Oil storage tank, base control valve

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Oil storage tanks are important equipment in petrochemical production, used for storing liquid hydrocarbon materials. The materials stored in oil tanks are flammable, explosive, or toxic; therefore, oil tank areas are generally considered major hazard sources. In recent years, oil storage tanks have become facilities that are subject to strict supervision; as a result, many regulations, laws, and standards have been introduced, and engineering design standards and procedures have been updated or supplemented. Among these new regulations, laws, and standards, improving the level of monitoring is a key aspect. The monitoring of petrochemical product storage tanks mainly includes two types of measures: 1) Continuous detection and indication of operating parameters such as level, temperature, and pressure, along with alarm functions for exceeding set limits ； Continuous detection, continuous indication, and over-limit alarm for combustible gases, toxic gases, fires, etc. 2) Interlock shutdown occurs when the operating parameters exceed the safe operating limits, as well as in case of a fire in the tank area; the fire protection system is activated simultaneously. Among the aforementioned monitoring measures, detection, indication, and alarm are the basic measures, while interlock shutdown is the key measure. Interlock shutdown is a measure that is particularly emphasized in new regulations, laws, and standards, and it is also a key focus of safety inspections. Interlock shutdown mainly involves shutting off the feed or discharge to the storage tank, or shutting off both feed and discharge simultaneously. According to safety analysis, promptly shutting down the tank base control valves for feeding and discharging remotely is the most effective and thorough method. Therefore, the requirements for installing root control valves are emphasized in the new standard specifications and regulations. The tank root control valve includes a feed valve and a discharge valve. Since petrochemical storage tanks do not have simultaneous feeding and discharging, a single control valve is often used to operate both the feeding valve and the discharging valve. The tank root control valve is primarily used to interlock and shut off feed when the liquid level is too high, and to interlock and shut off discharge when the liquid level is too low. In the event of serious accidents such as leaks or fires in the tank area, both feeding and discharging are stopped simultaneously. Although regulations, laws, and standards also impose certain requirements on tank root control valves, this information is fragmented, not systematic, and the requirements are incomplete; there are also many inconsistencies among them. During implementation, differences in understanding among technicians led to a variety of design approaches. Continuous disputes between design institutes and clients, as well as between enterprises and safety management authorities, cause a great deal of trouble for those involved in design, construction, production, and safety supervision. This undoubtedly affects the safety reliability, operational availability, and cost-effectiveness of tank root control valves. The tank root control valve performs multiple functions, and there are various requirements for it. The tank root control valve is a relatively complex unitized equipment set. To enable the device to function properly, it is necessary to take into comprehensive account its overall performance as well as the performance of various components such as the valve body, actuator, control devices, signal types, and control cables, and to carry out a detailed design that takes all these factors into consideration. In response to this, the author has conducted extensive research, exploration, and practice, and would like to share some preliminary insights here for reference, criticism, and correction. 1 Design Strategy Before proceeding with the design of the tank root control valve, it is essential to have a proper design strategy. The author believes that at least the following points should be included: 1) Clarify the process requirements. The tank root control valve may need to meet multiple process requirements, which should be clarified one by one. Only by understanding the process requirements can one determine the functions of the control valve, and only then can the desired functions be fulfilled completely and correctly. 2) Clarify reliability requirements and measures. Based on the application of control valves, clarify the safety function requirements and non-safety function requirements. Based on HAZOP and LOPA analysis reports, it was determined whether the tank root control valve performs safety functions at the SIL1 to SIL3 level. If it incorporates safety functions, its reliability must meet the corresponding SIL level; its configuration must be fail-safe, and it should be controlled by a Safety Instrumented System (SIS). 