What is a pressure vessel? How many types of pressure vessels are used in various industries?

Pressure vessels, as leak-proof containers designed to store liquids or gases at varying pressures, play a crucial role in numerous industries. They come in different shapes and sizes, including spherical, conical, and cylindrical geometries. Their applications span across industries such as oil and gas, chemicals, petrochemicals, distillation towers, nuclear reactors, natural gas storage systems, and hot water tanks. As investments increase, the market volume of pressure vessels is expected to grow, with the emergence of composite materials as alternatives to traditional metals like steel.

Working Principle of Pressure Vessels

The engineering behind pressure vessels focuses on achieving and maintaining specific pressure levels required for various applications. These vessels can deliver pressure directly through valves and gauges or indirectly through heat transfer. They are capable of handling an extensive range of pressure levels, depending on the size and type of pressure vessel. The size of pressure vessels can vary greatly, ranging from 5to 20 liters.

Applications of Pressure Vessels in Different Industries

Pressure vessels find extensive use across various industries, with the oil and gas, chemical, and energy sectors being the primary contributors to their demand.

Oil and Gas Industry

Pressure vessels serve as receivers for physical and chemical processes carried out at high temperatures and pressures in the oil and gas industry. Pressure vessels and distillation columns are commonly purchased from the same suppliers and have comparable building methods. In this industry, materials including carbon steel and stainless steel are frequently utilized. In this industry, distillation trays and vessel internals are also crucial parts of pressure vessels.

Chemical Industry

In the chemical industry, pressure vessels play a crucial role in facilitating processes such as chemical reactions that result in significant changes in the container contents. These vessels enable product combination, division, and transformation, and multiple types of pressure vessels can be employed simultaneously in chemical operations.

Energy (Power Generation) Industry

Pressure vessels are widely used in the energy industry for a variety of applications, including the containment of hazardous gases in metalworks and oil refineries. Reactor Pressure Vessels (RPVs) are specialized pressure vessels used in nuclear power reactors that are essential for withstanding high temperatures, pressures, and neutron radiation. It's crucial to remember that not every power reactor uses a reactor pressure vessel.

Types of Pressure Vessels

Pressure vessels are designed in various types to meet specific requirements across different industries. Here are some common types:

Process Vessels: Process vessels (tanks) are designed to simply hold and store liquids and they are used for an integrated operation in petrochemical facilities, refineries, gas plants, oil and gas production facilities, and other facilities.

Autoclaves: Autoclaves are large vessels that are pressurized and brought to high temperatures. They are usually cylindrical since the rounded shape is better able to safely withstand high pressures. Autoclaves are designed to hold items that are placed inside and then the lid is sealed. They are crucial when it comes to biological applications and sanitizing processes.

High-Pressure Vessels: They are the most durable vessels on the market which are capable of working under the heaviest loads and they provide the best resistance to corrosion, temperature, and pressure. The high-pressure vessels are usually made of stainless steel. Typical functions for the high-pressure vessel: high-speed mixers, chemical reactors, and supercritical extraction systems.

Expansion Tanks: The expansion tanks are designed to adjust for changes in the amount of hot water in heating systems and changes in water flow rate, and to maintain the static pressure produced by the pump at the utilization level in sanitary hot water systems.

Heat Exchangers: A heat exchanger is a device that transfers heat from one medium to another. Heat exchangers are most commonly used in industrial facilities such as iron and steel, petroleum, petrochemical, gas, power plants, food, pharmaceuticals, leather, textiles, air conditioning, ships, and marine industries.

Water Pressure Tanks: In a water well system the pressure tank produces water pressure by using compressed air to force down on the water. Because of this pressure, water is forced out of the tank through the pipes when a valve is opened.

Vacuum Tanks: A vacuum tank is part of a system that filters air or fluids through suction, out-gassing, pumping, or a combination of techniques. Vacuums use pressure to prevent contamination, purify, dehydrate, and even power.

