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3. Redesign the process to make the hazard development slower
4. Redesign the process to eliminate the hazard or to incorporate additional
Independent Protection Layers (and bring the risk to a tolerable level)
5. Prevent the hazard (reduce the risk) by mechanical locking of valves
6. Design an Inhibit SIF
3.1
Review whether the optimum SIF has been specified
In some cases, the SIF designed is not the only possible solution, and from a required
response time viewpoint, is not the optimum. It might well be logical that a high pressure
scenario should be prevented by a PSHH (high pressure trip). But by their nature, pressure
increases are often fast, and therefore the process safety time will be short. In the case of
gas blowby hazards, where a vessel with a gas/liquid interface is allowed to drain so that
high pressure gas passes to systems rated for low pressure liquid, the best solution for the
SIF may well be a LSLL (low liquid level trip), rather than a high pressure trip downstream.
Some companies identify “candidate SIFs” at the time of SIL assessment, while others prefer
an approach where the SIF is not yet designed. It’s important that the responsible party for
the production of the SRS understands the hazard well, the issues of speed of hazard
development, and alternative designs. The purpose of a SIL assessment is not to design
solutions, but in particularly difficult cases such as these, some guidance should be noted, or
actions assigned to ensure that an appropriate solution is designed.
3.2
Review/optimise the parameters used for the Process Safety time calculation
This paper recommends that clear terms of reference are defined for Process Safety Time
calculations. Assigning a task to the Process department to calculate the PST without clear
parameters has been shown to yield inconsistent results.
Relatively complex dynamic process modelling needed for overpressure scenarios needs to
be set-up in terms of the process state and response from other equipment or systems.
IEC61511-1 defines the Process Safety Time as the time to the occurrence of the hazardous
event, but at what overpressure do we consider this to be? We could consider any of the
following criteria:
• Exceed design Pressure
• Prevent relief valve operation
• Exceed Design pressure +15%
• Exceed test pressure (not recommended)
Furthermore, the PST modelling should generally be based on worst case levels in vessels
or other process conditions that would make the hazard development faster (e.g. if a control
valve fail open is the cause, assume failed to 100% open). No “help” should be modelled
from process control functions or relief valves. The PST should represent the maximum time
in which the SIF needs to operate independently of other protection layers to prevent the
hazard.
So then, how can the PST modelling be optimised? A number of factors may be considered,
or conservative assumptions adjusted, including: