What is a Pressure Vessel
Pressure vessels are containers designed to hold high-pressure gases and liquid for industrial or general use. The material used for their construction is dependent on the owner’s requirement or usage. In the oil refinery industry, for example, the standard material used in the fabrication of pressure vessels is carbon steel or stainless steel.
Pressure vessel usage applies to different industries. Their usage is very crucial in the refining process, where they are used as boilers, reactors, and heat exchangers. In our daily domestic environment, hot water storage, for example, is a pressure vessel. Pressure vessels are also essential in pharmaceutical, food processing, and in the hospitals where they help improve our lives.
Pressure Vessel Design and Construction
Pressure vessels usually have to deal with high internal pressure, the weight of its content, cyclic reaction, wind pressure, and so on. Because of such loading, their design must withstand failure and be leakproof. Their design usually comes in cylindrical or spherical shapes. The spherical types are typically more durable, but more challenging and costly to manufacture. As a result, the cylindrical shapes are preferred in most cases.
The design of most pressure vessels used in the industry today is per ASME BPVC (Boiler Pressure Vessel Code), Sec VIII. This code consists of standard rules an engineer or a fabricator is required to follow in their design and construction. Some other countries have also established their own safety rules governing the design and fabrication of pressure vessels.
The steps in the design process require gathering the necessary information about the pressure vessel. Depending on usage and specification, one of the code requirement as per ASME Sec VIII applies.
The three divisions of the code requirement are:
Division 1: This division is also known as the design by rule approach. The rule describes the design principles and other conditions that should be followed in the design and construction of vessels capable of holding pressure between 15 psi to 3000 psi.
Division 2: The requirement for this division is more stringent than the one above because it covers the rules for the construction of pressure vessels with operating pressures between 3000 psi to 10000psi.
Division 3: This division defines the guidelines for vessels required to withstand higher operating capacity exceeding 10000 psi. Unlike Division 1 and 2, it does not establish a maximum pressure or minimum pressure limit.
Establishing the design code will set the required dimension for the allowable thickness of the pressure vessel. The code also covers vessel components such as nozzles and flanges.
Why Pressure Vessels Fail
Vessel failures, usually undesired and unexpected, can have disastrous consequences when it suddenly loses its ability to carry its intended load. Several research papers on pressure vessel failures have traced the possible causes to incorrect design parameters, poor execution of quality control during fabrication and welding, and changes in service condition.
The effect of these inadequacies can result in pre-existing conditions that remain after design and manufacture, or later induced after some time-in-service to cause damage. In general, the failure of a pressure vessel is linked to either pre-existing or service-induced conditions.
According to a publication from The National Institute of Standard and Technology, the type of pre-service deficiencies that can be present before equipment enters service are:
- Inadequate considerations for design details such as lack of flexibility and sharp changes in thickness
- Improper materials either by wrong design selection or mistakes in identification; this includes both base materials and welds or other joint materials.
- Material production flaws that occur during production including lamination and lap in manufactured products, segregation, shrinks, cracks, and bursts in cast products.
- Undetected defects in the base material and the fabrication joints.
- Incorrect heat treatments and cleaning procedures.
A pre-existing condition can not cause immediate reliability and safety concerns. Still, they can sometimes be the underlying issue for a later in-service problem. For example, a vessel operating after a few years of service may suddenly experience an unexpected failure due to brittle fracture.
The cause of this sudden failure may have been initiated by a small pre-existing crack that was undetected during the initial inspection, or the defect was in a location that was uninspected. Then it slowly grows during in-service operation to cause a fracture.
Pressure vessels operate in corrosive and high operating temperature/pressure environment when they are in-service. Also, they experience both physical and chemical changes because of the combination of numerous aggressive process streams and startup/shutdowns. Prolonged exposure to these conditions leads to:
- Metal loss due to corrosion/erosion
- Subsurface and surface connected cracking
- Creep and hydrogen attack
- Metallurgical changes
These mechanisms can cause failures of a vessel in several ways. Some of these mechanisms initiate at the surface of the equipment. At the same time, some are difficult to detect and show no material damage before they eventually cause a fracture. Several other conditions can affect these mechanisms in some manner. For example, the cyclic stress responsible for fatigue can arise from mechanical sources such as pressure cycling or stresses produced from thermal differential or equipment start and stop operation.
Also, the heat effect during welding can result in metallurgical in-homogeneity, which could result in tensile residual stresses near the weld area and heat affected zone, causing cracking. Another vital element of causation is that of human intervention, which fails to occur at the right time.
For example, a simple activity such as re-coating an equipment that has shown signs of corrosion is overlooked. As a result, the affected areas may start to erode, prompting the wall thickness to reduce below the design thickness. Consequently, the material loses its load-bearing capability and fails when the load moderately increases.
How to Prevent Pressure Vessel Failure
Pressure vessel failures can be prevented by avoiding the mistakes that cause them to fail. The mistake areas can be in the design, material selection, fabrication, inspection, operation, and the chain of interrelated responsibilities.
According to a bulletin from the National Board of Boiler and Pressure Vessel Inspectors, “the first approach to failure avoidance is an excellent design.” That includes both the design of the structure and the welded joints. Even though the code requirements will govern many aspects of the design, the vessel designer has a significant influence, especially when it comes to stress concentration.
In many instances, the possibility of failure often exists long before the failure occurs. While there is no way to ascertain when the failure will occur, there is still an opportunity to intervene and prevent failure. With this in mind, a clear understanding of the distinction between failure mechanism and their consequences is crucial. That understanding will enhance the ability to identify the opportunities for intervention and effective forms of prevention.
Achieving the expected service life of a pressure vessel, and consequently preventing failures requires diligence on the part of the owner/user in operating and maintaining the equipment within established limits. In general, periodic inspection programs are critical for failure prevention.
Whenever a crack indication is found in the inspection, the structural integrity and safety of the vessel for continued operation should be evaluated. Research suggests that most repair welds are made under less ideal fabrication conditions. For this reason, careful attention to all aspects of welding and inspection must be exercised to avoid any circumstance that may cause failure.
After reviewing over a dozen published papers for this article, the general concern is that vessels will remain unsafe unless carefully designed, operated, and inspected at regular intervals. In the design and production phases, for instance, excellent communication and teamwork involving the designer, the engineer, the fabricator, and the owner will ultimately reduce the conditions that may cause failure in the future.