Pressure Vessel Applications

Most often, pressure vessels, also called air pressure tanks or boilers, are mostly used for applications pertaining to the food and beverage, chemical, pharmaceutical, plastics and oil and fuel industries. They can also be used for heating and cooling.

These pressure vessels are containers used to hold gases and liquids at different pressure, either low or high, but quite different from the ambient pressure. Examples of pressure vessels are glassware, autoclaves, compressed gas cylinders, vacuum, refrigeration, or custom-designed laboratory vessels.

History of Pressure Vessels

In 1495, Leonardo da Vinci came with the idea and designed air pressured containers for lifting purposes. However, the present-day application of pressure vessels was not devised until the advent of steam engines. In steam engines, pressure vessels were used as boilers, where steam was produced and contained for powering the cylinder.

[High Pressure Storage Vessel]
Pressure Vessels – B.E. Peterson, Inc.

At that time, in all external combustion engines, pressure vessels were major equipment. In the 16th century, they were even used for hauling up heavy weights underwater. However, pressure vessels were also the most unreliable part of external combustion engines, as the technology for manufacturing pressure vessels was in the formative stage. Moreover, the quality of materials used for making vessels was poor. The failure of pressure vessels was usually catastrophic, causing damage to the machines and killing operators. They were the source of explosions in an array of industries on a daily basis.

The first effort to modernize pressure vessels and improve their safety emerged in the United States. In 1911, American Society of Mechanical Engineers (ASME), which was established in 1880, came up with the idea of standardizing codes and inspection methods for making pressure vessels. From this, they came up with the Boiler and Pressure Vessel Code (BPVC). In 1914, they printed the first rules and code standards for boiler and pressure vessel construction.

In 1919, answering the call for pressure vessels that could withstand 10,000 psi, manufacturers put out a new pressure vessel design on the market. To help the tanks endure such high pressure, manufacturers spirally wound steel wires around the tank, and added steel rods for additional reinforcement. With this design, there was no rupture of the pressure vessels even under stressful conditions.

At the same time, welding replaced rivets, as industries, such as chemical plants and petroleum refineries, needed pressure steel vessels that could endure high temperature too. BPVC recognized how welding could make pressure vessels stronger and thus, safer, so welding was included in the fabrication code. It was the beginning of the modern manufacturing technique, as nearly all of them use welding for joining metal plates in vessels.

Over time, to make boilers and pressure vessels safer, engineers have designed different testing techniques and methods. For example, today, it is mandatory that all pressure vessels go through inspection after fabrication and before being sold. Traditional tests are mainly destructive; however, the modern tests are non-destructive. These new inspections include radiography and phased array ultrasonic. Moreover, to make sure pressure vessels safer and up to code, manufacturers have crafted new assessment and inspection methods. These have been a major player in promoting and achieving safety standards. Modern assessing and inspection methods, such as finite element analysis, can identify stress points. Manufacturers also use stronger materials, including better stainless steel and steel, than they did early on. If they so choose, they strengthen those steels and stainless steels.

The future of boiler and pressure vessel design is quite bright. As engineers continue to develop new ways to handle stress and pressure, the vessels will continue to become more stable. Likewise, it’s likely that they will continue to come up with stronger and more durable materials. With these improvements, engineers will be able to push for higher standards and more stringent codes, thus pushing us forward.

How Pressure Vessels Work

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 750°F. A pressure tank can hold anywhere from 20 gallons to several hundred thousand gallons.

Materials Used in Pressure Vessels

The reliability of pressure vessels depends on the materials used. Six different types of materials are used in manufacturing pressure vessels. These are:

Titanium Metal

It has the capability to retain its structure
Highly resistant to corrosion
Low maintenance is needed
It has a good tensile strength
The melting point of this material is higher than that of steel and aluminum
Highly compatible
It is non-toxic in nature

Nickel Alloys 

Nickel alloys give the best corrosion resistance
Protects from a thermal explosion
Nickel alloys are best to use in harsh environments
This material is reliable and can last long
Nickel alloys offer the best oxidation and carburization resistance

Stainless Steel 

Pressure vessels are required to be robust and steady. Stainless steel is the best option for that purpose.
Stainless steel is extremely resistant to chemicals.
Stainless steel shows good corrosion resistance.
Stainless steel 304L has excellent weldability.
This material has the capacity to withstand all humid conditions, sunlight exposure, or even high temperatures.

Aluminum Material

Aluminum can maintain high tensile strength.
This material is a lot easier and more cost-effective for machines than a stainless steel one.
Aluminum is also a good material as it has a large coefficient of expansion compared to other metals.

Carbon Steel 

Carbon steel has a high capacity to retain its strength.
The tensile strength of carbon steel is exceptional.
Carbon steel can be recycled easily.
It has the capacity to resist vibrations and shocks.

