
A pressure vessel is a container designed to hold gases or liquids at a
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
substantially different from the
ambient pressure
The ambient pressure on an object is the pressure of the surrounding medium, such as a gas or liquid, in contact with the object.
Atmosphere
Within the atmosphere, the ambient pressure decreases as elevation increases. By measuring ambient atmosp ...
.
Construction methods and materials may be chosen to suit the pressure application, and will depend on the size of the vessel, the contents, working pressure, mass constraints, and the number of items required.
Pressure vessels can be dangerous, and fatal accidents have occurred in the history of their development and operation. Consequently, pressure vessel design, manufacture, and operation are regulated by engineering authorities backed by legislation. For these reasons, the definition of a pressure vessel varies from country to country.
The design involves parameters such as maximum safe operating pressure and temperature,
safety factor
In engineering, a factor of safety (FoS) or safety factor (SF) expresses how much stronger a system is than it needs to be for its specified maximum load. Safety factors are often calculated using detailed analysis because comprehensive testing i ...
,
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
allowance and minimum design temperature (for brittle fracture). Construction is tested using
nondestructive testing, such as
ultrasonic testing,
radiography
Radiography is an imaging technology, imaging technique using X-rays, gamma rays, or similar ionizing radiation and non-ionizing radiation to view the internal form of an object. Applications of radiography include medical ("diagnostic" radiog ...
, and pressure tests. Hydrostatic pressure tests usually use water, but pneumatic tests use air or another gas. Hydrostatic testing is preferred, because it is a safer method, as much less energy is released if a fracture occurs during the test (water does not greatly increase its volume when rapid depressurisation occurs, unlike gases, which expand explosively). Mass or batch production products will often have a representative sample tested to destruction in controlled conditions for quality assurance.
Pressure relief devices may be fitted if the overall safety of the system is sufficiently enhanced.
In most countries, vessels over a certain size and pressure must be built to a formal code. In the United States that code is the
ASME Boiler and Pressure Vessel Code (BPVC)
The ASME Boiler & Pressure Vessel Code (BPVC) is an American Society of Mechanical Engineers, American Society of Mechanical Engineers (ASME) standardization, standard that regulates the design and construction of boilers and pressure vessels. The ...
. In Europe the code is the
Pressure Equipment Directive
The Pressure Equipment Directive (PED) 2014/68/EU (formerly 97/23/EC) of the EU sets out the standards for the design and fabrication of pressure equipment ("pressure equipment" means steam boilers, pressure vessels, piping, safety valves a ...
. These vessels also require an authorised inspector to sign off on every new vessel constructed and each vessel has a nameplate with pertinent information about the vessel, such as maximum allowable working pressure, maximum temperature,
minimum design metal temperature, what company manufactured it, the date, its registration number (through the National Board), and
American Society of Mechanical Engineers
The American Society of Mechanical Engineers (ASME) is an American professional association that, in its own words, "promotes the art, science, and practice of multidisciplinary engineering and allied sciences around the globe" via "continuing edu ...
's official stamp for pressure vessels (U-stamp). The nameplate makes the vessel traceable and officially an ASME Code vessel.
A special application is
pressure vessels for human occupancy, for which more stringent safety rules apply.
Definition and scope
The ASME definition of a pressure vessel is a container designed to hold gases or liquids at a
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
substantially different from the
ambient pressure
The ambient pressure on an object is the pressure of the surrounding medium, such as a gas or liquid, in contact with the object.
Atmosphere
Within the atmosphere, the ambient pressure decreases as elevation increases. By measuring ambient atmosp ...
.
The Australian and New Zealand standard "AS/NZS 1200:2000 Pressure equipment" defines a pressure vessel as a vessel subject to internal or external pressure, including connected components and accessories up to the connection to external piping.
This article may include information on pressure vessels in the broad sense, and is not restricted to any single definition.
Components
A pressure vessel comprises a shell, and usually one or more other components needed to pressurise, retain the pressure, depressurise, and provide access for maintenance and inspection. There may be other components and equipment provided to facilitate the intended use, and some of these may be considered parts of the pressure vessel, such as shell penetrations and their closures, and
viewport
A viewport is a polygon viewing region in computer graphics.
In computer graphics theory, there are two region-like notions of relevance when rendering some objects to an image. In textbook terminology, the ''world coordinate window'' is the area ...
s and
airlocks on a pressure vessel for human occupancy, as they affect the integrity and strength of the shell, are also part of the structure retaining the pressure. Pressure gauges and safety devices like pressure relief valves may also be deemed part of the pressure vessel.
There may also be structural components permanently attached to the vessel for lifting, moving, or mounting it, like a foot ring, skids, handles, lugs, or mounting brackets.
