In the fluid industry, storage tanks are widely used as storage devices. Whether for petroleum, natural gas, chemical products, or liquefied gases, tanks play an indispensable role. To ensure the safe and efficient operation of storage tanks, instruments must be used to measure key parameters such as liquid level, temperature, density, and pressure. These instruments not only help accurately calculate the volume and mass inventory of stored liquids but also enable the control of inlet and outlet valves and the measurement of inflow and outflow rates. Therefore, reasonable setup and configuration of storage tank instruments are crucial to ensuring the safe operation of the fluid industry. This article will detail the principles of instrument settings for different types of storage tanks and relevant safety regulations to ensure safe and reliable tank operation.
Storage tanks are essential components in the fluid industry, primarily used to store various liquid or gaseous media. These media may be flammable, explosive, or toxic, making safe tank operation extremely important. Instrumentation is key to ensuring the safe operation of storage tanks. By accurately measuring and monitoring parameters such as liquid level, temperature, and pressure inside the tank, accidents can be effectively prevented, ensuring personnel safety and normal equipment operation.
Liquid level is one of the most basic parameters in tank operation. Accurate level measurement can prevent tank overflow or run-dry conditions, avoiding safety incidents caused by abnormal levels. For example, if a tank overflows, it may cause fires, explosions, or environmental pollution; if it runs dry, it may damage equipment and affect production efficiency.
Temperature measurement is equally important for the safe operation of storage tanks. Many physical properties of liquids and gases change with temperature, such as density, viscosity, and vapor pressure. By monitoring the tank temperature, it is possible to ensure that the medium is stored and transported within a safe temperature range, preventing dangerous situations caused by excessively high or low temperatures.
For pressurized tanks, pressure measurement is essential. Excessive pressure may cause the tank to rupture or explode, while too low pressure may affect the transport efficiency of the medium. By installing pressure gauges and sensors, pressure changes inside the tank can be monitored in real time, and appropriate measures can be taken promptly to maintain safe operation.
Tanks typically store flammable and hazardous materials, and their instrumentation must be closely related to the characteristics of the media. Below are some basic requirements for tank instrumentation:
Atmospheric and Low-Pressure Tanks: Should be equipped with level gauges, thermometers, and high-level alarms. For tanks with a volume of 10,000 m³ or more, a high-high level alarm must also be set and interlocked with the feed pipeline control valve to prevent overflow.
High-Level Alarm Setting: Should correspond to the tank's design storage level.
High-High-Level Alarm Setting: Should be calculated based on the maximum feed rate over 10–15 minutes plus a 0.3 m safety margin.
Low-Level Alarm: Should ensure that tank run-dry does not occur within 10–15 minutes.
Thermometer Installation Position: For horizontal and spherical tanks, the thermometer should be installed to measure the liquid phase temperature even at the lowest level, and should be easy to observe and maintain.
Low-Pressure Tanks: Should be equipped with pressure gauges to monitor internal pressure changes.
Installation Requirement: The pressure gauge must be directly connected to the tank, without any other fittings or pipelines connected in between.
After clarifying the basic principles of tank instrumentation, we now explore specific instrumentation requirements for various tank types. The fluid industry uses a variety of tanks, each with different structures, media, and working environments, requiring unique instrumentation setups. Understanding these specific requirements is key to ensuring safe and efficient tank management. Below, we analyze dome roof tanks, horizontal tanks, ambient gas tanks, liquefied hydrocarbon sphere tanks, floating roof tanks, LNG tanks, and liquid ammonia tanks, focusing on instrumentation details and key points.
Instrumentation for dome roof tanks mainly includes level control, pressure control, level interlock, and temperature alarm systems. Specific requirements include:
Level Measurement: Typically uses radar, servo, magnetostrictive, or RF admittance level gauges.
Pressure Control: Usually nitrogen-sealed with segmented pressure control.
Temperature Alarm: Installed to monitor temperature variations in the tank.
Instrumentation generally includes level control, level interlock, and pressure interlock; temperature alarms may also be necessary.
Level Gauges: Common types include float-type, magnetostrictive, RF admittance level gauges, level switches, and local level indicators.
Pressure Interlock: A pressure interlock system should be installed to ensure automatic action in case of abnormal pressure.
Ambient gas tanks (e.g., for compressed air) typically require pressure and temperature instruments. When necessary, a level measurement device should be installed at the tank's bottom for dewatering operations to ensure safety.
Instrumentation requirements are stricter:
Temperature Measurement: Both local and remote thermometers must be installed and capable of measuring liquid phase temperature at minimum liquid level. Must also be easy to monitor and maintain.
Safety Devices: Must be equipped with safety valves, level gauges, pressure gauges, and thermometers.
