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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 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, it is necessary to fully consider the specific requirements of the production process as well as the parameters of the container itself. Storage tanks are important facilities in petrochemical enterprises, used to store liquid and gaseous raw materials, products, and intermediates. The quantity of materials stored in petrochemical storage tanks far exceeds the threshold quantity for major hazard sources; thus, the potential risks are significant and the situation is highly hazardous. Even a minor oversight could lead to a catastrophic accident. Statistics show that the accident rate of storage tanks is much higher than that of high-temperature, high-pressure, and continuously reacting facilities in petrochemical enterprises. Clearly, storage tanks are a key focus in the safety management of 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? Let’s look at a case study first: 1. Case of overflow – malfunction of the level gauge. There was a solvent storage tank in which an overflow occurred during the filling process; 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 level gauges and level alarms should fall under the scope of change management. The change procedures must be strictly followed; it is not permissible to arbitrarily disable the level monitoring function. 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. 2 Level alarm settings for storage tanks This is an example of level alarm settings for storage tanks, excerpted 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 Farms in Petrochemical Storage and Transportation Systems” SH/T 3007-2014 also provides detailed provisions regarding the setting of liquid level alarm values for storage tanks: 5.4.2 High- and low-level liquid level alarms shall be installed in the automatic control system, and shall comply with the following requirements: a) The set value for the high-level liquid level alarm shall not exceed the designed maximum storage 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. 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 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 applicable to large-capacity tanks, but it is not entirely suitable 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 regarding the installation of interlocks and emergency shut-off valves in tank areas: 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 trigger the shutdown of the pumps. II SH3136-2003 Code for Safe Design of Spherical Storage Tanks for Liquefied Hydrocarbons 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 subjected to truck loading and unloading under intermittent operation, an automatic high liquid level interlock emergency shutdown device for feeding should 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; Code for Design of Oil Depots GB50074-2014 8.3.10 Emergency shut-off valves shall 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 outside the dike, cut-off measures shall be taken outside the dike to prevent leaks of flammable and combustible liquids from escaping into the tank farm. 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. Fully cryogenic liquefied hydrocarbon storage tanks should also be equipped with vacuum relief systems and high/low temperature detectors, 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 Fully pressurized 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. 5 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 liquid 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. Course Update | April 19-21 [Shanghai] 2024 Training Course on Process Package Development, Design, and Construction Drawings. 6 AQ3036-2010: Specifications for the Installation of Safety Monitoring Equipment in Tank Areas for Hazardous Chemicals and Major Hazard Sources. 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; this includes systems for automatic shutdown or transfer of materials, as well as spray 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 be installed simultaneously.