It is thought that the market volume of pressure vessels used in many different industries will increase with new investments made in the coming years. The weighted part of the pressure vessels produced are manufactured with metal materials. Steel is the most widely used material among metals in pressure boiler production. In recent years, composite materials are expected to be a new alternative. Pressure vessels are containers that hold high-pressure gases or liquids and have wide applications in industries including oil & gas, chemicals, petrochemicals, distillation towers, nuclear reactor vessels, natural gas storage systems, and hot water storage tanks.
What is Pressure Vessel?
Pressure vessels are leak-proof containers that store liquid or gas. Pressure vessels of various sizes and shapes have been produced for different purposes. Generally, preferred geometries are spherical, conical and cylindrical. A typical model is the combination of a long cylinder with two heads. Pressure vessels work at internal pressures higher or lower than air pressure. Besides, the operating temperatures of these systems differentiate.
How it works?
Pressure vessels are designed to work by reaching the level of pressure required to make an application function, like holding air in a scuba tank. They can deliver pressure either directly through valves and release gauges, or indirectly via heat transfer. Potential pressure levels range from 15 psi up to around 150,000 psi, while temperatures are often above 400°C (750°F). A pressure tank can hold anywhere from 75 liter (20 gallons) to several thousand liter.
Applications of Pressure Vessels in Industry
Pressure vessels are used in many different industries, but 3 industries cover most of the market. These industries are the oil and gas industry, chemical industry and the energy industry.
Oil and Gas Industry
In the Oil and Gas industry, a pressure vessel is often used as a receiver where physical and chemical processes take place at high temperatures and pressures. Although the columns are used for a different purpose, they are similar in construction. Distillation columns are used to divide feed streams or streams into multiple sources, based on feed part boiling points. In general, pressure vessels and columns are purchased from the same manufacturers because of their similar construction process. Carbon steel and stainless steel are the two most commonly used materials for construction in the oil & gas industry. A pressure vessel also requires other components in addition to the external body to become usable, such as vessel internals, distillation trays. Such components are highly complex and they require specifications which are very different from those necessary for the manufacture of pressure vessels which are supplied by specialist suppliers.
It is a pressure vessel in which a process (chemical reactions) is carried out, which leads to a fundamental change in the content of the container. These can be processes such as combining one or more products to create a new product, dividing one product into one or more different products, removing directions of an existing product to create something else. Also, many different types of pressure vessels can be used simultaneously in the chemical industry.
Energy (Power Generation) Industry
There are a number of different causes why the energy sector needs pressure vessels overall. One of the main reasons why they are needed within the energy sector is to contain harmful gases. Often in locations such as oil refineries as well as metalwork’s excess gas needs to be stored. Also, Nuclear power plants use special pressure vessels which named Reactor Pressure Vessels (RPVs). RPVs are large cylindrical steel vessels containing a core, cooling water and generated steam, that requires high reliability to withstand high temperatures and high pressures, and neutron irradiation, which makes the RPV the most critical pressure boundary in the nuclear power plant. But keep in mind, not all power reactors have a reactor pressure vessel.
Pressure Vessel Types
Process Vessel: Process vessel (tank) are designed to simply hold and store liquids and they are used for an integrated operation in petrochemical facilities, refineries, gas plants, oil and gas production facilities, and other facilities.
Autoclaves: Autoclaves are large vessels that are pressurized and brought to high temperatures. They are usually cylindrical since the rounded shape is better able to safely withstand high pressures. Autoclaves are designed to hold items that are placed inside and then the lid is sealed.
High Pressure Vessels: They are the most durable vessels at the market which are capable of working under heaviest loads and they provide the best resistance to corrosion, temperature and pressure. The high pressure vessels are usually made of stainless steel. Typical functions for high pressure vessel: high speed mixers, chemical reactors and supercritical extraction systems.
Expansion Tanks: The expansion tanks are designed to adjust for changes in the amount of hot water in heating systems and changes in water flow rate, and to maintain the static pressure produced by the pump at the utilization level in sanitary hot water systems.
