A pressure vessel refers to a sealed device that contains gas or liquid and bears a certain pressure. As an indispensable key equipment in industrial production, it is widely used in petroleum, chemical, energy, pharmaceutical, food and other industries. Since pressure vessels usually store high-pressure, high-temperature, flammable, explosive or toxic media inside, the consequences would be disastrous in the event of rupture. Therefore, countries around the world have extremely strict laws and regulations governing the design, manufacture, use and inspection of pressure vessels.
1. Definition and Classification Criteria
Basic Characteristics
- Working Pressure (p): greater than or equal to 0.1 MPa (gauge pressure).
- Volume (V): greater than or equal to 0.03 m³ (30 liters), and the inner diameter (maximum geometric dimension for non-circular sections) is greater than or equal to 150 mm.
- Medium: gas, liquefied gas, or liquid whose maximum working temperature is higher than or equal to its standard boiling point.
2. Classification Methods
The classification of pressure vessels is rather complex. They are generally classified by pressure level, hazard level of the medium, structural form and function in the production process.
A. By Pressure Level
- Low Pressure Vessel (L): 0.1 ≤ p < 1.6 MPa
- Medium Pressure Vessel (M): 1.6 ≤ p < 10.0 MPa
- High Pressure Vessel (H): 10.0 ≤ p < 100.0 MPa
- Ultrahigh Pressure Vessel (U): p ≥ 100.0 MPa
B. By Hazard Level of Medium
- Group 1 Medium: chemical media of extreme or high toxicity, explosive media, liquefied gases.
- Group 2 Medium: media other than Group 1 (e.g., ordinary water, air, etc.).
C. By Structural Form
- Single-layer Vessel: fabricated by rolling and welding single-layer steel plates, the most common type.
- Multi-layer Vessel: constructed by wrapping multiple thin plates or strip winding, used for high-pressure service.
- Clad Vessel: composed of a base layer (bearing pressure) and a clad layer (corrosion resistance).
D. By Function in Process
- Reaction Pressure Vessel (R): used to carry out physical and chemical reactions of media.Examples: reactors, reaction kettles, synthesis towers.
- Heat Exchange Pressure Vessel (E): used to accomplish heat exchange between media.Examples: shell-and-tube waste heat boilers, heat exchangers, coolers.
- Separation Pressure Vessel (S): used for fluid pressure balance, buffering and gas purification/separation.Examples: separators, filters, drying towers.
- Storage Pressure Vessel (C): used for storing and containing gases, liquids, liquefied gases and other media.Examples: various storage tanks, buffer tanks.
3. Main Components
Although pressure vessels vary in shape, their basic structure generally includes the following parts:
- Shell: the main body of the pressure vessel, used for storing materials or conducting chemical reactions.
- Head: component that closes both ends of the shell; common types include elliptical, dished, spherical and flat heads.
- Flange: used to connect the shell with the head or with pipelines, facilitating disassembly and maintenance.
- Nozzle and Opening: used for medium inlet/outlet, instrument installation, etc.
- Sealing Elements: such as gaskets and O-rings, to prevent medium leakage.
- Support: bears the weight of the vessel and fixes it to the foundation, e.g., saddle supports, skirt supports.
- Safety Accessories (Critical):
- Safety valve: automatically relieves pressure when overpressure occurs.
- Rupture disc: ruptures instantly to release pressure under overpressure (often used for toxic or viscous media).
- Pressure gauge: indicates internal pressure.
- Level gauge: shows internal liquid level.
- Thermometer: monitors internal temperature.
4. Common Materials
Pressure vessels have high requirements for material strength, toughness, corrosion resistance and weldability.
- Steel: the most widely used, such as carbon steel (Q345R), low-alloy steel (16MnR), stainless steel (304, 316L).
- Non-ferrous Metals: such as titanium, aluminum, copper, used in special corrosion-resistant environments.
- Non-metallic Materials: such as fiberglass reinforced plastic (FRP), plastics, used in low-pressure corrosion-resistant applications.
5. Safety and Supervision
Pressure vessels are classified as special equipment, subject to strict national supervision:
- Design: must be designed by a qualified entity and submitted to relevant authorities for approval.
- Manufacture: manufacturers must hold a pressure vessel manufacturing license.
- Inspection:
- Factory Inspection: hydrostatic test or pneumatic tightness test is mandatory.
- Periodic Inspection: during service, a comprehensive inspection must be conducted by a third-party inspection agency at regular intervals (e.g., every 3–6 years) to ensure safety.
