What are Pressure vessels?

A pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially higher or lower than the ambient pressure. Examples include glassware, autoclaves, compressed gas cylinders, compressors (including refrigeration), vacuum chambers and custom designed laboratory vessels.

ASME(American Standards of Mechanical Engineers); a?pressure vessel?is defined as "a vessel in which the pressure is obtained from an indirect source or by the application of heat from an indirect source or a direct source.

Types of Pressure Vessels according to its Purpose

Storage Vessels.?Storage vessels are used to hold liquids and gases for industrial purposes. The vessel may be used to contain fluids in a later process, or for storing finished products such as compressed natural gas (CNG) and liquid nitrogen. Carbon steel is the most commonly used material for storage tanks.

Heat Exchangers.?

An environmental heat exchanger, as defined by the ASME Code, is a single-chamber pressure vessel that exchanges energy with the surrounding atmosphere. The heat exchanger consists of one or two headers and a number of tubes separating the working fluid from the atmosphere. The tubes may or may not be finned. The heat exchanger is a single-chamber vessel because none of the components is designed to be operated independently of the other components.

Heat exchangers are used to transfer heat between two or more fluids. They are commonly used in the food, pharmaceutical, energy, and bio-processing industries. The operation of heat exchanger equipment depends on the thermal and flow properties of the fluids involved in heat exchange, and on the thermal property of the conductive partition (for indirect contact heat exchangers). Materials in a heat exchanger experience stress from the temperature difference of the hot and cold fluids, and the internal pressure containing the fluids.

Boilers.

Boilers are heat transfer equipment that utilizes fuel, nuclear or electrical power as sources of heat. They are typically composed of an enclosed vessel that allows heat transfer from the source to the fluid. They are primarily used to heat liquids. Most of the time, phase transformation of the fluid from liquid to vapor phase occurs inside the boiler. The vapor generated by the boiler is used for various heating applications and in power generation. Steam boilers generate steam at an elevated pressure to accelerate the blades of the turbine. Hence, the boiler vessel must have high strength to endure such high pressures and thermal stress. For the majority of materials, strength decreases with increasing temperature.

Process Vessels.

Process vessels are a broad classification of pressure vessels. These are containers where industrial processes occur, such as mixing and agitation, decantation, distillation and mass separation, and chemical reaction. The change in the internal pressure of a process vessel depends on the nature of the process carried out and the transformation of the substances involved. Among the special types of process vessels are the following:

Distillation columns:

Distillation columns?allow the separation of a mixture of liquids based on the difference in their volatilities. In this process, the mixture is heated to a temperature where the more volatile component transforms into the vapor phase. The height of the vessel depends on the column internals (packings or trays).

Decanters:?

Decanters? allow separation of a solid-liquid or liquid-liquid mixture. The denser component settles at the bottom of the vessel. This type of vessel has a narrow cross-sectional area or oriented horizontally.

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Industrial mixers:

Industrial mixers? are pressure vessels that are equipped with motor-powered blades to homogenize and emulsify a single or multiple substances. The substances mixed perhaps both solids or liquids, or a combination. Agitating equipment operates at varying speeds depending on the extent of homogeneity.

Chemical reactors:

Chemical reactors are enclosed tanks used to contain the reactants and catalysts during a chemical reaction. They are equipped with agitators to facilitate molecular contact among the reactants. A chemical reaction is usually carried out in a jacketed vessel to absorb the heat generated. Depending on the heat of the reaction, the reactants may release heat (exothermic) or absorb heat (endothermic) during the reaction. As the reactants are converted into products, the internal pressure increases if gaseous products are generated and increases even more at higher temperatures.

Types of Pressure Vessels :

Cylindrical Pressure Vessels:

Cylinders are widely used for storage due to their being less expensive to produce than spheres. However, cylinders are not as strong as spheres due to the weak point at each end.This weakness is reduced by hemispherical or rounded ends being fitted. If the whole cylinder is manufactured from thicker material than a comparable spherical vessel of similar capacity, storage pressure can be similar to that of a sphere.

