Pressure Vessels

Pressure Vessels

In the coming years, many industries are planning new facilities and equipment upgrades, and because of that, the demand for pressure vessels is expected to climb. Right now, most pressure vessels are still built from metal, and steel is the material manufacturers turn to the most. It has a long track record, it’s tough, and it holds up well under high pressure.

Pressure vessels may not look complicated from the outside, but they’re used everywhere. You’ll find them in oil and gas plants, chemical and petrochemical operations, distillation towers, and nuclear facilities. They’re also used for storing natural gas and even in large hot-water units. Because these sectors keep expanding and modernizing, the need for safer and more efficient pressure vessel designs is growing right along with them.

United Cooling Systems designs and manufactures high-quality pressure vessels for industrial applications, including oil & gas, chemical, and power sectors. Our vessels meet ASME standards, offering durable materials, precise fabrication, and reliable performance under high pressures and temperatures, ensuring safety, efficiency, and long-term operational excellence For more detailed information on our products or to discuss your specific needs, feel free to contact us today.

What Is a Pressure Vessels?

Pressure vessels are leak-proof containers used to store liquids or gases. They come in many sizes and shapes depending on their purpose. The most common shapes are spherical, conical, and cylindrical. A typical pressure vessel is built as a long cylinder with two end heads. These vessels operate at internal pressures that are either higher or lower than normal air pressure, and their operating temperatures can also vary.

How Does It Work?

A pressure vessel works by reaching the pressure level needed for a specific application—for example, storing air in a scuba tank. Pressure can be delivered directly through valves and release gauges or indirectly through heat transfer. These vessels can handle pressures from about 15 psi up to roughly 150,000 psi, and temperatures can be above 400°C (750°F). Their capacity can range from around 75 liters (20 gallons) to several thousand liters.

Applications of Pressure Vessels

Pressure vessels are used in many industries, but three sectors make up most of the demand: the oil and gas industry, the chemical industry, and the energy industry.

1. Oil and Gas Industry: In the oil and gas sector, pressure vessels are commonly used as receivers where physical and chemical processes occur under high temperatures and high pressures.
   
   Although different types of columns are used for different purposes, their construction is very similar. Distillation columns, for example, separate feed streams into multiple products based on boiling points. Because pressure vessels and columns share similar construction methods, they are usually supplied by the same manufacturers.
   
   Carbon steel and stainless steel are the two most common materials used in this industry. A pressure vessel also needs internal components—such as vessel internals and distillation trays—to function properly. These parts are highly specialized and require detailed specifications, so they are usually supplied by companies that focus only on such components.
2. Chemical Industry: In the chemical industry, a pressure vessel is used to carry out chemical reactions that fundamentally change the contents inside the container. These reactions may involve combining materials to form a new product, breaking a material into different products, or removing certain parts of an existing product to produce something else. Many types of pressure vessels can be used at the same time in chemical processing plants.
3. Energy (Power Generation) Industry: The energy sector relies on pressure vessels for several reasons. One major use is the containment of harmful or excess gases. Facilities such as oil refineries and metalworking plants often need safe storage for these gases.
   
   Nuclear power plants use a special type of pressure vessel called a Reactor Pressure Vessel (RPV). An RPV is a large cylindrical steel vessel that holds the reactor core, cooling water, and the generated steam. It must be extremely reliable, since it is exposed to high temperatures, high pressures, and neutron irradiation. This makes the RPV the most critical pressure boundary in a nuclear plant. However, not all types of power reactors use a reactor pressure vessel.

