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This section delves into the essence of pressure tanks, their creation process, and their roles in a range of applications.
Pressure tanks are specially designed containers for storing, controlling, and transporting gases, vapors, or liquids at pressures above atmospheric levels, often known as elevated pressures.
Pressure vessels are closed containers utilized for storing liquids, vapors, and gases at much higher or lower pressures than the surrounding atmosphere. They are prevalent in numerous industries such as petrochemical, oil and gas, chemical, and food processing. Pressure vessels encompass items such as reactors, flash drums, separators, tanks, and heat exchangers.
Pressure vessels are designed to operate within specified temperature and pressure limits, known as safety standards. Their design, production, and inspection are performed by certified professionals following rigorous standards to avoid accidental leaks or emissions posing environmental hazards. Important standards include the API 510 Pressure Vessel Inspection Code and ASME Section 8 of the Boiler and Pressure Vessel Code.
The main distinction between pressure tanks and pressure vessels lies in the maximum allowable operating pressure (MAOP) they can handle. Pressure vessels are engineered to manage pressures significantly exceeding atmospheric conditions, often surpassing 15 PSI, with MAOPs reaching up to 3000 PSI or more in specific scenarios. In contrast, pressure tanks are primarily meant for static liquid storage, typically featuring an MAOP of 15 PSI or lower, and often functioning at or below atmospheric pressure.
Pressure vessel design and safety regulations are governed by ASME Section VIII, covering Divisions 1, 2, and 3. In Europe, comparable safety measures are established by BS5500 and other national guidelines. Meanwhile, pressure tanks are built according to industry-specific standards and pressure thresholds as follows:
In summary, pressure vessels are typically completely enclosed, whereas pressure tanks can be open or sealed, particularly in vertical cylindrical designs. For example, a fuel tank might feature an open top with a breather accommodating ambient pressure. Moreover, pressure tanks generally have a larger capacity than pressure vessels, with the latter requiring more caution during maintenance due to the potential for hazardous explosions.
The production of pressure tanks involves these critical stages:
Pressure tanks are typically crafted from steel and stainless steel, due to steel's durability, robustness, and capacity to endure major stress and extreme conditions. Stainless steel adds additional benefits, including ease of cleaning and corrosion resistance, making it suitable for hygiene-sensitive environments such as food and drink production, medical uses, or laboratory conditions.
Besides steel and stainless steel, pressure tanks can also be constructed from materials like aluminum, carbon fiber, titanium, zirconium, and various high-strength polymers. Based on the application requirements, tanks might also feature coatings composed of polymers, rubber, metals, or ceramics to enhance structural integrity and prevent leakage.
When selecting a material, consider these factors:
The design calculations for pressure vessels encompass several vital parameters. These elements are crucial for determining the shell and head wall thickness.
Design Pressure: This parameter is essential for defining the vessel's specifications. It is chosen based on the maximum operating pressure, considering variations during start-ups, emergency shutdowns, and process fluctuations. Design pressure must exceed the system's maximum operating pressure and influences the vessel’s pressure relief mechanisms to prevent explosions. It is generally advised that design pressure be 5-10% above peak operating pressure. For vessels potentially exposed to vacuum conditions, design pressure should accommodate a full vacuum (-14.7 psi).
Maximum Allowable Working Pressure (MAWP): MAWP is the highest pressure at which the vessel is safely designed to operate at its set temperature. It is the utmost pressure that the weakest vessel part can sustain at this temperature. Established by the American Society of Mechanical Engineers (ASME), MAWP ensures safety by preventing operation beyond this level, thus lowering explosion risks. Unlike design pressure, which considers operational conditions, MAWP is a more overarching metric based on the vessel's material thresholds. While risks like corrosion or wear can lessen MAWP, design pressure is dictated by operational conditions and may be equivalent to or below MAWP.
