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Liquid storage tanks are widely used in the chemical industry. During the design of process systems, it is often necessary to install level indicators and level alarms to ensure the stable and safe operation of the storage tanks and related equipment. For the storage tanks in the unit that require close monitoring or protection, low-low liquid level alarms and high-high liquid level alarms must also be installed, along with corresponding interlocks. When setting the high-level alarm value, the specific requirements of the production process and the parameters of the container itself should be taken fully into account. 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. So, what are the requirements for the high liquid level alarm and high-high liquid level alarm settings in storage tanks, as well as for the setting of the emergency shut-off valve? Can enterprises change it without permission? In this article, let’s explore it together. Let’s start with a case: an overflow incident caused by a level gauge failure. There was a solvent storage tank, and during the process of feeding material into the tank, an overflow occurred; flammable solvents leaked outside the tank. Fortunately, no fire broke out. The accident investigation found that before the overflow, the remote level gauge of the tank malfunctioned. The storage tank originally had a high liquid level alarm, but it was modified during a recent repair, and the high liquid level alarm no longer functions properly (this is an inappropriate change). Lesson from this accident: It is very important to maintain high liquid level alarms and ensure that the level gauges are functioning properly. Factories need to regularly test level gauges to ensure they are functioning properly. Level gauges always have a certain failure rate, with a failure typically occurring once every 10 years. Therefore, for solvents with high hazard levels, redundant level gauges or level switches should be considered if necessary. Any changes to the level gauge and level alarms should fall under change management; the procedures for making changes must be strictly followed, and the level monitoring function must not be discontinued arbitrarily. Bypassing the liquid level alarm must undergo change review. Bypassing the liquid level alarm casually may lead to overflow, and even cause fires and explosions. Liquid level alarm settings for storage tanks: These are examples of liquid level alarm settings for storage tanks taken from API standards. The times mentioned are merely for illustration; appropriate adjustments can be made according to actual circumstances. The \"Code for Design of Tank Areas in Petrochemical Storage and Transportation Systems\" SH/T 3007-2014 also provides detailed regulations regarding the setting of liquid level alarm values for storage tanks: 5.4.2 High and low liquid level alarms shall be installed in the automatic control system, and they shall comply with the following requirements: a) The set height for the high liquid level alarm of a storage tank shall not be higher than the designed maximum liquid level for that tank ； b) The set level for the low liquid level alarm in the storage tank should not be lower than the designed minimum liquid level for storage in that tank. Storage tanks of various capacities have different height-to-diameter ratios, depending on the process requirements or site layout. Based on the available information, the author concludes that for storage tanks used commonly, the height-to-diameter ratio of large-capacity tanks is close to 1:1, whereas that of small-capacity tanks is often greater than 1. For small-capacity tanks, at the same capacity, the higher the height-to-diameter ratio, the more significant the liquid level changes, which is beneficial for improving the display accuracy of on-site level gauges. In actual production, the \"Design Code for Tank Areas in Petrochemical Storage and Transportation Systems\" is highly suitable for large-capacity tanks, but it is not entirely appropriate for small-capacity tanks with special process requirements. For small-capacity tanks, it is advisable to set the alarm values as a percentage of capacity, in order to accommodate the reaction time of operators and the interlock valves. Some specification requirements for installing interlocks and emergency shut-off valves in tank areas: I. SH/T 3007-2014 Standard for the Design of Tank Areas in Petrochemical Storage and Transportation Systems, 5.4.1 Storage tanks with a capacity greater than 100 m3 shall be equipped with continuous level measurement and transmission instruments. 5.4.2 High and low liquid level alarms shall be provided in the automatic control system, and the following requirements shall be met: a) The set point for the high liquid level alarm in the storage tank shall not be higher than the designed maximum liquid level of the tank ； b) The set level for the low liquid level alarm in the storage tank should not be lower than the designed minimum liquid level for storage in that tank. 5.4.3 Storage tanks for liquids of Category I and II toxicity, storage tanks for Class A B and Class B A flammable liquids with a capacity of 3000 m3 or more, and storage tanks for other liquids with a capacity of 10000 m3 or more shall be equipped with high-high liquid level alarms and interlocks; the high-high liquid level alarm shall 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 shut down the pump. II. SH3136-2003 Code for Safe Design of Spherical Storage Tanks for Liquefied Hydrocarbons 5.3.1 Spherical tanks for liquefied hydrocarbons shall be equipped with local and remote level gauges; however, glass plate level gauges should not be used. The level gauge used should be safe and reliable, and the number of openings on the liquefied hydrocarbon spherical tank should be minimized as much as possible. 5.3.2 Liquefied hydrocarbon spherical storage tanks shall be equipped with a high liquid level alarm and a high-high liquid level interlock. 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 spherical storage tanks that are operated continuously in single-component liquefaction or refining plants, the interlock requirements shall be determined based on the needs of the upstream and downstream process streams. 6.1 Emergency shut-off valves should be installed at the liquid inlet and outlet of spherical liquefied petroleum gas storage tanks, and their location should be as close as possible to the spherical tank. 3. Design Code for Petroleum Depots GB50074-2014 8.3.10 Emergency shut-off valves should be provided at appropriate locations along the pipelines carrying flammable and combustible liquids, on the shore side. 13.2.2 When oil-containing wastewater pipes within the fire dike of the tank farm exit the dike, cut-off measures shall be taken outside the dike to prevent leaking flammable and combustible liquids from escaping into the tank farm. 15.1.4 The level measurement instruments used for high and low high/low level alarm signals in storage tanks shall be separate continuous level meters or level switches, and alarms and interlocks shall be provided in the automatic control system. IV. Code for Fire Protection Design of Petrochemical Enterprises GB50160-2008 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 systems to cut off feed when the level reaches a high value. Fullly frozen liquefied hydrocarbon storage tanks should also be equipped with vacuum relief facilities and high/low temperature monitoring, and should be connected to an automatic control system. 