Pressure Vessels: Design and Types

Chapter 1: Understanding Pressure Vessels

Pressure vessels are enclosed structures designed to hold liquids, gases, or vapors at pressures significantly differing from the surrounding atmosphere. They play a crucial role across diverse industries, such as petrochemical, oil and gas, chemical processing, and food manufacturing. Typical examples include reactors, flash drums, separators, and heat exchangers.

Pressure vessels must adhere to several standards and regulations. The ASME Boiler and Pressure Vessel Code (BPVC) is highly regarded, offering detailed guidelines for the design, construction, installation, testing, inspection, and certification of boilers, pressure vessels, and nuclear plant components. Specifically, ASME BPVC Section VIII specializes in pressure vessels and is segmented into three parts:

• Division I covers all pressure vessels intended for operation at internal or external pressures exceeding 15 psig. These vessels can be either fired or unfired, with pressure arising from an external source or through direct or indirect heating. Engineers employ a design-by-rule approach here. This division is grounded in normal stress theory.
• Division II addresses pressure vessels designed to operate at internal or external pressures up to 10,000 psig. The standards for materials, design, and non-destructive testing in Division II are stricter than those in Division I, requiring more advanced calculations but permitting vessels to endure higher stresses. Engineers use a design-by-analysis approach. Unlike Division I, it is based on maximum distortion energy theory.
• Division III outlines the essential requirements and prohibitions for pressure vessels operating at pressures above 10,000 psig.

Another significant standard is the API 510 - Pressure Vessel Inspection Code: In-Service Inspection, Rating, Repair, and Alteration. This standard emphasizes the ongoing maintenance, inspection, and repair of operational pressure vessels, ensuring their sustained integrity and safety.

Pressure vessels must be operated within prescribed safety parameters, like maximum allowable working temperature and pressure. All operations involving pressure vessels should be conducted by certified professionals, as any accidental release or leakage could present substantial hazards to the environment.

Chapter 2: What are the Different Types of Pressure Vessels?

Pressure vessels are specialized containers designed to safely hold liquids, gases, or vapors at pressures significantly different from ambient pressure. These critical components are foundational in industries such as chemical processing, oil and gas, power generation, pharmaceuticals, and food and beverage manufacturing. Pressure vessels can be categorized based on their function (purpose of use) or geometric shape, each with specific advantages and industry applications. Understanding the various types of pressure vessels, along with their construction materials and pressure ratings, is essential for equipment selection, plant safety, and regulatory compliance.

Types of Pressure Vessels According to Purpose or Function

• Storage Vessels: Storage vessels are pressure vessels designed for the temporary or long-term holding of liquids, vapors, and gases under pressure. These tanks are widely used across petrochemical plants, refineries, and industrial gas operations to store fluids for subsequent processing or containing finished products such as compressed natural gas (CNG), liquefied petroleum gas (LPG), and liquid nitrogen. Storage pressure vessels are engineered considering factors like maximum allowable working pressure (MAWP), temperature, and material compatibility to ensure safety and integrity.

  

  
  

• Heat Exchangers: Pressure vessel heat exchangers are used to efficiently transfer thermal energy between two or more process fluids, all while maintaining mechanical integrity under internal pressure. These units are integral in sectors such as food processing, pharmaceuticals, power plants, and bioprocessing, enabling temperature regulation and energy conservation. Industrial heat exchangers experience combined stresses from temperature gradients, thermal cycling, and internal pressurization. Compliance with standards like ASME Section VIII and rigorous material selection ensure safe operation and longevity in corrosive or high-temperature environments.

  

  
  

• Boilers: Boilers are pressurized vessels that use fuel, nuclear, or electrical energy to generate heat. The core function of an industrial boiler is to heat water or other fluids, often inducing phase changes from liquid to vapor (steam generation). High-pressure steam produced by boilers is critical for power generation, thermal heating, and process operations. Boiler design incorporates high-strength alloy steels to withstand intense thermal and pressure stresses, and strict adherence to codes like ASME Boiler and Pressure Vessel Code ensures operational safety and efficiency. Water-tube boilers, fire-tube boilers, and electric boilers are common types adapted to specific process needs.

