Pressure vessel


A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.
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, corrosion allowance and minimum design temperature. Construction is tested using nondestructive testing, such as ultrasonic testing, radiography, 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. 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. In Europe the code is the Pressure Equipment Directive. 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, and American Society of Mechanical Engineers's official stamp for pressure vessels. 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 substantially different from the ambient pressure.
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 viewports 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

Types by function:
  • – pressure vessel that stores air delivered from a compressor
  • *
  • * – system used in tall buildings and marine environments to maintain water pressure.
  • *
  • *
  • *
  • *
  • *
  • *
  • *
  • *
  • *
  • *
  • *
  • *
  • storage
  • storage
  • 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 receivers, boilers 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 operations, oil refineries and petrochemical plants, nuclear reactor vessels, submarine 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, chlorine, and LPG.
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 loads. The pressure hull of a submarine also carries the hull structural and maneuvering loads.

Design

Working pressure

The working pressure, i.e. the pressure difference between the interior of the pressure vessel and the surroundings when in operation, 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 instability and collapse.

Shape

Pressure vessels can theoretically be almost any shape, but shapes made of sections of spheres, cylinders, ellipsoids of revolution, and cones 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 hemispherical, ellipsoidal, or dished. More complicated shapes have historically been much harder to analyze for safe operation and are usually far more difficult to construct.
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, 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.

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 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 the thickness of the walls scales with the radius of the tank, the mass of a tank scales with the volume of the gas held. 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.

Spherical vessel

For a sphere, the minimum mass of a pressure vessel is
where:
  • is mass,
  • is the pressure difference from ambient,
  • is volume,
  • is the density of the pressure vessel material,
  • is the maximum working stress that material can tolerate.
Other shapes besides a sphere have constants larger than 3/2, 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
  • W is the middle cylinder width only, and the overall width is W + 2R

    Cylindrical vessel with semi-elliptical ends

In a vessel with an aspect ratio 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 energy. For a stored gas, PV is proportional to the mass of gas at a given temperature, thus
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 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.