For credible deviation/consequence combinations, the HAZOP-Team considers what safeguards/mitigating features exist and whether these sufficient or not, depending upon the severity of the expected outcomes. Valid safeguards have to be independent from the systems/devices which are causing the deviation under analysis.
Typical safeguards are:
- Independent systems (e.g. SSV, ESD)
- PSV’s / TSV’s
- Alarms in manned Control Room (only when operator / dispatcher is granted with a sufficient intervention time and there is a clear procedure for action)
Devices generally NOT considered as valid Safeguards:
- Local Alarms
- Alarms in Control Room when the time gap between Alarm activation and Consequence is not sufficient for operator / dispatcher effective intervention
- Indication without Alarm (even if in Control Room); Check valves are not considered as a means for eliminating backflow, but credit may be given for reduction of backflow
Typical examples of Safety Instrumented Safeguards are:
- High High Level Trips
- High High Pressure automatic shutdowns
- Low Low Level shutdowns
Non-Instrumented Safeguards are functions which do NOT initiate an automatic trip or shutdown action by themselves, but that can either prevent or help control or mitigate the issue and may be considered as an Independent Layer of Protection (ILP). Typical examples of Non-Instrumented Safeguards are:
- High Level Alarms
- Mechanical Overfilling
- High Pressure vents (PSVs)
- Procedures (clearly defined, tested and auditable)
- Check valves are not considered as a means for eliminating backflow, but credit may be given for reduction of backflow