Heat Exchangers: A heat exchanger is a device which transfers heat from one medium to another it is the most commonly used heat exchanger in industrial facilities such as iron and steel, petroleum, petrochemical, gas, power plants, food, pharmaceuticals, leather, textiles, air conditioning, ships, and marine industries.
Water Pressure Tanks: In a water well system the pressure tank produces water pressure by using compressed air to force down on the water. Because of this pressure, water is forced out of the tank through the pipes inside your home when a valve is opened.
Vacuum Tanks: A vacuum tank is part of a system that filters air or fluids through suction, outgassing, pumping, or a combination of techniques. Vacuums use pressure to prevent contamination, purify, dehydrate, and even power.
ASME Pressure Vessels: Theyalso known as ASME boilers, are any pressure vessel with an ASME stamp. The ASME stamp indicates the vessel has undergone inspection and meets stringent ASME VIII code standards. In addition, the ASME stamps offers end-users information about the ASME boiler and its manufacturer.
Thin-Walled Pressure Vessels: A thin-walled pressure vessel is one in which the skin of the vessel has a thickness that is much smaller than the overall size of the vessel, and the vessel is subjected to internal pressure that is much greater than the exterior air pressure.
Boilers: Theyare closed pressure vessels used to heat fluids, mostly water. These heated fluids are used for cooking, power generation, central heating, water heating and sanitation.
Pressure Vessel Production
Scientifically, the solution to the problem of reducing strain has a simple geometric answer: a sphere. The engineering solution obviously isn’t that simple. Spherical pressure vessels are incredibly difficult to build. Although NASA may choose to manufacture perfectly spherical, cryogenic carbon fiber tanks properly, most applications need a simpler, more realistic solution. Most used design is a configuration of a long, two-headed cylinder. The steel cylindrical pressure vessel matches the demands of various pressure vessel applications, these vessels are meticulously engineered to favor ease of production, while retaining a robust and resilient geometry.
The cylindrical middle section can be easily built from a rectangular steel piece while the absence of perpendicular edges provides a better distribution of the tension. While hemispheric heads provide better distribution of pressure, shallow heads are often used instead. Known as “dished” heads in the sector, they represent an important balance between strain minimization and manufacturability. These heads are much easier to form and can become slightly thicker and reach the same pressure resistance. In general, dished heads have one of two geometries: semi- elipsoidal or torispherical. Torispherical heads consist of a plate with a fixed radius that connects to the cylinder with a toroidal joint. The relative ease of production has made torispherical heads the most common head shape of the pressure vessel, finding use in recompression chambers, distillation towers, petrochemical plants and a variety of storage uses.
Semi elliptical heads are another choice used on a
regular basis. These are deeper, more spherical, and durable than a
torispherical head, and thus more costly to construct, but can handle more challenging
applications than torispherical heads. Semi elliptical heads are best suited to
applications with slightly higher pressure where the entire length of the
cylinder is still important.
The thicknesses determined by the relevant equations are minimal to which should be added various allowances, including allowances for corrosion, erosion, material supply tolerances and any fabrication thinning. The thicknesses calculated by the equations in principle are minimal, to which various allowances should be applied, including allowances for corrosion, erosion, tolerances of material supply and any fabrication thinning.
The spectrum of materials used in pressure vessels is wide and includes but is not limited to:
- Carbon steel (with less than 0.25% carbon)
- Carbon manganese steel (giving higher strength than carbon steel)
- Low alloy steels
- High alloy steels
- Austenitic stainless steels
- Non-ferrous materials (aluminum, copper, nickel and alloys)
- High duty bolting material
In order to comply with the production standards, the following material properties must be known in the selected materials. The designs made without knowing these features are very likely to have problems during long use. Therefore, much attention should be paid to the choice of materials.
- Elongation and reduction of area at fracture
- Notch toughness
- Ageing and embrittlement under operating conditions
- Fatigue strength
Design stresses are adjusted using safety factors applied to material properties, including:
- Yield strength at design temperature
- Ultimate tensile strength at room temperature
- Creep strength at design temperature
Welders and manufacturers must keep in mind the following points to guarantee that their pressure vessels fulfill all the requirements of industrial applications:
- Vessel weight and contents
- Ambient and operational temperatures
- Static and dynamic pressures
- Residual and thermal stress
- Reaction forces
Steps Involved in Pressure Vessel Fabrication
Before construction starts, the manufacturer is often required to submit fully dimensioned drawings of the main pressure vessel shell and components for approval by the purchaser and inspecting authority. In addition to showing dimensions and thicknesses, these drawings include the following information:
- Design conditions.
