Pressure vessel classification and pressure vessel characteristics

Pressure vessels, in a broad sense, refer to all closed equipment that withstands the pressure of liquid media.

Pressure vessels are widely used in industry, particularly in the petrochemical industry and other industrial production processes. They are pressure-bearing equipment with specific functions, used to complete reactions, heat transfer, mass transfer, separation, storage, and transportation. They are not only key equipment in the energy, petrochemical, military, and scientific research sectors, but are also very common in the civilian sector, such as gas or liquefied petroleum gas storage tanks, various accumulators, heat exchangers, separators, and large-scale pipeline projects.

1. Definition of pressure vessel

According to the "Regulations on Safety Technical Supervision of Pressure Vessels," equipment is considered a pressure vessel only if it meets the following three conditions: a maximum operating pressure of no less than 0.1 MPa, a volume of no less than 25 liters, a working medium of gas or liquefied gas, and a liquid with a maximum operating temperature above the standard boiling point (the boiling point at normal pressure). Equipment that does not meet these conditions is considered a normal pressure vessel.

Pressure vessels are mostly cylindrical, with a few being spherical or other special shapes. Cylindrical vessels are typically composed of a cylinder, a head, a pipe, a flange, and other components. The cylinder wall thickness increases with increasing operating pressure.

2. Classification of pressure vessels

Pressure vessels can be classified in a variety of ways according to different standards:

1. Classification by manufacturing method : can be divided into welded containers, forged containers, riveted containers, cast containers and combined containers.

2. Classification by material : divided into steel containers, non-ferrous metal containers and non-metallic containers.

3. Classification by wall thickness : There are two types: thin-walled containers and thick-walled containers.

4. Classification by geometric shape : mainly includes spherical containers, cylindrical containers, conical containers, etc.

5. Classification by pressure bearing method : divided into internal pressure vessels that bear internal pressure and external pressure vessels that bear external pressure.

6. Classification by design pressure level :

   • Low-pressure container (L): 0.1 MPa ≤ p < 1.6 MPa

   • Medium pressure vessel (M): 1.6 MPa ≤ p < 10 MPa

   • High-pressure vessel (H): 10 MPa ≤ p < 100 MPa

   • Ultra-high pressure vessel (U): p ≥ 100 MPa

7. Classification based on comprehensive factors (such as pressure, volume, medium hazard and importance in production) :

   • Class I container : a low-pressure container containing non-flammable or non-toxic media; or a low-pressure separation/heat exchange container containing flammable or toxic media.

   • Class II containers : meet any of the following conditions: medium-pressure containers; low-pressure containers containing highly toxic media; low-pressure reaction vessels and storage tanks containing flammable or toxic media; low-pressure waste heat boilers with an inner diameter of less than 1 meter.

   • Class III containers : meet any of the following conditions: high-pressure or ultra-high-pressure containers; medium-pressure reaction vessels, storage tanks or tank trucks containing flammable or toxic media; large low-pressure containers and medium-pressure containers containing highly toxic media; medium-pressure waste heat boilers or low-pressure waste heat boilers with an inner diameter greater than 1 meter.

8. According to the working temperature range : there are high temperature containers (200-500℃), normal temperature containers (natural ambient temperature) and low temperature containers (-20～-253℃).

9. Classification by process function :

   • Reaction pressure vessel : used to complete the physical and chemical reactions of the medium, such as reactors, generators, decomposition pots, etc.

   • Heat exchange pressure vessel : used for heat exchange of media, such as waste heat boilers, heat exchangers, coolers, etc.

   • Separation pressure vessels : used for medium pressure balance, purification and separation, such as separators, filters, oil collectors, etc.

   • Storage pressure vessels : used to contain gaseous, liquid or liquefied raw materials for production or daily life, such as various storage tanks and tank trucks.

10. Classification by installation method :

    • Fixed pressure vessel : has a fixed installation and use location, and the process conditions and operators are relatively fixed.

    • Mobile pressure vessels : They not only withstand internal or external pressure during use, but are also subject to impact caused by the shaking of the internal medium during handling, as well as external impact and vibration loads caused by transportation. Therefore, they have special requirements in terms of structure, use and safety.

3. Characteristics of pressure vessels

1. Harsh operating conditions : reflected in three aspects: load, temperature, and medium. ① Load: In addition to static loads, low-cycle fatigue loads are often also encountered. ② Temperature: Operation may occur in high or low temperature environments. ③ Medium: The medium is diverse, including air, water vapor, hydrogen sulfide, liquefied petroleum gas, liquid ammonia, liquid chlorine, various acids, and alkalis, many of which are corrosive, toxic, flammable, or explosive.

2. Accident susceptibility : ① Prone to overload: The pressure inside the container may rise rapidly due to operational errors or abnormal reactions, often leading to damage without being noticed; ② Complex local stress: Stress concentration is easily generated at structural discontinuities such as openings and pipes, and repeated pressurization and unloading may cause damage; ③ The existence of hidden defects: Defects such as tiny cracks generated during manufacturing may expand during use, or cause sudden failure under specific conditions; ④ Strict operating conditions.