Types
*
*
*
**
**
**
***
*** – system used in tall buildings and marine environments to maintain water pressure.
***
**
*
*
*
*
*
*
*}
**
**
**
**
**
**
**
**
**
**
*
Dissolved gas storage
*Fired pressure vessels
*
Liquefied gas (vapour over liquid) storage
*
Permanent gas storage
*
Supercritical fluid storage
*Internal pressure vs external
*Types by construction method
*Types by construction material
Uses

Pressure vessels are used in a variety of applications in both industry and the private sector. They appear in these sectors as industrial
compressed air
Compressed air is air kept under a pressure that is greater than atmospheric pressure. Compressed air in vehicle tires and shock absorbers are commonly used for improved traction and reduced vibration. Compressed air is an important medium for t ...
receivers,
boiler
A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, centra ...
s and
domestic hot water storage tanks. Other examples of pressure vessels are
diving cylinders,
recompression chambers,
distillation towers,
pressure reactors,
autoclaves, and many other vessels in
mining
Mining is the Resource extraction, extraction of valuable geological materials and minerals from the surface of the Earth. Mining is required to obtain most materials that cannot be grown through agriculture, agricultural processes, or feasib ...
operations,
oil refineries and
petrochemical
Petrochemicals (sometimes abbreviated as petchems) are the chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable s ...
plants,
nuclear reactor
A nuclear reactor is a device used to initiate and control a Nuclear fission, fission nuclear chain reaction. They are used for Nuclear power, commercial electricity, nuclear marine propulsion, marine propulsion, Weapons-grade plutonium, weapons ...
vessels,
submarine
A submarine (often shortened to sub) is a watercraft capable of independent operation underwater. (It differs from a submersible, which has more limited underwater capability.) The term "submarine" is also sometimes used historically or infor ...
and
space ship habitats,
atmospheric diving suits,
pneumatic reservoirs,
hydraulic reservoirs under pressure,
rail vehicle air brake reservoirs,
road vehicle air brake reservoirs, and storage vessels for high pressure
permanent gases and liquified gases such as
ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
,
chlorine
Chlorine is a chemical element; it has Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between ...
, and
LPG (
propane
Propane () is a three-carbon chain alkane with the molecular formula . It is a gas at standard temperature and pressure, but becomes liquid when compressed for transportation and storage. A by-product of natural gas processing and petroleum ref ...
,
butane
Butane () is an alkane with the formula C4H10. Butane exists as two isomers, ''n''-butane with connectivity and iso-butane with the formula . Both isomers are highly flammable, colorless, easily liquefied gases that quickly vaporize at ro ...
).
A pressure vessel may also support structural loads. The passenger cabin of an airliner's outer skin carries both the structural and maneuvering loads of the aircraft, and the
cabin pressurization
Cabin pressurization is a process in which conditioned air is pumped into the aircraft cabin, cabin of an aircraft or spacecraft in order to create a safe and comfortable environment for humans flying at high altitudes. For aircraft, this air i ...
loads. The pressure hull of a submarine also carries the hull structural and maneuvering loads.
File:Cylindrical-research-autoclave-illustration.jpg, Cylindrical research autoclave illustration
File:Nasa decompression chamber.jpg, NASA decompression chamber
File:Water well tank.JPG, A pressure tank connected to a water well and domestic hot water system.
File:Propane tanks large.jpg, A few pressure tanks, here used to hold propane
Propane () is a three-carbon chain alkane with the molecular formula . It is a gas at standard temperature and pressure, but becomes liquid when compressed for transportation and storage. A by-product of natural gas processing and petroleum ref ...
.
File:Bergisch Gladbach - Papiermühle Alte Dombach 07 ies.jpg, A pressure vessel used as a kier.
File:CST-100 pressure vessel.jpg, A pressure vessel used for The Boeing Company's CST-100 spacecraft.
Design
Working pressure
The working pressure, i.e. the pressure difference between the interior of the pressure vessel and the surroundings is the primary characteristic considered for design and construction. The concepts of high pressure and low pressure are somewhat flexible, and may be defined differently depending on context. There is also the matter of whether the internal pressure is greater or less than the external pressure, and its magnitude relative to normal atmospheric pressure. A vessel with internal pressure lower than atmospheric may also be called a
hypobaric vessel or a
vacuum vessel. A pressure vessel with high internal pressure can easily be made to be structurally stable, and will usually fail in tension, but failure due to excessive external pressure is usually by
buckling
In structural engineering, buckling is the sudden change in shape (Deformation (engineering), deformation) of a structural component under Structural load, load, such as the bowing of a column under Compression (physics), compression or the wrin ...
instability and collapse.
Shape
Pressure vessels can theoretically be almost any shape, but shapes made of sections of spheres, cylinders,
ellipsoid
An ellipsoid is a surface that can be obtained from a sphere by deforming it by means of directional Scaling (geometry), scalings, or more generally, of an affine transformation.
An ellipsoid is a quadric surface; that is, a Surface (mathemat ...
s of revolution, and
cone
In geometry, a cone is a three-dimensional figure that tapers smoothly from a flat base (typically a circle) to a point not contained in the base, called the '' apex'' or '' vertex''.
A cone is formed by a set of line segments, half-lines ...
s with circular sections are usually employed, though some other surfaces of revolution are also inherently stable. A common design is a cylinder with end caps called
heads. Head shapes are frequently either hemispherical or dished (
torispherical). More complicated shapes have historically been much harder to analyze for safe operation and are usually far more difficult to construct.
File:Biogasholder and flare.JPG, Spherical gas container.
File:Ресивер хладагента FP-LR-100.png, Cylindrical pressure vessel.
File:Diffuser Head.jpg, Picture of the bottom of an aerosol spray can.
File:ABC Fire Extinguisher.jpg, Fire Extinguisher with rounded rectangle pressure vessel
Theoretically, a spherical pressure vessel has approximately twice the strength of a cylindrical pressure vessel with the same wall thickness, and is the ideal shape to hold internal pressure.
