We do NOT use a risk matrix of any kind in the PHAs (HAZOPs) is we get our way. On 20% of the PHA/HAZOPs we do, we have to use a matrix and we manage such analysis as well as anyone can (we have led 10,000 PHAs/HAZOPs of entire units). But, yes in 1/2 of the times, the team will make the matrix location match their qualitative judgement. By the way, their qualitative judgment is usually more accurate (we feel) that using numbers or matrices. Note that ALL numbers that folks use in Matrices are coming from some other source and if you track those sources down, Most are either entirely expert opinion on the “average” value to use and many others are “adjusted by expert opinion”. I know this because I’m on such committees that come with the numbers/factors that others use.
If you get your team well calibrated (usually happens on the first day of meetings for a group of folks) and the team leader is good (not sure what % of time this is true; we have seen many poor leaders who were nonetheless leading PHA/HAZOP teams), then the teams qualitative judgment is better than having a team qualitative judge which numbers provided by others to use for their site. Also note that site-specific data is what we prefer and normally the only decent source of such data is embedded in the expert opinion.
Finally note that the main reason I was on the first LOPA method development was to allow us a good way and rationale for deleting the use of risk matrices from qualitative analysis. The reason Martin Gollin was involved was to help Arco Chemical have a good way to defend against arbitrary over-specification of SIF (and avoid spec of high SIL). The reason Art Dowell did a parallel development at Rohm & Haas (now Dow) was to have an approved methodology “after” (not within) HAZOP but before QRA (to which he could resolve 90+% of recommendations that called for further analysis).
The key to note is that everyone wants to do an accurate “assessment of risk”… but there is really no such thing since the data has error factors of an order of magnitude on either side of the average. But as technical types, we have trouble admitting we can’t really do something accurately. So, then another goal is to be consistent. Can LOPA (or any consistent factoring) help with this? Sure. But, you can also get your HAZOP teams just as consistent within a company WITHOUT LOPA.
Note that I am NOT pushing use of LOPA, yet I am one of the co-originators, have perhaps trained more LOPA folks than others have, and I help write the books. There is a reason I do not overly push it. But, I do think LOPA is better to use than QRA, in 95% of the cases that folks want to use QRA…. that is because the error factors do not support the use of QRA, in my opinion… but now i just said “your baby is ugly” to a lot of risk analysts (mainly consultants) around the world. One way to see my last point is to go back to basic rules for establishing significant digits…. once you look at the error factor for the data we use, you will see that the significant digit is the exponent; only. So there is no such thing as 2.3 x 10-3 in risk assessment. There is only 10-2 +/- 1.
Ask yourself why no one shows the error range in the data? The uncertainty? I wish they would because otherwise it is misleading. I wish we had in the first LOPA book (instead of only stating this issue in general).