3) Clarify availability requirements and measures. Identify the usability requirements based on the valve’s application. When the tank root control valve is used as a feed control valve for a downstream unit or an outlet control valve for an upstream unit, its closure will cause the connected unit to stop operating; therefore, the availability of such tank root control valves for this purpose must be given top priority. This needs to be implemented in aspects such as the selection of actuators, the provision of power sources, the redundant configuration and structure of solenoid valves, and the control signal mechanism. 4) Clarify fire protection requirements and measures. Clarify the fire protection requirements in accordance with the intended use and standard specifications, as well as regulatory requirements. When the tank root control valve has fire safety requirements, fire protection measures must be taken for the valve. In the event of a fire, it is still possible to remotely control the valves to close within a limited time frame. 2 Valve Type Selection 2.1 General Requirements The special location and functions of the tank bottom control valve dictate specific requirements for it. It must meet at least the following requirements: tight closure and low leakage ； The valve has a small mass, reducing its impact on the tank ； The valve has a small size, reducing the space required for installation. 2.2 Valve type analysis According to the above requirements, the available valve types include gate valves, ball valves, and butterfly valves, each with its own characteristics. 2.2.1 Gate Valves Gate valves are straight-stroke valves that feature mature technology, reliable operation, low cost, and tight sealing ； However, it has a large volume, especially in terms of height, which requires significant installation space as well as large-scale maintenance facilities. The valve stem has a long stroke, resulting in a longer time required to close the valve. Gate valves are classified into single-gate and double-gate valve types based on the number of gate plates and the cutting mechanism. Single-plate gate valves have a simple structure, small size, and light weight, but require a large force to open. Double-plate gate valves offer better sealing performance and require less force to open, but they have a more complex structure, are larger in size, and heavier in weight. Single-plate gate valves are classified into wedge gate valves and flat gate valves based on the shape of the gate plate. When compared, wedge gate valves provide a tighter seal than flat gate valves, but they are more difficult to manufacture and are more expensive. Plate gate valves are further divided into ordinary types and those with guide holes. The type with guide holes is suitable for pipelines under ball-passing conditions, or for pipelines whose process medium contains solid particles or residues, or where the medium tends to crystallize or solidify. 2.2.2 Ball valves Ball valves are quarter-turn valves characterized by their small size, short stroke length, brief closing time, tight shutoff, and reliable operation. They also require little installation space, as well as simple installation and maintenance facilities. However, they are difficult to manufacture and process, and their prices are high; especially large-diameter ball valves are very expensive. 2.2.3 Butterfly Valve The butterfly valve is a rotary-motion valve; it combines the advantages of gate valves and ball valves while overcoming their disadvantages. However, despite their seemingly simple design, butterfly valves require a high level of manufacturing and processing expertise. Otherwise, excessive vibration of the valve disc may lead to a high failure rate and low availability of the control valve ； It may not close properly, resulting in significant leakage and an inability to effectively perform the cutting function. Therefore, butterfly valves are not widely used at present. However, with the improvement of manufacturing technology, butterfly valves have a bright future. 2.3 Selection of valve types for the root control valves of atmospheric pressure tanks and low-pressure tanks Based on the above analysis and comparison, the author’s recommendations regarding the selection of valve types for the root control valves of atmospheric pressure tanks and low-pressure tanks are as follows: 1) For storage tanks that hold substances such as gasoline, kerosene, and diesel, which do not cause significant corrosion, contain no solid impurities, and have high viscosity, it is appropriate to use ordinary single-plate gate valves as the root control valves. When the valve port diameter is less than DN 150, a single-plate wedge gate valve is recommended ； When the valve port diameter is not less than DN 150, a single-plate gate valve is recommended. 2) For storage tanks used to hold media such as benzene, which have strong solvency for organic materials like rubber, double-disc gate valves should be used as the control valves at the tank base, in order to prevent the medium from dissolving and damaging the sealing rings between the valve discs and the valve seats, thereby avoiding internal leakage of the valve. 