Pressure Vessel

[Chapter 1] What Are Pressure Vessels?

Pressure vessels are enclosed containers that are used to keep liquids, vapours, and gases at a pressure that is much greater or lower than atmospheric pressure. They're employed in a variety of sectors, including petrochemical, oil and gas, chemical, and food processing. Pressure vessels include reactors, flash drums, separators, and heat exchangers, to name a few.

Each pressure vessel must be operated within its design temperature and pressure limitations, which are the safety limits of the pressure vessel. Because the inadvertent release and leaking of its contents is a hazard to the surrounding environment, the design, building, and testing of pressure vessels are carried out by trained individuals and are controlled by rules. The American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME BPVC) Section VIII and the American Petroleum Industry (API) 510 Pressure Vessel Inspection Code are two well-known standards.

[Chapter Two] Types of Pressure Vessels

The function or shape of pressure vessels can be used to classify them.

Variety of Shapes and Sizes Storage Vessels, according to their purpose. 

• Storage Vessels - For industrial applications, storage tanks are used to hold liquids and gases. The tank might be used to store finished goods like compressed natural gas (CNG) and liquid nitrogen, or it could be utilised to contain fluids in a later process. The most prevalent material for storage tanks is carbon steel.
• Heat exchangers - Are a type of heat exchanger that is used to transfer Heat exchangers are devices that allow heat to be transferred between two or more fluids. The food, pharmaceutical, energy, and bioprocessing sectors all employ them. The thermal and flow characteristics of the fluids engaged in heat exchange, as well as the thermal property of the conductive partition, all influence the functioning of heat exchanger equipment (for indirect contact heat exchangers). The temperature differential between the hot and cold fluids, as well as the internal pressure enclosing the fluids, put stress on the materials in a heat exchanger.
• Boilers - Boilers are heat-transfer devices that use fuel, nuclear, or electrical energy as heat sources. They are usually made up of an enclosed vessel that permits heat to be transferred from a source to a fluid. They're mostly used to warm up drinks. Inside the boiler, phase transition of the fluid from liquid to vapour occurs often. The boiler's vapour is utilised for a variety of heating purposes as well as power generating. Steam boilers produce steam at a high pressure to accelerate the turbine blades. As a result, the boiler vessel must be extremely strong in order to withstand such high pressures and temperatures. The strength of the bulk of materials diminishes as the temperature rises.
• Process vessels - Process vessels are a type of pressure vessel that falls into a broad category. Mixing and agitation, decantation, distillation and mass separation, and chemical reactions are all carried out in these containers. The nature of the process and the transformation of the chemicals involved determine the change in the internal pressure of a process vessel. The following are examples of particular types of process vessels:

1. Distillation columns allow a mixture of liquids to be separated depending on their different volatilities. The combination is heated to a temperature where the more volatile component converts to the vapour phase in this procedure. The height of the vessel is determined by the internals of the column (packings or trays).
2. Decanters allow a solid-liquid or liquid-liquid combination to be separated. The denser component settles toward the vessel's bottom. This vessel has a limited cross-sectional area and is horizontally orientated. 
3. Industrial mixers are pressure containers with motor-driven blades that are used to homogenise and emulsify a single or several substances. The components blended might be solids, liquids, or a mixture of both. Depending on the degree of homogeneity, agitating machinery runs at different speeds.
4. Chemical reactors are enclosed tanks that are used to keep reactants and catalysts separate during a chemical reaction. They have agitators to make molecular interaction between the reactants easier. To absorb the heat generated by a chemical reaction, it is normally carried out in a jacketed vessel. The reactants may emit heat (exothermic) or absorb heat (endothermic) throughout the reaction, depending on the heat of the reaction. If gaseous products are produced when the reactants are converted to products, the internal pressure rises, and it rises much more at higher temperatures.

Pressure Vessels Come in a Variety of Shapes and Sizes Geometry dictates its existence.