  

  
  

• Process Vessels: Process vessels are a broad classification of industrial pressure vessels designed for specialized chemical or physical transformations. These include equipment for mixing, agitation, decantation, distillation, extraction, and chemical reactions. The internal operating pressure often fluctuates based on reaction dynamics, process requirements, and changes in physical state. Various subtypes of process vessels enable precise control over industrial manufacturing and material handling:

  • Distillation columns enable the efficient separation of mixtures based on differences in volatility. They are essential in refining, petrochemical, and pharmaceutical industries. Two major types:

    • Flash distillation rapidly depressurizes a heated liquid mixture, causing partial vaporization. The lighter components separate in the flash chamber, facilitating efficient product recovery.
    • Column distillation (fractional distillation) utilizes internals such as trays or packing to maximize liquid-vapor contact for enhanced separation efficiency. Column design influences separation quality, throughput, and allowable pressure limits.

    

    
    
  • Decanters and gravity settlers utilize pressure vessels for phase separation of solid-liquid or immiscible liquid mixtures. Their geometry and design promote effective settling and removal of denser materials, improving product yield and process reliability.

  • Industrial mixers are specialized pressure vessels equipped with motorized blades and agitation systems. Used to homogenize, blend, or emulsify a range of liquid or semi-solid substances, mixing vessels are engineered for batch or continuous processes that may require elevated temperature and pressure. Optimized mixing enhances product consistency, quality, and process throughput.

    

    
    
  • Chemical reactors are pressurized vessels that safely contain, mix, and facilitate chemical reactions. Features like baffling, agitation, and catalyst supports ensure optimal reaction conditions and maximize yield. Reactors can be batch or continuous, with pressure and temperature control critical for process safety, selectivity, and efficiency.

• As chemical reactions progress, internal vessel pressure may increase due to vapor or gas production, particularly at higher temperatures. Safe design and pressure relief systems are paramount to preventing over-pressurization.

  The following are common chemical reactor types requiring pressure-resistance:

  • Jacketed reactors feature an integrated jacket for regulated heating or cooling. Circulating utilities like chilled water or steam through the jacket maintain optimal reaction temperatures, supporting both exothermic and endothermic reactions. Reactor temperature control increases product yield, prevents thermal runaway, and improves plant safety.

    

    
    
  • Packed bed reactors contain fixed catalyst beds and process fluid flow. Their cylindrical pressure vessel design must withstand the combined load of catalyst weight and internal operating pressure, ensuring high conversion efficiency—critical in chemical synthesis and petrochemical refining.
  • Fluidized bed reactors suspend solid catalysts using high-velocity process streams, enhancing heat and mass transfer rates. The vessel’s robust design accommodates fluidization forces, ensuring uniform temperature, optimal reactant distribution, and increased reaction throughput.

Types of Pressure Vessels According to Geometry

• Spherical Pressure Vessels: Spherical pressure vessels, often known as \"spherical tanks\" or \"Horton spheres,\" are preferred for storing large volumes of highly pressurized gases such as LPG, LNG, and ammonia due to their uniform stress distribution and structural efficiency. Their geometry minimizes the use of construction material for a given volume and optimizes pressure resistance, leading to enhanced safety and durability. These vessels are common in gas storage terminals, chemical plants, and refineries, although fabrication is more complex than for other shapes.

  

  
  

• Cylindrical Pressure Vessels: Cylindrical pressure vessel tanks consist of a tubular shell capped with end heads. The cylindrical configuration is the most widely used vessel shape in industrial applications due to its versatility, ease of fabrication, and cost-effectiveness. Applications include air receivers, process tanks, autoclaves, and reactor shells. However, their structure is subject to higher localized stress compared to spheres, requiring thicker walls or reinforced construction for high-pressure service.