However, a spherical shape is difficult to manufacture, and therefore more expensive, so most pressure vessels are cylindrical with 2:1 semi-elliptical heads or end caps on each end. Smaller pressure vessels are assembled from a pipe and two covers. For cylindrical vessels with a diameter up to 600 mm (NPS of 24 in), it is possible to use seamless pipe for the shell, thus avoiding many inspection and testing issues, mainly the nondestructive examination of radiography for the long seam if required. A disadvantage of these vessels is that greater diameters are more expensive, so that for example the most economic shape of a , pressure vessel might be a diameter of and a length of including the 2:1 semi-elliptical domed end caps.
Scaling
No matter what shape it takes, the minimum mass of a pressure vessel scales with the pressure and volume it contains and is inversely proportional to the
strength to weight ratio of the construction material (minimum mass decreases as strength increases).
Scaling of stress in walls of vessel
Pressure vessels are held together against the gas pressure due to tensile forces within the walls of the container. The normal (tensile)
stress in the walls of the container is proportional to the pressure and radius of the vessel and inversely proportional to the thickness of the walls. Therefore, pressure vessels are designed to have a thickness proportional to the radius of tank and the pressure of the tank and inversely proportional to the maximum allowed normal stress of the particular material used in the walls of the container.
Because (for a given pressure) the thickness of the walls scales with the radius of the tank, the mass of a tank (which scales as the length times radius times thickness of the wall for a cylindrical tank) scales with the volume of the gas held (which scales as length times radius squared). The exact formula varies with the tank shape but depends on the density, ρ, and maximum allowable stress σ of the material in addition to the pressure P and volume V of the vessel. (See below for the exact equations for the stress in the walls.)
Spherical vessel
For a
sphere
A sphere (from Ancient Greek, Greek , ) is a surface (mathematics), surface analogous to the circle, a curve. In solid geometry, a sphere is the Locus (mathematics), set of points that are all at the same distance from a given point in three ...
, the minimum mass of a pressure vessel is
:
,
where:
*
is mass, (kg)
*
is the pressure difference from ambient (the
gauge pressure), (Pa)
*
is volume,
*
is the density of the pressure vessel material, (kg/m
3)
*
is the maximum working
stress that material can tolerate. (Pa)
Other shapes besides a sphere have constants larger than 3/2 (infinite cylinders take 2), although some tanks, such as non-spherical wound composite tanks can approach this.
Cylindrical vessel with hemispherical ends
This is sometimes called a "bullet" for its shape, although in geometric terms it is a
capsule.
For a cylinder with hemispherical ends,
:
,
where
*R is the Radius (m)
*W is the middle cylinder width only, and the overall width is W + 2R (m)
Cylindrical vessel with semi-elliptical ends
In a vessel with an
aspect ratio
The aspect ratio of a geometry, geometric shape is the ratio of its sizes in different dimensions. For example, the aspect ratio of a rectangle is the ratio of its longer side to its shorter side—the ratio of width to height, when the rectangl ...
of middle cylinder width to radius of 2:1,
:
.
Gas storage capacity
In looking at the first equation, the factor PV, in SI units, is in units of (pressurization) energy. For a stored gas, PV is proportional to the mass of gas at a given temperature, thus
:
. (see
gas law)
The other factors are constant for a given vessel shape and material. So we can see that there is no theoretical "efficiency of scale", in terms of the ratio of pressure vessel mass to pressurization energy, or of pressure vessel mass to stored gas mass. For storing gases, "tankage efficiency" is independent of pressure, at least for the same temperature.
So, for example, a typical design for a minimum mass tank to hold
helium
Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
(as a pressurant gas) on a rocket would use a spherical chamber for a minimum shape constant, carbon fiber for best possible
, and very cold helium for best possible
.
Stress in thin-walled pressure vessels
Stress in a thin-walled pressure vessel in the shape of a sphere is
:
,
where
is hoop stress, or stress in the circumferential direction,
is stress in the longitudinal direction, ''p'' is internal gauge pressure, ''r'' is the inner radius of the sphere, and ''t'' is thickness of the sphere wall. A vessel can be considered "thin-walled" if the diameter is at least 10 times (sometimes cited as 20 times) greater than the wall thickness.

Stress in a thin-walled pressure vessel in the shape of a cylinder is
:
,
:
,
where:
*
is
hoop stress, or stress in the circumferential direction
*
is stress in the longitudinal direction
* ''p'' is internal gauge pressure
* ''r'' is the inner radius of the cylinder
* ''t'' is thickness of the cylinder wall.
Almost all pressure vessel design standards contain variations of these two formulas with additional empirical terms to account for variation of stresses across thickness, quality control of
welds and in-service
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
allowances.
All formulae mentioned above assume uniform distribution of membrane stresses across thickness of shell but in reality, that is not the case. Deeper analysis is given by
Lamé's theorem, which gives the distribution of stress in the walls of a thick-walled cylinder of a homogeneous and isotropic material. The formulae of pressure vessel design standards are extension of Lamé's theorem by putting some limit on ratio of inner radius and thickness.
For example, the
ASME Boiler and Pressure Vessel Code (BPVC)
The ASME Boiler & Pressure Vessel Code (BPVC) is an American Society of Mechanical Engineers, American Society of Mechanical Engineers (ASME) standardization, standard that regulates the design and construction of boilers and pressure vessels. The ...