3) For storage tanks holding highly toxic liquid media, double-disc gate valves should be used as the control valves at the tank base to improve sealing performance and reliability. 4) For storage tanks used to hold crude oil, residue oil, sludge oil, and other media that may contain solid impurities or have high viscosity, it is advisable to use single-plate flat gate valves with guide holes as the control valves at the tank base. 2.4 Pressure Vessels: Pressure vessels used for storing liquids such as liquefied petroleum gas, propylene, and ammonia; the author recommends that ball valves be used as the control valves at the base of these vessels. 3 Selection of actuator type: The selection of actuators for plug control valves shall comply with SH/T 3005—2016 \"Code for Selection and Design of Automation Instruments in Petrochemical Industries\". In addition, special attention should be paid to complying with the relevant regulations regarding this valve as stipulated by **, local authorities, and the standards of our corporate group. For example, AQ 3053—2015 \"Safety Technical Regulations for Vertical Cylindrical Welded Steel Storage Tanks\" stipulates that the actuators that can be used for the control valves at the base of vertical steel storage tanks include pneumatic actuators, electric actuators, and electro-hydraulic actuators. The \"Interim Provisions on the Safety Technology Management of Liquefied Hydrocarbon Spherical Tank Areas of Sinopec Group Corporation\" (Sinopec Safety [2010] No. 635) and the \"Provisions on the Selection and Design of Emergency Shutoff Valves for Liquefied Hydrocarbon Spherical Tanks\" (Sinopec [2011] Construction No. 518) stipulate that the actuators for the control valves at the base of spherical tanks can be pneumatic actuators, electric actuators, or electro-hydraulic actuators; pneumatic actuators should be given priority. Pneumatic actuators are technologically mature, easy to maintain, inexpensive, and can achieve fail-safe mode. However, its size, weight, and the amount of work required for piping and wiring are all substantial. Electric actuators are relatively mature in technology; they are small in size and light in weight, though their price is high, and the amount of work required for piping and wiring is minimal. Current mainstream products are not capable of operating in a fail-safe mode; only a few manufacturers offer fail-safe products with pre-stored energy, and these are mostly limited to valves with small diameters, short strokes, and linear motion. Reliability requires further verification. Electro-hydraulic actuators are technically complex, capable of operating in a fail-safe mode, and are expensive; currently, most of them are imported products. Based on the above analysis, the recommendations for selecting the control valve for the tank root are as follows: if it is to serve the purpose of providing interlock protection, a pneumatic actuator should be given priority ； If the interlock protection function is not required, a pneumatic actuator or an electric actuator can be used. 4 Selection and Implementation of Failure Modes for the Bottom Valve Control Valve: The failure modes of the bottom valve control valve should be chosen based on the valve’s purpose and functions, and achieved through comprehensive measures. Since electric valves are primarily used in fail-safe modes, the engineering design for implementing failure modes in electro-hydraulic and electric valves is simple, whereas the engineering design for achieving failure modes in pneumatic valves is more complex and results in a higher error rate; therefore, the following discussion focuses mainly on the implementation of failure modes in pneumatic valves. 4.1 Tank root control valve performing safety functions: For tank root control valves that perform safety functions (with a safety integrity level of SIL1–SIL3), the failure mode should be set to fail-closed (FC). There are two ways to implement the FC mode: select a single-acting spring-return single-cylinder actuator ； A double-acting dual-cylinder actuator is selected, equipped with an emergency air tank. Based on the current level of manufacturing, and taking into account factors such as reliability, availability, and cost, the author recommends that for valves with a diameter of less than DN300, a single-acting spring-return single-cylinder actuator should be used ； Valves with a diameter of DN300 or larger shall be equipped with a double-acting dual-cylinder actuator, along with an emergency air tank. 4.2 Tank root control valves for operating non-safety functions: For such control valves, it is advisable to select the fail-safe mode (FL) as the fault mode in order to improve availability. Implementation of FL mode: A double-acting dual-cylinder actuator is used. When the storage tank serves as a feed tank for downstream units or a storage tank for upstream units, it is advisable to equip the base control valve with an emergency air tank. This ensures that the valve can maintain its position reliably in the event of an air supply interruption, and it also allows for remote shutdown of the valve in case of an accident, thereby improving operational availability and safety reliability. 