EQUIPMENT FAILURE MODES

A reliability engineer s first design priority is successful operation. Great effort must be made to ensure that things work. This priority is certainly logical for most systems as failure mode is not relevant. 

In safety instrumented systems, however, the failure mode is very important. It makes a difference if the system fails and causes a false trip versus a failure that prevents the automatic protection. 

Actual failures of instruments can be classified as fail-safe, fail-danger, or another failure mode. Such failure modes will be defined in this chapter in the context of an individual instrument. Note that sometimes the application must be understood before these classifications can be made. It must be remembered that the safety instrumented function may or may not fail when one instrument has failed. A redundant architecture may compensate for instrument failures. 

Instrumentation equipment can fail in different ways. We call these failure modes. Consider a two-wire pressure transmitter. This instrument is designed to provide a 4 - 20 milliamp signal in proportion to the pressure input. Detailed failure modes, effects, and diagnostic analyses of several of these devices reveal a number of failure modes frozen output, current to upper limit, current to lower limit, diagnostic failure, communications failure, and drifting/erratic output among perhaps others. These instrument failures can be classified into failure mode categories when the application is known. 

If a single transmitter (no redundancy) were connected to a safety PLC programmed to trip when the current goes up (high trip), then the instrument failure modes could be classified as shown in Table 6-1. 

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Eault tree analysis (ETA) is a widely used computer-aided tool for plant and process safety analysis (69). One of the primary strengths of the method is the systematic, logical development of the many contributing factors that might result ia an accident. This type of analysis requires that the analyst have a complete understanding of the system and plant operations and the various equipment failure modes. 

Frequency Phase 1 Perform Qualitative Study, Typically Using HAZOP, FMEA, or What-if Analysis. To perform a qualitative study you should first (1) define the consequences of interest, (2) identify the initiating events and accident scenarios that could lead to the consequences of interest, and (3) identify the equipment failure modes and human errors that could contribute to the accident... 

An FMEA is a qualitative, systematic table of equipment, failure modes, and their effects. For each item of equipment, the failure modes and root causes for that failure are identified along with a worst-case estimate of the consequences, the method of detecting the failure and mi "ation ofits effects. Tables 3.3.5-2 and 3.3.5-3 present partial examples ofFMEAs addressing the Cuoling Tower Chlorination System, and the Dock 8 HF Supply System. 

Human operator errors are not usually examined in a FMEA, but the effects of human error are indicated by the equipment failure mode. FMEAs rarely investigate damage or injury that could arise if the system or process operated successfully. Because FMEAs focus on single event failures, they are not efficient for identifying an exhaustive list of combinations of equipment failures iliat iead to accidents. 

A uniform definition of a failure and a method of classifying failures is essential if data from different sources are to be compared. The anatomy of a failure includes the initiating or root cause of a failure that is propagated by contributory causes and results in a failure mode—the effect by which a failure occurs or is observed. Modes include failure to operate, no output, failure to alarm on demand. The end result of a failure sequence is the failure effect, such as no fluid is pumped to the absorber, or a tank overflows. As discussed in Appendix A of IEEE Std. 500-1984, only the equipment failure mode is relevant for data that are needed in a CPQRA. The failure model used in this book is based upon those in the IEEE publication and IPRDS. ... 

Figure 2.1 Active equipment failure modes. Reprinted from ANSI IEEE Std. 500-1984, with permission of the IEEE Standards Department.

It should be noted tliat FMECA identifies single failure modes tliat eitlier directly result in or contribute significantly to important accidents. Human/operator errors are generally not examined in a FMECA however, tlie effects of a misoperation are usually described by an equipment failure mode. It should also be noted that FMECA is not efficient for identifying combinations of equipment failures tliat lead to accidents. 

Effects. For each identified failure mode, the PrHA team should describe the anticipated effects of the failure on the overall system or process. The key to performing a consistent FMEA is to assure that all equipment failures are analyzed using a common basis. Typically, analysts evaluate effects on a worst-case basis, assuming that existing safety levels do not work. However, more optimistic assumptions may be satisfactory as long as all equipment failure modes are analyzed on the same basis. 

The statistical data used should cover aU the relevant causes of initiating events and all the relevant equipment failure modes. 

Graceful degradation An equipment failure mode in which the system suffers reduced capability, but does not fail altogether. 

Failure Mode and Effects (and Criticality) Analysis (FMEA/FMECA) are structured methodologies for the identification and analysis of the effects of latent equipment failure modes on system performance. This is a bottom-up process starting with the failure of a constituent/subsystem and investigating the effect of this on the system. It should be conducted by a team of experts with cross-functional knowledge of the analysed system, process or product. The methodology consists of the following steps ... 

Structural element, equipment (failure mode) Local inelastic energy absorption factor D,l (design/re-evaluation)... 

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