Design Temperature: Temperature affects the maximum allowable stress on a vessel, as material strength declines at higher temperatures and becomes fragile at very low temperatures. To calculate maximum allowable pressure, the vessel must not function at temperatures exceeding the design temperature. This design temperature is set higher than the highest operating temperature but lower than the lowest temperature the vessel may encounter.
When setting the design temperature, several guidelines should be adhered to. It is recommended to set the design temperature 500 degrees Fahrenheit above the highest operating temperature and 250 degrees Fahrenheit below the lowest operating temperature. For vessels operating in the range of -30°C to 345°C, a maximum design temperature allowance of 250°C is advised. Additionally, potential disturbances that might significantly impact the tank's temperature should be considered by designers.
Maximum Tolerated Stress: Maximum tolerated stress is obtained by applying a safety factor to maximum allowable stress. This safety factor compensates for potential deviations from ideal parameters during the pressure vessel's construction and operation.
Joint Effectiveness: The ASME Boiler and Pressure Vessel (BPV) Code classifies welded joints into four categories. Joint effectiveness measures the relative strength of the base material compared to the welded joint. Typically, welded joints exhibit lower strength. Without complementary inspection and radiographic testing, welded joints are deemed weaker due to defects like porosity. The ASME BPV Code Section supplies guidelines for evaluating joint efficiency.
Corrosion Allowance: Estimating corrosion allowance follows various guidelines that may differ by manufacturer. Generally, a corrosion allowance varies from 1.5 to 5 mm. It is advised to allocate 0.25 to 0.38 mm annually or roughly 3 mm over a decade. For high corrosion rate environments, allowances may extend to 8.9 mm. In less corrosive environs, around 3.8 mm is typical. Air receivers and steam drums frequently have a corrosion allowance of 1.5 mm. In heat exchanger systems, where wall thickness affects heat transfer efficiency, the corrosion allowance should be minimized.
The vessel's shell and heads are formed through forging, rolling, and welding metal plates. The plate thickness, which determines wall thickness, is meticulously calculated in line with prior criteria. Subsequently, various auxiliary equipment, devices, and accessories are added to ensure optimal pressure vessel operation.
Post-weld heat treatment alleviates stresses accrued during welding and forming processes.
Pressurized containment vessels can be constructed to align with the standards set by the American Society of Mechanical Engineers (ASME). Commonly referred to as ASME tanks, these vessels are crafted to comply with ASME's strict criteria for design, construction, fabrication, operation, and maintenance. To retain ASME certification, these pressure tanks must undergo routine inspections. While ASME certification isn't always mandatory, numerous manufacturers comply with these standards to assure quality and customer safety.
Pressure tanks may also adhere to an assortment of local and international standards, including UL, ASTM International, TUV, USDA, ISO, Canada, and the European Union, among others. Additionally, many manufacturers produce non-code vessels. For the latest and most relevant information tailored to a customer's needs, it is advisable to consult a seasoned pressure tank manufacturer.
Pressure vessels are engineered to operate at specific pressure levels determined by their intended functions, such as air storage in scuba tanks. They manage pressure either directly through valves and gauges or indirectly through heat transfer. Pressure conditions can vary widely, from 15 psi to 150,000 psi, with temperatures frequently.
Essential considerations when selecting pressure tanks include:
Material selection is vital in pressure tank manufacturing and significantly impacts cost. Options include carbon and low alloy steels, nickel alloys like Monel, Inconel, and Hastelloy, various stainless steels including series and high-temperature grades, duplex stainless steels like 2205 and 2507, and aluminum. Material selection hinges on specific process conditions and the tank's purpose, making it crucial to choose the best-suited material for the tank's envisioned use and specifications.
Design is a crucial aspect of developing a new pressure tank, as it must withstand demanding industrial conditions. Welded construction plays a key role, as imperfections can lead to severe failures. BEPeterson ensures all vessels meet ASME standards using the latest version of Compress software. Advanced fabrication methods refine designs, confirming thorough consideration of cost implications.
