285 / 1143
2.3
Consequence
In order to estimate consequence, the extent of expected overpressure should be evaluated.
In some cases this is straightforward, while in others detailed process modelling is needed.
In order to avoid unnecessarily conservative assumptions being made, there can be
significant benefit to be had from modelling very large overpressure scenarios. When
modelled, the extent of overpressure due to the release of a
finite
gas-cap from a riser (as
opposed to the fully packed riser scenario) may be significantly less than initially estimated.
The net result is that the actual risk may be lower than originally thought, and lower SIL
levels may be assigned.
Significant overpressures can be expected to result in potentially catastrophic ruptures of
process equipment. The ignition probability selected for the loss of containment of
hydrocarbon will be an important factor driving which SIL level is assigned. Multiple data
sources exist for ignition probability, including references [4] and [5]. A challenge in the case
of significant overpressure events is the selection an appropriate ignition probability. The
temptation to assume that all such high energy events have a 100% probability of ignition
will lead to over-conservative results.
Even where relief system capacity is exceeded, it can be beneficial to consider the reduction
in consequence when the relief valves do operate. Then two consequences, with different
frequencies based on the relief system reliability, can be considered. The net result is a
lower SIL because the overall risk is less than assuming that the relief system does not
operate.
Significant overpressure events will often need to be modelled in order to realistically
evaluate the likely extent of overpressure. This modelling should be prepared in advance of
SIL assessment to ensure a realistic, not over-conservative, levels of risk are estimated.
3
How to address very short Process Safety Time
So back to our problem – the Process department have returned a PST calculation which
the Instrumentation department advise cannot be achieved by the SIF design selected.
Solutions can be considered to speed up to the shutdown valve closure and hence the SIS
response time, or indeed the SIS response time might be set with less margin to the PST
than 50%. The temptation may also be to go back to the SIL assessment to see if this
inconveniently quick developing hazard can be considered to be not credible.
If the above solutions don’t resolve the issue, the process can grind to a halt. The
development of SRS and SIF design needs to be a collaborative process between
disciplines, where the Functional Safety Manager may need to play an active role to ensure
a successful outcome.
Other, sometimes more difficult, solutions need to be contemplated:
1. Review whether the optimum SIF has been specified
2. Review/optimise the parameters used for the Process Safety time calculation