The question of what is a pressure vessel, playing a central role in the operational continuity of industrial facilities, is technically answered as containers designed to store or process fluids with an internal pressure greater than the external pressure (atmospheric pressure) by more than 0.5 bar. These structures are not merely storage units; they are thermodynamic systems with high energy potential. The 0.5 bar threshold is accepted as a critical limit in international standards and local legislation for an equipment to be evaluated under pressure vessel status.When performing the classification of pressure vessels, engineers base their criteria on the equipment’s geometry, function, and the characteristics of the loads it is exposed to. This classification determines which standards (ASME Sec VIII, EN 13445, etc.) will be applied during the design phase. Asem Process creates an unshakeable safety margin by incorporating not only internal pressure but also external influences into its design calculations.
| Classification Criterion | Categories and Technical Details |
| Functional Structure | Storage tanks, process reactors, heat exchangers. |
| Geometric Form | Cylindrical, spherical, conical, or elliptical structures. |
| Wall Thickness | Thin-walled ($D/t > 20$) and thick-walled pressure vessels. |
| Thermal State | Fired (boilers) and unfired pressure vessels. |
| Material Structure | Carbon steel, alloy steels, stainless steel, composite, and non-ferrous metals. |
The design of a pressure vessel requires a comprehensive analysis of the loads acting upon it. While internal pressure is the primary design parameter, wind loads (especially for vertical vessels), seismic loads, the equipment’s own weight (dead load), the weight of the fluid inside, and operational vibrations are factors that determine the design life. Since the turbulent flow of wind can lead to fatigue stresses in tall vessels, these dynamic loads must be calculated with precision in leg and support designs.
Types of Pressure Vessels in Industrial Applications
In the modern production ecosystem, pressure vessel types are optimized according to the chemical nature of the process and physical requirements. Every sector, from oil refineries to food processing plants, uses equipment suitable for its own standards. Asem Process aims for maximum efficiency by offering high-engineering solutions within this diversity.
Air Tanks and Compressor Systems
Air tanks, the most critical component of compressed air systems, dampen pressure fluctuations from the compressor and store energy potentially. The safety of these tanks directly affects the operation of the facility. Compressor air tank inspection is vital for detecting corrosion buildup and fatigue in weld seams. Air tanks are generally manufactured from high-quality boiler plates such as P265GH, and their internal surfaces can be protected against corrosion with special coatings.
Industrial Autoclaves
Autoclaves, where sterilization and chemical reactions are carried out under high pressure and temperature, are distinguished from others by their complex lid mechanisms. Periodic control of autoclaves includes functional tests of the lid locking systems and sealing elements. These devices, used for pasteurization in the food industry and disinfection in the medical field, are evaluated in the high-risk group within the category of pressure vessels.
Expansion Tanks and Hydrophore Systems
Expansion tanks, which balance the volumetric increase caused by the heating of water in heating systems, are passive safety elements that prevent the system from exploding. Expansion tank maintenance is based on the integrity of the membrane inside the tank and keeping the pre-charge air pressure at the correct level (slightly above static pressure). A neglected expansion tank can cause water hammer shocks and pipe bursts in the installation.
Industrial Boilers and Heating Centers
Steam boilers, thermal oil boilers, and hot water systems are high-risk areas where energy is transferred. Periodic control of the boiler room covers not only the inspection of the vessel but the compliance of the entire heating center (burner, chimney, ventilation, safety devices). Automatic feeding systems that activate in case of low water levels and safety valves that discharge during overpressure are the fundamental protection mechanisms of the system.
Cryogenic Systems: Management of Ultra-Low Temperatures
Cryogenic technologies have revolutionized industrial logistics by enabling the storage and transport of gases in the liquid phase. Cryogenic systems cover equipment operating at temperatures near absolute zero (between -150°C and -273°C). The design of these systems has much more complex insulation and material requirements compared to standard pressure vessels.Asem Process produces high-performance cryogenic tanks for the safe storage of substances such as liquid nitrogen (LIN), liquid oxygen (LOX), liquid argon (LAR), and liquefied natural gas (LNG). These tanks operate on the “tank-in-tank” (double-walled) principle. The inner tank is manufactured from austenitic stainless steel (e.g., 304L or 316L) that does not undergo embrittlement at low temperatures, while the outer shell is usually made of carbon steel.