Pressure Vessel Types

1. Process Vessels: Process vessels (tanks) are designed simply to hold and store liquids. They are used as part of integrated operations in petrochemical facilities, refineries, gas plants, oil and gas production facilities, and similar industrial sites.
2. Autoclaves: Autoclaves are large vessels that operate under pressure and high temperatures. They are usually cylindrical because the rounded shape can safely withstand high pressures. Autoclaves are designed to hold items placed inside before the lid is sealed.
3. High-Pressure Vessels: High-pressure vessels are the most durable types on the market. They can operate under extremely heavy loads and offer strong resistance to corrosion, temperature, and pressure. They are commonly made from stainless steel. Typical uses include high-speed mixers, chemical reactors, and supercritical extraction systems.
4. Expansion Tanks: Expansion tanks are designed to compensate for changes in the volume of hot water in heating systems and variations in water flow. They also help maintain the static pressure created by the pump in sanitary hot water systems.
5. Heat Exchangers: A heat exchanger transfers heat from one medium to another. They are widely used in industrial facilities, including iron and steel plants, petroleum and petrochemical industries, gas processing facilities, power plants, food production, pharmaceuticals, leather, textiles, HVAC systems, ships, and marine applications.
6. Water Pressure Tanks: In a water well system, a pressure tank creates water pressure by using compressed air that pushes down on the water. This pressure forces water through the pipes in a home whenever a valve is opened.
7. Vacuum Tank: A vacuum tank is part of a system that filters air or fluids using suction, outgassing, pumping, or a combination of these processes. Vacuum systems use pressure differences to prevent contamination, purify, dehydrate, and even provide power.
8. ASME Pressure Vessels: ASME pressure vessels, also known as ASME boilers, are any vessels that carry an ASME stamp. This stamp shows that the vessel has been inspected and meets strict ASME Section VIII code requirements. It also provides users with important information about the vessel and its manufacturer.
9. Thin-Walled Pressure Vessels: A thin-walled pressure vessel has a wall thickness much smaller than its overall size. It is designed to withstand internal pressures significantly higher than the outside air pressure.
10. Boilers: Boilers are closed pressure vessels used to heat fluids, usually water. The heated fluid is then used for power generation, cooking, central heating, water heating, or sanitation.

Pressure Vessel Production

Design

Scientifically, the ideal shape for minimizing stress is a sphere. However, the engineering reality is more complicated. Spherical pressure vessels are extremely difficult to build. While organizations like NASA may manufacture perfectly spherical carbon-fiber tanks for cryogenic uses, most industries require designs that are easier and more practical to produce. The most common solution is a long cylinder with two end heads. Cylindrical steel vessels meet the needs of many applications and are engineered to balance manufacturability with strong, reliable geometry.

The cylindrical shell can be made easily from a rectangular steel plate, and the absence of sharp edges helps distribute stress more evenly. Although hemispherical heads offer the most uniform pressure distribution, many manufacturers use shallow “dished” heads instead. These heads are easier to form, can be made slightly thicker, and still provide the required pressure resistance.

In general, dished heads come in two main shapes: semi-ellipsoidal and torispherical. Torispherical heads are formed from a plate with a fixed radius that joins the cylinder through a toroidal transition. Their simple production makes them the most commonly used head type, found in recompression chambers, distillation towers, petrochemical equipment, and various storage applications.

Semi-elliptical heads are deeper, more rounded, and more durable than torispherical heads. They cost more to manufacture but handle higher pressures and are suitable for applications where the full cylinder length remains important.

The thickness determined by design equations represents the minimum value; additional allowances must be added. These include allowances for corrosion, erosion, material supply tolerances, and any thinning that occurs during fabrication.

Material Selection

A wide range of materials can be used in pressure vessels, including:

• Carbon steel (with less than 0.25% carbon)
• Carbon–manganese steel (offering higher strength than carbon steel)
• Low-alloy steels
• High-alloy steels
• Austenitic stainless steels
• Non-ferrous materials (aluminum, copper, nickel, and their alloys)
• High-duty bolting materials

To meet production standards, manufacturers must understand the key properties of the materials they select. Designs made without accurate material data can lead to serious problems over long-term operation. Material selection must consider:

1. Elongation and reduction of area at fracture
2. Notch toughness
3. Aging and embrittlement under service conditions
4. Fatigue strength
5. Availability

Design stresses are set by applying safety factors to material properties such as

1. Yield strength at design temperature
2. Ultimate tensile strength at room temperature
3. Creep strength at design temperature

Welders and manufacturers must also consider:

• Corrosion
• Vessel weight and contents
• Ambient and operating temperatures
• Static and dynamic pressures
• Residual and thermal stresses
• Reaction forces

Pressure Vessel Manufacturing Process

Before construction begins, the manufacturer typically submits fully dimensioned drawings of the vessel shell and components for approval by the purchaser and the inspecting authority. These drawings include:

• Design conditions
• Welding procedures
• Key weld details
• Heat-treatment procedures
• Non-destructive testing requirements
• Test pressures

Manufacturers must maintain full traceability of all materials used. Every plate, component, and weld must be identifiable and traceable to its origin.

