Brief overview of the fault tree analysis method
The FTA method can diagnose the reliability and security of processes, mainly those that can be a potential source of hazardous
and catastrophic events (8–10). Analysis starts from a top unwanted negative event and then developing all causal intermediate
events in a descending way until reaching the final independent events of origin. From this analysis, a representative logical
information flow diagram is obtained, in which special symbols depicting logical operators are used (11, 12).
FTA is useful for qualitative analysis because it reveals the various combinations of failures and cause-and-effect relationships
that provoke the unwanted top event, as for quantitative analysis in case of calculating the top event occurrence probability
from those to which the basic independent event can arise.
Brief overview of the failure mode and effect analysis method
FMEA can be used to discover and analyze all potential failure modes conceivable for a process or system and the effects they
can cause as well as to determine how to correct or mitigate those (13–15). Correction is based on the evaluation of the degree
of severity and criticality as well as their occurrence and cause detection levels. From there, the actions to be taken are
determined.
A list in tabular form is then developed (16). For the WPP process, this list was organized in the same consecutive order
of process stages, using the columns as follows (numbers correspond to columns matching the FMEA form):
1. Process equipment and components
2. Equipment and components function or operation
3. Related failure modes including those of nonimmediate detection
4. Effects of each failure mode
Table I: Severity index assignment criteria.
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5. Severity index of each effect (S). For the WPP case, an assignment table was elaborated, which takes into account the impact on the WPP process and also on
subsequent processes (see Table I).
6. Effect criticality, scoring a symbol if S ≥ 8
7. Potential causes ascribable to each failure mode
8. Cause occurrence probability index (O) through an assignment table found elsewhere (16)
9. Current existing controls
10. Control nondetection index (D) through an assignment table found elsewhere (16)
11. Risk priority number index calculation (RPN = S × O × D) for each cause, using maximum S value in case of several effects in a particular failure mode. RPN is used as a ranking of potential failure causes for assigning
priority
12. Recommended corrective actions for avoiding failure causes.
In corrective actions, changes for improving process and its controls were taken into account. Such actions were defined for
cases in which RPN > 100, as prioritized and for some particular situations in which RPN ≤ 100 also were considered critical.
Process improvement evaluation
Table II: Sampling plan for water pretreatment and purification system performance qualification.
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Corrective actions executed from FMEA final results were considered as major changes to the WPP process. Therefore, according
to house quality assurance (QA) policy, process revalidation was carried out according to a protocol specially elaborated
for this purpose (17). After installation and operational qualifications, performance qualification was conducted for verifying
change effectiveness during 15 days using offline measurements of those key variables better characterizing process functioning
and based on a sampling plan summarized in Table II.
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