Inspectioneering Journal

Fracture Analysis

By Cliff Knight, P.E., President and Chief Engineer at KnightHawk Engineering, Inc. This article appears in the September/October 2011 issue of Inspectioneering Journal.

The morning meeting at the plant was a tough one for you. As an area engineer you are not satisfied with the information you are receiving from your team’s investigation into a major compressor wreck that has happened once again. The conclusion from the team has always been corrosion fatigue, and suggestions have been made to change the material to a more exotic type. The cost of the impeller would be more than five times the original equipment manufacturer (OEM) and have a long delivery time. One of the aspects of the work conducted by all the “high powered” experts that really bugs you is that all the sister plants around the world with the same process have the same impeller material, and yet do not experience these failures. Also the plant has a long history of running in this service with this material in other pumps and compressors. So in your mind “things just don’t add up”. The words “corrosion fatigue” resonate in your mind. Also, there is no doubt in your mind that the team is one of the best in the business.

Any area engineer should take a broad base look at the facts and ask questions. Questions were asked about this not happening with other pieces of equipment in sister plant using the same materials. In fact, the team was correct with their conclusions. The cause of the problem was corrosion fatigue. However,the key term here is fatigue. Fatigue translates to the fact that in the impeller reverse loading occurs, which means in practical terms that a dynamic stress was present.The fact that the impeller failed suggests the endurance limit was exceeded. After putting all the facts together it’s time to “dig deeper” to see what is really going on. For the impeller to have failed, the endurance limit must have been exceeded. You ask the team to show you the Goodman Diagram so you can see the interaction of the steady state and dynamic stresses. The team does not produce a diagram, because none was ever developed. The reason is the team focused on “corrosion” as being the major player in this “corrosion fatigue” problem.

A typical allowable dynamic stress in an impeller on the Goodman diagram is 1⁄4 of the tensile stress of the material. This assumes the material is good, and meets the ASTM standard for the material. However, corrosion can cause pitting, and reduce the endurance limit by another factor of two to five. Does this mean we have found the root cause of the failure? The answer is a flat “No!” No Goodman, Campbell, or interference diagram was developed.

The next step is to look at the process, and determine the exact details of what may be different. There must be some reason for the change. To do this requires evaluating the transient and steady state operation of the compressor. This might require additional instrumentation be incorporated into the process to better capture the process transient events.

In this particular problem, the molecular weight of the process changed during a transient period of operation when the plant was running at a higher capacity. This caused an excitation of the cavity acoustics, which ultimately led to the excitation of the impeller blades. A forcing function was present that matched a natural frequency of the impeller. Higher level analysis determined the impeller would have failed anyway, even without “derated” endurance conditions present. In other words, the dynamic stresses were so high that they would exceed the endurance limit of the metal with no corrosion.

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