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Pressure Vessels: Types, Design, Supports, Applications, Materials



What are Pressure Vessels?

Pressure Vessels are closed, leak-tight vessels, often cylindrical or spherical, intended to keep fluids (gases or liquids) at a pressure significantly different (higher or lower) than the ambient pressure. The other parts of Pressure Vessels comprise the following: a shell/head, nozzles, flanges, gaskets, platforms and ladders, and a baseplate. They are typically constructed of carbon or stainless steel and put together by welding plates. However, in some circumstances, other materials, such as aluminium, copper, non-metals, etc., are also used as pressure vessel materials. Even though most pressure vessels resemble long cylinders with two heads at each end, they can also be shaped like spheres or cones. With a few exceptions, the pressure within is often higher than outside.

As with steam boilers, the fluid inside the vessel may change state, or chemical reactors may combine with other reagents. High pressures, high temperatures, and occasionally flammable fluids or highly radioactive chemicals are common characteristics of pressure vessels. Due to these risks, the design must ensure that there will be no leaks. These vessels must also be adequately built to withstand the operating temperature and pressure.

It is necessary to note that a pressure vessel rupture can result in significant property loss and bodily harm. Pressure vessel design is fundamentally concerned with plant safety and integrity. The safe operating ranges established by the design temperature and pressure for each pressure vessel must be followed. Because the unintentional release and leaking of its contents threaten the environment, pressure vessel design, construction, and testing are carefully carried out by trained experts and are subject to rules. The American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME BPVC) Section VIII and the American Petroleum Industry (API) 510 Pressure Vessel Inspection Code are two of the more well-known standards.

Types of Pressure Vessels:

According to their use, pressure vessels are categorized as:

Strong Vessel

Storage tanks hold liquids and gases for industrial applications. The vessel may hold fluids during a later process or keep finished commodities like liquid nitrogen and compressed natural gas (CNG).

Heat Exchangers:

Heat is transferred between two or more fluids using heat exchangers. They are frequently used in the food, pharmaceutical, energy, and bioprocessing industries. The temperature, flow, and thermal properties of the fluids involved in heat exchange, as well as the thermal property of the conductive partition, all affect how heat exchanger equipment operates (for indirect contact heat exchangers).


Boilers are heat-transfer mechanisms that generate heat from fuel, nuclear, or electrical energy. They generally consist of an enclosed vessel that transfers heat from the source to the fluid. They are mainly employed to warm liquids. Fluid often changes states inside the boiler, going from liquid to vapour. Numerous heating applications, as well as power generation, use the vapour that the boiler creates. To speed the turbine blades, steam boilers produce steam at high pressure. As a result, the boiler vessel must be powerful to withstand high pressures and thermal stress. For most materials, strength typically decreases as temperature rises.

Process Vessels:

Pressure vessels fall within the broad category of process vessels. These vessels are employed in industrial processes like agitation, mixing, decantation, distillation, mass separation, and chemical reaction. The internal pressure of a process vessel varies depending on the process utilized and how the substances being processed change. Here are a few illustrations of several process vessel types:

Types of Process Vessel

A mixture of liquids can be separated using distillation columns depending on the variance in their volatilities. In this process, the mixture is heated until the more volatile component enters the vapour phase. The internals of the column determines the vessel’s height (packings or trays).

Decanters enable the separation of a mixture of a liquid and a solid. The denser component sinks to the bottom of the vessel. This form of the vessel either has a limited cross-sectional area or is horizontally oriented.

Industrial mixers are pressure vessels with motorized blades that may homogenize and emulsify one or more substances. The mixture may have a mix of liquid and solid components. Depending on the level of homogeneity, agitating machinery runs at various speeds.

Reactants and catalysts are kept separate during a chemical reaction in chemical reactors, which are closed tanks. They have agitators to promote molecular contact between the reactants. The heat produced during a chemical reaction is typically absorbed in a jacketed vessel. Depending on the heat of the response, the reactants can either emit heat (exothermic) or absorb heat (endothermic). The internal pressure increases when the reactants are converted into the final products when gaseous products are created, and it grows much more at higher temperatures.