3. Widely used and requiring continuous operation : Pressure vessels typically require long-term continuous operation and cannot be shut down for maintenance. Unexpected downtime can severely impact the production and livelihood of an entire production line, a whole factory, or even a region, leading to significant direct and indirect economic losses.

4. Structural composition of pressure vessels

Pressure vessels come in a variety of shapes, the most common being cylindrical, spherical, and conical. Their pressure-bearing capacity is related to their wall thickness. Taking a cylindrical pressure vessel as an example, its main components include:

1. Cylinder : It is the main body that forms the pressure space. Its size is determined by the process requirements. In addition to cylindrical, its shape can also be conical or spherical.

2. Head : It is the component that closes the end of the cylinder. Common shapes include spherical, elliptical, dished and flat cap. When connecting to the cylinder, spherical or elliptical heads are usually used. Flat cap heads are generally not allowed.

3. Flange : It is a key component for connecting containers and pipelines, and is sealed by bolting and tightening gaskets.

4. Sealing element : Placed between the flange or head and the end of the cylinder, tightened by bolts to ensure sealing. Metal (such as copper, aluminum, mild steel), non-metal (such as asbestos, rubber), or combined (such as iron-clad asbestos) sealing elements are selected according to the working pressure, medium, and temperature.

5. Openings and connections : To meet the needs of process, maintenance and installation of accessories (such as pressure gauges and safety valves), manholes, sight glass holes, material holes and various connection ports are opened on the cylinder or head.

6. Support : It is the basic component that supports and fixes the container. The form depends on the installation method. Common types include saddle type, support type, suspension type, skirt type, etc. Spherical containers mostly use column or skirt type supports.

5. Selection principles for pressure vessel steel

Pressure vessels are widely used and operate under complex conditions, making proper material selection a critical and complex step in their design. Many accidents stem from improper material selection. For example, steel with poor weldability is prone to cracking; improper selection of certain stainless steels can lead to intergranular corrosion or stress corrosion; and ferritic steels used at low temperatures are susceptible to brittle failure if their transformation temperature exceeds the operating temperature. Therefore, material selection must comprehensively consider operating conditions (wall temperature, pressure, corrosiveness of the medium, embrittlement effects, flammability, explosiveness, and toxicity). Materials with satisfactory mechanical and physical properties, corrosion resistance, and processability (especially weldability) must be selected, while also considering cost-effectiveness and availability. It is important to note that the vast majority of pressure vessel materials are put into use after rolling, forging, forming, welding, and heat treatment.

1. Principles for selecting quality technology :

   • ① The selection must be made based on the equipment operating conditions (pressure, temperature, medium characteristics), material welding and processing performance, heat treatment requirements and container structure.

   • ② On the premise of meeting condition ①, strive for economic rationality: a. When the plate thickness is less than 8mm, carbon steel is preferred (except for multi-layer containers); b. When the stiffness or structural design is mainly based, ordinary carbon steel (such as Q235A, Q235B) is used as much as possible; when the strength design is mainly based, 20R (or 20g), 16MnR, etc. are used in turn; c. When the stainless steel thickness is greater than 12mm, lining, composite or surfacing structure are given priority; d. Avoid using stainless steel as heat-resistant steel ≤500℃ as much as possible; e. Minimize the varieties and specifications of pearlite heat-resistant steel and do not use it in heat-resistant or hydrogen-resistant occasions ≤350℃ unless necessary.

   • ③ General Selection Guide: a. Carbon steel is used for medium-pressure vessels and non-pressure parts with normal pressure, low pressure, low corrosion, and small wall thickness; b. Low-alloy high-strength steel is used for pressure vessels with a wall thickness ≥8mm and low corrosion; c. Pearlitic heat-resistant steel is used for high-temperature hydrogen/hydrogen sulfide corrosion resistance or heat-resistant applications of 350–650°C; d. Stainless steel is used for high-corrosion, iron ion contamination resistance, and heat-resistant/low-temperature applications >500°C or <-100°C; e. Austenitic stainless steel that does not contain stabilizing elements and has a carbon content greater than 0.03% should not be used in intergranular corrosion environments after welding or hot working above 400°C.

   • ④ Steel materials must comply with relevant standards.

   • ⑤ Steel used for standard parts (such as flanges, pipe fittings, manholes, and liquid level gauges) must comply with the requirements of relevant national or industry standards.

2. Economic selection principle :
   Equipment costs are largely influenced by material prices. Pressure vessel steel prices vary significantly. Taking the price of carbon steel Q235-A as a benchmark (1), the relative prices of other steels are roughly as follows: 16MnR (approximately 1.4), 20R (20g) (approximately 1.8), chrome steel (1Cr13, 2Cr13) (approximately 5.1), and high alloy steel 0Cr18Ni10 (approximately 14.1).
   However, price isn't the only factor. High-performance materials can offer advantages such as thinner walls, lighter weight, and longer lifespan, resulting in better overall economic benefits. Economic analysis also needs to consider national resource availability, prioritizing the use of readily available, domestically produced materials while minimizing the use of expensive, scarce, or imported materials.