(UG-27) formulas are:
Spherical shells: Thickness has to be less than 0.356 times inner radius
:
Cylindrical shells: Thickness has to be less than 0.5 times inner radius
:
:
where ''E'' is the joint efficiency, and all others variables as stated above.
The
factor of safety is often included in these formulas as well, in the case of the ASME BPVC this term is included in the material stress value when solving for pressure or thickness.
Shell penetrations
Also sometimes called hull penetrations, depending on context, shell penetrations are intentional breaks in the structural integrity of the shell, and are usually significant local stress-raisers, so they must be accounted for in the design so they do not become failure points. It is usually necessary to reinforce the shell in the immediate vicinity of such penetrations. Shell penetrations are necessary to provide a variety of functions, including passage of the contents from the outside to the inside and back out, and in special applications for transmission of electricity, light, and other services through the shell. The simplest case is gas cylinders, which need only a neck penetration threaded to fit a valve, while a submarine or spacecraft may have a large number of penetrations for a large number of functions.
Penetration thread
The screw thread used for high pressure vessel shell penetrations is subject to high loads and must not leak.
High pressure cylinders are produced with conical (tapered) threads and parallel threads. Two sizes of tapered threads have dominated the full metal cylinders in industrial use from in volume.
For smaller fittings, taper thread standard 17E is used,
with a 12% taper right hand thread,
standard Whitworth 55° form with a pitch of 14 threads per inch (5.5 threads per cm) and pitch diameter at the top thread of the cylinder of . These connections are sealed using thread tape and torqued to between on steel cylinders, and between on aluminium cylinders.
For larger fittings, taper thread standard 25E is used. To screw in the valve, a higher torque of typically about is necessary,
Until around 1950, hemp was used as a sealant. Later, a thin sheet of lead pressed to a hat form which closely fitted the external threads, with a hole on top was used. The fitter would squeeze the soft lead shim to conform better with the grooves and ridges of the fitting before screwing it into the hole. The lead would deform to form a thin layer between the internal and external thread, and thereby fill the gaps to create the seal. Since 2005,
PTFE
Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene, and has numerous applications because it is chemically inert. The commonly known brand name of PTFE-based composition is Teflon by Chemours, a spin-off fro ...
-tape has been used to avoid using lead.
A tapered thread provides simple assembly, but requires high torque for connecting and leads to high radial forces in the vessel neck, and has a limited number of times it can be used before it is excessively deformed. This could be extended a bit by always returning the same fitting to the same hole, and avoiding over-tightening.
All cylinders built for working pressure, all diving cylinders, and all composite cylinders use parallel threads.
Parallel threads for cylinder necks and similar penetrations of pressure vessels are made to several standards:
* M25x2
ISO parallel thread, which is sealed by an O-ring and torqued to on steel, and on aluminium cylinders;
[
* M18x1.5 parallel thread, which is sealed by an O-ring, and torqued to on steel cylinders, and on aluminium cylinders;][
* 3/4"x14 BSP parallel thread,] which has a 55° Whitworth thread form, a pitch diameter of and a pitch of 14 threads per inch (1.814 mm);
* 3/4"x14 NGS (NPSM) parallel thread, sealed by an O-ring, torqued to on aluminium cylinders, which has a 60° thread form, a pitch diameter of , and a pitch of 14 threads per inch (5.5 threads per cm);
* 3/4"x16 UNF, sealed by an O-ring, torqued to on aluminium cylinders.[
*7/8"x14 UNF, sealed by an O-ring.]
The 3/4"NGS and 3/4"BSP are very similar, having the same pitch and a pitch diameter that only differs by about , but they are not compatible, as the thread forms are different.
All parallel thread valves are sealed using an elastomer
An elastomer is a polymer with viscoelasticity (i.e. both viscosity and elasticity) and with weak intermolecular forces, generally low Young's modulus (E) and high failure strain compared with other materials. The term, a portmanteau of ''ela ...
O-ring at top of the neck thread which seals in a chamfer or step in the cylinder neck and against the flange of the valve.
Pressure vessel closures
Pressure vessel closures are pressure retaining structures designed to provide quick access to pipelines, pressure vessels, pig traps, filters and filtration systems. Typically pressure vessel closures allow access by maintenance personnel.
A commonly used maintenance access hole shape is elliptical, which allows the closure to be passed through the opening, and rotated into the working position, and is held in place by a bar on the outside, secured by a central bolt. The internal pressure prevents it from being inadvertently opened under load.
Placing the closure on the high pressure side of the opening uses the pressure difference to lock the closure when at service pressure. Where this is impracticable a safety interlock may be mandated.
An airlock is a room or compartment which permits passage between environments of differing atmospheric pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
or composition, while minimizing the changing of pressure or composition between the differing environments. It consists of a chamber with two airtight doors or hatches arranged in series, which are not opened simultaneously. Airlocks can be small or large enough for one or more people to pass through, which may take the form of an antechamber.
An airlock may also be used underwater to allow passage between the air environment in a pressure vessel, such as a submarine
A submarine (often shortened to sub) is a watercraft capable of independent operation underwater. (It differs from a submersible, which has more limited underwater capability.) The term "submarine" is also sometimes used historically or infor ...
or diving bell
A diving bell is a rigid chamber used to transport divers from the surface to depth and back in open water, usually for the purpose of performing underwater work. The most common types are the open-bottomed wet bell and the closed bell, which c ...
