Pressure Vessel Failures

Pressure vessels and pressure piping used in refineries, chemical processing plants, water treatment systems of boilers, low pressure storage tanks commonly used in process, pulp and paper and electric power plants operate over a broad range of pressures and temperatures and experience a variety of operating environments. Shell, head, attachments, and piping are some of the components that commonly fail.

Design errors, fabrication errors, corrosion, and improper maintenance are some of the causes of failures. Brittle fracture, stress corrosion cracking, fatigue, welding problems, erosion, creep, stress rupture, and hydrogen embrittlement are among some of the common failure modes found in pressure vessel components. Failure analysis can identify the root cause or causes that have contributed to your pressure vessel failure.

Corrosion Failures

Corrosion is chemically induced damage to a material that results in deterioration of the material and its properties. This may result in failure of the component. Several factors should be considered during a failure analysis to determine the affect corrosion played in a failure. Examples are listed below:

** Type of corrosion.
** Corrosion rate.
** The extent of the corrosion.
** Interaction between corrosion and other failure mechanisms.

Corrosion is is a normal, natural process. Corrosion can seldom be totally prevented, but it can be minimized or controlled by proper choice of material, design, coatings, and occasionally by changing the environment. Various types of metallic and nonmetallic coatings are regularly used to protect metal parts from corrosion.

Hydrogen Embrittlement

When tensile stresses are applied to a hydrogen embrittled component it may fail prematurely. Hydrogen embrittlement failures are frequently unexpected and sometimes catastrophic. An externally applied load is not required as the tensile stresses may be due to residual stresses in the material. The threshold stresses to cause cracking are commonly below the yield stress of the material.

High strength steel, such as quenched and tempered steels or precipitation hardened steels are particularly susceptible to hydrogen embrittlement. Hydrogen can be introduced into the material in service or during materials processing.

Hydrogen Embrittlement Failures

Tensile stresses, susceptible material, and the presence of hydrogen are necessary to cause hydrogen embrittlement. Residual stresses or externally applied loads resulting in stresses significantly below yield stresses can cause cracking. Thus, catastrophic failure can occur without significant deformation or obvious deterioration of the component.

Very small amounts of hydrogen can cause hydrogen embrittlement in high strength steels. Common causes of hydrogen embrittlement are pickling, electroplating and welding, however hydrogen embrittlement is not limited to these processes.

Hydrogen embrittlement is an insidious type of failure as it can occur without an externally applied load or at loads significantly below yield stress. While high strength steels are the most common case of hydrogen embrittlement all materials are susceptible.