Failure Investigations: How the Causes of Failures are Identified
When the cause of a failure of a structure or mechanical equipment is not apparent, a failure investigation is often conducted. While the details of how a failure investigation is conducted can vary depending on the individual conducting the investigation and the particular equipment or structure, the cause of nearly all failures can be grouped into one (or more) of the following five categories:
Material Issues
Fabrication Issues
Improper/Incorrect Design
Improper Operation
Maintenance Issues
Based on assumptions about the likely cause of the failure, investigations may sometimes focus on one of the above areas. Failures, however, are sometimes due to issues in more than one of these areas. A comprehensive investigation, therefore, will consider all of these areas to ensure the correct cause of the failure is identified.
This article discusses these five categories of potential causes of failures. Examples are provided as well as how a failure investigation identifies or rules out these potential causes.
Material Issues
There are various material issues that can result in a failure. These include: inappropriate material for the equipment’s intended service, incorrect material used in construction, and material being out of spec. Examples of these are: using material with poor creep properties at elevated temperatures, using bolts with a yield and ultimate tensile strength lower than what has been specified for the project, and material that supposedly meets an ANSI/ASTM standard but is proven not to.
Material issues that could have caused a failure are typically identified by conducting a series of tests and examinations known as a metallurgical investigation. A typical metallurgical investigation will consist of performing tensile tests (to determine the yield and ultimate strength of the material), determining the chemical composition of the material, and examining fracture surfaces using microscopy. Microscopy can include the use of a scanning electron microscope. All of these examinations and tests are used to determine what material the equipment or structure was actually fabricated from, and whether or not it was defective.
Fabrication Issues
Incorrect or defective fabrication can also lead to failures. Examples of fabrication issues that can cause failures include: using a fillet weld smaller than what is specified, incomplete joint penetration of welds, insufficient bolt preload, incorrect assembly, and failure to lubricate joints.
Fabrication issues that could have caused a failure are usually identified through a visual examination and a comparison of how the equipment or structure was supposed to be fabricated with how it actually was put together. Sometimes identifying these issues requires microscopy performed during a metallurgical investigation.
Improper/Incorrect Design
While the details of a design will vary widely between different types of equipment and structures, nearly all design related issues that can lead to a failure are either an engineering error or omission. An engineering error is just that, an error or mistake. This could be using the wrong equation when sizing a component, making a math error, incorrectly interpreting a Code requirement, or using the wrong Code or Standard during design in the first place.
Engineering omissions occur when the engineer forgets to consider something in the design of the equipment or structure. Examples of this are: not evaluating cyclic loading (fatigue), not specifying bolt preload when necessary, or not considering the response of a structure subjected to dynamic loads caused by vibrating equipment.
Sometimes the engineering “mistake” is not an error or omission on the part of the design engineer but not designing for what would reasonably be considered a rare or unanticipated load. Examples of this would be a fork truck impacting a piece of equipment, a fire adjacent to a structure, or a wind load that exceeds the design load specified by the appropriate Standard.
Design issues leading to failure are generally identified through a review of the engineering design calculations, project requirements, specifications, and applicable Codes and Standards.
Improper Operation
Improper operation of equipment can also lead to a failure. These include: operating the equipment outside of its design parameters, unintentional upset loads (overloads), bypassing safety measures, and operating the equipment beyond its design life.
Examples of improper operation that can lead to failure are: operating a pressure vessel below its MDMT (Minimum Design Metal Temperature), loading a storage rack beyond its rated capacity, heating up a pressure vessel faster than specified in the operations manual, circumventing automatic shutoff valves, and using equipment that has obviously exceeded its useful design life.
Identifying operational issues as the cause of a failure typically entails having an understanding of the operation of the equipment, understanding the consequences of improper operation of the equipment, examining the equipment or structure for evidence of improper use, talking to operators, and conducting a review of operation and instrumentation records.
Maintenance Issues
Poor maintenance, or a lack thereof, can cause a failure of equipment and structures. Examples include: not painting or treating carbon steel surfaces to prevent corrosion, failure to keep drain lines clear, not changing lubricants on a regular basis, and not properly cleaning equipment.
Maintenance issues leading to a failure are generally identified by inspecting the equipment or structure for telltale signs of the consequences of poor maintenance. Examples of these signs include: corrosion, discoloration or deterioration due to overheating, clogged piping, and seized valves.
Closing Remarks
Regardless of the details and type of structure or equipment, failure investigations look at the same potential sources, and typically perform many of the same tasks in ascertaining the cause of a failure. Identifying the cause of a failure entails investigating and understanding the materials, fabrication, design, operation, and maintenance of the structure or equipment. Tasks typically consist of performing visual examinations, reviewing documents, and performing materials testing.