Failure Modes and Effects Analysis AF ailure Modes and Effects Analysis FMEA

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					 Failure Modes and Effects Analysis

A Failure Modes and Effects Analysis (FMEA)




                                                                               leaks thru
tabulates failure modes of equipment and their effects




                                                            rupture
on a system or plant. The failure mode describes how




                                                                      sticks
equipment fails (open, closed, on, off, leaks, etc.).
The effect of the failure mode is determined by the
system’s response to the equipment failure. An
FMEA identifies single failure modes that either         open          closed
directly result in or contribute significantly to an
accident. Human operator error are usually not
examined directly in an FMEA; however, the effects               FC
of a misoperation as a result of human error are
usually indicated by an equipment failure mode. An FMEA is not efficient for
identifying an exhaustive list of combinations of equipment failures that lead to
accidents.
                                    Purpose
The purpose of an FMEA is to identify single equipment and system failure
modes and each failure mode’s potential effect(s) on the system or plant. This
analysis typically generates recommendations for increasing equipment reliability,
thus improving process safety.

                              Types of Results
An FMEA generates a qualitative, systematic reference list of equipment,
failure modes, and effects. A worst-case estimate of consequences resulting from
single failure is included. The FMEA may be easily updated for design changes or
system/plant modifications. FMEA results are usually documented in a column-
format table. Hazard analysts usually include suggestions for improving safety in
appropriate items in the table.
    Failure and Failure Mode
•   Failure: The termination of an item’s
    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
    functions.
     Classification of Failures
• Sudden versus gradual failures
• Hidden versus evident failures
• According to effects (critical, degraded or
  incipient)
• According to severity (catastrophic, critical,
  marginal or negligible)
• Primary failure, secondary failure and
  command fault
Classification of Failure Modes
1. Demanded change of       • Fail to open on
   state is not achieved.     command
                            • Fail to close on
                              command
2. Change of conditions     • Leakage through the
   or states.                 valve in closed
                              position
                            • Leakage to the
                              environment
            Examples of Equipment Failure Modes Used in an FMEA

Equipment Description                Example Failure Modes

Pump, normally 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 side, 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.
    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 time 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 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 Technique


   Scope        Perparation   Evaluation    Documentation

Simple/Small
   System        2 to 6 hr    1 to 3 days     1 to 3 days

Complex/Large
   Process      1 to 3 days   1 to 3 days    2 to 4 weeks
Analysis Procedure

(1)defining the study problem,

(2)performing the review, and

 (3)documenting the results.
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
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
     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 interfaces.
   • 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
     analyst 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 relationship 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 Worksheet

DATE:                                      PAGE:                of
PLANT:                                     SYSTEM:
REFERENCE:                                 ANALYST(S):
Item Identification Description   Failure Modes   Effects   Safeguards   Actions
FMEA-PC
        (Primatech, Inc, Columbus, Ohio)

HAZOOPtimizer
        (A. D. Little, Cambridge, Massachusetts)

SAFEPLAN
        (Du Pont, Westlake Village, California)
Standard word processing and spreadsheet software programs can also help
analysts document the results of FMEA studies.
                      Example
An FMEA study is performed to address safety hazards to plant
personnel in a DAP process. The DAP process schematic is
presented in Figure 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.
UNLOADING




                                                                                     UNLOADING
 STATIONS




                                                                                      STATIONS
            ~




                                                                                 ~
                               AMMONIA                 PHOSPHORIC
                               SOLUTION               ACID STORAGE
                             STORAGE TANK                 TANK              L1
                L1

                                      F1                F1




                                                                     ENCLOSED            OUTDOORS
                                                                     WORK AREA




                                 ~~~~~~~~~~~~~~~~




                                                                                 STATIONS
                                                                                 LOADING
                                       DAP STORAGE TANK



    Figure 6.7 DAP process schemativ for the FMEA example.
                Diammonium phosphate (DAP)
PHOS. ACID excess    off-spec. Product


NH3        excess    residual NH3 release


BOTH        excess   T    P
  Table 6.21 Sample Pages from the FMEA Table for the DAP Process Example
DATE:     1/21/91                       PAGE:      5     of     20
PLANT:    DAP Plant                    SYSTEM:        Figure 6.7
REFERENCE: Reaction System             ANALYST(S): Mr. Ray Johnson
Item Identification      Description     Failure           Effects            Safeguards        Actions
                                         Modes
                                                                                             Consider
4.1   Valve B on the   Motor-operated, Fails open Excess flow of           Flow indicator    alarm/shutdown
                                                                                             of the system for
      phosphoric       Normally open,             phosphoric acid to the   in the            high phosphoric
      acid solution    Phosphoric acid            reactor                  phosphoric acid   acid flow
      line             service                                             line
                                                  High pressure and high                     Consider
                                                                                             alarm/shutdown
                                                  temperature in the       Reactor relief    of the system for
                                                  reactor if the ammonia   valve vented to   high
                                                  feed rate is also high   the atmosphere    pressureand
                                                                                             high
                                                                                             temperature in
                                                   May cause a high level Operator           the reactor
                                                   in the reactor or the  observation of
                                                   DAP storage tank       the DAP storage Consider
                                                                          tank            Alarm/shutdown
                                                   Off-specification                      of the System
                                                                                          for high level in
                                                   Production (i.e., high                 the DAP
                                                   Acid concentration)                    storage tank
  Table 6.21 (cont’d)

 DATE:   1/21/91                      PAGE:   6   of     20
 PLANT:  DAP Plant                    REFERENCE: Figure 6.7
 SYSTEM: Reaction System              ANALYST(s): Mr. Ray Johnson
                                                                        Failure
Item         Identification                Description                  Modes

4.2    Valve B on the phosphoric Motor-operated, normally open, Falis closed
       acid solution line        phosphoric acid service


4.3    Valve B on the phosphoric Motor-operated, normally open, Leak (external)
       acid solution line        phosphoric acid service


4.4    Valve B on the phosphoric Motor-operated, normally open, Rupture
       acid solution line        phosphoric acid service
 Table 6.21 (cont’d) (續)

          Effects              Safeguards                      Actions

No flow of phosphoric    Flow indicator in the   Consider
acid to the reactor      phosphoric acid line    alarm/shutdown of the system for
                                                 low phosphoric acid flow
Ammonia carry-over to    Ammonia detector and
the DAP storage tank     alarm                   Consider using a closed tank for
and release to the                               DAP storage and/or ensure
enclosed work area                               adequate ventilation of the enclosed
                                                 work area

Small release of         Periodic maintenance    Verify periodic maintenance and
phosphoric acid to the                           Inspection is adequate for this
enclosed work area       Valve designed for acid valve
                         service

Large release of         Periodic maintenance    Verify periodic maintenance and
phosphoric acid to the                           Inspection is adequate for this
enclosed work area       Valve designed for acid valve
                         service

				
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