When acquiring a pressure tank, it's essential to ensure compliance with applicable industry standards and specifications:
Manufacturers perform numerous tests to assess the tank's load-bearing capacity and performance under extreme conditions. These evaluations include Ferrite Testing, Hydro-testing, Positive Material Identification (PMI), Helium Leak Testing, Dimensional Inspection with a FaroArm, alongside Radiography and Non-Destructive Examination (NDE) methods like Penetrant Testing (PT), Ultrasonic Testing (UT), and Magnetic Particle Testing (MT). These inspections provide insightful data regarding the tank’s strength and reliability.
This chapter will explore the various classes and types of pressure tanks.
This tank ensures a steady pressure in a well water system. In a residential well system, a pump—either submersible or jet—pulls water from the ground into the home. The well pressure tank also acts as a storage reservoir, holding water for the household and triggering the pump when the water level drops and needs replenishing. Below, three types of pressure tanks in this category are examined.
Commonly known as galvanized or stainless steel pressure tanks, these units store both water and air together. They usually have an interior coating to prevent corrosion, often referred to as "glass-lined" or "epoxy-coated."
In this type of tank, air and water are not separated. Although less common today, these tanks are still used. Compressed air is employed to apply pressure to the water, achieving the necessary pressure for your home. While these tanks are often larger than other types, their capacity can be comparable to that of smaller tanks. This can result in the pump cycling on and off more frequently, which may lead to premature wear and failure. Additionally, these tanks are susceptible to water-logging due to air loss through corroded pinholes.
These tanks feature two distinct compartments: one for air and one for water. A rubber diaphragm, which is permanently affixed inside the tank, separates these compartments and adjusts with the water level.
As water is pumped into the lower compartment, it pushes the diaphragm upward into the air chamber. This movement triggers a switch that turns off the pump.
While this system is effective, it has some potential drawbacks. One issue is the possibility of the diaphragm detaching from the tank’s interior, which allows water to enter the air chamber. If this happens, the tank will need to be replaced. Another concern is when the diaphragm folds over itself, leading to inaccurate readings and potentially causing premature pump burnout and tank failure.
These tanks feature two separate chambers for air and water. The bladder, resembling a large balloon filled with air, expands and contracts to activate the pump. Unlike single-chamber or diaphragm tanks, bladder tanks are self-contained and avoid the issues associated with those types. Moreover, since air and water are kept apart, there is no mixing, which eliminates the need for additional air. Bladder tanks also generally have a longer lifespan compared to diaphragm tanks.
A steam drum is an essential component of a water-tube boiler, located at the top of the water tubes. It serves as a reservoir for both water and steam, collecting the steam produced by the water tubes and separating it from the water. The density difference between hot and cold water facilitates the accumulation of the hotter water. Consequently, the steam within the steam drum becomes saturated.
The steam collected from the top of the drum is distributed to various processes. To produce superheated steam, the saturated steam is further heated, typically for use in steam turbines. Saturated steam is drawn from the drum's upper part and directed through a superheater before it returns to the furnace. A mixture of water and steam enters the steam drum through riser tubes, where a demister removes water droplets to produce dry steam. The steam drum is equipped with a safety valve, a water level indicator, and a level controller. Additionally, feed water is supplied to the steam drum through a dedicated feed pipe running along its length.
A steam drum can be utilized either independently or in conjunction with a mud drum or feed water drum positioned at a lower level. Boilers that feature both a steam drum and a mud or feed water drum are known as bi-drum boilers, whereas those with just a steam drum are referred to as mono-drum boilers. Generally, bi-drum boilers are designed for lower pressure applications, while mono-drum boilers are intended for higher pressure applications.
Steam drums come in various types, including:
Three and Four Drum Boilers: While these designs are traditional, they continue to be utilized in various industries today.
