Description of the Method
A FMEA is used to examine each potential failure mode of a process to determine the effects of the failure on the system. A failure mode is the symptom, condition, or fashion in which hardware fails. It may be identified as a loss of function, a premature function (function without demand), an out-of-tolerance condition, or a physical characteristic, such as a leak, observed during inspection. The effect of a failure mode is determined by the system's response to the failure.
A FMEA has three steps: (1) defining the process, (2) performing the analysis, and (3) documenting the results. Defining the process for study and documenting the results can be performed by a single person. The analysis itself must be performed by a team.
DEFINING THE PROCESS. This step identifies the specific vessels, equipment, and instrumentation to be included in the FMEA and the conditions under which they are analyzed. Defining the problem involves establishing an appropriate level of resolution for the study and defining the boundary conditions for the analysis.
The required level of resolution determines the extent of detail needed in a FMEA. The choices for the level of resolution range from the subcomponent level to the system level. To satisfy PSM Rule requirements, most FMEAs should be performed at the major component level. This level provides the best trade-off between the time necessary to perform the analysis and the usefulness of the information gained from it.
Defining the analysis boundary conditions requires the following.
1. Identifying the system or process to be analyzed.
2. Establishing the physical boundaries of the system or process.
3. Establishing the analytical boundaries of the system or process.
4. Documenting the internal and interface functions.
5. Documenting the expected performance of the system, process, or equipment item; the system or process restraints; and the failure definitions of the equipment items, the process, or the system.
6. Collecting up-to-date information identifying the process equipment and its functional relationship to the system.
Functional narratives about the system or process should include descriptions of the expected behavior of the system or process and the equipment components for each operational mode. Narratives should describe the operational profiles of the components and the functions and outputs of each.
To assist in the review, block diagrams should be constructed which illustrate the operation, interrelationships, and interdependencies of functional components for each equipment item. All interfaces should be indicated in these block diagrams.
PERFORMING THE ANALYSIS. The FMEA should be performed in a deliberate, systematic manner to reduce the possibility of omissions and to enhance completeness. All failure modes for one component should be addressed before proceeding to the next component. A tabular format is recommended for recording results. A FMEA worksheet is produced by beginning at a system boundary on a reference drawing and systematically evaluating the components in the order in which they appear in the process flow path. A worksheet such as that shown in Fig.1 should be completed for each equipment item, as follows.
Failure Mode. The PrHA team should list all of the equipment item and interface failure modes. Given the equipment's normal operating condition, the team should consider all conceivable malfunctions.
Cause(s). If desired, the root causes of the failure mode should be identified. Identification of root causes provides information helpful for ranking hazards.
Operational Mode. If the equipment being analyzed is subject to different modes of operation, each operational mode should be identified and analyzed separately.
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.
Failure Detection Method. The means of failure detection should be identified, such as visual or warning devices, automatic sensing devices, sensing instrumentation, or other indicators. The main purpose of identifying failure detection methods is to determine whether the failure mode is "hidden," i.e., not detectable for some period of time. If there is no means to detect failure, "none" should be entered into the worksheet.
Compensating Provisions. For each identified failure mode, the PrHA team should describe any design provisions, safety or relief devices, or operator actions that can reduce the likelihood of a specific failure or mitigate the consequences.
Severity Class. The severity of the worst consequence should be specified as follows.
Category I - Catastrophic - May cause death or loss of system or process.
Category II - Critical - May cause severe injury, major property damage, or major system damage.
Category III - Marginal - May cause minor injury, minor property damage, or minor system damage.
Category IV - Minor - Is not serious enough to cause injury, property damage, or system damage, but may result in unscheduled maintenance or repair.
Remarks/Actions. For each identified failure mode, the PrHA team should suggest actions for reducing its likelihood or mitigating its effects. The actions suggested for a particular piece of equipment may focus on the causes or effects of specific failure modes or may apply to all of the failure modes collectively.
If the team discovers that a single item failure is not detectable, the FMEA should be extended to determine if the effects of a second failure in combination with the first could have 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. Fig.1 shows a typical worksheet used in performing a FMEA.
Fig.1 Example FMEA Worksheet
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. Fig.2 presents estimates of the time needed to perform a PrHA using the FMEA method (CCPS, 1992).
Fig.2 Time Estimates for Using the Failure Mode and Effects Analysis Method
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.
Example Failure Mode and Effects Analyses
Fig.3 Partial FMEA for Dock 8 HF Supply System
Fig.4 Partial FMEA for the Cooling Water Chlorination System