Failure Modes and Effects Analysis

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Failure Modes and Effects Analysis A Failure
Modes and Effects Analysis (FMEA) tabulates
failure modes of equipment and their effects on a
system or plant. The failure mode describes
how equipment fails (open, closed, on, off,
leaks, etc.). The effect of the failure mode is
determined by the systems response to the
equipment failure. An FMEA identifies single
failure modes that either directly result in or
contribute significantly to an accident. Human
operator error are usually not examined directly
in an FMEA however, the effects of a
misoperation as a result of human error are
leaks thru
rupture
sticks
open closed
FC
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.
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Purpose The purpose of an FMEA is
to identify single equipment and system
failure modes and each failure modes 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.
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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
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Resource Requirements Using the
FMEA approach requires the following data and
information sources a system or plant equipment
list or PID, knowledge of equipment function and
failure modes, and 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.
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Time Estimates for Using the FMEA Technique
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Analysis Procedure

• (1)defining the study problem,
• (2)performing the review, and
• (3)documenting the results.

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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.

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(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.

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Table 6.19 Typical Format for an FMEA Worksheet
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FMEA-PC (Primatech, Inc, Columbus,
Ohio) HAZOOPtimizer
(A. D. Little, Cambridge, Massachusetts) SAFEPLA
N (Du Pont, Westlake Village,
California) Standard word processing and
spreadsheet software programs can also help
analysts document the results of FMEA studies.
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Example Using the DAP reaction system
presented in the Checklist Analysis example
(Section 6.2), an FMEA study is performed to
address safety hazards to plant personnel. The
DAP process schematic is repeated here as Figure
6.7 readers should refer to Section 6.2 for the
description of the DAP process. 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.
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UNLOADING STATIONS
UNLOADING STATIONS


PHOSPHORIC ACID STORAGE TANK
AMMONIA SOLUTION STORAGE TANK
L1
L1
F1
F1
ENCLOSED WORK AREA
OUTDOORS

LOADING STATIONS
DAP STORAGE TANK
Figure 6.7 DAP process schemativ for the FMEA
example. Diammonium phosphate
(DAP)
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PHOS. ACID excess off-spec.
Product NH3 excess
residual NH3 release BOTH excess
T P
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Table 6.21 Sample Pages from the FMEA Table for
the DAP Process Example
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Table 6.21 (contd)
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Table 6.21 (contd) (?)