What to Expect During a Failure Investigation
So, your pressure vessel or equipment has experienced a structural failure. But the question remains what caused the failure and what can be done so it doesn’t happen again?
While you may understand the operation of your vessel or equipment very well, you may need to consult with someone outside of your company to determine the cause of the failure. So, you hire an outside person or firm to conduct a failure investigation. What can you expect?
Potential Causes of Failure
There are a variety of things that can cause a structural failure of a pressure vessel or other equipment. While numerous, they can generally be grouped into one of the following categories:
Material issues
Fabrication and weldment issues
Inadequate or erroneous design
Excessive or abnormal/unexpected loading
Poor or improper maintenance
Sometimes the cause of the failure can fit nicely into one of above categories. Other times, the problem could be considered to fit into more than one of the above categories. Likewise, the failure may be the result of more than one issue. Therefore, it is important that the failure investigation should consider all of the potential reasonable causes of the failure.
Procedure for Conducting a Failure Investigation
The basic procedure for conducting a failure investigation is as follows:
Gathering of information
Visual examination and photographic documentation
Metallurgical/materials testing
Engineering investigation including evaluation and analysis of the design, loading, and maintenance of the vessel
Summary and conclusions of findings
Warranty Claims and Lawsuits
Now, before you let the person or company conducting the investigation loose, there is one very important question to be asked.
Is there a potential of the results of the investigation turning into a warranty claim or a lawsuit to recover damages?
If so, all potential interested parties need to be contacted prior to any material samples are taken and tested, or anything else done that permanently changes the condition of the vessel or equipment. This is to avoid the possibility of one of the interested parties claiming the evidence was corrupted prior to their being able to examine it. This is called spoilage of evidence.
If the investigation may result in a warranty claim or lawsuit, expect the investigation to take significantly more time. This is because the number of people involved has just increased dramatically. Now you have the other parties’ lawyers, insurance companies, and all of their forensics experts involved. If this is the situation you find yourself in, it is likely that the decision of who is to conduct the failure investigation on your behalf will be made by your company’s attorneys or your insurance company.
While the above steps involved in a failure investigation are largely self-explanatory, there are some details about each worthy of comment.
Gathering of Information
The gathering of information is important in determining the cause of the failure. The types of information typically requested include: drawings, design calculations, inspection records, maintenance records, operational data, and records of how the vessel or equipment was operated over its life. A lack of this information being available may result in not being able to determine the cause of the failure with an acceptable level of certainty.
Visual Examination and Photographic Documentation
The visual examination usually includes photographic documentation and will center around documenting the failure of the vessel and corresponding damage. The primary goal of the visual examination will be to determine where the structural failure began (crack initiation).
Since the location of crack initiation is significant to the investigation, care must be taken to preserve the integrity of this location. If the damage to the vessel or equipment is not confined to a local area, the location of the crack initiation may not be readily apparent. If this is the case, moving the vessel or “cleaning up the site” has the potential of either damaging the areas of interest or making determining where the failure started more difficult.
Oftentimes, however, leaving the site as-is after a failure is not an option. In these cases, sufficient photographic documentation of the failure and its surrounding area should be made prior to moving the evidence to another area.
Care should be made not to mar any fracture surfaces. Putting the broken pieces back together to view them should not be done. Doing so can damage the fracture surfaces and inhibit the failure investigation.
Material Testing
Once crack initiation has been determined (or surmised) and other areas of interest identified, a plan for material testing will be generated. This plan is referred to as a test protocol and will include a list of tasks that will be performed such as hardness testing, chemical testing, and sections of the fracture surface to examine under a microscope. These tasks are considered destructive tests in that they will destroy or permanently alter areas of interest.
If the investigation is not confined to being an in-house investigation but rather includes other interested parties that may be held liable, the creation of the test protocol will be a joint effort. Usually the person or company acting on behalf of the owner of the pressure vessel or equipment will create a preliminary test protocol. The experts for the other interested parties will then comment on or make additions (e.g. request additional testing) to the protocol until there is a consensus. This is all to be decided prior to cutting up the vessel or equipment.
Once the test protocol is finalized, a location of where the testing will occur (usually at the laboratory of the expert representing the owner) and date(s) of the testing set. If there are interested parties outside of the owner, their experts (and possibly attorneys) will usually want to be present during the testing.
Engineering Investigation and Evaluation
While a considerable amount of failures can be attributed to a material or fabrication issue, such as a defective weldment, an engineering investigation into the design and operation of the vessel or equipment should not be neglected. This is because, the results of a metallurgical investigation alone may not tell the whole story or may be misleading as to the root cause of the failure.
The design, loading, operation, and history of the vessel or equipment will usually determine the extent of the engineering investigation. This could be simply a review of the design calculations for accuracy or could include a finite element analysis to investigate a scenario that was not considered when the vessel was designed. How the vessel was operated and maintained over its history will also be evaluated. All of this is important in getting a complete picture of what caused the failure in addition to preventing failures in the future.
Identification of Root Cause of Failure
Once all of the evidence has been analyzed and evaluated, conclusions are drawn and the root cause of the failure identified.
Sometimes no one, single issue or problem can be considered to have caused the failure but rather the combined effects of several issues led to the failure.
While not common, there are times when the cause of failure may not be determined with an acceptable amount of certainty. This can happen when there is a lack of information, the evidence has been corrupted, or the location of where the failure started could not be identified with certainty.
Summary
To summarize, failure investigations generally follow the same procedure of gathering information, performing materials testing, and evaluating design and operating conditions. Given this, failure investigations of pressure vessels and other structural equipment are usually undertaken by metallurgists and/or mechanical engineers. If the investigation may result in a warranty claim or lawsuit, all interested parties need to be contacted at the beginning of the investigation. Lastly, while a considerable amount of failures are the result of a materials or fabrication issue, an engineering evaluation should also be performed to ensure the complete story and root cause of the failure is correctly identified.