ASME Pressure Vessels: They also known as ASME boilers, are any pressure vessel with an ASME stamp. The ASME stamp indicates the vessel has undergone inspection and meets stringent ASME VIII code standards. In addition, the ASME stamps offer end-users information about the ASME boiler and its manufacturer.

Thin-Walled Pressure Vessels: A thin-walled pressure vessel is one in which the skin of the vessel has a thickness that is much smaller than the overall size of the vessel, and the vessel is subjected to internal pressure that is much greater than the exterior air pressure.

Boilers: They are closed pressure vessels used to heat fluids, mostly water. These heated fluids are used for cooking, power generation, central heating, water heating, and sanitation.

Pressure Vessel Production

PressureVessel Design

When it comes to reducing strain, a spherical shape is scientifically optimal. However, building perfectly spherical pressure vessels is complex and often reserved for specialized applications. The most used design is a configuration of a long, two-headed cylinder. The steel cylindrical pressure vessel matches the demands of various pressure vessel applications, these vessels are meticulously engineered to favor ease of production while retaining a robust and resilient geometry.

The cylindrical middle section can be easily built from a rectangular steel piece while the absence of perpendicular edges provides a better distribution of the tension. While hemispheric heads provide a better distribution of pressure, shallow heads are often used instead. Known as “dished” heads in the sector, they represent an important balance between strain minimization and manufacturability.

These heads are much easier to form and can become slightly thicker and reach the same pressure resistance. In general, dished heads have one of two geometries: semi-ellipsoidal or torispherical. Torispherical heads consist of a plate with a fixed radius that connects to the cylinder with a toroidal joint. The relative ease of production has made torispherical heads the most common head shape of the pressure vessel, finding use  in re-compression chambers, distillation towers, petrochemical plants and a variety of storage uses.

Semi elliptical heads are another choice used regularly. These are deeper, more spherical, and durable than a torispherical head, and more costly to construct, but can handle more challenging applications than torispherical heads. Semi elliptical heads are best suited to applications with slightly higher pressure where the entire length of the cylinder is still important.

The calculated thicknesses based on the relevant equations are the bare minimum, and it is necessary to incorporate additional allowances to ensure the structural integrity and longevity of the pressure vessel. These allowances encompass factors such as protection against corrosion and erosion, accommodating material supply variations, and accounting for any potential thinning that may occur during the manufacturing process.

Material Selection for Pressure Vessels

The spectrum of materials used in pressure vessels is wide and includes but is not limited to:

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

To comply with the production standards, the following material properties must be known in the selected materials. The designs made without knowing these features are very likely to have problems during long use. Therefore, much attention should be paid to the choice of materials.

• Elongation and reduction of area at fracture
• Notch toughness
• Aging and embrittlement under operating conditions
• Fatigue strength
• Availability

Design stresses are adjusted using safety factors applied to material properties, including:

• Yield strength at design temperature
• Ultimate tensile strength at room temperature
• Creep strength at design temperature

Welders and manufacturers must keep in mind the following points to guarantee that their pressure vessels fulfil all the requirements of industrial applications:

• Corrosion
• Vessel weight and contents
• Ambient and operational temperatures
• Static and dynamic pressures
• Residual and thermal stress
• Reaction forces

Steps Involved in Pressure Vessel Fabrication

Before construction starts, the manufacturer is often required to submit fully dimensioned drawings of the main pressure vessel shell and components for approval by the purchaser and inspecting authority. In addition to showing dimensions and thicknesses, these drawings include the following information:

• Design conditions.
• Welding procedures to be applied
• Key weld details
• Heat treatment procedures to be applied
• Non-destructive test requirements
• Test pressures.

The manufacturer is generally required to maintain a positive system of identification for the materials used in construction so that all material in the completed pressure vessel can be traced to its origin. The creation of plates in rollers or dished ends is a hot or cold process, depending on the material, thickness, and finished dimensions. The standard regulates the allowable mounting tolerances. These tolerances limit the stresses caused by roundness and misalignment of the joint.