• Spherical Shape - Because of their sturdy construction, spherical pressure vessels are suitable for containing high-pressure fluids, but they are difficult and expensive to manufacture. There are no weak places since the internal and exterior load is equally distributed over the sphere's surface. Per unit volume, they have a lower surface area. If a pressure vessel of the same volume is produced, spherical vessels will use less material than cylindrical vessels. In comparison to other forms, the spherical vessel's reduced surface area allows for less heat transmission from the hotter body.
• Pressure Vessels with a Cylindrical Shape. Because of their flexibility and lower cost of production, cylindrical pressure vessels are the most often utilised vessels. However, because of the fittings used to connect the heads, they do not have the same strength as spherical vessels. A cylinder called a shell and end closures called heads to make up cylindrical vessels. The weld line is the link between the head and the shell. The tangent line marks the start of the head's curvature.
• Hemispherical Heads - The following are several varieties of cylindrical vessel heads. Because the pressure is spread evenly throughout the head's surface, hemispherical heads are perfect for handling high-pressure fluids and enclosing large-diameter vessels. They have a simple radial shape and a larger internal volume, but fabricating and joining them to the shell is more complex. Hemispherical heads, in comparison to other head geometries, require the smallest wall thickness to manage the same internal pressure. The radius of a hemispherical head is equal to the radius of the cylindrical vessel's cross-section. The head's depth is equal to half of its diameter.
• Torispherical Heads - can withstand pressures of less than 15 bar. Among the heads, they are the easiest and cheapest to make. Because of their flatter profile, they are employed for pressure vessels with height limits. They are made up of parts of a torus and a sphere. The knuckle, which is toroidal in form, is the transition between the cylinder and the dish. The radius of the torus is equal to the radius of the knuckles, and the radius of the sphere is equal to the radius of the crown.
• Ellipsoidal heads - Have a depth that is a small percentage of their width. Its radius changes according to the ratio of the main and minor axes, which is typically 2:1. The wall thickness of the ellipsoidal head and its shell is the same. Because of its height-to-weight ratio, this sort of head is suitable for confining high-pressure gases. It can withstand pressures of up to 15 bars. Ellipsoidal heads are pressure-resistant and have high overall strength, making them cost-effective due to their lower thickness need.

Orientation of the vessel

A cylindrical vessel's axis might be either vertical or horizontal.

When there is a limited amount of floor space, vertical vessel orientation is employed:

• When the volume of the vessel is little.
• Because the fluid is dispersed across a smaller cross-sectional area, it provides for effective mixing in mixing tanks.
• When the ratio of gas to liquid is large.
• For simpler component removal, use liquid-liquid separation. 

Horizontal vessel orientation is used when:

• Horizontal vessel orientation is utilised in heat exchangers because it makes cleaning easier.
• Low downward velocity is necessary for settling tanks and flash drums. Entrainment is less at low speeds.

[Chapter 3] Materials Selection for Pressure Vessels

The following are the factors for choosing the right material for pressure vessels:

• Can match the demands of a certain application in terms of strength. During the service life of a pressure vessel, materials must endure particular internal and external pressures, as well as structural stresses.
• Corrosion resistance is a term used to describe the ability of a material to Because a pressure vessel is intended to be trustworthy in severe settings, this is one of the most significant qualities.
• Investment Return. Throughout the lifespan of the pressure vessel, material, fabrication, and maintenance costs must be addressed. Economic assessments are carried out to select the most cost-effective material. If the purchase of a pressure vessel is beneficial, the Return on Investment must be studied and appraised.
• Fabrication and maintenance are simple. Metal sheets must be machinable and weldable because they are moulded into forms to generate the geometry of pressure vessels. Internals of vessels must be simple to install.
• Availability. Standard pressure vessel material sizes must be widely available in the manufacturer's location.

The following are some of the most regularly utilised pressure vessel construction materials:

• Carbon Steel. Steel with a carbon content of up to 2.5 per cent is known as carbon steel. Carbon steel containers are noted for their great tensile strength while having a thin wall thickness, making them ideal for a variety of applications. They're designed to withstand collision and vibration. Carbon steel, on the other hand, is difficult to bend and shape due to its tremendous strength. Because it does not contain chromium, it is more prone to corrosion and rusting than stainless steel.
• Stainless steel is the material of choice. Stainless steel is a form of steel with a greater chromium percentage, ranging from 10.5 to 30%, and a lower carbon content, as well as trace levels of nickel. Their high chromium concentration is responsible for their remarkable chemical, corrosion, and weathering resistance. To inhibit oxygen penetration into the metal's bulk, a thin, inert chromium oxide coating is created on the surface. It, like carbon steel, has a high strength-to-weight ratio and a thin wall thickness. Due to its higher ductility and elasticity, it is simpler to shape than carbon steel.
• Hastelloy. Hastelloy is a nickel, chromium, and molybdenum alloy that was developed by Haynes International, Inc. for the first time. It is widely utilised in the petrochemical, energy, and oil and gas industries for reactors, pressure vessels, and heat exchangers. It can be utilised in nuclear reactors as a material. It resists corrosion, cracking, and oxidising and reducing chemicals well. It retains its tensile strength even when exposed to extreme temperatures. Because of its ductility, it can be easily welded, moulded, and shaped. Its service life can extend several decades with careful maintenance, increasing its cost-efficiency.
• Nickel Alloys are a kind of nickel alloy. Nickel alloys are corrosion and weathering resistant, as well as resistant to thermal expansion. The addition of chromium to the nickel alloy boosts its heat resistance even further. Nickel alloy pressure vessels are often utilised in the oil and gas sector, cryogenic applications, and other extreme environments. It has a longer service life as well. It is, however, harder to work with and costs more to fabricate. Nickel alloy purity is critical for maintaining its strength and dependability.
• Aluminium. Aluminium is noted for having a high strength-to-density ratio, which indicates that metal is both strong and light. It is less expensive and easier to work with than stainless steel. It is also resistant to corrosion. In laboratory-scale applications, aluminium jars are frequently utilised. It is not appropriate for high-pressure applications, however, because it has a density of one-third that of stainless steel.
• Titanium. The titanium also has a thin wall thickness and excellent strength and stiffness. It is non-toxic and has strong corrosion resistance and biocompatibility. Its melting point is greater than steel and aluminium, making it suited for high-temperature applications. It also has high thermal conductivity and allows for efficient heat transmission, making it an excellent choice for heat exchangers.