Hastelloy Material

Hastelloy is a well-suited material to use in gas, oil, petrochemical, and chemical applications.
Good corrosion resistance
This alloy is long-lasting and can work for several years.
It is crack resistant.
It is available in many types, such as C276, S, C, B2, and many more.

Types of Pressure Vessels

Process tanks are designed to simply hold and store liquids.

Autoclaves, such as grease kettles, use a combination of pressure and steam to create substance-producing chemical reactions. Their goal is to deliver both pressure and elevated temperatures to processes that need them. Examples include: medical sterilization, rubber vulcanization and curing and synthetic crystal growth.

High pressure vessels are the strongest type available, use the highest psi and offer the best corrosion, temperature and pressure resistance. They usually operate at pressure levels between 10,000 psi and 150,000 psi. Typically, high pressure vessels are made of stainless steel. Common high-pressure vessel roles include: high speed mixers, chemical reactors and supercritical extraction systems.

Expansion tanks are found in every residential closed water heater. There, they absorb excess water pressure so that the heated water has room to expand. By relieving pressure, expansion tanks also make pipe damage less likely.

Heat exchangers transfer heat for applications in: HVAC, chemical, power, petrochemicals and petroleum, sewage treatment and space heating. Usually, this heat transfer takes place between one solid object and one fluid, or between two or more individual liquids. A great example of a heat exchanger is the heat sink. This passive heat exchanger transfers heat from a mechanical or an electronic object, like a PC motherboard circuit, to a fluid medium, like liquid coolant or air. In the fluid medium, the heat dissipates and the circuit stays cool.
Storage vessels are designed to store contents under pressure in such a way that you can readily access them when they’re needed. Examples of storage vessels include: propane tanks, gas tanks, hot water tanks and air tanks.

Water pressure tanks are found in wells. They are designed to help deliver water from the well through and out the faucet. When a resident turns on the faucet, the air pressure stored in the water tank gets the message to force the water through. The air pushes the water until its pressure drops to a predetermined point, usually around 40 to 60 psi. Once it hits this point, the water pump gets the message to turn on, and it pulls water into the house. When the resident turns the faucet off, the pressure builds back up to its default level.

Vacuum tanks provide important, mostly short term, support to sewage applications.

ASME pressure vessels, also 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 are pressure vessels designed with a wall thickness, or shell thickness, no more than 10% (by ratio) the diameter thickness. Thin wall pressure vessels are available as spherical pressure vessels and cylindrical pressure vessels. Mainly, they are used to store and transport liquids and gases. They may also serve as components of rockets and hypersonic drag balloon skins.

Boilers are closed pressure vessels used to heat fluids, mostly water. These heated fluids are used for cooking, power generation, central heating, water heating and sanitation. In the US, “boiler” is synonymous with “furnace.”

Pressure Vessel Equipment Components

Pressure vessels generally consist of: the main container, safety valve fittings and pressure vessel closures. The main container may be any shape. However, it’s usually a cylinder, sphere or cone. These simple shapes work better and are easier to analyze than more complex ones.

Possible additional components include: agitation systems or propellers for mixing, detachable lids, removable lids, heating and cooling systems, ladders, observation sight glass and stairs.

Design and Customization

Production Process
Metal used in any pressure vessel manufacturing process is usually first cold rolled, rather than hot rolled. Also, to increase tensile strength and temperature resistance, metalworkers frequently temper, quench or galvanize them.

Once the metals are ready, pressure vessels are usually manufactured using one of three processes: forging, brazing and welding. All three processes utilize heat to join metal pieces together, but they all use it differently.

Forging forms metal parts through the application of heat and pressure.

Brazing refers to the process of joining two metals by filling the space between them with a non-ferrous metal.

Welding is a process in which two similar pieces of metal are heated until their edges melt together and they fuse.

Manufacturers design tanks from a variety of durable metals or high-strength plastics that can maintain their shape and properties under pressure. Examples include: stainless steel, zirconium, carbon steel, titanium, niobium, nickel alloy(s) and fiberglass.

Considerations and Customization
During design and before fabrication, engineers must determine sensitive design components like: pressure level, temperature, material components, size and shape. They can make custom pressure vessels with any one of these components personalized. In addition to regular safety info, manufacturer info and certification stamps, they can also inscribe special information on your tank, such as ownership details.

Pressure Vessels within the Oil Refinery and Chemical Industry

Pressure vessels are purpose-built, reinforced enclosures that are fabricated for the containment of internal or external pressures. In the oil refinery and chemical industry, pressure vessels are used extensively either to hold liquids and vapours/gases or in process plants for boilers, heat exchangers, and reaction vessels. Intrinsically, pressure vessels in the oil and chemical industry are exposed to harsh operating conditions either by the virtue of the fluid they are handling or the atmosphere they are installed in (or both). Some of the inherent challenges for the pressure vessel in the oil and chemical industry are.

