Reliability Assessment: A Guide to Aligning Expectations, Practices, and Performance. Daniel Daley
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Название: Reliability Assessment: A Guide to Aligning Expectations, Practices, and Performance
Автор: Daniel Daley
Издательство: Ingram
Жанр: Здоровье
isbn: 9780831190613
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After RBDs have been assembled and calculations completed, you will have an initial estimate of the inherent reliability that is reasonable to expect. If the calculated reliability does not meet requirements or expectations, either the configuration can be changed (e.g., adding redundancy) or different (more reliable) components can be selected. By inserting the new configuration or characteristics of new components into the model and re-running the calculations or software, it will be possible to estimate the improvement.
Once a configuration and list of component choices have been finalized, it is possible to perform lifecycle cost comparisons to evaluate if the cost of changes is justified by the reduction in lifecycle costs (resulting from fewer and/or shorter outages or by lower maintenance costs).
If initial project design procedures account only for system integrity (e.g., structural or pressure retaining capability) and not for reliability and availability performance, the owner will have to “take what he gets” for those two performance areas.
Another element of reliability mentioned in the fictional account described above is that of initial construction or assembly. It is possible to design a system to be reliable, but then lose a portion of the benefits of all that cost and effort when the system is constructed. Inherent reliability depends on things being assembled in a manner that does not introduce additional defects. All too often, shortcuts made to meet schedule or due to misunderstandings in how things should be assembled lead to the inclusion of defects. The example of pipe stress on the nozzles of rotating equipment is one that many reliability engineers have faced. Inadequate door seals that allow liquid intrusion and ultimately cause corrosion are another common example. The list is endless, but the solution is strict controls during construction.
Harvesting All the Inherent Reliability
As mentioned earlier, the inherent reliability is the maximum possible reliability performance, but it is possible to perform much worse. The portion of the inherent reliability that is actually harvested or achieved is a result of:
•How well the system is operated
•How well it is maintained
•How well it is inspected
An automobile is a good example of a device that has a usable life that is determined by how it is operated. For example, some vehicles last several hundred thousand miles for an original owner. Yet, the exact same models frequently last only tens of thousands of miles when they are traded from hand to hand. If the owner drives the vehicle conservatively, sees that it is regularly maintained, and is sensitive to unusual noises or behaviors; it is possible to achieve a long and reliable life. If the owner accelerates too quickly, rides the brakes, and is insensitive to minor problems until they turn into major problems; the car is likely to be less reliable and to have a shorter life.
Although failures that are caused by poor operation are typically charged to the equipment rather than to the operator, a significant portion of the reduced reliability is not the fault of the equipment. For instance, if the MTBF (Mean Time Between Failure) for a device is two years and every other failure is caused by mis-operation, then the equipment MTBF should be four years. If the MTBF of a device is two years and every second failure is due to mis-operation and every third failure is due to a power failure or an upstream instrument failure, the MTBF of the device should be six years. If you are blaming the device and, as a result, you are focusing your attention on the device only, you will never achieve the desired improvement.
In order to achieve the desired improvement and to harvest the full inherent reliability, it is important to clearly recognize the source of failures.
In addition to mis-operation, it is possible to cause failures or allow failures to occur because of inadequate maintenance or inspection. Let’s look at a few simple examples.
The “Path to Failure” is a series of causes and effects that ultimately lead to a failure. At the very beginning of the path is a Systemic Cause that creates a trap for some unsuspecting individual. The next step is a Human Cause leading to a Physical Cause and finally setting up a Failure Mechanism and, ultimately, a defect that will result in a failure. (The following diagram shows a cause-effect flow in which each effect sequentially becomes the cause of the following effect.)
A Failure Mechanism is a form of deterioration that ultimately produces a defect. For instance, for any mechanical device, the only possible failure mechanisms are corrosion, erosion, fatigue, or overload. Let’s take corrosion as an example. If a corrosion circuit exists (cathode – anode – electrode), there will be visible signs. First, it should be possible to see two dissimilar metals being joined by a liquid electrolyte, or the products of corrosion (rust) should be evident. If operators, craftspersons, and inspectors are keeping their eyes open, they should be able to recognize this failure mechanism at work. If this failure mechanism is allowed to go on working for a long enough period to result in a defect and a failure, it is not the fault of the device. It is the fault of the humans who operate, maintain, or inspect the device. In order to harvest all the inherent reliability, people need to:
•Know what they are looking for (e.g., understand failure mechanisms)
•Be placed by design and discipline in a position where deterioration or defects are evident (e.g., follow organized rounds in a disciplined manner)
•Keep their eyes open
Taken one step further, after a failure mechanism has been at work for a period of time, a defect will form. But the presence of a defect does not automatically result in a failure. Often nature “throws the dice” for some period of time after a defect has formed but before a failure occurs. By this I mean that several circumstances may need to be present to result in a failure. For example, corrosion may weaken a pipe, but the piping system may also have to experience unusual but not unexpected pressure increases before a failure will occur. This aspect of “forgiving nature” or a grace period between defect and failure provides another opportunity to prevent a failure. But, as with the case of active failure mechanisms, people need to play an active role in finding and removing defects.
Well-designed programs for operations, maintenance, and inspection are one of the keys to harvesting all the inherent reliability of a system. Poorly-designed programs allow systems to operate at some level less than possible based on the inherent reliability.
Maintaining or Improving Inherent Reliability during Modification and Renewal
There are two distinctly different paradigms surrounding the aging of systems and equipment. One paradigm is best described by this description of an aging system, “This plant is unreliable because it is getting old.” The other paradigm is the complete opposite, “We have been working with this unit for a long time, so we have worked out all the bugs and know how to stay ahead of the problems.” In the first case, aging is used as an excuse for poor reliability. The equipment is managing the personnel. In the second case, aging is used as a reason why reliability is good. The personnel are managing the equipment.
In addition to the short-term or day-to-day concerns affecting reliability, there are long-term concerns. For instance, most units go through some form of modernization, expansion, or renewal process during their life. These events are often used as opportunities to enhance reliability. Sometimes, however, the reliability after the event is worse than before.
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