Classification Of Pressure Vessels Engineering Essay

The benefit of using pressure vessels is the possibility of holding a higher amount of a specific gas, which would occupy a smaller space maintained at a different pressure in a liquid state. They are considered to be indispensable for modern life.

This method would save costs in transportation, in material consumption for the manufacturing of the container, and usage of space in the storage.

Pressure Vessels Importance and Accidents (Pressure Vessel Code)

If the design or conditions for the pressure vessels are not the right ones, a small accident can present a lethal result, that is why a regulating committee of engineers was created. They make sure that a legislation for the specific country is being followed.

Known from history that many pressure vessel disasters have happened, one of the most important issues for the design is safety, the pressure release system, the installment, etc. Those pressure vessels have failed without any warning, creating deadly project-outs. They are known as high energy storage containers.

Most accidents have happened because of a mistake of the engineers in the pressure release system, that couldn’t handle a high amount of pressure, or the usage of an improper container as a pressure vessel, without being certified or inspected. Also, the improper welding of a vessel in 2004 of a 50.000lb with a violent explosion in a chemical plant, since it was modified by the company illegaly, by highly reducing the strength, by projecting pieces of the vessel to 1000ft away.

Pressure Vessel Code

In places such as Texas, there is no regulation for the pressure vessels, which is used everywhere else, founded by the American Society of Mechanical Engineers since 1915, providing fundamental safe cards for pressure vessels, including design, fabrication, welding procedures and pressure relief. It is very important to prevent the accidents by using this code, to protect industrial facilities and the population close to those areas.

History of pressure vessels

Large pressure vessels were invented during the industrial revolution, particularly in Great Britain, to be used as boilers for making steam to drive steam engines.http://upload.wikimedia.org/wikipedia/commons/4/45/Popular_Science_Jan_1919_p27_-_10%2C000psi_wrapped_fuel_tank.JPG

A 10,000 psi (69 MPa) pressure vessel from 1919, wrapped with high tensile steel banding and steel rods to secure the end caps.

In an early effort to design a tank capable of withstanding pressures up to 10,000 psi (69 MPa), a 6-inch (150 mm) 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. Abbotts & Co. has been building pressure vessels since it began back in 1870.It began by making boilers for the British admiralty, and continues.Today manufacturing a diverse range of vessels for a wide range of industries.

Back in the beginning pressure vessels and boilers were manufactured from riveted steel plates (boilerplate), today electric welding has made that process obsolete in all but the most traditional restoration workshops.

Classification of pressure vessels

The pressure vessels are classified as follows :

1: According to dimensions:

The pressure vessels, according to the dimensions are classified as thin and thick shells.

The ratio of internal diameter and wall thickness is the factor which differentiate between thin and thick shells.

If the ratio d/t is more than 10, then it is called thin shell and if this ratio is less than 10 it is said to be thick shell.

The example of the thin shells are pipes, boilers and storage tanks while the thick shells are used in pressure cylinders, Gun barrels, etc.

2: According to end construction:

The pressure vessels according to end construction are classified as open end and closed end.

A simple cylinder whit a piston is an example of closed end vessel.

In case of open end vessels the circumferential stress is induced in addition to the circumferential stress.

Filament wound pressure vessel.

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 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, pneumatic reservoirs, hydraulic reservoirs under pressure, rail vehicle airbrake reservoirs, road vehicle airbrake reservoirs, and storage vessels for liquified gases such as ammonia, chlorine, propane, butane, and LPG

Shapes and sizes of a Pressure Vessel

Pressure vessels can theoretically be almost any shape, but shapes made of sections of spheres, cylinders, and cones are usually employed. 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. Theoretically, a spherical pressure vessel has approximately twice the strength of a cylindrical pressure vessel. 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. A disadvantage of these vessels is that greater breadths are more expensive, so that for example the most economic shape of a 1,000 litres (35 cu ft), 250 bars (3,600 psi) pressure vessel might be a breadth of 914.4 millimeters (36 in) and a width of 1,701.8 millimeters (67 in) including the 2:1 semi-elliptical domed end caps.

Boiler drums, heat exchangers, chemical reactors, and so on, are generally cylindrical. Spherical vessels are used for large gas or liquid containers, gas-cooled nuclear reactors, containment buildings for nuclear plant, and so on.

The size of pressure vessels vary greatly from the large cylindrical vessels used for high pressure gas storage to the small size used as hydraulic units for aircraft. Some are buried in the ground or deep in the ocean, but most are positioned on ground or supported in platforms.

Construction Materials

Theoretically almost any material with good tensile properties that is chemically stable in the chosen application could be employed. However, pressure vessel design codes and application standards (ASME BPVC Section II, EN 13445-2 etc.) contain long lists of approved materials with associated limitations in temperature range. 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 materials, such as filament wound composite using carbon fibre held in place with a polymer. Due to the very high tensile strength of carbon fiber 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. 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.