Insulation Technologies and Vacuum Performance
The success of a cryogenic system is measured by how well it can prevent heat gain from the external environment. Three main methods are applied in the space between the inner and outer tanks to minimize heat transfer:
- Vacuum Insulation: Reduces heat transfer via conduction and convection almost to zero.
- Perlite Filling: Perlite, a volcanic mineral used in conjunction with a vacuum, prevents heat transfer by radiation.
- Multi-Layer Insulation (MLI): Also known as “super insulation,” this method uses reflective foils and low-conductivity spacers.
| Parameter | Cryogenic Storage Standards |
| Design Temperature | -196 °C (Standard for Liquid Nitrogen). |
| Insulation Type | Vacuum + Perlite or Multilayer (MLI). |
| Inner Tank Material | Stainless Steel (EN 10028-7 / AISI 304-316). |
| Outer Tank Material | Carbon Steel (EN 10025 / EN 10028-3). |
| Design Codes | EN 13458, EN 13530, ASME, PED. |
Maintaining the vacuum level is critical for the operational safety of cryogenic tanks. Loss of vacuum can lead to sudden heat gain, causing the liquefied gas to rapidly change phase and increase pressure to dangerous levels. Therefore, safety devices (safety valves and rupture disks) are provided redundantly for both the inner and outer tanks in the system.
2014/68/EU and the Pressure Equipment Directive Framework
The Pressure Equipment Directive (2014/68/EU) (PED), which came into force within the framework of European Union harmonization laws, regulates all processes from the design to the placing on the market of these devices. This directive categorizes equipment according to their energy potential to determine the depth of inspection.According to the PED directive, equipment is classified from Category I (low risk) to Category IV (high risk). In this classification, the type of fluid (flammable, explosive, toxic, or non-hazardous), the volume or nominal diameter of the equipment, and the maximum allowable working pressure (PS) are the determining criteria.
- Category I: The manufacturer’s own declaration and internal production control are sufficient.
- Category II-IV: A Notified Body is involved; design approval, material verification, and final tests are subject to independent audit.
Asem Process carries out its production in full compliance with these regulations and certifies them with the CE mark. The directive requires non-destructive testing (NDT) and hydrostatic tests to be performed completely in the final assessment of the equipment.
Periodic Inspection and Audit Methodology of Pressure Vessels
It is inevitable that equipment put into operation will lose its strength over time due to reasons such as corrosion, wear, or fatigue. To manage these risks, the periodic inspection of pressure vessels emerges as a legal obligation. The regulation issued based on the Occupational Health and Safety Law No. 6331 stipulates that these controls must be performed by competent mechanical engineers.The periodic inspection period for pressure vessels is at least once a year, unless otherwise specified in the relevant standards. However, it may be recommended from an engineering perspective to shorten this period, considering the intensity of equipment use, the corrosive effect of the fluid inside, and environmental conditions.
What is a Hydrostatic Test? Application and Safety Parameters
The question of what is a hydrostatic test, which is the most critical stage of the audit process, can be summarized as testing the equipment using water at a pressure above the operating pressure. The basic principles of the hydrostatic test are:
- Test Pressure: Unless otherwise stated in the standards, it is applied as 1.5 times the operating pressure. For example, an air tank operating at 10 bar is tested at 15 bar.
- Fluid Selection: Water is preferred due to its incompressible property and safety. Tests performed with gas (pneumatic) are much more dangerous due to the enormous energy that would be released at the moment of explosion and are only performed with special precautions in mandatory cases (processes where water is not allowed).
- Waiting Time: Pressure is increased gradually and maintained at the test pressure for at least 30 minutes (15-30 minutes in some standards).
- Criteria: No leakage, sweating, or permanent deformation (swelling, bending) should occur in the equipment during the test.
Non-Destructive Testing (NDT) Alternatives
In cases where the hydrostatic test could damage the equipment or make the production process impossible (e.g., massive spherical tanks or vessels containing chemicals that react with water), non-destructive testing methods are used. Asem Process expertly applies the following NDT techniques in its quality control processes:
- Radiographic Testing (RT): Detection of internal defects by taking X-ray films of weld seams.
- Ultrasonic Testing (UT): Material thickness measurement and crack analysis using sound waves.
- Magnetic Particle (MT) and Penetrant Testing (PT): Making surface cracks visible.
