Pressure Vessel 101: Design, Types, and Safety Standards

On July 14, 2023, the U.S. Chemical Safety Board launched an investigation at a facility in Plaquemine, Louisiana where a pressure vessel explosion occurred. The incident led local officials to order a shelter-in-place for affected residents within a half-mile of the plant. Although no injuries were reported, it is important to emphasize that caution should still be prioritized when working with pressure vessels. This is despite the fact that documented serious or fatal incidents have decreased over the years.

Knowing how these vessels are used, the different designs, the types available, and the standards they need to meet keep operations running safely. In addition to covering these critical details, I’ll also discuss the legal requirements and restrictions that apply to these vessels so your facility stays compliant and avoids costly mistakes.

Key Takeaways

• Regular inspection and maintenance of pressure vessels is both a regulatory requirement and a critical safety measure that can prevent catastrophic failures and protect both personnel and assets.

• The ASME Boiler and Pressure Vessel Code, along with NBIC standards, provide essential guidelines for design, construction, and maintenance that must be strictly followed to ensure safe operation.

• Material selection and proper design calculations are fundamental to pressure vessel safety, as they must withstand both regular operating conditions and potential extreme scenarios.

• A comprehensive risk management program, including emergency procedures and staff training, is essential for facilities operating pressure vessels, as it helps prevent accidents and ensures proper response to potential incidents.
  
  
  

What Is a Pressure Vessel?

Pressure vessels are enclosed containers designed to hold gases or liquids at a pressure substantially different from ambient pressure. These specialized containers are engineered to safely contain pressurized substances, making them essential in various industrial applications. The pressure differential between the inside and outside of the vessel creates unique engineering challenges that require careful consideration in design and operation. 

Pressure Vessel Primary Uses

• Storage of gas and liquids: Pressure vessels store gases and liquids under controlled conditions to prevent leaks and maintain stability. They are commonly used in industries like oil & gas, chemical processing, and water treatment. 

• Heat exchange processes: Heat exchangers within pressure vessels facilitate heat transfer between fluids without direct contact. They are essential in power plants, HVAC systems, and industrial heating and cooling applications. Their design optimizes energy efficiency and thermal regulation in various processes.

• Reactor vessels in chemical processes: Reactor vessels provide a controlled environment for chemical reactions, mixing, and synthesis under specific pressure and temperature conditions. Used in pharmaceutical, petrochemical, and polymer industries, they ensure reaction efficiency, product consistency, and safety.
  
  
  

Categories and Types of Pressure Vessels 

Pressure vessels come in various configurations and are classified by orientation and pressure levels, depending on operational needs. Each category goes into detail about a vessel’s physical attributes, including its functions, and features. It is best to learn about how each is optimized for specific applications:

According to Purpose

Storage Vessels

Storage vessels are engineered to safely contain gases or liquids under constant pressure for long periods. They feature robust construction with minimal internal components to maximize storage capacity. Hence, they are fundamental to operations in industries ranging from petroleum storage to industrial gas distribution.

Heat Exchangers

Heat exchanges use intricate internal designs to efficiently transfer heat between two separate fluid streams without mixing. They incorporate features like tube bundles or plate arrangements to maximize heat transfer surface area, making them essential in processes requiring precise temperature control, from oil refineries to food processing plants.

Boilers

Boilers convert water into steam through controlled heating under pressure for various industrial applications. They incorporate specific safety features like pressure relief valves and water level controls to prevent dangerous operating conditions. The generated steam provides thermal energy for power generation, process heating, or facility climate control.

Process Vessels

These vessels are designed to contain and control chemical reactions or physical changes under specific pressure conditions. They often include specialized internal components like agitators, baffles, or catalytic surfaces to facilitate the desired transformations. These vessels form the core of many manufacturing processes, from pharmaceutical production to chemical synthesis.

Here is a quick summary of these pressure vessels:

Among these pressure vessels, I once read that boilers are among the most dangerous ones to work with, and I’ve heard firsthand accounts of just how bad things can get when they fail.

A colleague of mine had a near-miss years ago when a pressure relief valve malfunctioned on a boiler he was working near. The pressure built up far beyond safe levels, and if the backup safety features hadn’t kicked in, the situation could have been catastrophic.