Plates and dished ends are formed through hot or cold processes depending on the material, thickness, and final dimensions. Standards specify allowable fabrication tolerances, which limit stresses caused by out-of-roundness or misaligned joints.

How Steel Dished Heads Are Made

Producing metal dished heads involves two main stages. First, the metal plate is cut to the required thickness and shape using plasma cutters or industrial circular shears, usually computer-controlled. After cutting, the plate is shaped into a head using either a flanging process or a spinning process.

In the spinning method, the plate rotates on a hydraulic lathe and is pressed against a forming tool. The tool shapes the metal to create both the bowl radius and the knuckle radius in a single operation.

Flanging is a two-step method designed to speed up vessel assembly. The plate is first pressed into a basic cap shape, then formed with a pressure roller to create a straight flange where the head will connect to the cylinder.

Development of Composite Vessels

Composite pressure vessels are classified into four main types based on their construction principles:

1. Type 1 – Complete Metal: The cylinder is made entirely of metal.
2. Type 2 – Hoop Wrap: A metal cylinder is covered with a fiber-material belt-like hoop. Geometrically, the spherical ends (bottom and head) of the cylinder can withstand roughly twice the pressure of the cylindrical shell, assuming uniform metal wall thickness.
3. Type 3 – Totally Wrapped over Metal Liner: Diagonally wrapped fibers make the walls highly resistant to pressure, especially at the ends and around the metal collar. The metal liner is thin and close to the vessel contents.
4. Type 4 – All-Carbon Fiber: Made entirely of carbon fiber with an inner liner of polyamide or polyethylene. These vessels are much lighter and extremely strong, but carbon fiber is comparatively expensive.

Type 2 and Type 3 vessels emerged around 1995, while Type 4 vessels have been commercially available since at least 2016.

Welding Techniques for Pressure Vessels

Pressure vessels store and distribute liquids and gases under high pressure. Welding must be of exceptional quality to ensure the vessel withstands operational conditions. Proper surface preparation is essential for passing inspections the first time and avoiding costly rework.

Common welding defects include:

1. Porosity: Gas trapped in the molten weld pool creates bubbles that form voids as the weld solidifies. Porosity can result from improper welding techniques or unsuitable consumables.
2. Nitrides: Contaminants formed during plasma cutting with compressed air or nitrogen make edges brittle and can cause porosity, especially in gas metal arc welding. Nitrides can exist 0.005–0.010 inches below the surface and cannot be removed by brushing.
3. Inclusions: These occur when surface contaminants mix into the weld pool during solidification. In multipass welding, leftover slag can also create inclusions. Thorough cleaning with a wire brush before welding and between passes helps prevent this defect.

The American Society of Mechanical Engineers (ASME) has established rules for pressure vessel production. The ASME pressure vessel code covers materials, assembly, and safety requirements to ensure vessels are manufactured to function safely without risk of damage or injury. Proper welding preparation and excellent welding techniques are essential for safe, high-quality, and cost-effective pressure vessels.

Industry Standards for Pressure Vessels

The ASME Boiler and Pressure Vessel Code (ASME BPVC) is a global standard for pressure equipment. It provides requirements for producer certification and quality assurance and sets standards for design, materials, manufacture, inspection, testing, and operation of boilers and pressure vessels. This includes power boilers, heating boilers, nuclear facility components, fiber-reinforced plastic pressure vessels, and transport tanks. The ASME Code is recognized in over 100 countries, and adding the ASME certification mark enhances trust among business partners, end-users, and authorities.

