There is always a risk of faults and failures when installation technology is used for process automation, and even reliable and robust communication via fieldbus can be affected. To implement protective measures for fieldbus communication that are both effective and efficient, it is vital to pinpoint the actual level of availability and potential causes of failure.
So what is the experience with availability of fieldbus installations in practice? And which potential causes are actually responsible for failures? Without knowing the answers to these questions, it is hardly possible to increase fieldbus availability long-term by adopting specific protection measures. However, availability calculations are based on descriptions, assumptions, and observations from the theory of probabilities. And here at Pepperl+Fuchs, we have discovered from many years of interaction with users that these calculations are derived from sometimes unrealistic, sometimes even plain incorrect, assumptions. As such, the results are often not a true reflection of reality either.
Discrepancy between theory and practice
It is often, and wrongfully, the case that only the probability of component failure (i.e., the inverse of the actual failure rate) is used to calculate availability. The probability of component failure represents the potential for random failure due to ageing and wear of the component. However, this means that important systematic criteria are not being taken into account. In practice, these criteria play a decisive role in availability: When environmental influences and the mode of operation, and their effect are not accounted for in the calculation, a significant discrepancy arises between the mathematical theory and the effect in practice when it comes to process automation. However, a brief glance at alarm and failure statistics makes it very clear that it is precisely the effects of the mode of operation and environmental conditions that are responsible for faults much more frequently than random faults that cause component failure.
To increase availability long-term, it is therefore crucial to identify the typical causes of faults. As part of a long-term project fault conditions and their causes and effects on the fieldbus infrastructure were studied. The results of the project indicate that systematic failures are the most likely causes of failure in plant operation. In addition to the poor design and planning of segments or failures in the installation, typical causes of failure include:
* Shorts circuits, contact bounce, and overloading that occur as a result of work on the segment.
* Surges caused by lightning strikes and a reduction in the performance of lightning protection that has previously gone unnoticed.
* Water penetration in field devices and junction boxes as a result of environmental influences or mode of operation, i.e., through heavy rainfall, condensation due to high humidity levels, use of high-pressure cleaners, and incorrect or faulty seals.
If the assessment of fieldbus technology it reduced only to competition among manufacturers to provide lower failure rates, the assessment is not accounting for maximum availability and is thus puts the user at a disadvantage.
Heuristic assessment of availability
As the purely mathematical calculation of the actual availability of a plant is not a realistic representation, the actual failure rates can be evaluated in another way. The actual or possible fault conditions of plant operation must be recorded for each assessment. A table in which the causes, their probability of occurrence, and impact are recorded has proven a very practical means of assessment. The product of the probability of occurrence and the impact represents the risk indicator. This analytical approach similar to an FMEA analysis (Failure Mode and Effects Analysis) makes it possible to take all potential causes of failure into account.
In such a table, the failure scenarios are assigned to possible protection methods as a solution. Taking an heuristic approach, each solution – regardless of whether it is a particular feature, component, a redundant design, or an operating procedure to avoid failures – is given a key performance indicator calculated using the associated risk indicators. A high indicator means that the measure prevents several very critical failures. This approach makes it clear which action protects against which potential causes of failure. The extent of the failure risk can also be compared with the cost and complexity of the solution. The efficacy and efficiency of the protective actions are therefore guaranteed to the same degree.
Purely mathematical calculations are not suitable for representing the complex array of potential causes of failure in practice. Consequently, the measures derived are also not tailored to the actual failure itself. In practice, it is often the more expensive and, in fact, irrelevant measures and redundancy concepts that are implemented, which ultimately do little to improve availability.
The right solution for every failure
The correct method for determining availability and actual causes of failure is of little help, of course, without the technical solutions that can effectively protect against the fault scenarios identified. Based on the in-depth studies that it has conducted regarding fault conditions in the fieldbus infrastructure, Pepperl+Fuchs has developed a new generation of components, specifically tailored to typical failure scenarios.
The new intelligent components bring diagnostic capability into the field and thus allow continuous error detection. A wide range of new diagnostic functions and fault-handling features limit faults in such a way that the process automation system remains in operation thus increasing overall plant availability. As a result, all information can be transferred via normal fieldbus communication, without additional costs being incurred.
The main feature of the new technology is the well-known Advanced Diagnostic module. This module monitors the physical layer, detects notifications from the downstream components capable of diagnostics, and forwards the notifications to the control system.
The Segment Protector, which is capable of performing diagnostics, offers progressive short circuit protection with excellent fault isolation. The intelligent diagnostic function detects and isolates faults such as contact bounce at spurs.
The Advanced Diagnostic Gateway acts as an interface between the Advanced Diagnostic modules and the control system, responding quickly and reliably to each error. A new I/O function also allows the control system cabinet itself to be monitored.
The diagnostics surge protection offers lightning protection with a self-monitoring function. It protects the fieldbus and field devices from lightning strikes or voltage spikes and emits an alarm signal when its function reserve is exhausted.
The intelligent leakage sensor can be connected to field devices as well as in marshalling cabinets, providing fast and reliable detection of the slightest change in moisture.
All new components are suitable for Foundation Fieldbus H1 and Profibus PA protocols. The new technology is incredibly easy to implement. The components are simply connected and then run without any further configuration – without fieldbus addresses or integration within the control system. The diagnostic functions can be put into operation quickly, easily and economically.For more information contact Mark Bracco, Pepperl+Fuchs, +27 (0)87 985 0797, [email protected], www.pepperl-fuchs.co.za
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