6.3.12 The gas holder shall be equipped with upper and lower limit alarm devices, and it is advisable to have automatic interlock cut-off devices for the inlet and outlet pipes. 6.3.14 Full-pressure liquefied hydrocarbon storage tanks should be equipped with a secondary dehydration system that includes anti-freezing measures, and an emergency shut-off valve should be installed at the base of the tank. V. AQ 3053-2015 Safety Technical Specifications for Vertical Cylindrical Welded Steel Storage Tanks 6.13 Isolation Valves A main isolation valve should be installed at the point where the inlet and outlet pipes of the tank connect to 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 Level control accessories: Flammable liquid storage tanks shall be equipped with level gauges, high and low 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 alarms, high-high liquid level alarm devices, and emergency shut-off devices; interlocking between the high-high liquid level alarm and the emergency shut-off device should be implemented, 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. In the event of a high level alarm or a fire 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. VI. AQ3036-2010 Standards for the Installation of On-site Safety Monitoring Equipment in Tank Areas for Major Hazardous Chemicals 4.2.5 For the renovation of existing tanks, sensors that can be installed without first emptying the tanks should be given priority. 5 Requirements for the installation of interlock control equipment 5.1 Interlock automatic control equipment can be installed to monitor parameters such as the temperature, level, and pressure of storage tanks, as well as the ambient temperature, depending on actual conditions; this includes systems for automatic shutdown or transfer of materials, as well as spraying cooling equipment. 5.2 The emergency switching device shall take into account the impact on the safe operation of upstream and downstream devices, and shall provide functions for alarm communication with those devices as well as delayed execution. If necessary, emergency pressure relief or material recovery facilities should also be installed. 6.3.7 Large combustible 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 detection, alarm, and interlock control systems. 6.3.8 The high liquid level detection and control system for pressure storage tanks shall consist of software alarms and hardware alarms. The alarm control should adopt an OR gate logic structure. VII. Notice on Further Strengthening the Safety Management of Chemical Tank Areas, Safety Supervision General Administration Document No. 3 [2014] 68: Further Enhancing the Safety Management of Chemical Tank Areas (1) Further improve the monitoring and control facilities in chemical tank areas. In accordance with regulatory requirements, high and low liquid level alarms are installed in the storage tank; automatic interlocks are employed to shut off the tank’s feed valve when the liquid level is too high, and to stop material transfer when the liquid level is too low. Ensure that the alarm systems for leaks of flammable, explosive, toxic, and harmful gases are in good working condition. Emergency shut-off valves must be installed on key storage tanks such as large-scale ones, those for liquefied gases, and those holding highly toxic chemicals. VIII. Notice on Issuing Guidelines for Preventing Serious Accidents Involving Hazardous Chemicals and Fireworks and Firecrackers, Safety Supervision General Administration Document No. San (2016) 62: Strengthen risk management and control as well as the identification and rectification of potential hazards. 3. Proactively carry out special rectification efforts for hazardous chemicals, and implement comprehensive measures for key areas of prevention and control: (4) As of January 1, 2017, all hazardous chemical storage areas that constitute Class I or Class II major hazard sources and do not have an emergency shutdown (emergency isolation) function must cease to be in use ； IX. SH/T 3184-2017 Design Code for Automation Systems of Petrochemical Tank Farms 4.2.3.1 Two sets of continuous level measurement instruments shall be installed on the tank top, one of which is used to set high and low level alarms in the control system. 4.2.3.2 According to the process requirements, a control scheme should be established in the control system for high-high liquid level alarms and for interlocking the shutdown of the switch valves controlling the feed pipes to the storage tank. The signals shall be generated by another set of continuous liquid level measurement instruments as mentioned above; when a third set of liquid level instruments is required, continuous measurement instruments can be used, or liquid level switches can also be employed. 4.2.4.1 Two sets of continuous level measurement instruments shall be installed on the tank top, one of which is used to set high and low level alarms in the control system. 5.3.7.1 External-mounted ultrasonic level switches can be used for various liquid carbon steel storage tanks with unlined inner walls and no fouling layer. 5.3.7.2 For floating roof tanks or storage tanks for heavy oils, external-mounted ultrasonic level switches are recommended. 5.3.7.3 For liquefied hydrocarbon spheres, external-mounted ultrasonic level switches are recommended. 5.3.7.4 The installation of the ultrasonic sensor shall ensure that its sensing direction is unobstructed by pipes, components, etc. inside the tank, and it shall avoid the tank wall welds. Principle and features of the Dinghua Electronics ultrasonic external level switch. This ultrasonic external level switch is a completely contactless measuring device that is installed outside the tank to detect the liquid level. It utilizes Dinghua Electronics Co., Ltd.’s patented technology of \"micro-vibration analysis\", along with precise manufacturing processes and rigorous quality control, to ensure the reliability, stability, and durability of this external-mounted level switch. The performance features of this external-mounted level switch are as follows: 1) It can be installed online without shutting down the system, without the need to open the tank, use fire, or install flanges. 2) It is not affected by the viscosity of the liquid being measured, nor by its purity. 3) It is not affected by the density, pressure, temperature, etc. of the medium. 4) Not affected by the operating conditions inside the tank under test. 5) No mechanical moving parts, durable, and wear-free. 6) Environmentally friendly, leak-free, and release-free. Due to the above characteristics of this external-mounted level switch, it is suitable for applications with stringent safety requirements and demanding installation conditions ； Suitable for various liquid media.