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1、 Failure Modes and Effects AnalysisA Failure Modes and Effects Analysis (FMEA) tabulates failure modes of equipment and their effectson a system or plant. The failure mode describes howequipment fails (open, closed, on, off, leaks, etc.).The effect of the failure mode is determined by thesystems res
2、ponse to the equipment failure. AnFMEA identifies single failure modes that either directly result in or contribute significantly to anaccident. Human operator error are usually notexamined directly in an FMEA; however, the effectsof a misoperation as a result of human error areusually indicated by
3、an equipment failure mode. An FMEA is not efficient foridentifying an exhaustive list of combinations of equipment failures that lead toaccidents.open closedsticksruptureleaks thruFC PurposeThe purpose of an FMEA is to identify single equipment and system failuremodes and each failure modes potentia
4、l effect(s) on the system or plant. Thisanalysis typically generates recommendations for increasing equipment reliability,thus improving process safety. Types of ResultsAn FMEA generates a qualitative, systematic reference list of equipment,failure modes, and effects. A worst-case estimate of conseq
5、uences resulting fromsingle failure is included. The FMEA may be easily updated for design changes orsystem/plant modifications. FMEA results are usually documented in a column-format table. Hazard analysts usually include suggestions for improving safety inappropriate items in the table.Failure and
6、 Failure ModeFailure: The termination of an items ability to perform a required function.Failure Mode: The effects by which a failure is observed on the failed item. All technical items are designed to fulfill one or more functions. A failure mode is thus defined as non-fulfillment of one of these f
7、unctions.Classification of FailuresSudden versus gradual failuresHidden versus evident failuresAccording to effects (critical, degraded or incipient)According to severity (catastrophic, critical, marginal or negligible)Primary failure, secondary failure and command faultClassification of Failure Mod
8、es1.Demanded change of state is not achieved.2.Change of conditions or states.Fail to open on commandFail to close on commandLeakage through the valve in closed positionLeakage to the environment Examples of Equipment Failure Modes Used in an FMEA Equipment Description Example Failure Modes Pump, no
9、rmally operating Fails on (fails to stop when required) Transfers off (stops when required to run) Seal leak/rupture Pump casing leak/rupture Heat exchanger, high pressure on Leak/rupture, tube side to shell side tube side Leak/rupture, shell side to external environment Tube side, plugged Shell sid
10、e, plugged Fouling Resource Requirements Using the FMEA approach requires the following data and information sources: (1) a system or plant equipment list or P&ID, (2) knowledge of equipment function and failure modes, and (3) knowledge of system or plant function and responses to equipment failures
11、. FMEAs can be performed by single analysts, but these analyses should be reviewed by others to help ensure completeness. Staff requirements will vary with the size and complexity of equipment functions and failure modes and how the failures might affect other portions of the system or plant. The ti
12、me and cost of an FMEA is proportional to the size of the process and number of components analyzed. On the average, an hour is sufficient for analyzing two to four equipment items. As with any HE study of systems with similar equipment performing similar functions, the time requirements are reduced
13、 significantly due to the repetitive nature of the evaluations. Table 4.8 lists estimates of the time needed to perform an HE study using the FMEA technique.Time Estimates for Using the FMEA TechniqueAnalysis Procedure(1)defining the study problem,(2)performing the review, and(3)documenting the resu
14、lts.STEP 1 :Defining the study problem. This step identifies the specific items to be included in the FMEA and the conditions under which they are analyzed. Defining the problem involves (1)establishing an appropriate level of resolution for the study and (2)defining the boundary conditions for the
15、analysis. A detailed problem definition is a necessary ingredient to performing a thorough and efficient FMEA.(2)Defining the analysis boundary conditions includes:Identifying the plant and/or systems that are the subject of the analysis.Establishing the physical system boundaries for the FMEA. This
16、 includes the interfaces with other processes and utility/support systems. One way to indicate the physical system boundaries is to mark them on a system drawing that encompasses all equipment within the scope of the FMEA. These boundary conditions should also state the operating conditions at the i
17、nterfaces.Establishing the system analytical boundaries, including: (1)the failure modes, operating consequences, causes, or existing safeguards that will not be considered and (2)the initial operating condition or position of equipment. As an example of effects beyond the scope of the study, an ana
18、lyst may choose not to consider airplane crashes, earthquakes, or tornadoes as causes of failure modes. An example of an initial condition is specifying whether a valve is normally open or closed.Collecting up-to-date reference information that identifies the equipment and its functional relationshi
19、p to the plant/system. This information is needed for all equipment included within the system boundary and appropriate interfaces with the rest of the plant.Table 6.19 Typical Format for an FMEA WorksheetFMEA-PC (Primatech, Inc, Columbus, Ohio) HAZOOPtimizer (A. D. Little, Cambridge, Massachusetts)
20、SAFEPLAN (Du Pont, Westlake Village, California)Standard word processing and spreadsheet software programs can also help analysts document the results of FMEA studies.An FMEA study is performed to address safety hazards to plant personnel in a DAP process. The DAP process schematic is presented in F
21、igure 6.7. Each component of the reaction system is evaluated with the relevant information recorded in an FMEA table. The section of the FMEA table for Control Valve B in the phosphoric acid solution line is presented in Table 6.21.ExampleL1F1L1F1PHOSPHORICACID STORAGETANKAMMONIASOLUTION STORAGE TA
22、NKUNLOADING STATIONSUNLOADING STATIONSLOADING STATIONSDAP STORAGE TANKENCLOSEDWORK AREAOUTDOORSFigure 6.7 DAP process schemativ for the FMEA example. Diammonium phosphate (DAP)PHOS. ACID excess off-spec. ProductNH3 excess residual NH3 releaseBOTH excess T PTable 6.21 Sample Pages from the FMEA Table for the DAP Process ExampleTable 6.21 (contd)Table 6.21 (contd) (續)
