Analysis of the reliability and safety of process facilities requires an overall vision and a good knowledge of the foundations of thermodynamics, fluid dynamics, energy balances, maintenance strategies, Boolean algebra and probability calculus.
For about the last 15 years, the indispensable starting point in safety analysis and consequent risk assessment has been International Standard IEC 61511. This contains a set of internationally recognized rules and procedures, that have been validated for studying, designing, installing, managing and maintaining the Safety Instrumented Systems (SIS) used in the process industry.
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Identification of potential accidents in a process facility is based on a historical analysis of accidents that have already occurred in such facilities, but also from globally recognized databases.
Alongside a historical analysis, predictive and probabilistic methods must be used to identify any additional risks. The most common predictive methods used to detect the risks associated with process anomalies are based on such risk analysis techniques as PHA (Process Hazard Analysis), What-If/Checklist, HAZOP (Hazard & Operability Study), FMEA (Failure Mode & Effect Analysis) and others. Some of these methods, such as What-If/Checklist, are based on information from the past and on the compliance of events with best practices, though they do have some limitations as they cannot be applied to new technologies or to spheres for which they were not designed.
Other methods, especially more recently devised ones such as HAZOP, FMECA and PHA, are based on complex analytical schemes, enabling in-depth prior assessment of the risks, though the technical and financial resources required can be substantial. All of these methods include a process-facility Gap Analysis covering normal operating conditions, associated risk assessment and any preventive and mitigating measures required.
This approach can be used to describe the technical and management scenario for the safety instrumented systems installed in the process facility and assign a safety integrity level to each safety instrumented function (SIF). Then the safety function analysis assesses how the SIFs perform with regard to probability of failure, requests for action and degree of tolerance to hardware failures. Therefore, the IEC 61508 standard and the corresponding certificates presented by the suppliers come into play to verify whether this function ensures adequate risk reduction for the process facility. With this type of gap analysis, we can identify appropriate safety functions or corrective/adaptive actions for the process facility in order to achieve compliance. The next phase (SIS validation) defines the procedures and tests to carry out on existing process facilities in order to verify the effectiveness of the technical solutions adopted. Lastly, the end user can have all the test methods implemented on his own instruments so as to detect any hazardous errors not detectable by instrumental diagnostics, with the support of a database with the instrument data and test results.