  Choices for vessel heads—critical in pressure vessel design and strength—include:

  • Hemispherical heads are well-suited for high-pressure applications and large-diameter pressure vessels due to their optimal geometry for uniform stress distribution. These heads allow for lower wall thickness and material cost while offering maximum volumetric efficiency.

    

    
    

  • Torispherical heads provide a practical, cost-effective option for moderate-pressure applications (typically under 15 bars). With their flatter profile, they are favored in compact systems and equipment with height restrictions. Their design incorporates both crown and knuckle radii, balancing strength and manufacturability.

    

    
    

  • Ellipsoidal heads (also known as 2:1 elliptical heads) are excellent for containing high-pressure gases, as their geometry provides elevated strength and reduces required wall thickness. These heads are a standard choice for ASME-coded pressure vessels where efficiency and cost savings are important.

    

    
    

  

  
  

Vessel Orientation

Cylindrical pressure vessels are installed in vertical or horizontal orientations, with the choice driven by application requirements, space limitations, and process needs. The vessel’s position impacts maintenance, mixing efficiency, drainage, and overall accessibility.

• Vertical vessel orientation is preferred:
  • When maximizing floor space in compact facilities.
  • For processes requiring small vessel volume and footprint optimization.
  • In reactors or mixing tanks, improving agitation and uniform fluid distribution due to smaller cross-sectional areas.
  • Where a high gas-to-liquid ratio enhances separation efficiency.
  • In separators for efficient removal of immiscible liquid phases or slurries, aiding product recovery.
• Horizontal vessel orientation is suited for:
  • Shell-and-tube heat exchangers, where accessibility and ease of tube cleaning are paramount.
  • Settling tanks, surge drums, and flash vessels requiring low downward fluid velocity to minimize phase entrainment or foaming.

  

  
  

When selecting a pressure vessel, consider aspects such as operating pressure, temperature, vessel material (carbon steel, stainless steel, alloys), corrosion resistance, inspection standards (such as ASME, PED, or API), maintenance accessibility, and the intended process application. Working with leading pressure vessel manufacturers ensures compliance, custom solutions, and reliable service for your specific industry.

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Chapter 3: What factors should be considered when selecting materials for pressure vessels?

When choosing the right material for constructing pressure vessels, the following criteria should be considered:

• Can meet the strength requirements of a particular application. Materials must withstand specific internal and external pressures, and structural stresses during the pressure vessel‘s service life.
• Corrosion Resistance: This is one of the most important properties of a pressure vessel since it is expected to be reliable in harsh environments.
• Return of Investment: Costs of materials, fabrication, and maintenance must be considered during the lifecycle of the pressure vessel. Economic analyses are done to determine the best material which yields the least cost. The Return of Investment must be evaluated and assessed if the acquisition of a pressure vessel is profitable.
• Ease of Fabrication and Maintenance: Since metal sheets are formed into shapes to create the geometry of the pressure vessels, they must have good machinability and weldability. Vessel internals must be easily installed.
• Availability: Standard sizes for pressure vessel materials must be readily available in the region of the manufacturer.

Typical materials used in the construction of pressure vessels include:

• Carbon Steel: Carbon steel is a type of steel that has a higher carbon content of up to 2.5%. Carbon steel vessels are known for their high tensile strength for a minimal wall thickness, which is suitable for a wide range of applications. They are to impact and vibration. However, carbon steel is difficult to bend and form into shapes due to its high strength. It is also more susceptible to corrosion and rusting than stainless steel since it does not contain chromium.

