Materials Selection and Design of a Pressure Vessel
1) Provide a shortlist of materials suitable for use as construction of the pressure vessel based upon the major requirements as following:
a)Tensile Strength Properties
b)Temperature Properties
c)Impact Properties
d)Fatigue Properties
e)Creep Properties
f)Corrosion Properties
Briefly describe the materials shortlisted based on the above properties.
2) Rank and index the materials in terms of the suitability and meeting the operating requirements
3)Include and discuss other factors affecting choice of material such as cost, availability of materials, processing, welding, joining, etc.
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Background:
Requirements for Pressure Vessels
In order for the pressure vessel to function satisfactorily, a number of requirements have to be met. Some of the more important properties are detailed below:
A pressure vessel can be defined as a closed container, designed in such a way that the primary purpose is to hold fluids (gases or liquids) at pressures that are significantly different to ambient pressures. Pressure vessels are used in a wide variety of applications in both industry and the private sector. Pressure vessels are commonly used in petroleum refining applications, chemical process industry, nuclear industry, mining industry, power generation, food and beverage, and pharmaceutical industries. Specific examples of applications include domestic hot water tanks, carbonated beverage containers, diving cylinders, autoclaves, distillation towers, pressure reactors, autoclaves, submarine and space habitats, pneumatic and hydraulic reservoirs, and storage vessels for liquefied gases (ammonia, chlorine, propane, butane, LPG, etc). Although one of the most common materials for pressure vessel fabrication is steel, whereby cylindrical or spherical pressure vessel are fabricated through rolling, forging and welding techniques, stainless steels, non ferrous alloys and composite materials, such as filament wound composite using carbon fibres bound within a polymer matrix, have been employed.
The composite material may be wound around a metal liner, forming acomposite
overwrapped pressure vessel. Other very common materials include aluminium alloys, polymers such as PET in carbonated beverage containers and copper 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.
Tensile Strength Properties – a major requirement is that the chosen material of design must possess adequate tensile properties in order to withstand the pressures generated by the contained fluids
Temperature Properties – depending on the application pressure vessels may be subject to operation over a wide range of temperatures, ranging from elevated temperature, well above room temperature to cryogenic temperatures. Consequently, materials chosen for the given application must be able to withstand the designed operation temperature and or temperature range, with respect to retaining mechanical properties at elevated temperatures, resistance to fracture at low temperatures and resistance to thermal shocking and thermal fatigue when subject to temperature fluctuations
Impact Properties –impact resistance is defined as the ability for a material to withstand loading due to a sudden, intense blow, in terms of the amount of energy absorbed by the sample prior to failure. Many materials show a temperature dependent impact behaviour, whereby the materials undergo a ductile to brittle transition as the temperature is lowered.
Brittle materials require low impact energies to initiate fracture, compared to ductile
materials. Brittle materials, with poor fracture toughness is an undesirable characteristic for pressure vessel design, particularly vessels designed for lower temperature operations
Fatigue Properties – the constant pressurising / depressurising of vessels can lead to the generation of cyclic tensile stresses and thus induce fatigue. Fatigue failure is defined as failure of a material due to cyclic, repeated loading at stress levels well below that of the yield strength. Pressure vessel design considerations may include resistance to fatigue failure
Creep Properties – creep is defined as plastic deformation at high temperatures, resulting in rupture of the component, even though the applied stress is less than the yield strength at that temperature. For pressure vessels operating at elevated temperature, creep resistance may be a consideration in the design process
Corrosion Properties – The nature of the fluids contained within a pressure vessel, which may range from acidic, caustic, saline to highly corrosive gases, may result in the materials being exposed to many potential forms of corrosion, which could include stress corrosion cracking, intergranular attack, pitting, crevice corrosion, corrosion fatigue, general corrosion, galvanic corrosion, etc. Design of pressure vessels will have to take into account the susceptibility to various forms of corrosion that may occur for a given system, and methods to control / eliminate these forms of corrosion.
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