Classifying Pressure Vessels according to their Geometry:

Spherical Pressure Vessels:

Pressure vessels with spheres. Spherical pressure vessels are the greatest at storing high-pressure fluids because of their robust design, but they are time- and money-consuming to produce. Since the sphere’s surface is equally pressured from the inside to the outside, there are no weak places. Their surface area to volume ratio is smaller. Spherical vessels will require less material to build than cylindrical vessels if a pressure vessel of the same volume is needed. The spherical vessel’s smaller surface area than other designs will allow less heat transmission from the hotter body.

Cylindrical Pressure Vessels:

The most popular pressure vessels are cylindrical because they are more versatile and less expensive to manufacture than spherical containers. But they aren’t as robust as spherical vessels due to the fittings that join the heads together. A cylinder called the shell, and end caps called the heads to make up cylindrical vessels. The region where the head and shell are attached is known as the weld line. The head begins to curl at the tangent line.


Due to the equal distribution of pressure across the surface of the hemispherical head, handling high-pressure fluids and enclosing large-diameter vessels is possible. They have a more significant internal volume and a straightforward radial form, but they are more challenging to construct and link to the shell. Hemispherical heads require the most negligible wall thickness for the same internal pressure as other head designs. A hemispherical head has a radius equal to the cross-sectional radius of a cylindrical vessel. Half of the head’s circumference corresponds to its depth.

Vessel Types/Orientation:

  • The axis of a cylindrical vessel may be oriented either vertically or horizontally.
  • The vessel is oriented vertically:
  • When there is limited floor space.
  • When the vessel has a modest volume.
  • In mixing tanks because the fluid is spread over a smaller cross-sectional area, allowing for effective mixing.
  • Where there is a large ratio of gas to liquid.
  • In liquid-liquid separation, to make component removal simpler.
  • The vessel is oriented horizontally:
  • Since this orientation makes cleaning heat exchangers easier.
  • In flash drums and settling tanks where modest downward velocities are necessary. Low speeds exhibit less entrainment.

Design of Pressure Vessels:

The calculations for pressure vessel design make use of the factors listed below. These elements are essential for establishing the thickness of the head and shell walls.

Design Pressure:

The vessel requirements are computed using a figure known as the design pressure. It is derived from the maximum operating pressure, representing the expected pressure spikes during disturbance conditions, including startup, emergency shutdown, and process irregularities. It consistently exceeds the working pressure limit. The pressure relief system of a vessel is also based on this value to lessen the possibility of explosions. Vessels that may someday experience vacuum pressure must have a design pressure that can resist a complete vacuum.

Maximum Allowable Working Pressure:

As per its design temperature, the vessel must operate at the maximum allowable working pressure (MAWP), which is the most excellent permitted pressure as measured at the top of the equipment. At its design temperature, it is the highest pressure that the vessel’s weakest point can withstand. The American Society of Mechanical Engineers (ASME) has set a MAWP value. Companies use this value to ensure that the vessel won’t run above it to build safety precautions and prevent explosions. Different from design pressure is MAWP. The MAWP is a broad attribute that depends on the material’s physical constraints. Wear and corrosion reduce the material’s MAWP. The design pressure might not even be the same as the MAWP.

Design Temperature:

Given that strength declines with rising temperature and becomes brittle at very low temperatures, the temperature significantly impacts the maximum allowed stress. The maximum permissible pressure should not be exceeded while the pressure vessel operates. The design temperature is always more than the maximum operating temperature and lower than the minimum and maximum temperatures.

There are a few general guidelines to follow when determining the design temperature. The design temperature must be 500F below the operational temperature’s maximum and -250F below its lowest. Vessels operating between -30 and 3450C must be given a maximum limit of 250C. The designer must take into account the disturbances that have a significant impact on the pressure vessel’s temperature.

Maximum Allowable Stress:

The maximum permitted stress is calculated by multiplying the maximum stress that the material can withstand by a safety factor. The safety factor considers any deviations from the optimum design and operation of the pressure vessel.