Bi Drum Boilers: These boilers are utilized for power and steam generation but have largely been replaced by single drum boilers in power generation due to their lack of reheating capability. Single drum boilers are favored for their efficiency in plants with high heat rates. Despite this, bi drum boilers are still widely used in process steam generation due to their capacity to manage significant load variations and adapt to changing demands.
Single Drum Boilers: These boilers are predominantly used in power plants for generating electricity. They are capable of handling higher pressure limits than bi drum boilers due to reduced stress concentration. Single drum boilers feature a single drum with welded downcomers and are suitable for both reheat and non-reheat applications. They can be designed as corner tube boilers, where the downcomers create the structural framework, or as top-supported boilers, where the assembly is supported by an external frame and the top drum.
Storing ammonia is essential in many industries. Given its unique properties, ammonia typically needs to be kept in either high-pressure storage tanks or low-temperature refrigerated vessels. Managing substantial quantities of ammonia involves tackling several issues related to safety, health, and environmental impact.
Low-pressure ammonia storage is becoming more prevalent for two key reasons. Firstly, it involves a much lower capital expenditure per volume unit. Secondly, it provides enhanced safety compared to spherical storage systems, which operate at pressures exceeding atmospheric levels. Due to the widespread industrial production of ammonia, storing it at atmospheric pressure and -33°C has become a common standard.
For managing large volumes, such as up to 50,000 tonnes of ammonia, on-site storage tanks are utilized, maintaining a temperature of -33°C.
Known by various names including hot water storage tank, thermal storage tank, unit thermal storage tank, hot water cylinder, or heat storage tank, this device is used to store hot water for applications like space heating or domestic use.
Because of its large specific heat capacity, water is an excellent heat storage medium. This means that it can mass more heat per unit weight than other substances. Water is inexpensive and non-toxic. A well-insulated tank can keep stored heat for days, lowering fuel expenses. Hot water tanks may have a gas or oil burner system as well as electric immersion heaters. Some employ an external heat exchanger, such as a central heating system or warm water from another energy source. In the household environment, the most common are an electric immersion element, fossil-fuel burner, or a district heating scheme.
Water heaters designed for tasks such as washing laundry or bathing are equipped with thermostat controls that allow temperature settings ranging from 40 to 60°C (104 to 140°F). These heaters are connected to the home's cold water supply.
If the water in the tank has a high mineral content, such as limestone, boiling can lead to mineral buildup or scaling. Over time, this can cause leaks due to corrosion, a problem exacerbated by dissolved oxygen in the water, which accelerates the corrosion of both the tank and its fittings.
A process tank is a container or vessel used for mixing or batching chemicals, wastewater, feeds, or other substances, or for preparing these components to achieve a specific outcome. The term encompasses all related piping and fixtures necessary for the tank's intended function.
This chapter will explore the various uses and advantages of pressure tanks.
Pressure tanks are utilized across various industries for storing and transporting substances. These industries include food and beverage, chemicals, recreation, plastics and polymers, rubber, military and defense, water treatment and filtration, pharmaceuticals, oil, fuel, and energy.
Furthermore, they are frequently associated with well systems, from which residential residences and structures obtain their water supply. Other pressure tank applications include nuclear reactor vessel operation, autoclaves, recompression chambers, and road vehicle air brake reservoirs, powering of air tools, and the functionality of diving cylinders and other cylinders of gas utilized in medical, chemical, and industrial processes. The most common pressure tank applications are discussed below.
In the oil and gas industry, pressure vessels are widely used as receivers for high-temperature and high-pressure physical and chemical processes. These vessels serve various functions, but their construction generally follows similar principles. For example, distillation columns are utilized to separate feed streams into different components based on their boiling points. Due to the similarities in their manufacturing processes, pressure vessels and columns are often produced by the same manufacturers.
