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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. For external-mounted ultrasonic level switches, what are the requirements regarding the high-level alarm and very high-level alarm settings for storage tanks, as well as the settings for the emergency shut-off valve? Can enterprises change it without permission? In this article, let’s discuss it together. Let’s start with an example of an external-mounted level switch: an overflow incident caused by a malfunctioning level gauge. There was a solvent storage tank, and during the process of filling it, an overflow occurred; the flammable solvent 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 external-mounted level gauges and external-mounted ultrasonic level switch alarms should fall under change management; the change procedures must be strictly followed, and the level monitoring function must not be disabled arbitrarily. Bypassing the liquid level alarm must undergo change review. Bypassing the liquid level alarm arbitrarily may lead to overflow, and even cause fires and explosions. External-mounted ultrasonic level switch – Level alarm settings for storage tanks. This is an example of the settings for external-mounted level switches on 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 these alarms shall comply with the following requirements: a) The set height for the high liquid level alarm in 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 change in liquid level, which is conducive to 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: 1. 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 they shall comply with the following requirements: 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 trigger the interlocking closure of the control valve on 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 Safety 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 for flammable and combustible liquids, on the shore side. 13.2.2 When oil-containing wastewater pipes within the fire dike of the tank farm emerge from the dike, cut-off measures outside the dike shall be implemented 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 measuring instruments 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 at extremely high levels. 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 tank 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.