- PMI Testing: Verification of the chemical composition of the material (alloy accuracy).
Safety Devices and Instrumentation in Pressure Vessels
For a pressure vessel to be described as “safe,” it must be equipped with active protection systems in addition to its mechanical strength. Safety devices in pressure vessels are components that detect and intervene in abnormal pressure increases in the system.
| Device Type | Function and Operating Principle |
| Safety Valve | A mechanical valve that opens automatically when the pressure exceeds the set value (usually 1.1 x $P_{operating}$) and closes after discharge. |
| Rupture Disk | A single-use safety element that tears to open the path during sudden pressure increases. |
| Pressure Gauge | An indicator subject to periodic calibration that allows the operator to monitor system pressure instantaneously. |
| Pressure Switch (Presostat) | An electrical switch that stops the pump or compressor when pressure rises. |
| Blowdown Valve | A valve that prevents corrosion by discharging sediment and water accumulated in boilers or tanks. |
It is a legal requirement to verify safety valves on a test bench or on-site during annual periodic inspections, and to record spring stiffnesses and opening pressures. Asem Process guarantees the safe discharge of all fluid in the event of a possible failure by basing the selection of safety devices on the discharge capacity (capacity calculation) of the equipment.
Operational Traceability: Air Tank Periodic Inspection Form and Reporting
The legal validity of all inspections and tests performed is registered with a report prepared in accordance with the procedures. The air tank periodic inspection form is a technical document containing the observations made by the auditor in the field and the test results. This form must include the following information:
- Equipment Information: Brand, model, serial number, year of manufacture, volume, and operating pressure.
- Inspection Points: Condition of weld seams, signs of corrosion, paint and insulation quality, integrity of legs and supports.
- Test Data: Applied hydrostatic pressure value, waiting time, ambient temperature during the test, and the serial number of the calibrated pressure gauge used.
- Result and Recommendation: Whether it is “suitable” for the equipment to operate until the next inspection date, and any deficiencies that need to be rectified.
Asem Process ensures that businesses do not miss inspection periods by integrating this reporting process with digital tracking systems in its field services.
Safety in Boiler Rooms: A Holistic Approach
Heating centers, where pressure vessels are most densely located, require a special inspection regime. Periodic control of the boiler room evaluates not only the steam or hot water boiler but the occupational health and safety conditions of the entire environment.Critical elements observed in boiler rooms:
- Ventilation: Providing the fresh air necessary for the boiler’s burner, and ensuring the compliance of lower and upper vents with standards (TS 7363, TS 2192).
- Gas Detection: Presence of ex-proof gas detectors and the connection of these detectors to the solenoid valve at the main gas inlet.
- Explosion Hatches: In liquid and gas-fueled boilers, the functionality of mechanical hatches that discharge sudden pressure increases that may occur inside the furnace.
- Emergency Scenarios: Ensuring that emergency stop buttons outside the boiler room and fire extinguishing systems are in working order.
Quality Standards in Design and Manufacturing: The Asem Process Vision
The life of a pressure vessel or cryogenic system is determined by the very first design line and the quality of the selected material. Asem Process follows a globally recognized quality policy by basing its production processes on ASME, EN, and ISO standards.In material selection, the chemical corrosiveness of the fluid and the operational temperature range are analyzed down to the finest detail. Especially in cryogenic tanks, the stainless steel structure of the inner tank and thermal expansion cycles in the piping system are the main elements determining the fatigue life of the equipment. WPQR (Welding Procedure Qualification Record) and welder certifications applied during the manufacturing phase ensure that every seam meets the highest strength values.
Conclusion and Future Projection
Digital transformation in industry is also changing the face of pressure vessels and cryogenic systems. Now, “smart” tanks can report wall thickness, corrosion rate, and vacuum level instantaneously to central control systems via sensors. This allows periodic inspections to evolve from being merely calendar-dependent to a “condition-based predictive maintenance” model.However, no matter how much technology advances, physical inspections carried out with the awareness of periodic control of pressure vessels and what is a hydrostatic test will continue to be the unshakeable foundation of safety. Asem Process secures both the present and future of industrial facilities by carrying engineering excellence beyond legal regulations (2014/68/EU). A safe working environment is only possible with correctly designed, standard-compliant produced, and regularly inspected pressure equipment.