A BRIEF INTRODUCTION TO PRESSURE VESSELS

Pressure vessels are containers which designed to contain liquids and gases for industrial usage. Holding gases and liquids have a significant role in many kinds of industry, thus making them efficient and safe have become one of the highest priorities of engineering design and research areas. To understand the modern pressure vessel designs and usage, alongside with its features, a brief history of the engineering of pressure vessels throughout the world will be given in the article before further explaining its features, design and usage areas

1. HISTORY

Earliest documentation of a pressure vessel design was in the Codex Madrid I by Leonardo da Vinci, which he wrote in 1495. However, this design had neither the same usage area nor similar features as what can be called a modern pressure vessel. First ancestors of today’s pressure vessels were mainly designed in 1800s, where the industrial revolution started to took place. Yet, these early pressure vessels and boilers lacked material quality alongside with lacks of manufacturing techniques. They were dangerous and had led to many fatal accidents until finally getting into regulation in USA with the release of ASME Boiler and Pressure Vessel Code (BPVC).

Earliest documentation of a pressure vessel design was in the Codex Madrid I by Leonardo da Vinci, which he wrote in 1495. However, this design had neither the same usage area nor similar features as what can be called a modern pressure vessel. First ancestors of today’s pressure vessels were mainly designed in 1800s, where the industrial revolution started to took place. Yet, these early pressure vessels and boilers lacked material quality alongside with lacks of manufacturing techniques. They were dangerous and had led to many fatal accidents until finally getting into regulation in USA with the release of ASME Boiler and Pressure Vessel Code (BPVC).

After the release of codes in developed countries such as USA, Japan and European countries, advancements occurred in the area of pressure vessel designing and engineering which led to safer usage and better material quality.

2. FEATURES

2.1. SHAPES

Pressure vessels can have all kinds of shapes theoretically, but two main styles are the spherical and cylindrical types.

Spherical Pressure Vessels:
While being able to withstand more external pressure than its alternatives, spherical pressure vessels are somehow less efficient and therefore only have a limited area of use. However, it requires a lesser wall thickness then the cylindrical vessels. It is mainly used as a design for mini-submarines. But they are also used as large liquid and gas containers.
Cylindrical Pressure Vessels:
Even though the mini-submarines are made as spherical vessels, nearly every modern submarine is designed as a cylindrical pressure vessel. While they are relatively inefficient at withstanding external pressure as contrary to the spherical vessels, submarines designed with a shape of a cylinder have better maneuvering capabilities. Cylindrical pressure vessels theoretically have half the strength of a spherical vessel with the same wall thickness, however they are preferred over the spherical vessels because spherical vessels are harder to manufacture and cost more than the cylindrical pressure vessels.

2.2. MATERIALS

Key to build a good pressure vessel is choosing the right materials. To choose the right material which is optimal for the vessel, expected external pressure must be calculated while analyzing the different case scenarios which the vessel could encounter during its mission. To make a hull or a vessel to withstand the most pressure in the maximum depth, the easiest way is to increase the wall thickness with hard materials such as high-strength structural steel.
However, if the vessel does not need to go that deep, a less strong but cheaper material of construction such as mild steel may be chosen to lower the manufacturing expenses. There are also many alternatives to mild steel, although strength is not the only parameter that should be looked while choosing the material. Since one material is usually not perfect for all environments and circumstances, material of construction range is very wide for pressure vessels.
The material should satisfy the factors such as high corrosion resistivity, low cost, long life span, strength to weight and strength to density ratios. Even though most of the pressure vessels are made of steel, non-ferrous alloys such as titanium alloys and aluminium alloys are also used. Composite and polymer materials can also be used as construction materials in pressure vessels.
Possible Problems:
Corrosion is one of the most encountered problems in a submarine environment. Since manufacturing of stainless steel as construction material in a pressure vessel is expensive and hard, structural and mild steels that being used may have corrosion problems in water. To lower this possibility and make the life span of the surface material longer, other corrosion resistant materials such as aluminium, which has a higher corrosion resistivity in a marine environment then steel, can be chosen for long operations underwater.
Another common problem that occurs, is stress corrosion cracking, which mostly happens when the material encounters corrosion and stress at the same time. To solve this problem even before happening, tensile stress and composititon of the material must have been determined suitably to the planned working environment.
Fabrication problems due to poor welding may also occur, however they are encountered more rare in comparison to other possible problems.

3. USAGE

Pressure vessels are used widely throughout many areas with different uses in both industry and the private sector. While submarines make historically the most common and known examples of pressure vessels, modern storage tanks and compressed air receivers can also be given as usage of a pressure vessel. Many other examples of pressure vessels usage can be written, such as; distillation towers, pressure reactors, nuclear reactors, oil refineries and petrochemical industry.

4. DESIGN

Designing a pressure vessel is the most important task of engineers and scientists who work on that field of industry. While choosing the materials being the key factor, designing for a designated area of work, planning solutions to possible problems that might occur, ensuring the safety of workers and the pressure vessel itself are the main aims of the pressure vessel designing along with the aim of economic manufacturing.
After determining the work area of a vessel, suitable materials should be chosen first. It is important to foresee the mechanical loads which the pressured vessel will endure during its mission, thus materials engineers must then calculate the most important problems in materials selection, which is stress and fatigue.
Stress and fatigue are the main reasons for a failure of an industrial machine, yet alone in pressure vessels, these failures can be fatal. Therefore one must know the mechanical properties of the chosen material before applying it to work design. The most important mechanical properties are yield strength, ultimate tensile strength, reduction of area, fracture toughness and resistance to corrosion.
The chosen material should withstand mechanical loads such as external and internal pressures, dead weight and also seismic activity, which mostly occurs surprisingly. Other environmental difficulties such as wind, excessive loads of snow, and heavy raining should also be taken into account.

REFERENCES

  1. Hearn, E.J. (1997). Mechanics of Materials 1. An Introduction to the Mechanics of Elastic and Plastic Deformation of Solids and Structural Materials – Third Edition. Chapter 9: Butterworth-Heinemann. pp. 199–203. ISBN 0-7506-3265-8.
  2. Ross, C. T. (2014, March 27). An overview of pressure vessels under external pressure. Retrieved from https://doi.org/10.1533/9780857092496.1
  3. Chattopadhyay, S. (2005). Pressure vessels: Design and practice. Boca Raton: CRC Press.
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