  

  
  

• Stainless Steel: Stainless steel is a type of steel that has a higher chromium content of up to 10.5 – 30% and lower carbon content and trace amounts of nickel. They are known for their excellent chemical, corrosion, and weathering resistance which is attributed to their chromium content. A thin, inert chromium oxide film is formed at the surface to prevent oxygen diffusion to the bulk of the metal. Like carbon steel, it also exhibits high strength for a lower wall thickness. It is easier to form compared to carbon steel due to its increased ductility and elasticity.

  

  
  
• Hastelloy: Hastelloy is an alloy composed of nickel, chromium, and molybdenum which was the first alloy formulated by Haynes International, Inc. It is a widely used material for reactors, pressure vessels, and heat exchangers in the petrochemical, energy, and oil and gas industry. It can be used as a material for nuclear reactors. It has excellent corrosion resistance, cracking, and oxidizing and reducing agents. It maintains its strength at high temperatures. It is easily welded, and formed, and shaped due to its good ductility. With proper maintenance, its service life can last up to several decades which increases its cost-efficiency.

• Nickel Alloys: Nickel alloys offer good corrosion and weathering resistance, and protection against thermal expansion. The addition of chromium to the nickel alloy further increases its heat resistance. Pressure vessels constructed from nickel alloys are widely used in the oil and gas industry, cryogenic applications, and in other harsh environments. It also has a longer service life. However, it is difficult to work and has a higher fabrication cost. The purity of nickel alloys is important to protect their strength and reliability.

  

  
  

• Aluminum: Aluminum is known for its high strength-to-density ratio, which means it has high strength and lightweight at the same time. It is cheaper and more fabricated than stainless steel. It also has good corrosion resistance. Aluminum vessels are commonly used in laboratory-scale applications. However, it is not suitable for high-pressure applications since it has less density, which is one-third of stainless steel.

  

  
  
• Titanium: Titanium also offers high strength and rigidity for a minimal wall thickness. It has good corrosion resistance and biocompatibility, and it is also non-toxic. It has a higher melting point than steel and aluminum, hence it is ideal for higher temperature applications. It also has high thermal conductivity and facilitates efficient heat transfer, which is an ideal material for heat exchangers.

Chapter 4: What are the key considerations in the design of pressure vessels?

In designing a pressure vessel, the following parameters are essential for determining the wall thickness of the shell and heads:

• Design Pressure: The design pressure is a value in which the vessel specifications are calculated. It is derived from the maximum operating pressure, which is the anticipated surge in pressure during upset conditions such as start-ups, emergency shutdown, and process abnormalities. It is always higher than the maximum operating pressure. The pressure relief system of a vessel is also based on this parameter to minimize the risk of explosions. According to Towler, the design pressure should be overdesigned by 5-10% from the maximum operating pressure.

  For vessels that potentially can experience vacuum pressure, the design pressure must be set to resist one full vacuum (-14.7 psig).

• Maximum Allowable Working Pressure (MAWP): The MAWP is the highest permissible pressure measured at the top of the equipment at which the vessel must operate based on its design temperature. It is the highest pressure that the weakest part of the vessel can handle at its design temperature. MAWP value is designated by the American Society of Mechanical Engineers (ASME) and is used by industries to ensure that the vessel will not operate beyond this value to establish safety protocols and prevent explosions.

  MAWP is different from the design pressure. MAWP is an extensive property that is based on the physical limitations of the material. Corrosion and wear lower the MAWP of the material. The design pressure, on the other hand, is based on the operating condition of the process, and it may be lower than or equal to the MAWP.

• Design Temperature: The maximum allowable stress is highly dependent on the temperature, as strength decreases with increasing temperature and becomes brittle at very low temperatures. The pressure vessel should not operate at a higher temperature where the maximum allowable pressure is evaluated. The design temperature is always greater than the maximum operating temperature and lesser than the minimum temperature.

  There are several rules of thumb in evaluating the design temperature. Towler suggests that the design temperature must be 50°F from the maximum operating temperature and -25°F from the minimum operating temperature. For Turton, a maximum allowance of 25°C must be given for vessels that will be operating between -30 to 345°C. The disturbances that have a drastic influence on the temperature of the pressure vessel must be considered by the designer.