Joint Efficiency:

The strength of the welded plate divided by the strength of the virgin, unsealed plate gives the combined efficiency. The strength is often reduced at the welded junction. Without additional inspection and radiographic testing, welded joints are considered weaker due to potential flaws like porosity.

Corrosion Allowance:

When estimating the corrosion allowance, there are a few general guidelines that the manufacturer may or may not follow. Corrosion allowances should typically fall between 1.5 and 5 mm. Other recommendations state that the corrosion allowance should be 1.5 mm for stream drums and air receivers and 8.9 mm for corrosive situations. The corrosion allowance should be 3.8 mm for non-corrosive streams. In heat exchanger equipment, the corrosion allowance must be minimal since the heat transfer rate is influenced by the thickness of the wall.

Design codes used for Pressure Vessel Design:

Various codes are employed for pressure vessel design, building, and maintenance purposes. Below is a list of a few of them: EN 13445- used in Europe. ASME Code Section VIII, including ASME Code Section VIII Division 1: follows the US standard, designed by the formula; ASME Code Section VIII Division 2: which is intended by analysis; and ASME Code Section VIII Division 3: which follows alternative rules for building high-pressure vessels. Other codes include BS 5500, AD Merkblätter: as per the German standard, and the BS 4994 code.

Design Formula for Pressure Vessels:

The cylindrical shell’s basic design formula is σ =PD/2t.

t = PD/2 follows as a result. Where t is the shell’s thickness, Internal pressure is P, the Diameter of the shell is denoted by D, and σ refers to Tensile tension.

International design codes make modifications to this fundamental formula.

The following formula is used to determine the thickness of the cylinder for ASME Sec VIII: PR/SE-0.6P = t

The formula for calculating the thickness of the pressure vessel shell

where t is the shell’s thickness, P denotes the Internal pressure, R is the cylinder’s radius, S = tensile stress, and E = joint effectiveness. 

For design formulas for all portions of the vessel/cylinder, ASME Sec VIII Div 1 should be referred to.

Applications of Pressure Vessels:

In the oil and gas, fertilizer, power plant, petroleum refining, and chemical as well as petrochemical processing industries, all of these pressure vessels are widely employed, and power plants have various operating pressure and temperature requirements. Columns, boilers, separators, knock-out drums, towers, bullet tanks, reactors, and heat exchangers have commonly used pressure vessels. 

Some specific applications of pressure vessels include commercial pressurized air receivers, tanks for storing domestic hot water, cylinders for diving (Scuba diving), compressed air rooms, towers for distilling, autoclaves (used in the medical industry to sterilize), petrochemical facilities, and refineries for oil, vessels for nuclear reactors, hydraulic and pneumatic reservoirs, storage containers for liquid gases such propane, butane, propane, ammonia, and LPG.

An atmospheric storage tank is not a pressure vessel, though, as should be noted. 

How to Choose Appropriate Material for Pressure Vessels?

When selecting the ideal material to construct pressure vessels, the following considerations must be made:

  • If it can meet the strength requirements for a particular application. 
  • Materials must withstand various internal and external pressures and structural stresses over the course of the pressure vessel’s service life.
  • A pressure vessel’s ability to resist corrosion is one of its most essential qualities since it must be dependable under challenging circumstances.
  • Investment income: The costs of materials, fabrication, and maintenance must be considered across the pressure vessel’s whole existence. Economic analyses are conducted to select the best material that yields the lowest cost. 
  • The Return on Investment must be investigated and assessed to determine whether buying a pressure vessel is financially advantageous.
  • Simple fabrication and maintenance – Metal sheets must be well-machined and weldable since they are moulded to generate the geometry of the pressure vessels. This simplifies fabrication and maintenance. The vessel’s interior components must be easy to install.
  • Material available on the market: Materials for pressure vessels in the manufacturer’s location must be readily available in standard sizes.