5 Configuration of solenoid valves in pneumatic actuators: The solenoid valves are arranged according to the purpose and functions of the tank bottom control valves. 5.1 Solenoid valve configuration for tank bottom control valves performing safety functions must ensure reliability. The configuration is as follows: 1) Use 1 two-position four-way solenoid valve, or 1 two-position five-way solenoid valve, or 2 two-position three-way solenoid valves. 2) Use a single-electrocontrol solenoid valve. 3) Under normal conditions, the solenoid valve is energized and the control valve opens ； During interlocking, the solenoid valve loses power and the control valve closes ； The control valve closes when the air supply is interrupted. 4) The valve should be equipped with a control box. There are various wiring options for the cables from the control box to the control room. For example: when configuring 2 solenoid valves, an 8-core cable can be used at most, with 4 cores dedicated to the solenoid valves and 4 cores for valve position feedback ； A minimum of 5-core cable can be used: 2 cores for the solenoid valve, 2 cores for valve position feedback, with the neutral wire shared by both. Other configuration wiring can be inferred in the same way. The wiring scheme is determined based on the owner’s requirements, the selected control valves, and the needs of the control system. A 2-core cable is used for the remote control switch cable from the control box to the site. 5.2 The configuration of solenoid valves for the tank root control valves that are used to execute non-safety functions should prioritize availability. The configuration is as follows: 1) Use 1 two-position four-way solenoid valve, or 1 two-position five-way solenoid valve, or 2 two-position three-way solenoid valves. 2) Use a single-electrocontrol solenoid valve. 3) Under normal conditions, the solenoid valve is not energized, and the control valve is open ； During interlocking, the solenoid valve is energized and the control valve closes ； The control valve maintains its position in the event of a gas supply interruption. 4) The control box, on-site remote control switches, and cable wiring scheme are the same as above, so no further explanation is needed. 6 Fire Protection Requirements and Measures September 22-24 [Suzhou] 2023 Training Course on Selection and Design Calculations of Chemical Equipment, as well as Design and Selection of Chemical Process System Facilities 6.1 Requirements In the past, domestic standards, specifications, regulations, and laws basically did not address the fire protection aspects of instrumentation equipment. In recent years, some standards and regulations have set requirements in this regard. However, these requirements are mostly fragmented information; the concepts are incomplete and the requirements are not unified. Therefore, fire protection has become a highly controversial topic, with whether fire protection is required for the tank root control valve being the focus of that controversy. Due to the significant role of the tank root control valve, high safety and reliability requirements are necessary. Tank root control valves are large in size and numerous in quantity, resulting in high costs for fire protection measures. The current approach often falls into two extremes: one is the belief that fire prevention is necessary, and investments are made without hesitation to ensure complete fire protection in order to avoid disputes ； The other view is that it’s unnecessary to make everything fire-resistant; neither of these two approaches conforms to engineering design principles. The author believes that regarding whether fire protection is required for the tank root control valves, it is necessary to comply with ** and local safety regulations and laws, the industry’s standard specifications, as well as the requirements of the enterprise group, and strict provisions should be adopted. For example, in AQ3053—2015, regarding the fire protection requirements for the base control valves of vertical steel storage tanks: when the actuator is of electric type, its power cables, signal cables, and actuator itself must be provided with fire protection. The \"Interim Provisions on the Safety Technology Management of Liquefied Hydrocarbon Spherical Tank Areas of Sinopec Group Corporation\" (Sinopec Safety [2010] No. 635) specify the fire protection requirements for the control valves at the base of liquefied hydrocarbon spherical tanks: when the actuator is of electric type, its power cables, signal cables, and the actuator itself must be provided with fire protection. Pneumatic actuators are equipped with fusible plugs that automatically shut off in case of a fire. Fire-resistant sealing packing is used for the valves. \"Specifications for the Selection and Design of Emergency Shut-off Valves for Liquefied Hydrocarbon Spheres\" (Sinopec [2011] Jian 518): Regarding the fire protection requirements for the control valves at the base of