  

  
  
• Maximum Allowable Stress: The maximum allowable stress is obtained by multiplying a safety factor to the value of maximum stress the material can withstand. The safety factor accounts for possible deviations from the ideal construction and operation of the pressure vessel.
• Joint Efficiency: The ASME Boiler and Pressure Vessel (BPV) Code has four categories of welded joints:

  Category A	Longitudinal or spiral welds in the main shell, necks or nozzles, or circumferential welds connecting hemispherical heads to the main shell, necks or nozzles.
  Category B	Circumferential welds in the main shell, necks or nozzles or connecting a formed head other than hemispherical.
  Category C	Welds connecting flanges, tubesheets or flat heads to the main shell, a formed head, neck or nozzle.
  Category D	Welds connecting the communicating chambers or nozzles to the main shell, to heads or to necks.

  The joint efficiency is the ratio of the strength of the welded plate to the strength of the unwelded virgin plate. Generally, the strength is lower at the welded joint. Welded joints without further inspection and radiographic testing are assumed to be weaker due to defects such as porosity are potentially present. Joint efficiencies allowed under ASME BPV Code Sec. VIII D.1 is summarized in the table below:

  Joint Description	Joint Category	Joint Efficiency (Based on degree of radiographic examination)
  		Full	Spot	None
  Double-welded butt joint or equivalent	A, B, C, D	1.0	0.85	0.70
  Single-welded butt joint with backing strip	A, B, C, D	0.9	0.8	0.65
  Single-welded butt joint without backing strip	A, B, C	NA	NA	0.60
  Double full fillet lap joint	A, B, C	NA	NA	0.55
  Single full fillet lap joint with plug welds	B, C	NA	NA	0.50
  Single full fillet lap joint without plug welds	A, B	NA	NA	0.45

  

  
  
• Corrosion Allowance. There are several rules of thumb in estimating the corrosion allowance and maybe arbitrary to the manufacturer. Generally, corrosion allowance should range from 1.5 – 5 mm. According to Peters, Timmerhaus, and West, corrosion allowance should be 0.25 – 0.38 mm annually or 3 mm for 10 years. Meanwhile, Turton suggests that the corrosion allowance should be 8.9 mm for corrosive conditions, 3.8 mm for non-corrosive streams, and 1.5 mm for stream drums and air receivers. In heat exchanger equipment, corrosion allowance must be small because wall thickness affects the rate of heat transfer.

Fabrication of Pressure Vessels

The vessel's shell and heads are fabricated by forging, rolling, and welding metal sheets. The wall thickness of these components is determined through detailed calculations, taking into account the previously mentioned factors. To ensure the pressure vessel functions effectively, various auxiliary equipment, devices, and accessories are also installed:

• Nozzles allow the introduction and discharge of feed, products, and utilities. They are usually welded perpendicularly on the shell or head and away from the weld lines.
• Pressure relief valves as a safety feature during its operation
• Heating or cooling jacket for stirred reactors
• Support such as saddles, skirts, or legs that allow thermal expansion of the material in operation.

Post-weld heat treatment is performed to alleviate stresses induced during the welding and forming processes.

Pressure vessels can be fabricated either on-site or in a shop. Field-erected pressure vessels are too large to be assembled in a workshop and transported as a whole, so their parts are individually fabricated in a shop and then transported to the installation site. Assembly, welding, finishing, and accessory installation occur on-site. In contrast, shop-erected pressure vessels are smaller and can be fully assembled within a manufacturing facility. Once assembled, these vessels are transported to the site where only the installation of piping and minor adjustments are needed. The major phases of fabrication are completed in the shop before the vessel is delivered.

Chapter 5: What methods are used for quality testing and inspection of pressure vessels?