Cylindrical vessels are composed of a cylinder called shell and end closures called heads. The connection of the head to the shell is called the weld line. The curvature of the head begins at the tangent line.

The common heads of a cylindrical vessel are the following types.

Hemispherical heads:

?Hemispherical heads are a popular choice for pressure vessels due to their ideal shape, which divides the pressure in the vessel equally across the head’s surface. Also known as hemispherical tank heads or hemispherical dished ends, they’re commonly used on liquid and gas storage tanks. At tank components Industries, they construct multi-piece hemispherical heads from stainless steel, Hastelloy? and other nickel alloys in a variety of sizes and thicknesses.

Torispherical heads:

Torispherical heads are suitable for handling pressures less than 15 bars. They are the easiest and cheapest to fabricate among the heads. They are used for pressure vessels with height restrictions because of their flatter profile. They are formed from part of a torus and part of a sphere. The transition of the cylinder and the dish is called the knuckle that is in toroidal shape. The knuckle radius is equal to the radius of the torus, and the crown radius is equal to the radius of the sphere.

Ellipsoidal heads:

Ellipsoidal heads have a depth that is a fraction of the width of the head. Its radius varies between the major and minor axis, which is usually 2:1. The ellipsoidal head and its shell have the same wall thickness. This type of head is ideal for containing high-pressure gases due to its height-to-weight ratio. It can handle pressures greater than 15 bars. Ellipsoidal heads are resistant to pressure and have high overall strength, which makes them economical due to their reduced thickness requirement.

Spherical Pressure Vessels:

Spherical pressure vessels are ideal for containing high-pressure fluids due to their strong structure, but they are difficult and expensive to manufacture. The internal and external stress is evenly distributed on the sphere‘s surface, which means there are no weak points. They have a smaller surface area per unit volume. Spherical vessels will consume less amount of material than the cylindrical vessel if a pressure vessel of the same volume will be fabricated. The smaller surface area of the spherical vessel will also have less heat transfer from the hotter body compared to other shapes.

?Vessel Orientation:

The axis of a cylindrical vessel may be oriented vertically or horizontally.

Vertical vessel orientation?is used:

a.???????When the floor space is small.

b.??????When the vessel volume is small.

c.???????In mixing tanks because it allows efficient mixing since the fluid is distributed at a smaller cross-sectional area.

d.??????When gas to liquid ratio is high.

e.??????In liquid-liquid separation for easier removal of components.

Horizontal vessel orientation?is used:

a.???In heat exchangers since this orientation allows easier cleaning.

b.???In settling tanks and flash drums, where low downward velocity is required. Low velocities have less entrainment.

Vital codes and standards in Oil and Gas industry:

ASME: American Standards of Mechanical Engineers

API: American Petroleum Institute

ASTM: American Society for Trading and Materials

ANSI: American National Standards Institute

AWS: American Welding Society

AISI: American Iron and Steel Institute

AMCA: American Movement and Control Association

NACE: National Association of Corrosion Engineers

ISO: International Standard Organization?

ASME Section:

I.?Rules and Regulations

II. Materials

a.??????Ferrous

b.?????Non-ferrous

b.?????Specifications for Electrodes ad filler wares

c.??????Properties

(There is No Section III)

IV. Rules and construction of heating boilers

V. NDT Non-destructive Test (Radiographic, Penetronics, MPI-Mind particle test and UT- Ultrasonic test)

VI. Care and operation of heating boilers

VII. Care and operation of power boilers

VIII. Rules and construction of pressure vessels

IX. Building and bracing qualification

Important related API (American Petroleum Institute) codes:

API 510(for repair)

610 Centrifugal Pumps

611 Steam turbines

617 Axial/Centrifugal pumps

618/919 Displacement compressor

650 Storage tank

661 Fin fan cooler

674, 675 & 676 positive displacement pump

686 Machinery Installation

For more information please contact JMS-Johar Manufacturing Services; [email protected] or call on

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