, and the water environment outside. In such cases the airlock can contain air
An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
or water
Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
. This is called a floodable airlock or underwater airlock, and is used to prevent water from entering a submersible
A submersible is an underwater vehicle which needs to be transported and supported by a larger ship, watercraft or dock, platform. This distinguishes submersibles from submarines, which are self-supporting and capable of prolonged independent ope ...
vessel or underwater habitat
Underwater habitats are underwater structures in which people can live for extended periods and carry out most of the Circadian rhythm, basic human functions of a 24-hour day, such as working, resting, eating, attending to personal hygiene, and ...
. A similar arrangement is used on spacecraft to facilitate extravehicular activity
Extravehicular activity (EVA) is any activity done by an astronaut in outer space outside a spacecraft. In the absence of a breathable atmosphere of Earth, Earthlike atmosphere, the astronaut is completely reliant on a space suit for environme ...
.
Construction materials
Many pressure vessels are made of steel. To manufacture a cylindrical or spherical pressure vessel, rolled and possibly forged parts would have to be welded together. Some mechanical properties of steel, achieved by rolling or forging, could be adversely affected by welding, unless special precautions are taken. In addition to adequate mechanical strength, current standards dictate the use of steel with a high impact resistance, especially for vessels used in low temperatures. In applications where carbon steel would suffer corrosion, special corrosion resistant material should also be used.
Some pressure vessels are made of composite material
A composite or composite material (also composition material) is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or physical properties and are merged to create a ...
s, such as filament wound composite using carbon fibre
Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers ( Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon comp ...
held in place with a polymer. Due to the very high tensile strength of carbon fibre these vessels can be very light, but are much more difficult to manufacture. The composite material may be wound around a metal liner, forming a composite overwrapped pressure vessel.
Other very common materials include polymer
A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
s such as PET in carbonated beverage containers and copper
Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...
in plumbing.
Pressure vessels may be lined with various metals, ceramics, or polymers to prevent leaking and protect the structure of the vessel from the contained medium. This liner may also carry a significant portion of the pressure load.
Pressure Vessels may also be constructed from concrete (PCV) or other materials which are weak in tension. Cabling, wrapped around the vessel or within the wall or the vessel itself, provides the necessary tension to resist the internal pressure. A "leakproof steel thin membrane" lines the internal wall of the vessel. Such vessels can be assembled from modular pieces and so have "no inherent size limitations". There is also a high order of redundancy thanks to the large number of individual cables resisting the internal pressure.
The very small vessels used to make liquid butane fueled cigarette lighters are subjected to about 2 bar pressure, depending on ambient temperature. These vessels are often oval (1 x 2 cm ... 1.3 x 2.5 cm) in cross section but sometimes circular. The oval versions generally include one or two internal tension struts which appear to be baffles but which also provide additional cylinder strength.
Manufacturing processes
Riveted
The standard method of construction for boilers, compressed air receivers and other pressure vessels of iron or steel before gas and electrical welding of reliable quality became widespread was rivet
A rivet is a permanent mechanical fastener. Before being installed, a rivet consists of a smooth cylinder (geometry), cylindrical shaft with a head on one end. The end opposite the head is called the ''tail''. On installation, the deformed e ...
ed sheets which had been rolled and forged into shape, then riveted together, often using butt straps along the joints, and caulk
Caulk (also known as caulking and calking) is a material used to Seal (mechanical), seal Joint (building), joints or seams against leakage in various structures and piping.
The oldest form of caulk consisted of fibrous materials driven into ...
ed along the riveted seams by deforming the edges of the overlap with a blunt chisel to create a continuous line of high contact pressure along the joint. Hot riveting caused the rivets to contract on cooling, forming a tighter joint.
Welded
Large and low pressure vessels are commonly manufactured from formed plates welded together. Weld quality is critical to safety in pressure vessels for human occupancy.
Seamless
The typical circular-cylindrical high pressure gas cylinders for permanent gases (that do not liquify at storing pressure, like air, oxygen, nitrogen, hydrogen, argon, helium) have been manufactured by hot forging by pressing and rolling to get a seamless vessel of consistent material characteristics and minimised stress concentrations.
Working pressure of cylinders for use in industry, skilled craft, diving and medicine had a standardized working pressure (WP) of about in Europe until about 1950. From about 1975, the standard pressure rose to about . Firemen need slim, lightweight cylinders to move in confined spaces; since about 1995 cylinders for WP were used (first in pure steel).
A demand for reduced weight led to different generations of composite (fiber and matrix, over a liner) cylinders that are more vulnerable to impact damage. Composite cylinders for breathing gas are usually built for working pressure of .
Manufacturing methods for seamless metal pressure vessels are commonly used for relatively small diameter cylinders where large numbers will be produced, as the machinery and tooling require large capital outlay. The methods are well suited to high pressure gas transport and storage applications, and provide consistently high quality products.
Backward extrusion
Backward extrusion is a process by which the material is forced to flow back along the mandrel between the mandrel and die.