- Welding procedures to be applied
- Key weld details
- Heat treatment procedures to be applied
- Non-destructive test requirements
- Test pressures.
The manufacturer is generally required to maintain a positive system of identification for the materials used in construction so that all material in the completed pressure vessel can be traced to its origin. The creation of plates in rollers or dished ends is a hot or cold process, depending on the material, thickness and finished dimensions. The standard regulates the allowable mounting tolerances. These tolerances limit the stresses caused by roundness and misalignment of the joint.
Manufacturing Steel Dished Heads
There are two main stages of making dished heads made of metal. Firstly, the metal is sliced in the correct thickness and shape using plasma cutting machines or industrial circular shears which are commonly guided by computer. When cut to form, the metal is transformed into a head using a flanging process or a spinning process. In the spinning approach, the metal is rotated on a hydraulic lathe and pressed to a tool.
The tool forms the metal according to the desired head shape and enables the hinge radius and the bowl radius to be produced in one go. Flanging is a two-step process modeled to accelerate the final cylinder assembly: The steel is pressed cold into a shaped cap and then formed with a pressure roller so that it demonstrates a straight flange at the point where the cylinder is connected.
Development of composite vessels
Defines 4 types of composite cylinders to describe the specific making principles.
- Type 1 – Complete Metal: Cylinder made entirely of metal.
- Type 2 – Hoop Wrap: metal ring, covered with fiber-material belt-like hoop cover. For geometrical purposes, the spherical bottom and the head of a cylindrical cylinder can withstand twice the pressure as the cylindrical shell (assuming uniform metal wall thickness).
- Type 3 – Totally Wrapped, over Metal Liner: Diagonally wrapped fibers make the wall resistant to pressure right at the bottom and around the metal collar. The metal lining is thin and is close to the vessel water.
- Type 4 – A vessel made of all carbon fibre, with polyamide or polyethylene insulation inside liner. Features are a much lower weight and a very high resistance. The price of carbon fibre is comparatively high.
Type 2 and 3 cylinders came up around 1995. Type 4 cylinders are commercially available at least from 2016 on.
Pressure Vessel Welding Process
Pressure vessels are used for the high pressure storage and distribution of liquids and gases. Welding must be of exceptionally high quality on pressure vessels to withstand working conditions. Good surface preparation is crucial to passing first-time challenging pressure vessel welding inspections easily and protecting precious money in the process. It is possible that some errors occur during welding. These errors are mentioned below. It is common to use undamaged inspection tests to detect imperfection.
Porosity occurs when a gas enters the molten weld pool. As the source cools and solidifies, the gas creates bubbles that appear as voids during the inspection. Numerous problems can cause porosity in a weld. It is important to check that appropriate welding techniques are followed and that appropriate consumables are used.
Nitrides are a highly adherent contaminant created when plasma cutting with compressed air or nitrogen. They make the edges brittle and create porosity in some welding processes, especially gas metal arc welding. Because nitrides can exist 0.005 to 0.010 inch. below the surface of the material, you cannot remove them with brushes.
Inclusions often result from surface contaminants that become mixed into the weld pool and are trapped during solidification. In multipass welding applications, slag that is not completely removed can be a source of inclusions. Thorough cleaning with a suitable wire brush before welding and between passes is a very effective means of eliminating this type of defect.
The American Society of Mechanical Engineers (ASME) has established rules for the production of the pressure vessels. The ASME pressure vessel code includes materials, assembly and safety details to ensure that the manufacturing process for the pressure vessel meets industry needs and will function properly and without concerns about damage or injury to people working around them. Best welding preparation work and outstanding welding technique are important for building safe and profitable pressure vessels as well as satisfying your customers.