Safety Standards and Codes

In addition to ASME BPVC Section VIII, which governs design and manufacturing, pressure vessel users should follow other safety standards and codes such as

1. OSHA 1915 Subpart K: Covers vessels, drums, and containers.
2. API 510: For maintenance, repair, and alteration of pressure vessels.
3. API 572: For inspection procedures.

Authorized local inspection agencies regulate inspections and installations in accordance with local laws.

Handling by Trained Personnel

Because pressure vessels operate under high-risk conditions, only qualified personnel should handle, maintain, or operate them.

Inspection and Testing

Every pressure vessel must be checked during construction by an authorized inspector. Standards say inspections must happen at specific stages, from when the raw materials arrive to when the vessel is finished. Sometimes, the customer may ask for extra checks, like inspecting the internal parts of the vessel. The manufacturer decides on the welding process and creates small test pieces that match the actual materials and thickness of the vessel. Inspectors usually watch how these test pieces are made and tested, unless pre-approved samples are already available. Welders must pass tests to prove they can make welds just like the ones needed for the real vessel. Licensing authorities approve and maintain these qualifications.

Non-Destructive Testing (NDT)

Standards also specify how vessels are inspected without causing damage. The most common methods are:

1. Magnetic particle or dye penetrant testing:
   Dye penetrant finds cracks on the surface, while magnetic particle testing can also find flaws just under the surface.
2. Radiography (X-ray testing):
   X-rays reveal cracks or defects below the surface. Because it is expensive, it is usually only used for very important welds, like those in nuclear plants or submarines.
3. Ultrasonic testing:
   High-frequency sound waves pass through the metal to detect surface and internal defects.
   
   How much inspection is needed depends on the material, thickness, and difficulty of the weld. Some designs allow less inspection if the metal is thicker.
4. Pressure Testing
   Before delivery, most vessels must undergo a pressure test, watched by the inspector. Water is preferred because it cannot be compressed, making the test safer. If air is used, special precautions must be taken.
   
   The test pressure is usually 1.2 to 1.5 times the design pressure, applied slowly and held for a while to make sure the vessel is strong and safe.
   
   Once the vessel is delivered, the customer is responsible for operating it safely. Many countries also require regular inspections while the vessel is in use, especially for vessels containing hazardous materials.

Conclusion

Choosing the right pressure vessel depends on several factors. First, the process requirements must be fully understood so the design and material are suitable. Even with the right design and materials, production must be checked carefully using nondestructive testing.

After installation, vessels need regular maintenance. Pressure vessels built according to recognized standards, such as ASME, are safer and more reliable, giving confidence to both operators and inspectors.

FAQ

1. What is a pressure vessel used for?
A pressure vessel is used to store or contain gases or liquids under high or low pressure. They are widely used in industries such as chemical processing, oil & gas, power generation, and pharmaceuticals to safely handle pressurized fluids.

2. What is a Type 3 pressure vessel?
A Type 3 pressure vessel, often called a composite-over-metal vessel, typically has a metal liner (like steel or aluminum) with a composite outer layer for added strength and reduced weight. These vessels are used where lightweight and high-pressure resistance are required, such as in aerospace or high-performance fuel storage.

3. What is a pressure vessel and its types?
A pressure vessel is a container designed to hold fluids at pressures different from ambient pressure. Common types include:

1. Type 1: All-metal vessels
2. Type 2: Metal-lined, partially reinforced with composites
3. Type 3: Metal liner fully reinforced with composites
4. Type 4: Fully composite vessels without a metal liner

4. What is a pressure vessel as per ASME?
According to ASME (American Society of Mechanical Engineers), a pressure vessel is a container designed to operate at pressures significantly above or below atmospheric pressure, and it must meet specific safety and design codes defined in the ASME Boiler and Pressure Vessel Code (BPVC).

5. What is a Class 2 pressure vessel?
A Class 2 pressure vessel refers to a classification in certain regional codes (like some ASME divisions) indicating moderate pressure and temperature ratings, usually for industrial and commercial applications where full Class 1 compliance is not required.

6. What does ASME stand for?
ASME stands for the American Society of Mechanical Engineers, a professional organization that develops codes and standards for mechanical devices, including pressure vessels, boilers, and piping systems