Boilers operate under high pressure and temperature, and if something goes wrong, whether it’s a failure in pressure control or neglected maintenance, the results can be deadly.

According to Geometry

Spherical Vessels

Spherical Vessels are optimal for high-pressure applications such as liquefied natural gas (LNG) storage, high-pressure gas containment, and petrochemical industries. As opposed to cylindrical vessels, they cannot be oriented vertically or horizontally as their symmetrical shape distributes pressure evenly in all directions, reducing weak points and enhancing structural integrity. However, they are more expensive to manufacture due to their complex fabrication requirements such as specialized welding.

Cylindrical Vessels

Cylindrical Vessels are the most common designs for vessels and can be oriented vertically for space efficiency or horizontally for ease of transport, making them more versatile than spherical vessels. Such vessels are suitable for most industrial applications like refineries, chemical processing plants, and water treatment facilities as they are easier to construct. They are more space-efficient and cheaper to manufacture than spherical ones but experience much higher stress concentrations at their ends, requiring reinforcement. 

Pressure Vessel Orientation

Vertical Vessels

Vertical Vessels are commonly used in refineries, chemical plants, and water treatment facilities where height is an advantage. They are ideal for processes like distillation, absorption, and liquid-gas separation, where gravity helps separate liquids and gases naturally. Compared to horizontal vessels, they occupy less ground space but may require taller structures for installation.

Horizontal Vessels

Horizontal Vessels are widely used in bulk storage, natural gas processing, and oil refineries where large capacities are needed. They are preferred for gas-liquid separation and settling applications due to their larger vapor-liquid interface. Unlike vertical vessels, they require more ground space and thicker walls for high-pressure applications but are easier to install and maintain since their components are more accessible at ground level.

Design and Construction

I remember speaking with a pressure vessel inspector who shared that people often overlook the importance of the pressure vessel’s design during inspection and operation, which sometimes leads to accidents or missed inspection data. These containers operate under extreme pressure, temperature, and chemical conditions, making them vulnerable to catastrophic failure if not properly engineered.

A well-designed vessel prevents accidents, ensures compliance with safety regulations, and enhances operational effectiveness. Proper design and construction create the foundation for protection, making it a critical investment that directly impacts both safety and operational success. To make sure that design meets safety requirements, here are the things you should look into:

Vessel Material

• Carbon Steel: This is the most commonly used material for pressure vessels due to its cost-effectiveness and good mechanical properties. It performs well in standard temperature and pressure conditions where corrosion is not a major concern. However, it requires proper protective coatings and regular maintenance to prevent rust and degradation.

• Stainless Steel: Stainless steel offers superior corrosion resistance due to its chromium content. They are ideal for handling aggressive chemicals and processes involving food or pharmaceuticals. Their higher cost is offset by longer service life and reduced maintenance requirements.

• Special Alloys: Special alloys like nickel-based materials can withstand extreme temperatures and pressures that would compromise standard metals. These alloys maintain their mechanical properties in harsh operating conditions where other materials would fail. They are specifically engineered for challenging environments like high-temperature reactors or cryogenic storage.

• Composites: Composite materials combine fibers and resins to create lightweight yet strong pressure vessels. They are particularly valuable in transportation applications where weight reduction is crucial. Composites can also offer excellent corrosion resistance and fatigue properties, though they typically have temperature limitations.

Engineering Standards and Codes

Vessel material is just one of the things you should value in a pressure vessel. It’s worth noting that the vessel should also contain the following codes: The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code and the National Board Inspection Code (NBIC). 

ASME Code

The ASME certification mark ("U", "U2", "S", etc.) is stamped directly on the vessel during manufacture. This stamping indicates the vessel was constructed according to ASME standards, provides unique identification and tracking throughout the vessel’s lifetime, and includes vital information like:

• Manufacturer's name

• Serial number

• Maximum allowable working pressure

• Year built

• Design temperature

• Capacity

NBIC Code 

The NBIC "R" stamp appears when a pressure vessel has undergone repairs or alterations after its original construction. This stamp indicates that:

• Repairs were performed by an authorized facility

• Work complied with NBIC standards

• Proper documentation was maintained

• The repair company's "R" number is included

These markings are typically found on the nameplate or stamped directly on the vessel's shell, and they must remain visible and legible throughout the vessel's service life.