Corrosive Atmosphere
Corrosive/Erosive Fluid 
Extremes of Temperature
Extremes of Pressure

At the same time, the nature of the oil and chemical industry requires that pressure vessels be manufactured with a higher safety standard to avoid any failures as they can be catastrophic. As a result, engineering authorities have imposed strict rules on the production and operation of pressure vessels. Pressure vessels are built and inspected according to well-known regulations such as ASME BPVC Section VIII, BS5500 and API 510. 

Material selection for the fabrication of pressure vessels in the petrochemical industry is the most crucial step. Consideration is given to many factors such as:

Is the operating fluid corrosive?
Is the installation requirement ATEX or ex-proof?
What are the operating pressure and temperature rating of the process?
What is the maximum permissible pressure and temperature rating of the process?
Is the vessel for the stagnant process (storage tank) or flow process (boilers, heat exchangers)?

Keeping in view all the above and many more user-specific requirements, material selection is made, materials including:

Stainless steel
Carbon steels
Chrome-moly steels
Super duplex
High nickel alloys
Clad plates

NDT Techniques for Pressure Vessels

Pressure vessels have toxic and hazardous gases in them, therefore it is very likely that some accident may happen. To minimize this risk, non-destructive testing (NDT) techniques are there. Under the observation of the best operators and technicians, NDT techniques can be implemented.

-For the detection of surface defects following NDT techniques are used:

Visual Testing 

     A common NDT method is the visual testing of the vessel. It is used to look out for the defects such as corrosion or any visible physical damage. Visual testing requires good light and perfect eyesight of the inspector. The visual testing method has easy access to the surface. However, some sites can be challenging. If we want to enhance the visual method, the use of magnifying glass is suggested. Visual testing methods can also be done using computerized video systems. 

Magnetic Particle Testing 

     Another NDT technique that is used to detect surface defects is magnetic particle testing. This technique is only for materials that are ferromagnetic in nature, such as steel. This NDT technique is easy to carry, portable and inexpensive. Magnetic particle testing is used for detecting cracks, which are responsible for surface breakage of the material. This NDT technique is a two-stage process. First, a magnetic current is passed through the component. If there is any crack or defect present, it will interfere with the lines of magnetic flux. After that, magnetic particles are sprayed, which collects all the surface defects. The use of fluorescent magnetic inks can also be used for more visibility of particles.

Eddy Current Testing 

    A coil is placed near the pressure vessel surface and an electrical current is passed through it. This electric current will induce eddy current. If there is a defect on the vessel surface, it will hinder current flow. However, if the material of the vessel is non-magnetic, eddy current testing will give a measurement that will show the depth of the defect.

Dye Penetrant Testing 

   A penetrant, i.e., a liquid, is sprayed on the vessel surface to detect its surface flaws. To make defects more visible under UV light, a fluorescent chemical is also added.

-For internal defects of pressure vessels following NDT techniques are used:

Ultrasonic Testing 

    Ultrasonic testing requires an excellent surface finish because it uses high-frequency sound waves to detect cracks. The ultrasonic waves will be reflected on the cracked parts of the surface and give data on wall thickness. For its working, a coupling medium such as water or gel is required. An ultrasonic probe transfers sound waves through the component to detect defects. The automated technique of this testing is the best to use as the manual application has some speed limitations.

Radiographic Testing 

   This NDT technique works by detecting material loss. Radiations are applied on the component, and any alteration in the material can be detected and recorded on the film. Radiography is the most popular NDT technique as it can capture defects on the film and also provides a hard copy of it. Digital radiography is even better as it can give results on computer screens in seconds.

Safety and Compliance Standards of Pressure Vessels

To be considered a finished and functional tank, many pressure heaters must adhere to regulations by and be registered with the American Society of Mechanical Engineers (ASME). The ASME Boiler and Pressure Vessel Code (ASME Section VIII code) and standards and inspection codes set out by others, like the American Petroleum Institute (API) help ensure worker and building safety; because tanks are under extreme pressure, even the tiniest leak could cause a large explosion with shrapnel damage.

Due to the intricate nature involved in the design and fabrication of pressure vessels, several regulations and codes are put in place to ensure the safe fabrication and operation of pressure vessels, such as:

ASME VIII (Div. 1 & 2 plus holders of ASME U, U2, R and NB stamps)
API 650
DNV offshore rules
NORSOK standards
GOST plus SNIP standards
NACE MR0175/ISO 15156
PED 97/23/EC Compliance