The Complete Guide to Pressure Vessels
Discover everything about pressure vessels, including their designs and safety standards.
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Discover everything about pressure vessels, including their designs and safety standards.
Pressure vessels are specialized containers used to store or transport gases and liquids under pressure, either significantly higher or lower than the surrounding atmospheric pressure. These containers are highly regulated and must meet strict safety standards to ensure they can withstand the extreme pressures they are subject to. Examples of pressure vessels include boilers, compressed air tanks, propane tanks, heat exchangers, and chemical reactors.
In addition to steel, pressure vessels can also be made from aluminum or composite materials. Pressure vessels can be either spherical or cylindrical in shape and range in size from small tanks one person can carry to massive industrial vessels several stories tall. Regardless of their size or shape, all pressure vessels share one common goal: safely contain their contents at high pressures.
Several standards and regulations control every part of pressure vessels. However, the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) is a widely accepted standard.
As part of its responsibilities, the department designs, constructs, installs, tests, inspects, and certifies boilers, pressure vessels, and nuclear power plant components. Section VIII of the ASME BPVC governs pressure vessels. It has three divisions:
It encompasses all pressure vessels designed to operate at an internal or external pressure that may exceed 15 psig. It’s possible to use a fired or an unfired vessel and to obtain pressure through an external source or indirect heating. Engineers adapt the design-by-rule approach. Division I is based on normal stress theory.
This section covers all pressure vessels that are meant to operate with internal or external pressures up to 10,000 pounds per square inch. The requirements for materials, design, and nondestructive examination in Division II are more stringent than in Division I. It means that the calculations required are more detailed.
Engineers in this division use design-by-analysis to subject pressure vessels to higher stress levels. Unlike Division I, it is based on maximum distortion energy theory.
It identifies the required and not allowed pressure vessels for use above 10,000 pounds per square inch.
Another standard that maintains the integrity of pressure vessels is API 510 Pressure Vessel Inspection Code for In-Service Inspection, Rating, Repair, and Alteration.
There are different types of pressure vessels based on their shape or function. Types of pressure vessels include:
Storage vessels are pressure containers that hold liquids, vapors, and gases. It’s possible to reuse vessels for future processes or to store finished products like compressed natural gas (CNG) and liquid nitrogen.
Heat exchangers help to transfer heat between two or more fluids. Bioprocessing, food, pharmaceutical, and energy industries use them most often. Equipment for heat exchangers is designed according to the thermal and flow properties of the fluids involved in heat exchange and according to the thermal property of the conductive partition (for indirect contact heat exchangers). Heat exchanger materials are stressed by the temperature difference between hot and cold fluids and their internal pressure.
Boilers are machines that use different types of energy to create heat. Liquids are then turned into vapor using this heat. A boiler is usually composed of metal containers that transfer heat from a source to a fluid. Steam from the boiler is used for a variety of purposes. The boiler makes the steam go faster so it can turn the turbine blades. The boiler must be solid to handle all the pressure and heat. Most materials get weaker when they get hotter.
Process vessels are big containers where industrial processes happen, like mixing and stirring, decanting, distilling and separating chemicals, and chemical reactions. The pressure inside a process vessel changes depending on the process and the substances used.
Spherical pressure vessels are suitable for holding liquids under a lot of pressure. They are robust and not easy to break, but they are hard and costly to make. The pressure is spread out evenly on the sphere, so there are no weak points. Spheres also don’t take up as much space as other shapes. Spherical vessels will use less material than cylindrical vessels if they are the same size. Spherical vessels are also less likely to transfer heat than other shapes due to their smaller surface area.
Cylindrical pressure vessels have a cylindrical shell and one or more heads. The cylindrical shell is the main body of the pressure vessel. The heads are like end caps that cover the contents of the vessel and may have a flatter or more rounded profile. It reduces the cylindrical container’s weakness.
Cylindrical pressure vessels are among the most popular type because of their many possible applications. They’re less expensive to produce than spherical vessels but not as durable. It then requires cylindrical pressure vessels to have thicker walls to withstand equal amounts of pressure than a spherical vessel.
A cylindrical vessel may have a horizontal or vertical axis depending on its purpose.
For small floor space
For small-volume vessels
For efficient mixing of fluid in mixing tanks
When the gas-to-liquid ratio is high
For easier removal of components in liquid-liquid separation
In heat exchangers for easier cleaning
In settling tanks and flash drums to have less entrainment
Can meet the strength requirements – The materials used to make the pressure vessel must be strong enough to last during its service life. It includes withstanding specific internal and external pressures and structural stresses.
Corrosion resistance – This is essential for pressure vessels because they need to work well under challenging conditions.
Return of investment – A pressure vessel’s lifespan must consider materials, fabrication, and maintenance costs.
Ease of fabrication and maintenance – Metal sheets must be machinable and weldable to form the pressure vessels’ geometry. It will be easier to install the vessel’s internals this way.
Availability – The sizes for pressure vessel materials must be easy to find near the manufacturer.
The most popular materials used to construct pressure vessels are:
Carbon steel
Stainless steel
Hastelloy
Nickel alloys
Aluminum
Titanium
The following are testing methods employed to ensure the reliability and factor of safety of the pressure vessel.
A certified inspector inspects pressure vessels at least once every five years. They will check the vessel’s interior and exterior for any cracks, deformation, blistering, fluid leakage, corrosion, or other damages that might have occurred.
This tool uses high-frequency sound waves to detect surface or subsurface flaws and measure the pressure vessel’s wall thickness. The material absorbs the ultrasonic sound waves and turns them into an electrical signal via a transducer. If there happen to be any flaws present, the reflected waves will be disturbed.
A radiographic test involves using x-rays or gamma rays to create a picture of what lies beneath a pressure vessel’s surface. Discontinuities, holes, and density differences will change how waves reflect. The film will then be exposed after this.
This method produces consistent results; hence it’s widely used. However, radiation exposure is expensive and requires specialized training.
The machine uses magnetic current to detect surface imperfections in ferromagnetic materials. The inspector runs a magnetic current through the pressure vessel between two probes. If the material is free from defects, the flux flows smoothly through it. But if cracks or other abnormalities are present, some of the magnetic flux will leak out of material around those areas. These imperfections will be more visible once ferromagnetic particles—in either liquid suspension or powdered form—are applied to the hull’s exterior.
An inspector applies a small amount of liquid called a penetrant to an area with welded seams or plates that may have flaws. Once the penetrant settles after spraying, the excess is wiped from the surface before applying developer, revealing any penetrant that has seeped into cracks.
The ASME BPV Code requires strength and leak tests. Pressure testing uses either hydrostatic or pneumatic methods—the former uses water as a medium, while the latter uses air or nitrogen. The process works by removing all air within the vessel before filling it with test fluid until 1.5 times design pressure for hydrostatic testing and 1.2-1.5 times design pressures for pneumatic tests have been reached, internalizing said pressure, respectively. Industry experts recommend hydrostatic testing over pneumatic methods because of the safety risks posed by the potential energy levels in pneumatic testing.
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