Manufacturing Steel Dished Heads for Pressure Vessels

There are two main stages of making dished heads made of metal. Firstly, the metal is sliced in the correct thickness and shape using plasma cutting machines or industrial circular shears which are commonly guided by computers. When cut to form, the metal is transformed into a head using a flanging process or a spinning process. In the spinning approach, the metal is rotated on a hydraulic lathe and pressed into a tool.

The tool forms the metal according to the desired head shape and enables the hinge radius and the bowl radius to be produced in one go. Flanging is a two-step process modeled to accelerate the final cylinder assembly: The steel is pressed cold into a shaped cap and then formed with a pressure roller so that it demonstrates a straight flange at the point where the cylinder is connected.

Development of Composite Pressure Vessels

Defines 4 types of composite cylinders to describe the specific making principles.

• Type 1 – Complete Metal: Cylinder made entirely of metal.
• Type 2 – Hoop Wrap: metal ring, covered with fiber-material belt-like hoop cover. For geometrical purposes, the spherical bottom and the head of a cylindrical cylinder can withstand twice the pressure as the cylindrical shell (assuming uniform metal wall thickness).
• Type 3 – Totally Wrapped, over Metal Liner: Diagonally wrapped fibers make the wall resistant to pressure right at the bottom and around the metal collar. The metal lining is thin and is close to the vessel water.
• Type 4 – A vessel made of all carbon fiber, with polyamide or polyethylene insulation inside the liner. Features are much lower weight and very high resistance. The price of carbon fibre is comparatively high.

Type 2 and 3 cylinders came up around 1995. Type 4 cylinders are commercially available at least from 2016 on.

Pressure Vessel Welding Process

The welding process for pressure vessels plays a critical role in ensuring their high-quality construction, which is essential for withstanding the demanding conditions of high-pressure storage and distribution of liquids and gases. Achieving exceptional welding quality is vital not only for passing stringent pressure vessel welding inspections successfully but also for safeguarding valuable investments. However, errors can occur during the welding process, and it is important to be aware of them.

Porosity arises when gas enters the molten weld pool, leading to the formation of bubbles that manifest as voids during inspection. Several factors can contribute to porosity, such as improper welding techniques or the use of unsuitable consumables. It is crucial to ensure that appropriate welding techniques are followed and the correct consumables are used to mitigate porosity.

Nitrides are highly adherent contaminants that form when plasma cutting is performed with compressed air or nitrogen. These contaminants render the edges brittle and can induce porosity in certain welding processes, especially gas-metal arc welding. Removing nitrides is challenging because they can exist up to a depth of 0.005 to 0.010 inches beneath the material's surface, making it impossible to eliminate them using brushes.

Inclusions occur when surface contaminants become mixed into the weld pool and get trapped during the solidification process. In multipass welding applications, incompletely removed slag can also lead to inclusions. Thorough cleaning using an appropriate wire brush before welding and between passes is an effective method for minimizing this type of defect.

The American Society of Mechanical Engineers (ASME) has established comprehensive rules for pressure vessel production. The ASME pressure vessel code covers various aspects, including materials, assembly, and safety specifications. Its purpose is to ensure that the manufacturing process adheres to industry requirements, enabling pressure vessels to function effectively without compromising safety or causing harm to individuals working with or around them. Employing proper welding preparation techniques and employing excellent welding practices are vital for constructing safe, reliable, and profitable pressure vessels while meeting customer expectations.

Pressure Vessel Standards

Standards play a crucial role in ensuring the safety, quality, and reliability of pressure equipment and components. The ASME Boiler and Pressure Vessel Code (ASME Code) is widely recognized as a leading global standard in this field. It provides comprehensive criteria for producer certification and quality assurance. The ASME Code establishes standards for the design, materials, manufacturing, inspection, testing, and operation of various types of pressure vessels, including power boilers, heating boilers, nuclear facility components, fiber-reinforced plastic pressure vessels, and transport tanks. Accepted in over 100 countries, the ASME Code carries significant weight and adding the ASME certification mark to your pressure equipment instills greater trust among your business partners, end-users, and regulatory authorities.