Stainless steel and carbon steel are the two most often utilized building materials in the oil and gas industry. Other components, in addition to the external body, are required for a pressure vessel to be usable, like vessel internals and distillation trays. Such components are extremely complicated, necessitating standards that differ greatly from those required for the manufacturing of pressure vessels, which are provided by specialized vendors.
This type of pressure tank is used for processes such as chemical reactions, which result in significant transformations of the contents within the container. These operations might involve combining substances to create a new product, breaking down a product into one or more new products, or removing components from an existing product to produce a different item. In the chemical industry, various types of pressure vessels are often utilized simultaneously to achieve these processes.
Pressure tanks are essential in the energy sector for several reasons. They play a crucial role in managing hazardous gases, which are often stored in facilities such as oil refineries and metal works. Additionally, nuclear power plants employ specialized pressure vessels known as Reactor Pressure Vessels (RPVs). These large cylindrical steel containers house the reactor core, steam, and cooling water. RPVs must be extremely reliable to withstand high temperatures, pressures, and neutron irradiation, making them a critical component in a nuclear power plant. It is important to note, however, that not all power reactors use reactor pressure vessels.
Pressure tanks are utilized across a range of industrial applications, including nuclear reactors and mining operations. They also play a role in everyday processes such as heating water and distillation. These tanks are essential for safely storing liquid gases and are crucial for handling and transporting volatile chemicals like propane, ammonia, and LPG.
In water distribution systems, pressure tanks provide notable benefits, including extending pump lifespan and reducing hydraulic shocks when the pump is activated. As water is pumped into the tank, it compresses the air inside, which helps manage pressure more efficiently and protects the system from sudden pressure changes.
Elastic Deformation - Elastic buckling or instability is influenced by factors such as the vessel's shape, stiffness, and material properties. These factors collectively determine the vessel's resistance to buckling under applied loads.
Brittle Fracture - Brittle fractures can occur at low or moderate temperatures and are often observed in vessels made of low carbon steel. These fractures may be detected during hydrotesting in the 40° – 50°F range, especially where minor flaws are present.
Excessive Plastic Deformation - To prevent excessive plastic deformation and gradual collapse, the ASME's main and secondary stress limits are established. These limits ensure that the vessel maintains its structural integrity under operating conditions.
Stress Rupture - This phenomenon involves progressive fracture due to creep deformation, which is a time-dependent process, or fatigue resulting from cyclic loading. Creep and fatigue are distinct but can both lead to stress rupture in pressure vessels.
Plastic Instability - Also known as "incremental collapse," this phenomenon occurs due to cumulative cyclic strain or deformation. It leads to vessel instability caused by plastic deformation over time.
Stress Corrosion - Chlorides can induce stress corrosion cracking in stainless steels, whereas caustic environments may cause stress corrosion in carbon steels. Selecting the appropriate materials is crucial for preventing these issues.
High Strain - Low cyclic fatigue, often influenced by strain, is typically observed in materials with low strength and high ductility.
Despite these drawbacks, the benefits of using pressure tanks far outweigh the potential issues.
Pressure vessel closures are pressure-retaining devices that allow easy access to pipelines, pressure vessels, pig traps, filters, and filtration systems. Pressure vessel closures often enable maintenance staff access. An elliptical access hole form is typically utilized, allowing the closure to be inserted through the opening and twisted into the working position, and is kept in place by an outer bar secured by a central bolt. Internal pressure prevents it from being opened accidentally while under load.
Pressure tanks are vessels that are used to store or convey gasses, vapors, and fluids at pressures greater than atmospheric pressure, also known as high pressures. They come from pressure vessels which are sealed containers that store the same contents. Steel and stainless steel are the ideal materials for pressure tanks. Material selection, design industry standard, and pressure tests conducted are to be considered when choosing a pressure tank. Several various types of tanks may be considered as pressure tanks. These examples are hot water tanks, ammonia tanks, steam drums, process tanks, and refractory lined vessels.
Pressure vessels are enclosed containers used to 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...
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...