Materials Required for Construction of Pressure Vessels:

The following materials can be used in the construction of Pressure Vessels:

Carbon Steel: Carbon steel is a type of steel with a higher carbon content of up to 2.5%. Containers made of carbon steel are perfect for several applications due to their excellent tensile strength at low wall thickness. They are vulnerable to impact and vibration. But because of its incredible strength, carbon steel is difficult to bend and form into different shapes. It is also more prone to corrosion and rusting than stainless steel since it lacks chromium.



Stainless steel: A type of steel with a reduced carbon content, a higher chromium concentration of up to 10.5 to 30%, and trace amounts of nickel known as stainless steel. They exhibit excellent chemical, corrosion, and weathering resistance because of their high chromium concentration. A thin, inert layer of chromium oxide is produced at the metal’s surface to prevent oxygen from diffusing into the metal’s core. Like carbon steel, it has excellent strength despite its thin wall thickness. It is easier to form because it is more elastic and ductile than carbon steel.


Hastelloy: Made of nickel, chromium, and molybdenum, Hastelloy was the first alloy developed by Haynes International, Inc. It is widely used in the petrochemical, energy, and oil and gas industries to manufacture reactors, pressure vessels, and heat exchangers. This chemical has applications in nuclear reactors. It has remarkable resistance to corrosion, oxidizing and reducing agents, and cracking. Its strength is not weakened by high temperatures. It can easily be moulded and welded because of its excellent ductility. Proper maintenance can extend its service life for several decades, increasing its cost-efficiency.

Nickel -Alloys made of nickel protect against corrosion, deterioration, and thermal expansion. Chromium is added to the nickel alloy to further boost its heat resistance. Pressure vessels made of nickel alloy are often used in the oil and gas sector, cryogenic applications, and other harsh environments. Additionally, it has a longer service life. However, it is more expensive to manufacture and challenging to handle. Purity is essential for the durability and dependability of nickel alloys.
Aluminum is well known for having a high strength-to-density ratio, which makes metal incredibly robust and light in weight. Its production is less expensive than stainless steel. It also has good rust resistance. Vessels made of aluminium are frequently utilized in laboratory-scale applications. It is ineffective for high-pressure applications due to its reduced density—one-third that of stainless steel.

Titanium: Titanium offers excellent strength and stiffness while having a small wall thickness. It has excellent corrosion resistance, is non-toxic, and is biocompatible. It is ideal for applications involving greater temperatures since it melts at a higher temperature than steel and aluminium. Due to its excellent thermal conductivity and ability to effectively transmit heat, it is the ideal material for heat exchangers.


What are the Types of Vessel Supports?

Depending upon the way the pressure vessels are supported, there can be the following types of supports:

  • Vessels Supported on Lug Support
  • Vessels Supported on Skit- Pressure containers with tall, vertical, cylindric shapes are frequently supported by skirts. A support skirt is a cylindrical shell piece welded to the bottom head or the lower part of the vessel shell ( for cylindrical vessels). The skirt is typically long enough to offer sufficient flexibility, preventing significant thermal stresses at the junction of the shell and the skirt from being caused by the radial thermal expansion of the shell.
  • Vessels Supported on Leg- The legs welded to the lowest part of the shell usually support small vertical drums. Typically, the maximum support leg length to drum diameter ratio is 2:1. To offer extra local reinforcement and local distribution, reinforcing pads are first welded to the shell. The number of legs depends on the drum’s size and the load’s weight. Support legs are also included in spherical pressurized storage tanks. Cross bracing is utilised between the legs to counteract the effects of wind or earthquakes. Lungs may also act as a support for vertical pressure channels. The usage of lugs is usually restricted to pressure vessels with small to medium diameters (1 to 10 ft) and with moderate height-to-diameter ratios of 2:1 to 5:1. For stability against overturning loads, the lugs are usually fastened to horizontal structural components.
  • Vessels Supported on Saddle- Saddle support is often used to support horizontal drums at two points. To prevent excessive local stress at the support point in the shell, it disperses throughout a sizable portion of the shell. One saddle support is fixed, while the other is free to allow for the drum’s uninterrupted longitudinal thermal expansion.



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