The following is a quotation from a process safety expert.
I recall once working with a client on a small plant expansion. In the morning we reviewed the P&IDs. In the afternoon we conducted the HAZOP. Same drawings, same people, same room — but the meetings, both of which went well, were totally different.
Both P&ID reviews and HAZOPs (Hazard and Operability Studies) are time consuming and require the attention of the facility’s most experienced and knowledgeable people. In order to minimize this demand on such valuable resources management can be tempted to merge the two activities. After all, they appear to be quite similar — at least superficially.
But the two activities have completely different purposes and are conducted in very different ways. The purpose of a P&ID review is to make sure that the drawings of the piping, equipment and instrumentation are correct. In the early stages of a project the review team will make sure that the new or changed facility is operable. The team will check for items such as pump start-up piping, the presence of sufficient instrumentation and adequate valving. One way of carrying out the check is to work meticulously through the start-up and shutdown of the process, step by step, and to make sure that every action can actually be carried out. There need to be sufficient valves, instruments, vents, drains and bypass lines to ensure that each step can be carried out safely and efficiently.
Immediately prior to start-up, a P&ID review becomes effectively part of the operational readiness or Prestartup Safety process — the team checks to make sure that what is installed is what was called for in the design and that there have been no last-minute changes.
The purpose of a Hazard and Operability Study (HAZOP) is completely different; its purpose is to identify process-related hazards using phrases such as ‘Reverse Flow’ or ‘Wrong Material’ and to evaluate the likelihood, consequence and risk associated with those hazards. The HAZOP team works on the assumption that the P&IDs are correct. If they do find significant problems to do with P&ID accuracy then the HAZOP should be stopped, and only resumed when the drawings have been corrected.
It is the responsibility of both the review leader and the HAZOP team leader to make sure that the two activities are kept separate.