File:Die with billet.png, alt=Diagram showing a steel die in section with an aluminium billet inserted, Section of die with billet inserted
File:Aluminium cylinder extrusion.gif, alt=Animation showing cold extrusion of the cylinder by pressing a rounded end cylindrical mandrel into the billet, with the aluminium extruding between the sides of the die and the mandrel to form a blind tube, Backward extrusion process, showing the material flowing out of the die back along the mandrel
File:Cylinder open.png, alt= The blind tube of the cylinder after removal from the die. It consists of the base and walls of the cylinder, but is still open at the top, Extrusion product before trimming
File:Cylinder closed.png, alt= The cylinder has been closed at the top by further cold forming, and the neck is still closed, Section after closure of the top end
File:Cylinder machined with neck detail.png, alt= The cylinder neck has been machined, and the threaded hole for the cylinder valve is shown, Section showing machined areas of the neck in detail
Cold extrusion (aluminium):
Seamless aluminium cylinders may be manufactured by cold backward extrusion of aluminium billets in a process which first presses the walls and base, then trims the top edge of the cylinder walls, followed by press forming the shoulder and neck.
Hot extrusion (steel):
In the hot extrusion process a billet of steel is cut to size, induction heated to the correct temperature for the alloy, descaled and placed in the die. The metal is backward extruded by forcing the mandrel into it, causing it to flow through the annular gap until a deep cup is formed. This cup is further drawn to diameter and wall thickness reduced and the bottom formed. After inspection and trimming of the open end, the cylinder is hot spun to close the end and form the neck.
Drawn
Seamless cylinders may also be cold drawn from steel plate discs to a cylindrical cup form, in two to four stages, depending on the final ratio of diameter to cylinder length. After forming the base and side walls, the top of the cylinder is trimmed to length, heated and hot spun to form the shoulder and close the neck. The spinning process thickens the material of the shoulder. The cylinder is heat-treated by quenching and tempering to provide the best strength and toughness.
Spun from seamless tube
A seamless steel cylinder can also be formed by hot spinning a closure at both ends. The base is first closed completely, and trimmed to form a smooth internal surface before the shoulder and neck are formed.
Regardless of the method used to form the cylinder, it will be machined to finish the neck and cut the neck threads, heat treated, cleaned, and surface finished, stamp marked, tested, and inspected for quality assurance.
Composite
Composite pressure vessels are generally laid up from filament wound rovings in a thermosetting polymer matrix. The mandrel may be removable after cure, or may remain a part of the finished product, often providing a more reliable gas or liquid-tight liner, or better chemical resistance to the intended contents than the resin matrix. Metallic inserts may be provided for attaching threaded accessories, such as valves and pipes.
Development of composite vessels
To classify the different structural principles of gas storage cylinders, 4 types are defined.
* Type 1 – Full metal: Cylinder is made entirely from metal.
* Type 2 – Hoop wrap: Metal cylinder, reinforced by a belt-like hoop wrap with fibre-reinforced resin.
* Type 3 – Fully wrapped, over metal liner: Diagonally wrapped fibres form the load bearing shell on the cylindrical section and at the bottom and shoulder around the metal neck. The metal liner is thin and provides the gas tight barrier.
* Type 4 – Fully wrapped, over non-metal liner: A lightweight thermoplastic liner provides the gas tight barrier, and the mandrel to wrap fibres and resin matrix around. Only the neck which carries the neck thread and its anchor to the liner is made of metal.
Type 2 and 3 cylinders have been in production since around 1995. Type 4 cylinders are commercially available at least since 2016.
Winding angle of composite vessels
Wound infinite cylindrical shapes optimally take a winding angle of 54.7 degrees to the cylindrical axis, as this gives the necessary twice the strength in the circumferential direction to the longitudinal.
Hoop wound fibre reinforcement is wound at an angle of nearly 90° to the cylinder axis.
Safety
Overpressure relief
As the pressure vessel is designed to a pressure, there is typically a safety valve or relief valve to ensure that this pressure is not exceeded in operation.
There may be a rupture disc fitted to the vessel or the cylinder valve or a fusible plug to protect in case of overheating.
Leak before burst
Leak before burst describes a pressure vessel designed such that a crack in the vessel will grow through the wall, allowing the contained fluid to escape and reducing the pressure, prior to growing so large as to cause catastrophic fracture
Fracture is the appearance of a crack or complete separation of an object or material into two or more pieces under the action of stress (mechanics), stress. The fracture of a solid usually occurs due to the development of certain displacemen ...
at the operating pressure.
Many pressure vessel standards, including the ASME Boiler and Pressure Vessel Code and the AIAA metallic pressure vessel standard, either require pressure vessel designs to be leak before burst, or require pressure vessels to meet more stringent requirements for fatigue
Fatigue is a state of tiredness (which is not sleepiness), exhaustion or loss of energy. It is a signs and symptoms, symptom of any of various diseases; it is not a disease in itself.
Fatigue (in the medical sense) is sometimes associated wit ...
and fracture if they are not shown to be leak before burst.
Testing and inspection
Hydrostatic test
A hydrostatic test is a way in which pressure vessels such as Pipeline transport, pipelines, plumbing, gas cylinders, boilers and fuel tanks can be tested for strength and leaks. The test involves filling the vessel or pipe system with a liquid ...
(filled with water) pressure is usually 1.5 times working pressure, but DOT test pressure for scuba cylinders is 5/3 (1.66) times working pressure.
