Spherical tank is a structural form of pressure vessel, so it is the same as pressure vessel in the basic requirements of material selection. Compared with other pressure vessels, the spherical tank has its special characteristics: firstly, the volume of the spherical tank is large, and the length of the weld seam should be reduced as much as possible; the steel plate must have sufficient width, length and thickness; secondly, the safety requirements of the spherical tank are higher The occurrence of safety failure, which has great impact and hazard, requires higher plasticity and toughness reserves of the material; the third spherical tank is field welded, with many welds, high local stress, and high weldability requirements for the welding material.

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Because of the inherent characteristics of spherical tanks, special requirements must be put on the materials. The selection of spherical tanks should mainly consider the use conditions of the spherical tanks (such as design temperature, design pressure, structural characteristics, etc.), the welding performance of the materials, and the Manufacturing process and assembly welding requirements and economic rationality. Spherical tank steel should be calm steel, and boiling steel should never be used. The following problems should be solved.

(1) Intensity level

Strength mainly refers to the yield strength ReL and tensile strength Rm of a material. At present, there are international classifications based on yield strength as a material level and also tensile strengths as a material level classification. For example, Q 345R and Q370R are classified by yield strength. CF62, 610U2, 610F are classified according to tensile strength.

For spherical tanks of the same specification, the higher-level materials are used, the thickness of the spherical shell plate is thinner, and the weight of the spherical tank is lighter, but the higher-level materials have higher unit prices, are more difficult to form and group weld, and have poor welding performance Therefore, the selection of materials should be considered from the aspects of the volume of the spherical tank, the nature of the storage materials, and the thickness of the spherical shell plate.

(2) toughness index

Toughness is an important indicator to ensure that the material avoids cracks and rapid fractures. It is especially important for spherical tanks. There are many toughness indicators, including V-notch impact test, drop weight test (NDT temperature), wide plate tensile test considering "turning temperature", COD and K1C considering "fracture toughness", etc. Due to the variety of toughness indicators, they cannot be unified due to different test methods. However, countries around the world practically associate various toughness indicators with the absorbed energy of the V-notch impact test in order to achieve simple and convenient purposes. Spherical tanks should have a high toughness reserve for safety reasons.

(3) Plasticity index

Spherical tanks must undergo various deformations during the forming process. The good plasticity of steel is a prerequisite for the manufacture of spherical shell plates. It must have a certain plasticity, that is, a certain elongation (δ) and sectional shrinkage (ψ ).

(4) Solderability

Weldability indicators of steel are as follows.

①Carbon equivalent CE The formula of carbon equivalent recommended by the International Welding Institute (IIW) for low alloy steel is

Generally, the CE of low-alloy high-strength steel with tensile strength of 610 MPa is controlled to be less than or equal to 0.40.

② The crack sensitivity index Pc carbon equivalent has a certain relationship with the hardness of the heat-affected zone of welding. However, a large number of research results have pointed out that the possibility of crack occurrence is not comprehensive enough. Crack sensitivity index Pc. Pc is calculated as follows:

Where δ is the thickness of the steel plate, mm;

Additional resources:
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H-1 H content in the weld, mL / 100g.

The applicable range of formula (7-2) is: wc = 0.07% ~ 0.22%, ωMn = 0.40% ~ 1.40%, ωSi = 0 ~0.6%, ωCu = 0 ~ 0.5%, ωCr = 0 ~ 1.2%, ωv = 0 ~ 0.12%, ωNi = 0 ~ 0.05%, ωB = 0 ~ 0.005%, δ = 19 ~ 50mm, H = 1.0 ~ 50mL / 100g. A large number of test results show that the probability of crack occurrence is high under the condition of PC> 0.35, and the probability of crack occurrence is small when PC <0.30 (of course, the effect of preheating should also be considered). From PC, the preheating temperature to prevent cracks and its empirical relationship are

Where T is the preheating temperature to prevent cracks in the Y-shaped groove restraint test, ℃.

③ Crack sensitivity coefficient Pcm The crack sensitivity index Pc includes the two items of plate thickness and the amount of diffusible hydrogen in the weld metal, which has little relationship with the material. Therefore, these two items are omitted when considering the welding crack of the plate. After cancellation, the crack sensitivity coefficient Pcm was induced. Pcm is essentially a formula for calculating carbon equivalent, and is especially suitable for high-strength steels with a yield strength greater than 390 MPa.