The ASME Boiler and Pressure Vessel Code (ASME Code) is a leading standard for pressure equipment and components worldwide and provides criteria for producer certification and quality assurance. It sets standards for the design, materials, manufacture, inspection, testing and operation of boilers and pressure vessels (including power boilers, heating boilers, components of nuclear facilities, fibre-reinforced plastic pressure vessels, and transport tanks). Across over 100 countries, the ASME Code is accepted. The adding of the ASME certification mark to your pressure equipment encourages greater trust among your business partners, end users and authorities.
Adhere to Safety Standards and Codes: In addition to the ASME BPVC Standards Section VIII which governs the design and manufacture of pressure vessels, pressure vessel users should adhere to safety standards and codes such as OSHA (Occupational Safety and Health Administration) 1915 Subpart K for vessels, drums, and containers, API 510 Vessel Code for maintenance, repair, and alteration and API 572 for Inspection. Local jurisdiction authorized inspection agencies govern and regulate inspections and installations.
Allow Trained Personnel to Handle Pressure Vessels: It is critical that only qualified personnel be allowed to handle the vessels because of the high risk factor involved when dealing with the pressure vessels.
Inspection and Testing
During construction each pressure vessel shall be inspected by the inspecting authority. The standard defines the stages from the reception of the material to the completed vessel for which the inspection by that authority is compulsory. For instance, the customer may require extra inspection to check internals.
The manufacturer specifies the welding procedures used in the construction of the pressure vessel along with test pieces which are indicative of the materials and thicknesses used in the actual vessel. The inspecting authority must usually observe the creation and testing of such test pieces, unless previously authenticated test pieces are available.
Welders must pass approval tests designed to show that they are capable of making welds identical to those used in the actual vessel. A recognized licensing authority reaffirms those permits for welders.
The national standard determines the level of non-destructive inspection applied during construction. Usually the non-destructive testing is one or more of the following.
- Magnetic particle or dye penetrant (for weld surface flaws).
Dye penetrant testing only detects discontinuities which are on surface while magnetic particle testing detects not only surface cracks but also those imperfections that are very near to surface.
- Radiography (for weld internal flaws).
X-ray inspection can detect cracking and inclusions on the subsurface, but it is incredibly expensive. Usually, only X-ray monitoring is performed for critical weld joints such as at nuclear plants and submarines.
- Ultrasonic (for weld internal flaws).
Ultrasonic testing can detect surface and subsurface imperfections and is conducted by directing a high frequency sound wave through metal and welding.
The degree of non-destructive inspection depends on the material and thickness (i.e. depends on the difficulty of welding). Some standards use a “joint factor” approach that allows a reduced amount of non-destructive testing if the intended thickness is increased. This common factor is chosen and applied in the initial design phase.
Before delivery, most standards require a pressure test witnessed by the inspection authority. Since it cannot be compressed, water is the preferred test liquid. If air is the only test liquid possible, special precautions should be taken and consultations with the inspection authority and other relevant law enforcement agencies are needed. The test pressure is usually 1.2 to 1.5 times the design pressure, which is applied gradually and is held for a set period of time to demonstrate the pot adequacy.
When delivered and put into operation, the customer accepts liability for safe service. Legislation may also require periodic inspection over the life of the vessel and may require the intervention of the regulatory authority for certain essential contents.
To summarize, many criteria are important to choose the pressure boiler that best suits your own process. First of all, all the conditions of the process must be known very well. In this way, the most suitable design can be made according to the process and the most suitable material can be selected. However, even if all these steps take place properly, it may be necessary to check all steps of production with non-destructive testing after the production phase. It should not be forgotten to carry out maintenance for a certain period of time after the pressure vessel is started to be used. It should be noted that a pressure boiler manufactured according to the standards is safer. The standards recognized in many countries in this regard have been established by ASME.
High-Quality Engineering & Procurement
YENA Engineering provides pressure vessels with wide range of material grade and thicknesses. Our facilities are ASME U and S code certified manufacturers for related products.
YENA Engineering is able to provide heat exchangers for Shell and tube. We are pleased to offer custom design and manufacture of heat exchangers.
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