A typical pressure sensor consists of a pressure-retaining housing containing a diaphragm that is slightly moved by changes in fluid pressure. Some form of electronic device is mounted to the diaphragm that converts the movement or deformation of the diaphragm to an electrical signal that is proportional to the pressure being experienced.
While the failure of a pressure sensor may announce itself through the failure of a system, an entire plant, or some large asset, it is typically only the diaphragm that has failed. It is also possible that the small passage in the sensor tube leading to the diaphragm has become plugged with debris or corrosion.
There are several important elements to the failure of the pressure sensor described above. First, while the large scale effects of the failure may be most evident, it is only a small subcomponent of the pressure sensor that has failed. Second, in some cases, the diaphragm might have failed and in other cases the pressure tube might have become plugged. The failure modes are not the same and the ultimate solutions are different.
In the case of a cracked diaphragm, the crack may be the result of failure mechanism fatigue. The diaphragm could have reached the end of its fatigue life, or it may have cracked due to chloride stress cracking. Either the fluid unexpectedly became contaminated with chlorides or the designer might have failed to account for the normal presence of chlorides in the fluid when selecting the diaphragm material. In either case, the failure frequency and the useful life of the pressure sensor failing from two different failure modes are likely to be different from one another.
In the case of a plugged pressure tube, the behavior of the pressure sensing system leading up to the failure is likely to be different from the behavior of a system failing from a cracked diaphragm. Because of the partial plugging leading up to the event, the pressure being sensed is likely to be inaccurate. When a sensor diaphragm experiences a fatigue crack, the accuracy of the sensor leading up to the failure is good right up to the point it completely fails.
Further, the failure due to plugging will happen at a different time and with a differing frequency than the diaphragm failures. The plugging also may be related to an event where the fluid filter was under maintained or failed and allowed debris into the fluid system.
The point is that the actual failure mode involves a small subcomponent
or a part of a system. The behavior of the system leading up to the failure can provide insight into the impending failure. And the failure mechanism leading to the failure mode and failure are different in each case.
Addressing the incorrect failure mode or failure mechanism will not produce a solution to the problem.
To achieve the intended objectives of this book, it is necessary to have a very close focus on the specific components that are failing and causing the larger systems or complete assets to fail. An unfortunate aspect of many approaches to reliability is the idea that reliability programs can be successful by focusing exclusively on the reliability process or an approach that is based on people sitting around a table talking. Unless those people are inspecting a collection of failed components, their efforts will not be successful.
The following paragraphs continue with components and subcomponents and their relationship to failure modes and failure mechanisms. A few of the components have been selected because they most frequently fail, leading to the failure of a complete equipment item or system. Other components were selected because, while they are frequently the failure mode, they are relatively inexpensive and a more robust component could have been chosen at only a small increase in cost.
Tip from Critical Connections - Linking Failure Modes and Failure Mechanisms to Predictive and Preventive Maintenance by Daniel T. Daley.