Pressure Measurement: Local and remote pressure gauges must be installed with a separate high-pressure alarm. No other fittings or pipes should connect between the gauge and the tank.
Level Measurement: Both local and remote level gauges must be used. Glass plate gauges are not permitted. Instruments must be safe, reliable, and should minimize tank penetrations.
High-Level Alarm: Set to the tank's design storage level.
High-High-Level Alarm: Set below 90% of the tank's calculated volume.
High-High-Level Interlock: Detection elements should be independently installed (e.g., ultrasonic, tuning fork, float, or capacitive switches) and may be combined with high-high signals from radar or servo level gauges using a 2-out-of-3 logic to shut off feed lines. Elements should support online verification.
Valve Requirements: Valve materials for LPG should preferably be carbon steel, with properties equivalent to the tank body's low-temperature and H₂S corrosion resistance. Shutoff valves should be globe valves; if using gate or ball valves, cavity pressure relief is required. Design pressure must not be less than 2.5 MPa. Emergency shutoff valves must be installed near tank inlets/outlets.
Instrumentation includes level control, level interlock, and temperature alarm:
Level Measurement: Typically uses radar, servo, magnetostrictive, or RF admittance gauges. These must be installed with stilling wells; steel tape float gauges are also allowed.
Pressure Control: Internal floating roof tanks may require nitrogen seals and segmented pressure control.
Temperature Alarm: A temperature alarm system should be installed.
LNG tanks are usually double-walled with insulation between the inner and outer shells.
Level Measurement: Two independent level gauges are required. Due to LNG's unstable density, differential pressure gauges are not suitable.
Temperature Measurement: A bottom temperature monitoring system should be installed to measure surface temperatures and monitor the insulation and foundation heating systems.
Cryogenic Protection: For connections, flanges, valves, seals, or other non-welded joints, leakage of cryogenic media must be considered, and components must be cryogenically rated or otherwise protected.
Instrumentation requirements are strict:
Level Measurement: At least two different types of level instruments must be installed, along with high and low level alarm circuits. Interlocks with related process parameters should be provided if necessary.
Level Gauge Design: Should allow for replacement without affecting tank operation. For example, servo gauges should include shutoff valves and calibration chambers to allow maintenance by retracting the float into the chamber.
High-Level Alarms: Tanks should have two independent high-level alarms, which can be integrated into level gauges. Alarm points must allow operators enough time to stop inflow before exceeding the maximum allowable filling height. Alarms must be audible from operator locations and cannot be replaced by high-high level alarms.
Pressure Gauges: Except for floating roof tanks, all tanks must have pressure gauges with pressure taps above the highest liquid level.
Vacuum Jacket Equipment: Should have instruments or interfaces to monitor absolute pressure in the annular space.
Thermometers: Tanks must be equipped with thermometers to assist in temperature control during startup or as a method to verify or calibrate level gauges.
Cryogenic Foundations: Cryogenic tanks and equipment foundations may be affected by frost or soil freezing and should have temperature monitoring systems.
Instrument Failure Protection: Instrumentation for liquefaction, storage, and vaporization systems must allow the system to remain in a safe state during power or air supply failure, guiding operators to take appropriate actions.
Sealing and Isolation: All seals, isolators, and other measures must prevent flammable liquids from flowing along piping, conduits, or cables to ensure overall tank safety.
Beyond general tank instrumentation, special working conditions may require additional considerations due to the nature of the stored media or specific process requirements. Below are key points for common special scenarios.
When storing H₂S-containing hydrocarbons, materials must account for stress corrosion. The number of bottom interfaces should be minimized to reduce leakage risks.
Material Selection: Pressure-bearing parts must have excellent resistance to stress corrosion. Stainless steel or other corrosion-resistant materials should be used if necessary.
Interface Setup: Minimize the number of bottom interfaces without compromising process requirements.
When storing unstable substances like olefins or diolefins, precautions must be taken to prevent the formation of peroxides and polymers.
Preventing Peroxide Formation: Add antioxidants or stabilizers as needed.
Preventing Polymer Formation: Strict temperature control is required, with an alarm system to avoid overheating or prolonged high-temperature conditions.
Proper instrumentation setup is essential to ensure the safe and efficient operation of storage tanks in the fluid industry. Through reasonable configuration of level, pressure, temperature, and density measurement instruments, tank safety risks can be effectively controlled, and operational efficiency improved. At the same time, for different types of tanks and special working conditions, corresponding instrumentation strategies must be formulated to meet technical requirements and comply with safety regulations. With technological advancements, future tank instrumentation will become more intelligent and automated, providing more reliable safety assurance for the development of the fluid industry.