Spherical vessel

For a sphere, the mass of a pressure vessel is

M = {3 over 2} P V {rho over sigma},

where

Mis mass,

Pis the pressure difference from ambient

Vis volume,

rhois the density of the pressure vessel material,

sigmais the maximum working stress that material can tolerate.

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.

For a cylinder with hemispherical ends,

M = 2 pi R^2 (R + W) P {rho over sigma},

where

R is the radius

W is the middle cylinder width only, and the overall width is W + 2R

2:1 Cylindrical vessel with semi-elliptical ends

M = 6 pi R^3 P {rho over sigma}.

Stress in thin-walled pressure vessels

Stress in a shallow-walled pressure vessel in the shape of a sphere is

sigma_theta = sigma_{rm long} = frac{pr}{2t},

where sigma_thetais hoop stress, or stress in the circumferential direction, sigma_{long}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 cylinder wall. A vessel can be considered “shallow-walled” if the diameter is at least 10 times (sometimes cited as 20 times) greater than the wall depth.

Stress in a shallow-walled pressure vessel in the shape of a cylinder is

sigma_theta = frac{pr}{t},

sigma_{rm long} = frac{pr}{2t},

Almost all pressure vessel design standards contain variations of these two formulas with additional empirical terms to account for wall thickness tolerances, quality control of welds and in-service corrosion allowances.

For example, the ASME Boiler and Pressure Vessel Code (BPVC) (UG-27) formulas are:

Spherical shells:

sigma_theta = sigma_{rm long} = frac{p(r + 0.2t)}{2tE}

Cylindrical shells:

sigma_theta = frac{p(r + 0.6t)}{tE}

sigma_{rm long} = frac{p(r – 0.4t)}{2tE}

where E is the joint efficient, and all others variables as stated above.

Case study

Low-Pressure Chamber

The low-pressure chamber is an inexpensive pressure vessel used in soil-moisture and water-retention analyses. The chamber is safe for use to a maximum pressure of 1 bar (100 kPa). The low-pressure chamber is constructed of aluminum.

The large-volume chamber can accommodate several porous ceramic plates at once, allowing many soil samples to be analyzed concurrently. A quick-connect fitting allows easy connection to a compressor/manifold unit or other source of pressurized air.

Safety-relief features prevent over-pressurization, assuring safe operation of the system. A safety-relief valve releases pressure automatically if the pressure exceeds 1.3 bar (130 kPa), and can also be used to manually reduce the pressure inside the chamber. A blowout plug prevents over-pressurization of the chamber.

Specifications:

Dimensions: outside maximum: 16 in (41 cm) diameter x 20 in (50 cm) high

inside chamber: 12 in (30 cm) diameter x 12 in (308 cm) deep

Weight: 9 kg (20 lbs)

Pressure range: 0 to 1 bar (0 to 100 kPa)

Safety features: pressure-relief valve set to release at 1.3 bar (130 kPa) rubber blowout plug

Construction: aluminum pressure vessel and lid six clamping boltshttp://www.lisec.com/var/em_plain_site/storage/images/medien/bilder/maschinen/verbundglas/low-pressure_autoklave/32749-1-ger-DE/low-pressure_autoklave_medium.jpg

http://www.dlr.de/me/en/Portaldata/25/Resources/images/flugphysiologie/ukammer1.jpg

Low-pressure chamber components

Pressure chamber vessel

The pressure chamber is a low-pressure vessel which contains the regulated air pressure.

Lid : The lid closes and seals the pressure vessel. Six clamping bolts on the pressure chamber vessel hold the lid securely when the chamber is pressurized.

Lid alignment markers

The lid and pressure chamber are designed to fit together tightly and form an air-tight seal without the use of an O-ring or gasket. In order to form this seal, the lid must be aligned properly prior to closing and sealing the chamber. Triangular alignment marks, found on the top surface of the lid flange and on the side of the pressure chamber, are used to properly orient the lid.

Pressure hose connection

A quick-connect fitting is used to connect the pressurized air-supply hose to the pressure chamber. The quick-connect fitting allows the hose to be quickly and easily attached and removed.

Safety relief valve

The safety relief valve ensures that the extractor chamber is not over-pressurized. The valve is set to open and release pressure when the air pressure exceeds approximately 20 psi (1.3 bars or 130 kPa). The valve automatically resets when the excess pressure is released. The valve can also be operated manually to release pressure in the chamber by pulling the release ring, which opens the valve immediately.

Rubber blowout plug

The blowout plug is a secondary safety device which releases pressure if the pressure inside the chamber becomes unsafe. The rubber plug is blown out of the lid and the pressure is released immediately. The plug must then be reinstalled in the lid before the chamber can be pressurized again.

Outflow tube connections

The outflow tube connections allow attachment of outflow tubes to the porous plates inside the chamber. The outflow tubes carry water displaced from soil samples due to the air pressure inside the chamber.

Conclusion

in conclusion , 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, Japanese Industrial Standard (JIS), CSA B51 in Canada, 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.

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