[Chapter 4] Design of Pressure Vessels

The parameters utilised in pressure vessel design calculations are as follows. These factors are crucial in determining the thickness of the shell and heads' walls.

• Design pressure. The design pressure is a number that is used to compute the vessel's parameters. It's calculated from the maximum operational pressure, which accounts for expected pressure surges during start-ups, emergency shutdowns, and process irregularities. The maximum operating pressure is constantly exceeded. To reduce the danger of explosions, a vessel's pressure release mechanism is also dependent on this value. Towler recommends that the design pressure be 5-10 per cent higher than the maximum operating pressure. The design pressure for containers that may be subjected to vacuum pressure must be specified to withstand one complete vacuum (-14.7 psig).
• Working Pressure Maximum Allowable (MAWP). Based on its design temperature, the MAWP is the greatest allowed pressure measured at the top of the equipment at which the vessel must function. It is the maximum pressure that the vessel's weakest section can withstand at its design temperature. The American Society of Mechanical Engineers (ASME) defines the MAWP value, which is utilised by businesses to ensure that the vessel will not run over this value in order to implement safety standards and avoid explosions. The MAWP is not the same as the design pressure. MAWP is a broad property that is based on the material's physical constraints. Corrosion and wear reduce the material's MAWP. The design pressure, on the other hand, is determined by the process's operating conditions and may be less than or equal to the MAWP.
• Temperature to be used in the design. Because strength declines with rising temperature and becomes brittle at extremely low temperatures, the maximum allowed stress is significantly dependent on temperature. When determining the maximum allowed pressure, the pressure vessel should not be operated at a higher temperature. The design temperature is always higher than the maximum operational temperature while being lower than the lowest. When determining the design temperature, there are a few guidelines to follow. According to Towler, the design temperature should be 500 degrees Fahrenheit below the maximum operating temperature and -250 degrees Fahrenheit below the minimum working temperature. For boats operating between -30 and 3450C, a maximum limit of 250C must be granted, according to Turton. The designer must consider disturbances that have a significant impact on the temperature of the pressure vessel.
• Maximum Stress Permitted. The maximum allowed stress is calculated by multiplying the greatest stress the material can sustain by a safety factor. The safety factor accounts for deviations from the pressure vessel's ideal construction and operation. 
• Join Efficiency. The ASME Boiler and Pressure Vessel (BPV) Code divides welded connections into four categories:

The junction efficiency is the ratio of the welded plate's strength to the unwelded virgin plate's strength. The strength of the welded junction is usually lower. Welded joints are presumed to be weaker without additional examination and radiographic testing because flaws like porosity may be present. The table below summarises the joint efficiencies permitted under ASME BPV Code Sec. VIII D.1:

• Allowance for Corrosion. In evaluating the corrosion allowance, there are numerous rules of thumb that may be arbitrary to the manufacturer. Corrosion allowances should be between 1.5 and 5 mm in general. According to Peters, Timmerhaus, and West, a corrosion allowance of 0.25–0.38 mm per year, or 3 mm over ten years, is recommended. In the meanwhile, according to Turton, the corrosion tolerance for corrosive situations should be 8.9 mm, 3.8 mm for non-corrosive streams, and 1.5 mm for stream drums and air receivers. Because the rate of heat transfer is affected by the thickness of the wall in heat exchanger equipment, the corrosion allowance must be modest.