liquefied hydrocarbon spheres, all such valves must be equipped with fire protection measures; flame shields are the preferred option. These flame shields must comply with the UL1709:2017 Standard for safety rapid rise fire tests of protection materials for structural steel, and they should be able to resist hydrocarbon fires for 30 minutes at a temperature of 1,093 ℃. Fusible plugs should be installed in the pneumatic control lines or cylinders of pneumatic valves. Fire-resistant sealing packing is used for the valves. The valve body shall meet the fire resistance standards specified in API 607:2016 Fire test for quarter turn valves and valves equipped with nonmetallic seats, or API 6FA:2018 Standard for fire test for valves. The \"Guiding Opinions on the Campaign to Rectify Hazards in Tank Areas\" (Sinopec Safety [2016] No. 39) stipulate fire protection requirements for the root control valves of tanks: For tanks equipped with an independent SIS, fire protection measures must be taken for their root control valves. According to SH/T 3005—2016, the valve body of cut-off valves shall meet the fire resistance standards specified in API 607:2016 or API 6FA:2018. The author believes that, on the basis of meeting the above requirements, the design firm and the owner should conduct further risk assessments considering factors such as the properties of the storage medium, the amount of medium stored, and the operating principle of the valves. Through a comprehensive comparison based on risk, investment, and owner safety considerations, it is determined whether fire protection design is required for the tank foot control valves in scenarios other than those specified by standards. For example: some standards require electric valves to be fire-resistant, while no such requirement exists for other valves. The author believes that in such application scenarios, double-acting dual-cylinder pneumatic valves also require fire protection. This is because in both cases, there is the problem of the valve actuator failing during a fire, thereby preventing remote shutdown of the valve. 6.2 Measures If the valve is required to be fire-resistant, corresponding fire-resistant measures must be taken. Due to differences in the understanding of standards, regulations, and laws, the requirements regarding fire protection measures vary among these various standards, regulations, and laws. As a result, the fire protection measures implemented in actual engineering designs differ significantly, leading to many disputes regarding the compliance of such designs with relevant regulations and their fire safety viability. This undoubtedly introduces uncertainty into the fire prevention effectiveness, poses risks to safe production, and also makes the effectiveness of the investment unclear. The author believes that fire prevention measures should first comply with the requirements of local safety regulations and laws, the standard specifications of this industry, as well as the requirements of the enterprise group, with strict provisions being adopted for implementation. On this basis, fire prevention measures must be understood in their essence. Fire protection measures stem from the functional and performance requirements for root valve control in order to achieve safe, stable, and long-term production. Main functions and performance requirements: In the event of an accident, when the valve is located in the fire area, it should be possible to remotely close the tank bottom valve within a limited time, or the tank bottom valve should close automatically due to heat ； The valve body does not crack and leak, the internal components of the valve do not leak internally, and there is no external leakage at the valve cover. Fire prevention measures are the specific methods to ensure the aforementioned functions. Fire protection measures include valve body fire protection and actuator fire protection ； Secondly, this includes fire protection for the control signal cables and power cables of remote control valves. The main measures are as follows. 6.2.1 Fire protection measures for the valve body The fire protection measures for the valve body are as follows: 1) The pressure rating shall be no less than PN20 (ASME CL150). 2) The valve body material shall be at least cast steel. 3) The entire valve body shall comply with the fire resistance test standards of API 607:2016 or API 6FA:2018. 4) A metal-to-metal hard seal should be used between the valve seat and the valve core, or such a hard seal should be achieved between them in the event of fire and heat exposure. The leakage class shall comply with API 598:2016 Valve inspection and testing, or meet FCI 70-2 Class V. 5) The piping connection method shall be flange connection or welding; clamped connection methods are not permitted. 6.2.2 Fire protection measures for actuators The fire protection measures for actuators are as follows: 1) Electric and electro-hydraulic actuators shall be equipped with fire shields ； Flame-retardant cables should be used for power cables and control cables, or they should be provided with flame-retardant protection ； The electric actuator should be equipped with a dedicated power supply. 