Manufacturing Methods

Manufacturing must comply with recognized industry standards such as ASME Boiler and Pressure Vessel Code (BPVC), API (American Petroleum Institute), and ISO (International Organization for Standardization). The following are the key certified manufacturing methods:

• Welding and Fabrication 

• Forging and Casting 

• Cold and Hot Forming 

• Heat Treatment

• Non-Destructive Testing (NDT) and Quality Assurance

• Coating and Corrosion Protection

Design Calculation for Thickness

Proper design formulas ensure vessels can withstand intended operating pressures and stresses throughout their lifecycle. Wall thickness calculations must account for all possible loading conditions and material properties. The most widely used method for determining pressure vessel wall thickness is Barlow’s Formula, which considers internal pressure, vessel diameter, material strength, and weld quality:

t = (P * D) / (2 * S * E – 0.6 * P)

Where: 

• t is the required vessel wall thickness

• P is the internal pressure

• D is the inside diameter of the vessel

• S is the maximum amount of stress the material can endure

• E is the joint efficiency, accounting for factors like welding quality

Engineers must also factor in corrosion allowance to account for material degradation over time. Other external forces, such as wind loads and seismic activity, must also be considered to ensure the vessel remains stable and safe under all conditions.

Regulations and Requirements

Working with pressure vessels requires awareness of regulations, requirements, and even some restrictions. These guidelines were developed to ensure that safety standards are met by workers and industries alike. Such regulations are laid down in the following:

OSHA Guidelines for Pressure Vessel Safety

OSHA maintains a list of comprehensive resources for hazard recognition in pressure vessel operations. Among these resources is the OSHA Technical Manual (OTM), which provides detailed information on safety and hazard evaluations of pressure vessels, highlighting the consequences of leakage or rupture failures. The manual serves as a valuable tool for employers aiming to enhance workplace safety and ensure compliance with federal regulations.

The OSHA Technical Manual (OTM) provides detailed guidance on pressure vessel safety, particularly in Section IV, Chapter 3. This chapter offers comprehensive information on various aspects of pressure vessel safety, including:

• Types of Pressure Vessels: An overview of different designs and their specific applications.

• Common Failure Mechanisms: These are insights into typical failure modes such as cracking, corrosion, and fatigue, along with contributing factors.

• Examination Methods: These contain descriptions of inspection techniques like radiographic, ultrasonic, and magnetic particle testing to assess vessel integrity.

• Safety Assessment Procedures: These set guidelines for evaluating the safety of pressure vessels, including risk assessment and mitigation strategies.

Also, Appendix IV:3-1 within this chapter provides a template for recordkeeping, detailing the essential data to be maintained for steel vessels, and low-pressure storage tanks. This structured approach ensures thorough documentation, facilitating compliance with safety regulations and aiding in the prevention of potential hazards.

ASME Boiler and Pressure Vessel Code (BPVC)

The ASME Boiler and Pressure Vessel Code (BPVC), on the other hand, serves as a foundational framework to ensure the integrity and safe operation of pressure-containing equipment throughout its lifecycle, from initial concept to final installation. The code regulates design and construction through three divisions:

• Division I: This division covers vessels operating above 15 psig in pressure. The requirements apply to both fired and unfired vessels that use external pressure sources or indirect heating. The design methodology follows a design-by-rule approach that is based on normal stress theory.

• Division II: This division addresses vessels operating up to 10,000 psi with stricter material and testing requirements than Division I. The engineers must use a design-by-analysis methodology that enables higher allowable stress levels. The analysis is based on maximum distortion energy theory rather than the normal stress theory used in Division I.

• Division III: This division establishes requirements for vessels operating above 10,000 psi in pressure. The code specifically defines which vessels are allowed and prohibited at these extreme pressures. The requirements are complemented by API 510 standards that govern in-service inspection, rating, repair, and modification procedures.
  