Pressure Vessel Fabrication

The vessel's shell and heads are made of metal sheet that has been forged, rolled, and welded together. The wall thickness of a metal sheet is determined by a detailed calculation that takes into account the above-mentioned criteria. Auxiliary equipment, gadgets, and accessories are placed to enable the pressure vessel to perform its function:

• Nozzles allow feed, products, and utilities to be introduced and discharged. They are normally welded perpendicular to the weld lines on the shell or head.
• During operation, pressure relief valves are used as a safety element.
• Stirred reactors with a heating or cooling jacket
• Supports for thermal expansion of the material in use, such as saddles, skirts, or legs.

Heat treatment is applied after welding to reduce tension produced by joining and forming.

[Chapter 5] Pressure Vessel Quality Testing and Inspection

The following are the testing procedures used to assure the pressure vessel's dependability.

• Visual testing is an important aspect of pressure vessel maintenance. Once every five years, and before it is placed into service after being erected or repaired, it must be inspected. The inside and outside of the vessel structure are inspected by a skilled inspector. The inspector examines the whole vessel structure for cracks, distortion, blistering, fluid leakage, corrosion, and other faults. 
• Ultrasonic testing uses high-frequency sound waves to discover surface and subsurface faults, as well as to measure the pressure vessel's wall thickness. The substance absorbs the ultrasonic sound waves, which are then reflected back into an electrical signal via a transducer. If there are defects, the reflected waves are disrupted.
• Radiographic testing uses x-rays or gamma rays to provide a picture of the surface and subsurface of a pressure vessel. After passing through any discontinuities, holes, or differences in density, the reflected rays will be deformed and revealed in the film. Radiographic testing is extremely repeatable and requires just minor surface preparation. However, handling ionising radiation is more costly and needs a highly competent operator. 
• Magnetic particle testing detects surface discontinuities in ferromagnetic materials by using magnetic current. Between two probes, the inspector passes a magnetic current across the pressure vessel. The magnetic flux travels through the material without interruption if it is defect-free. The magnetic flux, on the other hand, seeps out of the material if there are fractures or other flaws. Once ferromagnetic particles, either in a liquid solution or powdered form, are put to the vessel, the flaw will become more obvious.
• Liquid Penetrant Testing is routinely employed on welded seams and plates. A little quantity of liquid, known as the penetrant, is applied by the inspector to a region suspected of having a fault. After spraying, the penetrant is allowed to settle before wiping away any excess penetrant on the surface. After that, the developer is used to expose the penetrant that has seeped into the fissures.
• Pressure Testing. The ASME BPV Code requires pressure testing to check for strength and leakage. There are two types of pressure tests: hydrostatic pressure testing and pneumatic pressure testing. Hydrostatic pressure testing employs water as the medium, while pneumatic pressure testing uses air or nitrogen. Because compressed liquid carries less energy than compressed gas, it is favoured for safety reasons. It operates by removing the air from the vessel and filling it with the test fluid until the internal pressure reaches 1.5 times the design pressure for hydrostatic testing and 1.2–1.5 times the design pressure for pneumatic testing. The fluid is then maintained in place for at least 10 minutes. The inspector then examines the system for fractures and leaks. To figure out where the fractures are coming from, fluorescent dyes or tracers are applied. Pressure testing is typically performed during a shutdown or as a validation test after a vessel has been repaired.

Summary

• Pressure vessels are enclosed containers that retain and store liquids, vapours, and gases at a pressure that is much greater or lower than atmospheric pressure. Some laws, such as ASME BPVC and API 510, control the design, building, repair, and testing of pressure vessels. These restrictions are in place to guarantee that the pressure vessel's functioning is safe.
• Storage tanks, boilers, heat exchangers, and process vessels are the different types of pressure vessels based on their functions. The shape of a pressure vessel can be either spherical or cylindrical. The heads of cylindrical vessels can be hemispherical, ellipsoidal, or torispherical, and their heads can be hemispherical, ellipsoidal, or torispherical. A pressure vessel's axis might be vertical or horizontal in orientation.
• Can satisfy the strength requirements of a specific application, corrosion resistance, return on investment, ease of fabrication and maintenance, and availability are the factors for material selection for a pressure vessel. Design pressure, maximum permitted working pressure, design temperature, maximum allowable stress, joint efficiency, and corrosion allowance are the important design factors for computing the specification of a pressure vessel.
• Metal sheets are forged, rolled, and welded together to form the pressure vessel. For the vessel to completely perform its purpose, auxiliary equipment and accessories are fitted.
• Visual testing, ultrasonic testing, radiographic testing, magnetic particle testing, liquid penetrant testing, and pressure testing are some of the procedures used to assess the durability of pressure vessels.