The research on crack sensitivity coefficient in Japan is very deep. According to the requirement of Pcm ≤ 0.20, low crack sensitivity steel has been studied, and it has been widely used in spherical tanks.

(5) Economical

Various special requirements are put on the steel for spherical tanks, which will certainly increase the cost economically. The price of steel for spherical tanks accounts for a large proportion of the total investment in spherical tanks. Therefore, in the selection of spherical tank materials, economics must be a key consideration.

(6) Corrosion resistance

The corrosion damage caused by storage materials to the spherical tank is very serious. Corrosion will damage the surface of the spherical tank, and cause corrosion pits, grooves and even cracks. Some even erode into the metal and change its structure, which deteriorates the mechanical properties of the steel. It may seriously cause the spherical tank to fail due to corrosion, or even cause a major accident.

There are many reasons for corrosion, so there are many types of corrosion, such as general corrosion, intergranular corrosion, pitting corrosion, alkali embrittlement, hydrogen embrittlement, stress corrosion, and corrosion fatigue. It should be noted that in recent years, H2S stress corrosion has become the main form of corrosion damage of spherical tanks, so the problem of corrosion resistance must be considered from the selection of materials. If H2S is contained in materials, steels with lower strength levels should be selected as much as possible

Comprehensive Guide to Storage Tank Types and Measuring Instrumentation

Jul 19 ,

Storage tanks are extensively used in the fluid industry for storing raw materials, finished products, and intermediates. They play an irreplaceable role in ensuring safe production, energy conservation, emissions reduction, and enhancing overall management levels.

Industrial storage tanks are generally made of steel, with material choices such as carbon steel, low-temperature steel, and stainless steel based on the properties of the stored medium and storage parameters like temperature and pressure. Other materials like fiberglass and plastic are not considered due to factors such as fire resistance and pressure tolerance.

1. Types of Storage Tanks

Storage tanks can be classified based on their structure into spherical tanks, horizontal tanks, dome tanks, external floating roof tanks, and internal floating roof tanks.

1.1 Spherical Tanks

Spherical tanks are large-capacity, pressure-resistant spherical storage containers widely used in the petroleum, chemical, and metallurgical industries to store liquefied petroleum gas (LPG), liquefied natural gas (LNG), liquid oxygen, liquid ammonia, liquid nitrogen, and other media, as well as compressed gases such as air, oxygen, nitrogen, and city gas. They are used for storage at normal, low, or cryogenic temperatures and for materials with a saturated vapor pressure greater than atmospheric pressure at storage temperature.

Normal Temperature Spherical Tanks: Used for liquefied petroleum gas, nitrogen, city gas, oxygen, etc., with higher pressure depending on the saturated vapor pressure of the liquefied gas or the compressor’s outlet pressure. The design temperature is greater than -20°C.

Low Temperature Spherical Tanks: These tanks have a design temperature of -20°C or lower, typically not lower than -100°C.

Cryogenic Spherical Tanks: With a design temperature below -100°C, often stored below the liquefaction point of the medium. They have low pressure, sometimes atmospheric, and require high insulation, often using a double-layer spherical shell.

1.2 Horizontal Tanks

Horizontal tanks have a smaller volume (generally less than 100 m³) and occupy a larger area. They are primarily used for storing chemicals such as acids and bases, and for small-capacity storage of other media within production facilities. The tank body is parallel to the ground and typically supported by saddle supports. They are pressure vessels that can withstand higher positive and negative pressures.

1.3 Dome Tanks

Dome tanks have a spherical crown top and cylindrical body. They are simple to manufacture and cost-effective, widely used in various industries. The most common volumes range from 1,000 to 10,000 m³, with the largest dome tank in China reaching 30,000 m³. Dome tanks are generally low-pressure or atmospheric storage tanks used for storing Class IIB and Class III liquids and, in some cases, Class IB and Class IIA liquids with specific storage requirements.

1.4 Floating Roof Tanks

Floating roof tanks consist of a floating roof that rests on the surface of the stored medium and a vertical cylindrical tank wall. The floating roof rises and falls with the storage level, reducing evaporation. They are used for storing Class IB and Class IIA liquids with saturated vapor pressure lower than atmospheric pressure. There are two types of floating roof tanks: external and internal.

External Floating Roof Tanks: Typically used for storing volatile petroleum products like crude oil, gasoline, or kerosene. They reduce evaporation losses and environmental pollution and decrease the risk of explosive gas formation.

