We don’t recommend using a risk matrix during PHAs/HAZOPs. matrices are good tools to teach risk concepts, but in a well led PHA/HAZOP they reduce quality more than they increase quality.
To see much more details, download and read this free paper:
We use a risk matrix and a risk tolerance criteria only when doing a LOPA (to address a confusing issue from a PHA/HAZOP) or when required to a QRA. One reason we might use LOPA is to confirm If a SIL-2 is needed (versus a SIL 1), though not always (and we never recommend a SIL 3… a different discussion). To read more on this side-topic, read: