Functional safety is used to reduce the risks associated with equipment under control. A hazard and risk analysis is carried out considering the information originating from the definition of the general scope (described in IEC 61508-1 clause 7.3).
Hazard and risk analysis phase
The aim of risk analysis is to identify what the hazards are, how often they might happen, and when they are tolerable, specifically for a system, process or component.
The recommended method is ALARP (As Low As Reasonably Practicable), where risk must be lowered as much as reasonably practicable. This concept is used to avoid spending limited resources to further reduce already relatively low or tolerable risks.
Risk analysis must be carried out at the first stages of the Functional Safety Lifecycle. If further decisions are made at later stages of the E/E/PE system or safety software lifecycle that may change the basis on which previous decisions were made, then a further hazard and risk analysis must be carried out.
Among the techniques of risk analysis the best-known ones are:
- HazOp (HAZard and OPerability analysis)
- LOPA (Layer Of Protection Analysis)
- PHA (Process Hazard Analysis)
Recommended in-depth study:
Risk analysis means that the manufacturer decides which technique is best to use in your specific case. Further, risk analysis is necessary to define the SIL to be achieved by each safety function.
The factors to be taken into account when assessing risk are:
- Probability for the initiating event to occur (such as the chain of events leading to the dangerous event, the possible triggers, the probabilities of the event tree)
- Individual or population subject to potential harm
- Estimating subject’s vulnerability
- Estimating avoidability of damage
The risk analysis process depends on the team carrying out the analysis. Different teams may lead to different results. For this reason, teams must include:
- People who know the methods to be applied;
- Process specialists;
- Maintenance staff;
The necessary factor is knowledge and experience regarding the subject being analysed.
Do you want to contribute to our page? Follow us on Linkedin
SIL allocation, as well as the risk analysis, are not mathematical processes, but depend on the team performing the analysis. Different teams may lead to different results. The necessary factor is knowledge and experience regarding the subject being analysed.
In the SIL assignment phase, the systems that can help raise the level of system protection are:
- Mitigation Systems: Measures to eliminate or minimise the impact of a negative event (e.g. physical barriers)
- Inpedent Alarms: Allow an operator to intervene;
- Continuous Control: Continuous monitoring by operators;
- Basic Process Control System (BPCS): Controls the process parameters and implements (non-safety) functions;
- IPL (Independent Protection Layer): Independent systems that independently implement safety actions (e.g. relief valve)
- Experienced and trained Operator: the presence of experienced and trained operators is considered an additional level of protection.
An example of SIL allocation method is Risk Graph.
As shown in the image above, it considers several factors:
- Ca: Minor injury
- Cb: Serious permanent injury to one or more persons; death of a person
- Cc: Death of more than one person
- Cd: Many people killed
- Fa: Rare or frequent exposure to the hazardous area
- Fb: Frequent permanent exposure to the hazardous area
- Pa: Possible under certain conditions
- Pb: Almost impossible
- W1: Very low Probability
- W2: Average probability
- W3: High probability