To verify the reliability of a pressure vessel, the following testing methods are utilized:

• Visual Testing is a critical part of the maintenance of pressure vessels. The frequency of inspection must be once every five years, and before it is put into service after being installed or repaired. A trained inspector checks the interior and exterior of the vessel structure. The inspector looks for cracks, deformation, blistering, leakage of fluids, corrosion, and other flaws in the entire vessel structure.
• Ultrasonic Testing utilizes high-frequency sound waves to detect surface and subsurface flaws and to measure the wall thickness of the pressure vessel. The ultrasonic sound waves are absorbed by the material and are reflected back into an electrical signal by means of a transducer. The reflected waves are disturbed if flaws are present.

• Radiographic Testing utilizes x-rays or gamma rays to produce an image of a pressure vessel‘s surface and subsurface. The reflected rays will be distorted once it passes any discontinuities, holes, and difference in density and will be exposed in the film. Radiographic testing is highly reproducible and requires minimal surface preparation. However, it is more expensive and requires a highly skilled operator to handle ionizing radiation.

  

  
  

• Magnetic Particle Testing uses magnetic current to detect discontinuities on the surface in ferromagnetic materials. The inspector runs a magnetic current through the pressure vessel between two probes. If the material is defect-free, the magnetic flux flows through the material without any interruption. However, if cracks or any other imperfections are present, the magnetic flux leaks out of the material. The imperfection will be more visible once ferromagnetic particles, either in a liquid suspension or powdered form, are applied to the vessel.

  

  
  

• Liquid Penetrant Testing is commonly used on welded seams and plates. The inspector applies a small amount of liquid, called the penetrant, to an area with a possible flaw. The penetrant is allowed to settle after spraying and then wiped to clean the excess penetrant on the surface. The developer is then applied to reveal the penetrant that has seeped into the cracks.

  

  
  
• Pressure Testing is required by the ASME BPV Code to test for strength and leaks. There are two methods of pressure tests: hydrostatic pressure testing uses water as a medium, while pneumatic pressure testing uses air or nitrogen. The latter is more preferred for safety purposes since compressed liquid contains less energy than compressed gas. It works by removing the air from the vessel, and the unit is filled with the test fluid until the internal pressure is 1.5 times the design pressure for hydrostatic testing and 1.2 – 1.5 times for pneumatic testing. The fluid is then continuously held for a minimum of 10 minutes. The inspector then looks for cracks and leaks in the system. Fluorescent dyes or tracers are used to determine where the cracks are originating. Pressure testing is usually done during a shutdown, or as a validation test after the vessel is repaired from damage.

Conclusion

• Pressure vessels are enclosed containers that hold and store liquids, vapors, and gases at a pressure significantly higher or lower than the ambient pressure.
• Design, construction, repair, and testing of pressure vessels are governed by some regulations such as ASME BPVC and API 510. Such regulations are made to ensure safety during the pressure vessel‘s operation.
• The types of pressure vessels according to their function are storage tanks, boilers, heat exchangers, and process vessels. A pressure vessel may be spherical or cylindrical. Cylindrical vessels are more common, and their heads may be hemispherical, ellipsoidal, or torispherical. The axis of a pressure vessel may be vertically or horizontally oriented.
• The criteria for material selection for a pressure vessel are: can meet the strength requirements of a certain application, corrosion resistance, return of investment, ease of fabrication and maintenance, and availability.
• The critical design parameters for calculating the specification of a pressure vessel are design pressure, maximum allowable working pressure, design temperature, maximum allowable stress, joint efficiency, and corrosion allowance.
• The pressure vessel is fabricated through forging, rolling, and welding the metal sheets. Auxiliary equipment and accessories are installed for the vessel to fully serve its purpose.
• The methods employed to test the reliability of pressure vessels are visual testing, ultrasonic testing, radiographic testing, magnetic particle testing, liquid penetrant testing, and pressure testing.

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