Responsibility Assignments

Persons in charge of upholding pressure vessel safety have varying responsibilities. These duties include conducting regular safety audits, enforcing training programs, and reviewing inspection reports to prevent accidents. They also ensure adherence to OSHA and ASME standards, responding to safety concerns, and implementing corrective actions when risks are identified.

• Safety Committee Members: Their primary duty is to review and establish safety protocols for operations across the facility. They conduct regular meetings to assess incident reports, analyze trends, and recommend systemic improvements. For example, they might review quarterly safety statistics and propose new lockout/tagout procedures after identifying recurring issues.

• Safety Coordinators: They serve as the day-to-day safety champions, conducting regular walkthrough inspections and maintaining safety documentation. Coordinators organize and track employee safety training, ensuring all pressure vessel operators maintain current certifications. A typical task includes performing weekly safety walks to verify proper pressure gauge functionality and emergency shutdown procedure postings.

• Laboratory Managers: They oversee the safe operation of pressure vessels within laboratory settings, ensuring proper maintenance schedules and operating procedures. Lab managers maintain equipment logs, coordinate with maintenance teams, and verify that all vessels operate within design parameters. For instance, they might establish standard operating procedures for an autoclave, including proper loading techniques and emergency response protocols.

• Plant Managers: They hold ultimate responsibility for facility-wide pressure vessel safety and regulatory compliance. Plant managers allocate resources for maintenance, approve safety program budgets, and ensure implementation of OSHA and ASME requirements. An example would be authorizing capital expenditure for replacing aging pressure relief valves identified during annual inspections.

Potential Risks and Hazards

To reduce the likelihood of incidents happening, determining the potential risks and hazards is crucial when working with large equipment like these vessels. The most common types of risks in pressure vessel safety are further categorized into (4) main failure mechanisms:

Type 1: Fatigue Failure

Fatigue failure occurs when repeated loading weakens a material over time, even if each load is below its yield strength. Similar to bending a paperclip back and forth until it snaps, small cracks form and grow with each cycle. Fatigue can result from:

• Fluctuating pressures

• Thermal cycling

• Vibrations 

Type 2: Creep Failure

Creep occurs when materials slowly deform under constant stress at high temperatures. Engineers prevent creep failure by selecting heat-resistant materials suited for high-temperature applications. In pressure vessels, prolonged exposure to heat causes:

• Vessel shape changes

• Wall thinning

• Potential rupture

Type 3: Brittle Fracture

Brittle fracture happens when materials break suddenly with little deformation, often due to low temperatures reducing ductility. To prevent failure, pressure vessels must operate above the ductile-to-brittle transition temperature with appropriate material selection. These factors increase the risk of brittle fracture:

• Rapid stress application

• Existing cracks 

• Unsuitable materials

Type 4: Corrosion

Corrosion weakens pressure vessels by degrading materials through chemical reactions with internal fluids or external environments. Using corrosion-resistant materials, protective coatings, and routine inspections helps mitigate the risk of corrosion. This often appears as:

• Wall thinning

• Pitting

• Stress corrosion cracking

Pressure Vessel Inspection

The inspector I spoke with also recalled a time when inspections used to be much more complicated. Not only was there a lack of standardized procedures, but inspecting gigantic vessels was especially difficult as some of its areas were hard to reach. However, as shown in the photo above, size merits no exception to a thorough inspection.

These inspections are crucial for preventing catastrophic failures and ensuring the safety of personnel and facilities. They also have to be performed by certified professionals. Here are the types of pressure vessel inspections:

Internal Inspections

Internal inspections are conducted inside the equipment and require dismantling or entering confined spaces to assess conditions. They involve checking for corrosion, material degradation, cracks, and structural integrity. 

During these inspections, shutdowns are required as well as special safety measures, such as confined space entry permits.  These processes often include:

• Visual Inspection: A basic but essential method where inspectors check for surface defects like cracks, corrosion, or leaks. It requires good lighting and sometimes magnification tools.

• Ultrasonic Testing: This uses high-frequency sound waves to detect internal flaws in materials. It’s non-destructive and provides precise measurements of thickness and defects.

