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Description:
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Ammonia refrigeration systems are widely used in industry . Demand of these
systems is expected to increase due to the advantages of ammonia as refrigerant and
because ammonia is considered a green refrigerant . Therefore , it is important to evaluate
the risks in existing and future ammonia refrigeration systems to ensure their safety .
LOPA (Layer of Protection Analysis ) is one of the best ways to estimate the risk .
It provides quantified risk results with less effort and time than other methods . LOPA
analyses one cause -consequence scenario per time . It requires failure data and PFD
(Probability of Failure on Demand ) of the independent protection layers available to
prevent the scenario . Complete application of LOPA requires the estimation of the
severity of the consequences and the mitigated frequency of the initiating event for risk
calculations .
Especially in existing ammonia refrigeration systems , information to develop
LOPA is sometimes scarce and uncertain . In these cases , the analysis relies on expert
opinion to determine the values of the variables required for risk estimation . Fuzzy
Logic has demonstrated to be useful in this situation allowing the construction of expert
systems .
Based on fuzzy logic , the LOPA method was adapted to represent the knowledge
available in standards and good industry practices for ammonia refrigeration . Fuzzy
inference systems were developed for severity and risk calculation . Severity fuzzy inference system uses the number of life threatening injuries or deaths , number of
injuries and type of medical attention required to calculate the severity risk index .
Frequency of the mitigated scenario is calculated using generic data for the initiating
event frequency and PFD of the independent protection layers . Finally , the risk fuzzy
inference system uses the frequency and severity values obtained to determine the risk of
the scenario .
The methodology was applied to four scenarios . Risk indexes were calculated
and compared with the traditional approach and risk decisions were made .
In conclusion , the fuzzy logic LOPA method provides good approximations of
the risk for ammonia refrigeration systems . The technique can be useful for risk
assessment of existing ammonia refrigeration systems . |