To ensure compliance and promote safety, it is important to adhere not only to the ASME BPVC Standards Section VIII, which governs the design and manufacture of pressure vessels but also to other relevant safety standards and codes. For example, OSHA (Occupational Safety and Health Administration) 1915 Subpart K provides guidelines for vessels, drums, and containers, while API 510 Vessel Code addresses maintenance, repair, and alteration procedures. Additionally, API 572 focuses on inspection requirements. Local jurisdiction authorized inspection agencies typically govern and regulate inspections and installations, so it is essential to comply with their directives.

Given the high-risk nature of working with pressure vessels, it is imperative to assign handling responsibilities solely to qualified personnel. Proper training and expertise are necessary to mitigate risks associated with pressure vessels. By ensuring that only trained individuals are authorized to handle these vessels, you can enhance safety and minimize the potential for accidents or incidents.

By adhering to the appropriate standards, codes, and regulations, and entrusting qualified personnel with pressure vessel handling, you can promote a culture of safety, protect against potential hazards, and maintain the integrity and reliability of your pressure equipment.

Inspection and Testing of Pressure Vessels

During the production of each pressure vessel, it is essential to conduct thorough inspections to ensure compliance with regulatory requirements. The standard outlines the various stages, starting from material reception to the completion of the vessel, for which inspection by an authorized authority is mandatory. Additionally, customers may request additional inspections to verify the internals of the vessel.

The manufacturer provides details of the welding procedures employed in constructing the pressure vessel, along with test pieces that represent the materials and thicknesses used in the actual vessel. The inspecting authority typically oversees the creation and testing of these test pieces, unless authenticated test pieces from previous productions are available.

Welders must pass approval tests to demonstrate their capability to produce welds of the same quality as those used in the actual vessel. These permits are typically reaffirmed by a recognized licensing authority to ensure welder competency.

The level of non-destructive inspection required during the construction phase is determined by national standards. Common non-destructive testing methods include magnetic particle or dye penetrant testing (to detect surface flaws in welds), radiography (to detect internal flaws in welds), and ultrasonic testing (to detect surface and subsurface imperfections by using high-frequency sound waves). The extent of non-destructive testing depends on the material and thickness, with some standards employing a "joint factor" approach that reduces the testing requirement if the intended thickness is increased. This factor is determined and applied during the initial design phase.

Before delivery, most standards mandate a pressure test witnessed by the inspection authority. Typically, water is the preferred test liquid as it cannot be compressed. However, if air is the only feasible test liquid, special precautions need to be taken, and consultation with the inspection authority and relevant law enforcement agencies is necessary. The test pressure is usually set at 1.2 to 1.5 times the design pressure and is gradually applied and held for a specific duration to demonstrate the vessel's adequacy.

Upon delivery and operation, the customer assumes responsibility for the safe usage of the pressure vessel. Legislative requirements may also necessitate periodic inspections throughout the vessel's lifespan, with certain critical aspects requiring intervention from regulatory authorities.

In summary, several factors must be considered when selecting a pressure vessel that best suits your specific process. Thorough knowledge of the process conditions is crucial to ensure optimal design and material selection. Even with proper implementation of these steps, it is still necessary to conduct non-destructive testing during the production phase. Furthermore, regular maintenance should be performed after the pressure vessel is put into service. It is important to emphasize that pressure vessels manufactured in accordance with recognized standards offer enhanced safety. ASME has established widely recognized standards in numerous countries to ensure compliance and promote safety in this regard.

High-Quality Engineering & Procurement

Anachem Lab specializes in delivering high-quality pressure vessels that cater to diverse material grades and thickness requirements. Our manufacturing facilities are certified by the ASME U and S codes, ensuring adherence to the highest standards in the industry.

At Anachem Lab, we are committed to delivering exceptional engineering and procurement services, ensuring that our clients receive reliable and top-of-the-line products tailored to their specific needs.