Limitations of Failure Mode and Effects Analysis

DOE-HDBK-1100-2004

catastrophic consequences. When a safety, redundant, or back-up component is evaluated, the

analysis should consider the conditions that generated the need for the component.

DOCUMENTING THE RESULTS.

A FMEA generates a qualitative, systematic reference list of

equipment, failure modes, and effects. The results of a FMEA are usually listed in tabular

format, by equipment item. Table 4.20 shows a typical worksheet used in performing a FMEA.

For each equipment item, the failure modes for that item and, if desired, the root causes for that

failure mode are identified. For each failure mode, a worst-case estimate of the consequences is

identified. This worst-case estimate assumes the failure of all protection against both the failure

itself and the undesired consequences of the failure. The method by which the failure is

detected is specified along with any compensating provisions. Finally, any suggestions for

improving safety are listed in the table.

The PSM Rule requires that a FMEA be performed by a team, all of whose members participate

in the analysis. The most practical means of performing the FMEA is to prepare blank

worksheets on viewgraphs or on a large display screen. For each equipment item, the PrHA

team reaches a consensus on its failure modes and their causes, effects, detection methods,

compensating provisions, severity (if desired), and any remarks or action items.

Staff requirements for a FMEA vary with the size and complexity of equipment items being

analyzed. The time and cost of a FMEA is proportional to the size of the process and

number of components analyzed. On average, an hour is sufficient to analyze two to four

equipment items. For processes or systems in which similar equipment items perform similar

functions, the time requirements for completing a FMEA are reduced. Table 4.21 presents

estimates of the time needed to perform a PrHA using the FMEA method (CCPS, 1992).

4.5.3 Limitations of Failure Mode and Effects Analysis

Human operator errors are not usually examined in a FMEA, but the effects of human error are

indicated by an 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 that lead to accidents.

4.5.4 Example Failure Mode and Effects Analyses

Partial FMEAs for the example processes described in Section 4.0 are shown in Tables 4.22

and 4.23.

4.6

Fault Tree Analysis

4.6.1 Description of the Method

FTA is a systematic, deductive failure analysis that focuses on a particular accident or undesired

event called the "top event" and develops the underlying sequence of events leading to the top

event. A separate FTA must be performed for each top event.