Internal Floating Roof Tanks: Combine features of dome and floating roof tanks. They have a fixed dome outside and a floating roof inside, providing advantages such as preventing contamination from external elements, reducing evaporation losses by 85%-96%, and lowering fire and explosion risks. They are suitable for storing high-grade gasoline, jet fuel, and toxic petrochemical products.

2. Tank Farm Classification

In the fluid industry, storage tanks are typically arranged in tank farms based on their usage or pressure levels.

By Medium and Pressure Levels: Classified into atmospheric tank farms, spherical tank farms, acid and alkali tank farms, and cryogenic tank farms.

By Usage: Divided into raw material tank farms, intermediate tank farms, and finished product tank farms.

Each tank farm is further subdivided into different tank groups based on tank types, medium properties, pressure levels, and safety requirements.

3. Common Measuring Instruments in Tank Farms and Storage

Storage tanks are essential in the fluid industry, and we generally measure their parameters such as liquid level, temperature, density, and pressure (for pressurized tanks) to calculate storage volume and mass, and control inlet and outlet valves and flow rates.They can be classified into intermediate tanks and trade tanks.

Intermediate Tanks: Monitored the level, temperature, and pressure to prevent accidents like tank overflow or vacuum formation.

Trade Tanks: High precision monitoring and measurement of level, temperature, density, volume, and mass are required for trade accuracy.

How to Select the Right Measuring Instrumentation?

Instrument selection should be based on environmental conditions, tank type, maintenance and operation cycles, and lifecycle costs to ensure optimal performance and cost-effectiveness.

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3.1. Level Instruments

Accurate level measurement is crucial for monitoring and calculating stored volume, and, if necessary, density compensation to determine storage mass.

3.1.1 Radar Level Transmitters

Radar level transmitters are non-contact instruments with no moving parts, providing reliable operation, especially suitable for high viscosity and highly corrosive media. They are ideal for large tanks, high-temperature, high-viscosity, and highly corrosive environments, offering high accuracy and fast response times. When installing, consider tank type, medium properties (dielectric constant, volatility, boiling, steam, dust, adhesion), and operating parameters (temperature, pressure, etc.).

3.1.2 External Ultrasonic Level Meters

External ultrasonic level meters consist of a main unit, probe, and metal structure, used for measuring liquid levels in rail tank cars, road tankers, and various storage tanks. They measure liquid level by transmitting ultrasonic waves through the tank wall and liquid, receiving echo signals from the liquid surface. They are ideal for high and low-level supplementary measurements without requiring tank penetration.

3.1.3 Magnetostrictive Level Meters

Magnetostrictive level meters feature high accuracy, installed on the top or side of the tank. They measure liquid levels using magnetic pulse waves. The float with a permanent magnet moves along the rod with the liquid level, creating a magnetic field that interacts with the pulse wave to generate a return pulse, indicating the level change. This type of level sensors are easy to install with little maintenance.

3.1.4 Capacitive and RF Admittance Level Meters

Capacitive level meters measure liquid levels based on changes in capacitance between electrodes and the conductive tank wall. They require stable dielectric constants in the measured medium.

Due to the tendency of capacitive electrodes to accumulate material during use, which generates additional capacitance and resistance directly affecting the performance of capacitive level meters, RF admittance level meters overcome this issue by using compensation technologies, available in rod, coaxial, cable, and heavy-duty cable forms, with various output signals and connection sizes.

3.1.5 Differential Pressure Level Transmitters

Differential pressure level measurement uses transmitters like blowdown, standard, single-flange, double-flange, and insertion flange transmitters based on specific conditions. Different types are suitable for specific applications.

4. Pressure Instruments

Pressure measurement in storage tanks is straightforward. Spherical tanks or pressurized dome tanks need pressure alarms or interlocks. Micro-positive pressure dome tanks or internal floating roof tanks generally require pressure measurement for nitrogen sealing. Industrial pressure transmitters are commonly used.

5. Flow Instruments

Flow instruments are generally not required in tank farms unless for metering purposes. Common instruments include mass flow meters and volumetric flow meters. Utility metering can use vortex flow meters or differential pressure devices with temperature and pressure compensation if needed.

By using these right instrumentation and following best practices in installation and maintenance, tank farms can achieve optimal automation and efficiency in storage operations.

Holykell has rich experience in this industrial automation. Reach out to us for more technical information.

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