Pressure vessel

A pressure vessel is a specialized container designed to hold gases or liquids. These substances are kept at a pressure that is significantly different from the ambient pressure of the surrounding environment.

This pressure difference can be much higher or much lower than the air around us. A vessel with internal pressure lower than the atmosphere is called a vacuum vessel or a hypobaric vessel. Because these containers hold large amounts of energy, they can be dangerous if they fail. Fatal accidents have occurred throughout history during their development and operation. For this reason, their design and use are strictly regulated by engineering authorities and laws.

The primary characteristic for designing a vessel is the working pressure. This is the difference between the internal pressure and the external surroundings during use. Engineers must also account for the maximum safe temperature and a safety factor. They must also consider a corrosion allowance and the minimum design temperature to prevent brittle fracture. The physical shape of the vessel is critical to its strength. While almost any shape is theoretically possible, engineers usually choose spheres, cylinders, ellipsoids, or cones.

These specific shapes are chosen because they are inherently stable under pressure.

Different shapes handle internal forces in different ways. A spherical vessel is the ideal shape for holding internal pressure. In fact, a sphere has approximately twice the strength of a cylindrical vessel with the same wall thickness. However, spheres are difficult and expensive to manufacture. Most common vessels are cylindrical with end caps called heads. These heads can be hemispherical, ellipsoidal, or dished, which is also known as torispherical.

Many smaller vessels are simply made from a pipe with two covers. For cylindrical vessels with a diameter up to 600 mm, manufacturers might use seamless pipe to avoid complex inspections of long seams.

To hold the contents safely, the walls must resist tensile forces. Tensile stress occurs within the walls as the gas pressure pushes outward. The normal stress in these walls is proportional to the pressure and the radius of the vessel. It is also inversely proportional to the thickness of the walls. This means that as a tank gets larger, the walls must get thicker to handle the stress. The minimum mass of a vessel depends on its pressure, its volume, and the strength-to-weight ratio of its material. For a sphere, the mass is determined by the pressure difference, the volume, the material density, and the maximum allowable working stress.

Safety is managed through strict international codes and rigorous testing. In the United States, engineers follow the ASME Boiler and Pressure Vessel Code (BPVC). In Europe, they follow the Pressure Equipment Directive. Every new vessel must be checked by an authorized inspector. Each vessel also carries a nameplate containing vital information. This plate includes the maximum allowable working pressure, the maximum temperature, and the manufacturer's name. It also lists the registration number and the official ASME U-stamp. This makes every vessel traceable and officially verified.

Before a vessel is put into service, it must undergo nondestructive testing. This includes methods like ultrasonic testing and radiography, which uses X-rays to inspect the material. One common method is the hydrostatic pressure test.

This test uses water to fill the container. Engineers prefer hydrostatic testing because it is much safer than pneumatic testing, which uses air or gas. If a fracture occurs during a water test, the water does not expand much in volume. In contrast, gases expand explosively during rapid depressurization. In some cases, a representative sample from a production batch is even tested to destruction to ensure quality.

Pressure vessels are essential to many different industries and technologies. They are used in industrial compressed air receivers, boilers, and hot water tanks. They are also found in diving cylinders, autoclaves, and nuclear reactor vessels. In the transportation sector, they serve as air brake reservoirs for rail and road vehicles. Even the passenger cabin of an airliner acts as a pressure vessel to carry structural and maneuvering loads.

In extreme environments, they are used for submarine habitats and even spacecraft. From storing chemicals like ammonia to keeping humans safe in the deep ocean, these vessels are vital to modern life.