• Radiographic Analysis: This uses X-rays or gamma rays to reveal hidden defects inside metal structures. It provides detailed imaging of cracks, voids, and weld quality and is mostly used for ensuring structural integrity in high-pressure systems.

• Hydrostatic Testing: This is a widely used method for testing the strength and leaks in pressurized systems and is often considered safer than pneumatic testing. It involves filling the equipment with water and pressurizing it beyond its normal operating level. Leaks, structural weaknesses, and material fatigue are typically detected during this process.

• Pneumatic Testing: This method uses air, nitrogen, or other gases instead of water to test for leaks. It is most useful for systems that cannot tolerate water exposure, such as gas pipelines, refrigeration systems, and delicate instruments. However, it is more dangerous than hydrostatic testing due to the compressibility of gases, which can cause explosive failures.

External Inspections

These inspections evaluate equipment from the outside, focusing on structural integrity, signs of leaks, surface corrosion, and external wear. These can be conducted while the equipment is in use. This inspection often includes:

• Infrared Thermography: This method detects temperature variations to identify heat loss, leaks, or insulation failures. It’s widely used in electrical, mechanical, and inspections.

• Ultrasonic Thickness Measurement: This measures the thickness of materials using ultrasonic waves to detect corrosion or thinning and helps assess structural integrity without dismantling equipment

• Dye Penetrant Testing: This applies a colored dye to the surface to reveal small cracks and defects. It works best for detecting flaws in non-porous metals, ceramics, and plastics.
  
  
  

Who Are Qualified Inspectors?

An experienced inspector I know had a nerve-wracking moment while performing a radiographic analysis inside a pressure vessel. The job was to check for hidden cracks in a high-pressure system using X-rays, a routine task for him. But as he was setting up, he realized his radiation dosimeter was reading higher than normal, turns out, there was a shielding issue, and he was being exposed. He got out immediately, but it shook him.

Radiation exposure isn’t something you feel in the moment, and without proper precautions, long-term effects can be serious. He told me that every time he steps into a vessel for this kind of work, he double-checks shielding, verifies all safety measures, and makes sure his dosimeter is always in sight. This is just one example of how pressure vessel inspections could put inspectors in danger, making safety very crucial in these processes, and shows that inspections should only be done by certified professionals, as listed below:

• National Board of Boiler and Pressure Vessel Inspectors: These inspectors are required for boiler and pressure vessel inspections in most regions. They ensure compliance with ASME and safety codes and their certification is essential for high-risk industrial equipment.

• API 510 Inspectors: They specialize in pressure vessel inspections to detect defects and ensure code compliance. They also assess structural integrity and safety risks. Such inspectors are common in refineries, chemical plants, and power facilities.

• API 570 Inspectors: They focus on piping inspections to identify corrosion, leaks, and structural weaknesses. These inspectors are essential for maintaining safe and efficient pipeline operations. They typically work in oil, gas, and industrial plants.

• API 653 Inspectors: They are in charge of inspecting storage tanks for corrosion, foundation issues, and structural integrity. Ensuring tanks comply with API standards and safety regulations is one of their duties. They are usually employed in petrochemical and bulk liquid storage industries.

• Certified Welding Inspectors (CWI): They assess welded joints for quality, strength, and compliance with industry standards. Such inspectors use visual and non-destructive testing methods. They are essential for pressure vessels, pipelines, and construction.

• State-Licensed Inspectors: Some states require additional licensing for boilers, pressure vessels, and structural inspections. These inspectors enforce local safety regulations and their approval is necessary for equipment operation and compliance.
  
  
  

Documentation in Pressure Vessel Inspection

Proper documentation of inspection findings is essential for traceability, compliance, and accountability. High-resolution photographs, including before-and-after images and infrared thermography, provide visual proof of defects and track deterioration over time. Aside from photographs, standardized inspection forms for documentation also include:

• Equipment details

• Inspection dates

• Findings

• Maintenance recommendations
  
  
  

Operations & Maintenance

Operations require systematic monitoring, inspections, and preventive maintenance activities to ensure safe performance. Maintenance programs cover routine tasks like valve servicing and scheduled procedures including internal inspections and testing. Here are the parameters to observe during operations and maintenance:

Safe Operating Parameters

Every pressure vessel has a Maximum Allowable Working Pressure (MAWP) that must never be exceeded to prevent structural failure. For example, ASME standards require pressure vessels to operate within 90% of their MAWP to maintain a safety margin. Other examples of safety parameters are the following:

• Temperature fluctuations must be carefully monitored, as excessive heat or cold can cause material expansion, embrittlement, or cracking. 

