We do not recommend a separate SIL assessment “session,” since it is already imbedded in the brainstorming during the PHA/HAZOP. In summary, we identify the need for SIF (if SIF is the right chose of protection layer for the scenario) during a PHA/HAZOP for each scenario disucssed (more than 90% of the scenarios do not need a SIF) and we also identify the SIL needed to reach tolerable risk if a SIF is chosen as an appropirate layer.

We will do a LOPA for any scenario where the PHA/HAZOP team is confused. LOPA is not required to determine if a SIF is needed or not; and is not required for a well-led PHA/HAZOP to determine if a SIL 1 or SIL 2 are needed. (And there is really no such thing as a SIL 3; at least it is so difficult to control the human errors of bypassing a SIL 3 that we never recommend them for continuous processes.) When the PHA/HAZOP team is confused by the complexity of a given scenario, we make a recommendation for further analysis by LOPA (or perhaps something else if LOPA is not going to answer whatever is confusing the team); we therefore do LOPA on about 3% of the scenarios; many of these are not related to SIF, but many are.

It is good to note that ALL PHA/HAZOPs are SIL determinations as well… even when you decide a SIL is not required or not the best solution.

Note that during a PHA/HAZOP, when we identify a SIF is needed and we state the SIL required, we are NOT going into detail; we are definitely not developing the SRS in the PHA/HAZOP… just stating the SIF needed and stating in general what it needs to do… maybe two sentences. Also, don’t let anything disrupt the flow of brainstorming… LOPA can disrupt the PHA/HAZOP brainstorming, if coupled. Finally, you don’t usually need a “LOPA session” since a team is not required, though sometimes it takes more than one head to complete a LOPA for some scenarios. Instead, we do need to save them up and do the LOPAs later; sometimes the LOPA analyst is the same person leading the PHA/HAZOP and so they (me, in some cases) do the LOPA during afternoon after the recommendation is raised to do the LOPA; only takes about 30 minutes per scenario on average, counting time to type up the summary.

To put these comments into perspective, note that I’m one of the three co-inventors of LOPA (1995-1996) and I co-authored the LOPA book (2001; AIChE/CCPS); and note that me and my staff have now led more than 10,000 unit-size PHA/HAZOPs and have performed many thousands of LOPA as well. We have, of course, identified 100,000+ SIF/SIL along the way (about 10 per unit reviewed, but this varies greatly) and we have afterwards done a great many SIL Verification for the final SIF designs. To find out more, visit:


or e-mail me: wbridges@piii.com