Pressure Tanks

Pressure tanks are vessels that are used to store, hold, and/or convey gasses, vapors and fluids at pressures greater than atmospheric pressure, also known as high pressures...
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This article presents all the information you need to know about Pressure Vessels. Read further and learn more about:
Pressure vessels are enclosed containers that hold liquids, vapors, and gases at a pressure significantly higher or lower than the ambient pressure. They are widely used in various industries such as petrochemical, oil and gas, chemical, and food processing industries. Equipment such as reactors, flash drums, separators, and heat exchangers are examples of pressure vessels.
Several standards and regulations governing every aspect of pressure vessels. The ASME Boiler and Pressure Vessel Code (BPVC) is the most popular set of universally acknowledged standards governing the design, construction, installation, testing, inspection, and certification of boilers, pressure vessels, and nuclear power plant components. The ASME BPVC Section VIII is the code dedicated to pressure vessels and has three divisions:
Another standard is the API 510 - Pressure Vessel Inspection Code: In-Service Inspection, Rating, Repair, and Alteration which deals with the maintenance, inspection, and repair of operating pressure vessels. It aims to revisit and preserve the integrity of the pressure vessels in service.
A pressure vessel must be operated below the maximum allowable working temperature and pressure, the pressure vessel’s safety limits. All activities involving pressure vessels must be carried out by qualified personnel because the accidental release or leakage of its contents poses a threat to the pressure vessel’s surrounding environment.
Pressure vessels may be classified according to their purpose or geometry.
Storage Vessels. Storage vessels are pressure vessels that temporarily hold liquids, vapors, and gases. 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.
Heat Exchangers. Heat exchangers are used to transfer heat between two or more fluids. They are commonly used in the food, pharmaceutical, energy, and bioprocessing 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. Oftentimes, 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 allow the separation of a mixture of liquids based on the difference in their volatilities. There are two types of distillation processes. The type of distillation process will greatly influence the design of the pressure vessel:
Flash distillation involves heating a highly pressurized liquid mixture stream followed by separation of the vaporization of the more volatile component inside a flash chamber. The heated mixture first passes through a valve, and the pressure drop across the valve will result in the partial vaporization of the fluid. The vapor will be collected in the overhead of the flash chamber, while the liquid will settle at the bottom.
In column distillation or fractional distillation, one or more liquid mixture streams enters in the column at one or more points. As liquid stream flows down the column through the holes of the column internals, it comes in contact with the rising vapor coming from the bottom of the column. The column internals such as trays, plates, and packings provide the surface for mass transfer between the liquid and the vapor phases. The height of the column vessel depends on the number of trays or height of packings contained inside the vessel.
Industrial mixers are pressure vessels that are equipped with motor-powered blades to homogenize and emulsify a single or multiple substances. The substances mixed may be a pure liquid mixture, a semi-solid mixture, or a solid-liquid mixture. Agitating equipment operates at varying speeds depending on the extent of homogeneity. The mixing tank may be subjected to elevated temperatures and pressures, depending on the final product requirements.
Chemical reactors are enclosed pressure vessels used to contain and/or stir the reactants, products, and catalysts during a chemical reaction. They are equipped with agitators or stirrers to facilitate the blending among the reactants, thereby increasing the molecular contact among them. Baffles are installed to avoid the swirling of the fluid and create a desirable flow pattern inside the reactor.
As the reactants are converted into products, the internal pressure increases if gaseous products are generated and increases even more at higher temperatures.
The following are the types of chemical reactors which utilize a pressure vessel:
Jacketed reactors maintain the temperature of the reactants, products, and catalysts during a chemical reaction. A utility fluid (e.g., cooling water, steam) flows through the jacket that wraps around the vessel to cool or heat the contents of the reactor.
The nature of the reaction is a critical consideration in designing reactors. Heat may be released (exothermic reaction) or absorbed (endothermic reaction) during a chemical reaction. Cooling or heating is important to provide favorable conditions for the reaction, thus maximizing product conversion and increasing efficiency, and to prevent uncontrolled increase or decrease in temperature during the reaction. Therefore, a jacketed reactor must be considered.
Packed bed reactors are cylindrical vessels that contain an immobilized bed of catalyst. The liquid or gaseous reactants flow from one side of the vessel, and the reaction takes place on the surface of the solid catalyst. Packed bed reactors provide high conversion per weight of catalyst and more contact area for the reactant and the catalyst. However, in these reactors, the cylindrical vessel must be able to support the weight of the catalyst bed.
Fluidized bed reactors also contain a bed of catalyst. In these reactors, the gaseous or liquid reactants pass through the bed at high velocities which suspend the solid catalyst inside the vessel and make it behave like a fluid. The fluidization of the catalyst allows thorough mixing of the reactants in all directions, resulting in attaining high reactant conversion and mass transfer rates, and uniform temperature across the reactor.