• Relief valves, which automatically release excess pressure, must be tested at least once a year per OSHA and ASME guidelines to ensure they function properly in an overpressure event.

Routine Maintenance and Inspections

Daily visual inspections help detect obvious problems like leaks, rust, or pressure gauge abnormalities. Documentation is critical. More detailed monthly inspections assess vessel components, including seals, welds, and connections, using non-destructive testing (NDT) methods like ultrasonic or radiographic analysis. There are two types of maintenance and inspections of pressure vessels:

• Reactive Approach – This method addresses failures after they occur, focusing on emergency response and repairs. If a vessel leaks or ruptures, operations are shut down, emergency protocols are activated, and repairs are made, often leading to high costs and safety risks.

• Preventive Approach – This method focuses on regular inspections and maintenance to catch issues like corrosion or fatigue before they become dangerous. By preventing failures, this approach extends equipment life, reduces downtime, and ensures compliance with safety regulations.

Part of the routine maintenance and inspection procedures for pressure vessels is implementing lockout tagout procedures. I recall a troubling incident where a maintenance team assumed that shutting down the main power was sufficient, neglecting to isolate and de-energize all energy sources.

Unfortunately, residual pressure within the system led to an unexpected release, causing serious injuries to two workers. This incident shows the critical importance of comprehensive LOTO procedures during maintenance activities.

According to OSHA's standard on the control of hazardous energy (29 CFR 1910.147), employers must establish practices and procedures to disable machinery or equipment, including pressure vessels, preventing the release of hazardous energy during maintenance.

Here are key steps to ensure compliance:

• Developing Energy Control Procedures: Clearly outline the scope, purpose, authorization, rules, and techniques for controlling hazardous energy.

• Training Employees: Ensure that all employees understand the energy control procedures and the importance of not tampering with lockout or tagout devices.

• Periodic Inspections: Regularly review and, if necessary, update the energy control procedures to maintain their effectiveness.

By diligently following these steps, organizations can significantly reduce the risk of accidents and ensure a safer working environment during pressure vessel maintenance.

Can I Create My Own Pressure Vessel?

No, unless you are qualified to create one. Questions about DIY or makeshift vessels commonly appear in online forums, raising important safety concerns about small-scale vessels that can pose significant risks if not properly designed and maintained. All vessels must comply with ASME's Pressure Vessel Code requirements for size and structure, regardless of their scale. 

Here is one Reddit user’s thoughts about homemade pressure vessels:

Pressure Vessel FAQs

What is the difference between a pressure vessel and storage tank? 

The former operate above ambient pressure, while the latter typically operate at atmospheric pressure. Storage tanks usually operate at or near atmospheric pressure with simpler construction requirements. The key distinction lies in their pressure rating, with pressure vessels being engineered to safely contain internal pressures that could cause catastrophic failure in standard storage tanks.

What is the difference between a pressure vessel and a compressor? 

A pressure vessel stores pressurized substances, while a compressor is a device that increases gas pressure. A compressor is an active mechanical device that increases the pressure of gases by reducing their volume. While compressors often work in conjunction with these vessels, they serve different functions in a pressurized system.

What is a type 3 pressure vessel? 

A type 3 pressure vessel typically refers to a composite vessel with a metal liner and full composite overwrap. This design combines the sealing properties of metal with the strength-to-weight advantages of composites. These vessels are commonly used in compressed natural gas vehicles and breathing air applications due to their lightweight characteristics.

Which pressure vessels are regulated and which are not regulated? 

Generally, vessels operating above 15 PSI are regulated, while those below may be exempt, depending on size and application. Exemptions may exist for very small vessels, certain research applications, or vessels operating at lower pressures. The specific regulatory requirements can vary by jurisdiction, application, and the potential hazards associated with the vessel's contents.