Operation standards
Pressure vessels are designed to operate safely at a specific pressure and temperature, technically referred to as the "Design Pressure" and "Design Temperature". A vessel that is inadequately designed to handle a high pressure constitutes a very significant safety hazard. Because of that, the design and certification of pressure vessels is governed by design codes such as the ASME Boiler and Pressure Vessel Code in North America, the Pressure Equipment Directive
The Pressure Equipment Directive (PED) 2014/68/EU (formerly 97/23/EC) of the EU sets out the standards for the design and fabrication of pressure equipment ("pressure equipment" means steam boilers, pressure vessels, piping, safety valves a ...
of the EU (PED), Japanese Industrial Standard (JIS), CSA B51 in Canada
Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...
, Australian Standards in Australia and other international standard
An international standard is a technical standard developed by one or more international standards organizations. International standards are available for consideration and use worldwide. The most prominent such organization is the International O ...
s like Lloyd's, Germanischer Lloyd, Det Norske Veritas, Société Générale de Surveillance (SGS S.A.)
Lloyd's Register Energy Nederland (formerly known as Stoomwezen)
etc.
Note that where the pressure-volume product is part of a safety standard, any incompressible liquid in the vessel can be excluded as it does not contribute to the potential energy stored in the vessel, so only the volume of the compressible part such as gas is used.
List of standards
* EN 13445
EN or En or en may refer to:
Businesses
* Bouygues (stock symbol EN)
* Island Rail Corridor, formerly known as the Esquimalt and Nanaimo Railway (reporting mark EN)
* Euronews, a news television and internet channel
Language and writing
* N, 1 ...
: The current European Standard, harmonized with the Pressure Equipment Directive
The Pressure Equipment Directive (PED) 2014/68/EU (formerly 97/23/EC) of the EU sets out the standards for the design and fabrication of pressure equipment ("pressure equipment" means steam boilers, pressure vessels, piping, safety valves a ...
(Originally "97/23/EC", since 2014 "2014/68/EU"). Extensively used in Europe.
* ASME Boiler and Pressure Vessel Code Section VIII: Rules for Construction of Pressure Vessels.
* BS 5500: Former British Standard, replaced in the UK by BS EN 13445 but retained under the name PD 5500 PD 5500 is a specification for unfired pressure vessels. It specifies requirements for the design, manufacture, inspection and testing of unfired pressure vessels made from carbon, ferritic alloy, and austenitic steels. It also includes material su ...
for the design and construction of export equipment.
* AD Merkblätter: German standard, harmonized with the Pressure Equipment Directive
The Pressure Equipment Directive (PED) 2014/68/EU (formerly 97/23/EC) of the EU sets out the standards for the design and fabrication of pressure equipment ("pressure equipment" means steam boilers, pressure vessels, piping, safety valves a ...
.
* EN 286 (Parts 1 to 4): European standard for simple pressure vessels (air tanks), harmonized with Council Directive 87/404/EEC.
* BS 4994: Specification for design and construction of vessels and tanks in reinforced plastics.
* ASME PVHO: US standard for Pressure Vessels for Human Occupancy.
* CODAP: French Code for Construction of Unfired Pressure Vessel.
* AS/NZS 1200: Australian and New Zealand Standard for the requirements of Pressure equipment including Pressure Vessels, boilers and pressure piping.
* AS 1210: Australian Standard for the design and construction of Pressure Vessels
* AS/NZS 3788: Australian and New Zealand Standard for the inspection of pressure vessels
* API 510.
* ISO 11439: Compressed natural gas (CNG) cylinders
* IS 2825–1969 (RE1977)_code_unfired_Pressure_vessels.
* FRP tanks and vessels Fibre-reinforced plastic, FRP (Glass-reinforced plastic, Fibreglass Reinforced Plastics, also known as GRP, or Glass Reinforced Plastics) is a modern composite material of construction for chemical plant, pulp and paper mill, and food and pharmaceut ...
.
* AIAA S-080-1998: AIAA Standard for Space Systems – Metallic Pressure Vessels, Pressurized Structures, and Pressure Components.
* AIAA S-081A-2006: AIAA Standard for Space Systems – Composite Overwrapped Pressure Vessels
A composite overwrapped pressure vessel (COPV) is a pressure vessel, vessel consisting of a thin, non-structural liner wrapped with a structural fiber Composite material, composite, designed to hold a fluid under pressure. The liner provides a bar ...
(COPVs).
* ECSS-E-ST-32-02C Rev.1: Space engineering – Structural design and verification of pressurized hardware
* B51-09 Canadian Boiler, pressure vessel, and pressure piping code.
* HSE guidelines for pressure systems.
* Stoomwezen: Former pressure vessels code in the Netherlands, also known as RToD: Regels voor Toestellen onder Druk (Dutch Rules for Pressure Vessels).
* SANS 10019:2021 South African National Standard: Transportable pressure receptacles for compressed, dissolved and liquefied gases - Basic design, manufacture, use and maintenance.
* SANS 1825:2010 Edition 3: South African National Standard: Gas cylinder test stations ― General requirements for periodic inspection and testing of transportable refillable gas pressure receptacles. ISBN 978-0-626-23561-1
History
The earliest documented design of pressure vessels was described in 1495 in the book by Leonardo da Vinci
Leonardo di ser Piero da Vinci (15 April 1452 - 2 May 1519) was an Italian polymath of the High Renaissance who was active as a painter, draughtsman, engineer, scientist, theorist, sculptor, and architect. While his fame initially rested o ...