2) The FL-type double-acting dual-cylinder actuator shall be equipped with a fire shield and an emergency air tank. The control signal cables shall be fire-resistant cables or provided with fire-resistant protection. Some regulations require that, in addition to a fire shield, this type of pneumatic actuator be equipped with a fusible plug in the cylinder or control air circuit as well. In the author’s opinion, there is no need to add a fusible plug; this is dictated by the operating principle of the double-acting, twin-cylinder pneumatic actuator. 3) The FC-type single-acting spring-return single-cylinder actuator does not require a fire shield; the control signal cable does not need to be a flame-retardant cable and is not provided with flame-retardant protection, instead a fusible plug is installed in the cylinder or the control air line. During a fire, the fusible plug melts, the actuator loses pressure, and the valve closes on its own due to the force of the spring. The melting point of the fuse plug should be 250 °C. Some regulations require that this type of pneumatic actuator be equipped not only with a fusible plug but also with a fire shield. The author believes that, merely to achieve fire safety objectives, it is sufficient to equip such pneumatic actuators with fusible plugs; there is no need for an additional fire shield, as the purpose of closing the valve has already been achieved. The fire resistance of the fire shield meets UL1709:2017 standards; it can resist hydrocarbon fires for 30 minutes at 1,093 °C, and the temperature inside the area protected by the fire shield does not exceed 70 °C (or the maximum operating temperature of the selected actuator). 4) Burial of cables underground is an effective measure for fire protection of cables; cables in the tank farm of new projects should be buried underground. 7 Selection of Other Accessories 7.1 Selection of Valve Covers and Sealing Gaskets For the valve cover types and sealing gaskets used in tank bottom control valves, the following recommendations are given: 1) For ordinary oils, enhanced flexible graphite (ρ≥1,360 kg/m3) sealing gaskets should be used, along with upper and lower sealing rings. Other sealing fillers selected must meet the requirements of API607:2016 or API6FA:2018. 2) When the process medium is highly toxic substances such as benzene or liquid ammonia, or when the wH2S concentration is ≥1×10-3, low leakage at the valve cover is required; the leakage standards shall comply with those specified in ISO 15848-2:2006 Industrial valves—measurement test and qualification procedures for fugitive emissions part 2: production acceptance test of valves. Straight-stroke valves can adopt the following low-leakage measures: for valves with a diameter of no more than DN 80, bellows sealing is used ； Valves with a diameter larger than DN 80 use double-layer sealing packing. 7.2 Handwheel and Valve Position Switch: The bottom of the tank control valve shall be equipped with a handwheel mechanism. The handwheel should have a clutch mechanism. Lead seals or locks are used on a daily basis to prevent accidental operations. The tank root control valve should be equipped with valve position switches, including an open position switch and a closed position switch ； An override timeout alarm should be provided at the control room operator station to ensure that the valve actually closes as commanded. 7.3 On-site remote control switch: In accordance with the safety requirements of the tank bottom control valve as well as standards such as AQ 3053—2015, the tank bottom control valve should have the capability to be shut down remotely from the control room as well as on-site. A on-site remote control switch is installed at the inspection entrance outside the fire dike of the tank area. The remote control switch should provide two pairs of contacts, one of which is connected to the valve control box to directly control the closing of the valve ； The other pair is connected to the control room for alarm, recording, and interlock control. It is recommended that this remote control switch be designed as part of the specification sheet/data sheet for the control valve at the tank base, and be provided together with the control valve set, in order to reduce the number of interfaces, ensure proper wiring and cabling, and accurately achieve the desired functions. 8 Conclusion The above mainly outlines the additional or special requirements for tank root control valves; the common requirements for control valves are not repeated here, and relevant standards for selection and installation should be followed. The author believes that regarding the technical requirements for tank root control valves, China is still in a process of continuous understanding, improvement, and standardization. With economic development, improved safety awareness, and advancements in manufacturing technology, the technical requirements for tank root control valves will also change, and the design solutions will continue to be improved.