liquids with toxicity levels I and II, tanks for flammable liquids of Class A B and Class B A with a capacity of 3000 m3 or more, and tanks for other liquids with a capacity of 10000 m3 or more shall be equipped with high-level liquid level alarms and interlocks. The high-level liquid level alarm shall trigger the interlock to close the control valve of the tank’s inlet pipeline. 5.4.4 The raw material storage tanks of the unit should be equipped with a low-low liquid level alarm, and this alarm should be linked to trigger the shutdown of the pump. 6.3.4 The pressure storage tank shall be equipped with a separate set of level monitoring instrument or level switch dedicated to high-high liquid level alarm and interlocked shutdown of the tank feed pipe valve. 10. \"Code for the Installation of Safety Monitoring Equipment at Tank Areas for Major Hazardous Sources of Hazardous Chemicals\" AQ 3036-2010: 5.1 Interlocked automatic control equipment for parameters such as the temperature, liquid level, pressure of storage tanks, and ambient temperature can be installed according to actual conditions; this includes equipment for automatic shutdown or transfer of materials, as well as spray cooling equipment. 6.3.7 Large flammable liquid storage tanks (over 5000 m3) and pressure storage tanks for hazardous chemicals (over 400 m3) shall be equipped with separate high-high liquid level monitoring, alarm, and interlock control systems. 11. \"Safety Technical Regulations for Vertical Cylindrical Welded Steel Storage Tanks\" AQ 3053-2015 6.13 Isolation Valves: A main isolation valve should be installed at the point where the inlet and outlet pipes of the storage tank meet the tank body. For large storage tanks, valves equipped with pneumatic, hydraulic, or electric actuators should be used. When the actuator is of electric type, its power cable, signal cable, and electric actuator shall be provided with fire protection. The cut-off valve should have automatic and manual shutdown functions, with manual shutdown including remote manual shutdown and on-site manual shutdown. 12.2.2 Flammable liquid storage tanks shall be equipped with level gauges, low and high level alarm devices, as well as high-high level alarm devices, in accordance with the requirements of relevant specifications and operational needs; the alarm signals and level information shall be transmitted to the control room. Storage tanks that are operated frequently should be equipped with automatic interlock emergency shut-off devices. Large tanks should be equipped with low and high liquid level alarm devices, high-high liquid level alarm devices, and emergency shut-off devices. Interlocking measures should be implemented between the high-high liquid level alarm and the emergency shut-off device, and interlocking buttons for the emergency shut-off valves should be provided outside the fire dike as well as at the control room operation stations. When a high level alarm or a fire occurs in the storage tank, it is possible to remotely or manually close the feed cut-off valve; once the cut-off valve is closed, the feed pump should be automatically shut down via interlock. Note: According to the \"Safety Technical Regulations for Vertical Cylindrical Welded Steel Storage Tanks\" AQ 3053-2015, large and medium-sized storage tanks refer to those with a nominal diameter of 30 m or more, or a nominal volume of 10,000 m3 or more. II. Summary By summarizing the above requirements, the questions raised earlier are answered from the perspective of compliance with current regulations and standards: 1. What type of interlocks are required for storage tanks? (1) An interlock that automatically stops feeding when the liquid level is too high; it is preferable that this interlock closes the valve at the bottom of the tank, and that valve should be capable of being operated from a distance to enable emergency shutdown. The purpose is to prevent oil spillage caused by an excessively high liquid level in the storage tank. (2) The interlock that automatically stops discharging at a very low liquid level mainly shuts down the discharge pump. The purpose is, firstly, to prevent the floating roof of the floating roof tank from sinking to the bottom and allowing air to enter the tank, thereby creating an explosive gas environment ； Second is to prevent damage caused by pump cavitation. 2. Which storage tanks require interlocks? (1) High liquid level interlock: ① Pressure tanks and tanks storing liquids of toxicity levels I and II shall be equipped with a high liquid level interlock (no requirement regarding tank capacity) ； ②Flammable liquid storage tanks of Class A B and Class B A with a capacity of 3000 m3 or more, Class B B flammable liquid storage tanks with a capacity of over 5000 m3, and other liquid storage tanks with a capacity of 10000 m3 or more are equipped with high-high level interlocks (requirements depend on the tank capacity and/or the fire hazard of the medium). (2) Low-low level interlock: Low-low level interlocks are installed in the raw material storage tanks of the facility, external floating roof tanks and internal floating roof tanks with a capacity of 50,000 m3 or more, as well as in the oil tanks of oil reserve facilities. Based on the risk analysis, it is recommended that floating roof storage tanks for Class A and B flammable liquids be equipped with a low-low level interlock to stop the discharge pump and prevent the floating disk from touching the bottom. 3. Should the interlocks be integrated into a standalone SIS? (1) For Class 1 or Class 2 major hazard sources involving toxic gases, liquefied gases, or highly toxic liquids, an independent Safety Instrumented System (SIS) must be installed ； (2) In other cases, it is necessary to decide whether to incorporate it into an independent SIS system based on the SIL assessment results. Furthermore, for storage tanks in a hazardous chemicals tank farm that are not covered by the above discussions but still constitute primary or secondary major hazard sources, a shut-off valve must be installed at the base of the tank to enable emergency shutdown.