Cylindrical Pressure Vessels. Cylindrical pressure vessels are composed of a cylindrical shell and a set of heads. The cylindrical shell is the body of the pressure vessel. The heads serve as the end caps or enclosure to the shell to cover the contents of the vessel. The heads may have a flatter or more rounded profile. The latter reduces the weakness of the cylindrical vessel.
Cylindrical pressure vessels are the most widely used vessel shape due to their versatility. They are much cheaper to produce than spherical vessels. However, they are generally weaker than spherical pressure vessels. They typically require thicker walls to achieve the same strength of spherical vessels bearing the same internal pressure.
The following are the types of pressure vessel heads:
The axis of a cylindrical vessel may be oriented vertically or horizontally.
The criteria for selecting the appropriate material of construction for pressure vessels are:
The commonly used materials of construction for pressure vessels are the following:
Aluminum. Aluminum is known for its high strength-to-density ratio, which means it has high strength and lightweight at the same time. It is cheaper and more fabricated than stainless steel. It also has good corrosion resistance. Aluminum vessels are commonly used in laboratory-scale applications. However, it is not suitable for high-pressure applications since it has less density, which is one-third of stainless steel.
The following are the parameters used in the design calculations of a pressure vessel. Such parameters are critical in evaluating the wall thickness of the shell and heads.
Design Temperature. The maximum allowable stress is highly dependent on the temperature, as strength decreases with increasing temperature and becomes brittle at very low temperatures. The pressure vessel should not operate at a higher temperature where the maximum allowable pressure is evaluated. The design temperature is always greater than the maximum operating temperature and lesser than the minimum temperature.
There are several rules of thumb in evaluating the design temperature. Towler suggests that the design temperature must be 50°F from the maximum operating temperature and -25°F from the minimum operating temperature. For Turton, a maximum allowance of 25°C must be given for vessels that will be operating between -30 to 345°C. The disturbances that have a drastic influence on the temperature of the pressure vessel must be considered by the designer.
Category A |
Longitudinal or spiral welds in the main shell, necks or nozzles, or circumferential welds connecting hemispherical heads to the main shell, necks or nozzles. |
Category B | Circumferential welds in the main shell, necks or nozzles or connecting a formed head other than hemispherical. |
Category C | Welds connecting flanges, tubesheets or flat heads to the main shell, a formed head, neck or nozzle. |
Category D | Welds connecting the communicating chambers or nozzles to the main shell, to heads or to necks. |
The joint efficiency is the ratio of the strength of the welded plate to the strength of the unwelded virgin plate. Generally, the strength is lower at the welded joint. Welded joints without further inspection and radiographic testing are assumed to be weaker due to defects such as porosity are potentially present. Joint efficiencies allowed under ASME BPV Code Sec. VIII D.1 is summarized in the table below:
Joint Description | Joint Category | Joint Efficiency (Based on degree of radiographic examination) | ||
---|---|---|---|---|
Full | Spot | None | ||
Double-welded butt joint or equivalent | A, B, C, D | 1.0 | 0.85 | 0.70 |
Single-welded butt joint with backing strip | A, B, C, D | 0.9 | 0.8 | 0.65 |
Single-welded butt joint without backing strip | A, B, C | NA | NA | 0.60 |
Double full fillet lap joint | A, B, C | NA | NA | 0.55 |
Single full fillet lap joint with plug welds | B, C | NA | NA | 0.50 |
Single full fillet lap joint without plug welds | A, B | NA | NA | 0.45 |
The shell of the vessel and its heads are constructed by forging, rolling, and welding the metal sheet. The thickness of the metal sheet is the wall thickness which is obtained by thorough calculation, considering the above-mentioned factors. For the pressure vessel to serve its purpose, auxiliary equipment and devices and accessories are installed:
Post weld heat treatment is done to relieve stress caused by joining and forming.
The fabrication of pressure vessels may be field-erected or shop-erected. Field-erected pressure vessels are too large to assemble inside a shop facility and be transported to the site. Thus, these vessels have their individual parts fabricated in a shop. These parts are delivered to the site wherein the pressure vessel is planned to be located. Welding, finishing, and installation of the accessories are performed on the site. On the other hand, shop-erected pressure vessels are much smaller in size that their components can be put together in a manufacturing facility. These vessels can fit inside a building or an enclosed facility. They are only delivered to the site after their assembly. The major phases of the fabrication are performed in the shop, and the installation of the vessel piping and minor adjustments are only made after the pressure vessel arrives on the site.
The following are testing methods employed to ensure the reliability of the pressure vessel.
Pressure tanks are vessels that are used to store, hold, and/or convey gasses, vapors and fluids at pressures greater than atmospheric pressure, also known as high pressures...
Stainless steel tanks are widely used in food, beverage, dairy, medicine, cosmetics, and other manufacturing processes where cleanliness and purity are important. These are also used in industrial plants for storing chemicals and gases where strong resistance from chemical degradation is required...