, the Codex Madrid I, in which containers of pressurized air were theorized to lift heavy weights underwater. However, vessels resembling those used today did not come about until the 1800s, when steam was generated in boilers' helping to spur the Industrial Revolution
The Industrial Revolution, sometimes divided into the First Industrial Revolution and Second Industrial Revolution, was a transitional period of the global economy toward more widespread, efficient and stable manufacturing processes, succee ...
. However, with poor material quality and manufacturing techniques along with improper knowledge of design, operation and maintenance there was a large number of damaging and often deadly explosions associated with these boilers and pressure vessels, with a death occurring on a nearly daily basis in the United States. Local provinces and states in the US began enacting rules for constructing these vessels after some particularly devastating vessel failures occurred killing dozens of people at a time, which made it difficult for manufacturers to keep up with the varied rules from one location to another. The first pressure vessel code was developed starting in 1911 and released in 1914, starting the ASME Boiler and Pressure Vessel Code (BPVC)
The ASME Boiler & Pressure Vessel Code (BPVC) is an American Society of Mechanical Engineers, American Society of Mechanical Engineers (ASME) standardization, standard that regulates the design and construction of boilers and pressure vessels. The ...
.
In an early effort to design a tank capable of withstanding pressures up to , a diameter tank was developed in 1919 that was spirally-wound with two layers of high tensile strength steel wire to prevent sidewall rupture, and the end caps longitudinally reinforced with lengthwise high-tensile rods. The need for high pressure and temperature vessels for petroleum refineries and chemical plants gave rise to vessels joined with welding instead of rivets (which were unsuitable for the pressures and temperatures required) and in the 1920s and 1930s the BPVC included welding as an acceptable means of construction; welding is the main means of joining metal vessels today.
There have been many advancements in the field of pressure vessel engineering such as advanced non-destructive examination, phased array ultrasonic testing and radiography
Radiography is an imaging technology, imaging technique using X-rays, gamma rays, or similar ionizing radiation and non-ionizing radiation to view the internal form of an object. Applications of radiography include medical ("diagnostic" radiog ...
, new material grades with increased corrosion resistance and stronger materials, and new ways to join materials such as explosion welding, friction stir welding, advanced theories and means of more accurately assessing the stresses encountered in vessels such as with the use of Finite Element Analysis
Finite element method (FEM) is a popular method for numerically solving differential equations arising in engineering and mathematical models, mathematical modeling. Typical problem areas of interest include the traditional fields of structural ...
, allowing the vessels to be built safer and more efficiently. Pressure vessels in the USA require BPVC stamping, but the BPVC is not just a domestic code, many other countries have adopted the BPVC as their official code. There are, however, other official codes in some countries, such as Japan, Australia, Canada, Britain, and other countries in the European Union. Nearly all recognize the inherent potential hazards of pressure vessels and the need for standards and codes regulating their design and construction.
Gallery
Alternatives
* Natural gas storage
* Gas holder
Depending on the application and local circumstances, alternatives to pressure vessels exist. Examples can be seen in domestic water collection systems, where the following may be used:
*Gravity-controlled systems which typically consist of an unpressurized water tank at an elevation higher than the point of use. Pressure at the point of use is the result of the hydrostatic pressure caused by the elevation difference. Gravity systems produce per foot of water head (elevation difference). A municipal water supply or pumped water is typically around .
* Inline pump controllers or pressure-sensitive pumps.
* In nuclear reactors, pressure vessels are primarily used to keep the coolant (water) liquid at high temperatures to increase Carnot efficiency. Other coolants can be kept at high temperatures with much less pressure, explaining the interest in molten salt reactors, lead cooled fast reactors and gas cooled reactors. However, the benefits of not needing a pressure vessel or one of less pressure are in part compensated by drawbacks unique to each alternative approach.
See also
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* - a small, inexpensive, disposable metal gas cylinder for providing pneumatic power
*
*
*
* – a device for measuring leaf water potential
Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure and mat ...
s
*
*
* or Knock-out drum
*
*
*
Notes
References
Sources
* A.C. Ugural, S.K. Fenster, Advanced Strength and Applied Elasticity, 4th ed.
* E.P. Popov, Engineering Mechanics of Solids, 1st ed.
* Megyesy, Eugene F. "Pressure Vessel Handbook, 14th Edition." PV Publishing, Inc. Oklahoma City, OK
Further reading
* Megyesy, Eugene F. (2008, 14th ed.) ''Pressure Vessel Handbook.'' PV Publishing, Inc.: Oklahoma City, Oklahoma, US. www.pressurevesselhandbook.com Design handbook for pressure vessels based on the ASME code.
External links
Use of pressure vessels in oil and gas industry
Basic formulas for thin walled pressure vessels, with examples
Educational Excel spreadsheets for ASME head, shell and nozzle designs
ASME boiler and pressure vessel website
Journal of Pressure Vessel Technology
Pressure vessel attachments
Image of a carbon-fiber composite gas cylinder, showing construction details
Image of a carbon-fiber composite oxygen cylinder
for an industrial breathing set
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Gas technologies