Alarm settings for liquid hydrocarbon spheres: temperature, pressure, and upper/lower limits for liquid level alarms

An article I came across online, which I’d like to share: Liquid storage tanks are widely used in the chemical industry. During the design of process systems, it is often necessary to install level alarms and indicators for these tanks, in order to ensure the stable and safe operation of the tanks as well as the associated 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 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 mistake 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 arbitrarily change the liquid level alarm values for storage tanks? In this article, let’s explore it together. Let’s first look at a case of overflow caused by a malfunctioning level gauge: There was a solvent storage tank, and overflow occurred while feeding material into the tank; the flammable solvent leaked outside the tank, but 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). Lessons learned from this incident: ◆ It is crucial to maintain high-level alarms and ensure that level gauges function properly. Factories need to regularly test level gauges to ensure they are functioning properly. ◆Level gauges always have a certain failure rate; typically, a failure may occur 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 change management; the change procedures 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 arbitrarily may lead to overflow, and even cause fires and explosions. Regulations and standards regarding tank level alarm settings. These are examples of tank level alarm settings 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. According to the data available for digital display instruments, the commonly used capacities for storage tanks are as follows: large-capacity tanks have a height-to-diameter ratio of around 1:1, while small-capacity tanks typically have a height-to-diameter ratio 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. There are certain regulatory requirements regarding the installation of interlocks and emergency shut-off valves in tank areas. Rules are necessary to ensure order; similarly, there are standards for setting the alarm values for tank levels. Changhui Instruments has compiled the relevant provisions from current regulations on the setting of such alarm values for tank levels, to provide reference for instrument technicians. ◆SH/T 3007-2014 Code for 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 remote liquid level measurement 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 toxicity grades I and II, 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 pump. http://yunrun.com.cn/upload/201812/20/201812201607161244.png ◆SH3136-2003 Safety Design Code for Spherical Storage Tanks of 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 shall be safe and reliable, and shall minimize the number of openings in the spherical tanks for liquefied hydrocarbons 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 flows. 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 ； ◆According to Clause 8.3.10 of the Design Code for Oil Depots GB50074-2014, isolation valves should be installed at appropriate locations along the shore where pipelines carrying flammable and combustible liquids pass, for use in emergency situations. 13.2.2 When oil-containing wastewater pipes within the fire dike of the tank farm emerge from 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 measuring 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. ◆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. Fully cryogenic liquefied hydrocarbon storage tanks should also be equipped with vacuum relief systems 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 featuring anti-freezing measures, and an emergency shut-off valve should be installed at the base of the tank. ◆AQ 3053-2015 Safety Technical Specifications for Vertical Cylindrical Welded Steel Storage Tanks 6.13 Isolation Valves A main isolation valve shall be installed at the part of the inlet and outlet pipes of the tank where they are adjacent 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; in addition, interlocks should be established between the high-high liquid level alarm and the emergency shut-off devices. Interlock 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 isolation valve; once the isolation valve is closed, the feed pump should be automatically shut down via interlock. ◆AQ3036-2010 Specifications 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 emptying the tank should be given priority. 5 Requirements for the installation of interlock control equipment 5.1 Interlock automatic control equipment can be installed as appropriate, based on actual conditions, to monitor parameters such as the temperature, liquid level, 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 such devices as well as delayed execution. If necessary, emergency ◆ pressure relief or material recovery facilities should also be installed. 6.3.7 For large-sized combustible liquid storage tanks (with a capacity of over 5,000 m³) and pressure storage tanks for hazardous chemicals with a capacity of over 400 m³, a separate high-high level detection, alarm, and interlock control system shall be installed. 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. ◆Notice on Further Strengthening the Safety Management of Chemical Tank Areas, Safety Supervision General Administration Order No. 368: Further Enhancing the Safety Management of Chemical Tank Areas (1) Further improve the monitoring and control facilities for 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 those for large-scale liquefied gases and highly toxic chemicals. ◆Notice on Issuing Guidelines for Preventing Serious Accidents Involving Hazardous Chemicals and Fireworks and Firecrackers, Safety Supervision General Administration Document No. San 62: Strengthen risk management and the identification and rectification of potential hazards. 3. Advance the special rectification efforts related to hazardous chemicals by fully implementing key preventive measures: (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 operations ； ◆SH/T 3184-2017 Specification for the Design of Automation Systems in 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 provided by another set of continuous liquid level measuring instruments as mentioned above; when a third set of liquid level instruments is required, continuous measuring instruments can be used, or liquid level switches can also be employed. 4.2.4.1 Two sets of continuous level measuring instruments shall be installed on the tank roof; one of them 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 should ensure that its sensing direction is unobstructed by pipes, components, etc. inside the tank, and it should be positioned away from the welds on the tank walls.

The liquid level in the tank is primarily determined based on the filling coefficient specified during design. The size of the storage tank is determined by taking into account the maximum amount that can be stored, and then the tank size is calculated using the filling coefficient. Many companies choose the alarm values for liquid level levels arbitrarily based on their process requirements, but this is not the case in reality; moreover, the interlock values must not exceed the maximum filling level. Temperature and pressure are primarily designed based on the properties of the medium.