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Common Pitfalls in Process Hazard Analysis

The room grew tense as the reliability engineer complained that the condenser being discussed was slated to be replaced during the next turnaround. The simulator model, the basis of the whole project, didn’t include this change. What had been a plant hazard analysis (PHA) quickly degenerated into a process review.

This is an excellent example of what can go wrong during such evaluations. So, let’s look at what you can do to make your PHA or hazard and operability (HAZOP) study go smoothly.

First, carefully consider the basis for the process change — the reason for the review. What’s the purpose? What’re the options? Of course, the particular situation may limit the choices. Is there a way to avoid safety problems, simplify a process, or reduce hazards? For instance, Can you improve mixing in a reactor and make it smaller? Or Can you eliminate recirculation lines (reducing hazardous inventories)? Which option offers the lowest total cost? Is this the safest option? Consider summarizing the choices in a table. The design team should present these thoughts in a report with a buy-in from the project leader before scheduling a PHA or HAZOP.

With the solution in hand, next account for the plant’s preferences — e.g., for a particular type of flow meter or for RTDs instead of thermocouples. Don’t completely rely on company standards. After all, one unit at the site may prefer steam tracing while another may only accept electric tracing. If these choices pose new safety issues, they need to be considered before the safety review.

Another often missed hazard is scheduling. Can your process change be implemented all at once or will several plant outages be required? Can the plant operate safely with the interim changes? How will other changes in parallel projects affect yours?

Then, of course, there’s operator training — you’ll want to avoid gaps in training. You also should assess how a change will affect operator workload.

Now, you’re ready for the process and instrumentation drawings (P&IDs). First confirm the battery limits and make sure the P&ID you start with is the latest revision. Don’t crowd your P&IDs; leave plenty of room for notes and equipment. When more detail is needed, notes can refer to other documents. Make sure old bubbles are removed. Use color, if possible, to highlight the changes. Don’t be afraid to add a second or third sheet to your drawing set.

Finally, prepare your presentation. Utilize all team members effectively. Don’t assume everyone is familiar with your process or project. Here’s what each attendee will need: 1) P&IDs; 2) process flow diagrams; 3) a material balance; 4) mechanical information; 5) the alarm/trip schedule; 6) the process description; and 7) the purpose of the review and the process changes.

Have reference material on-hand for consideration by your team. For instance, provide a drawing of the entire facility with the area affected by the hazard review highlighted. If equipment will change, create a table comparing the old and the new items. Make available summaries of past PHAs and HAZOPs of the area and accidents with excerpts of concerns listed for discussion during the review if necessary. Time-line via tables the maintenance history of relevant equipment.

Now you’re ready to do your review — but guard against common mistakes.

Hazard reviews often focus too much on operations without considering maintenance. When a new threat appears on the horizon, operation engineers prefer to slap in another instrument rather than use the ones they have in a new way. This adds to the maintenance workload.

Another problem is what I call “the boogie man effect.” Hazard reviews tend to produce unrealistic solutions and imaginary fears. In one case, a caustic scrubber was proposed for a remote unloading facility. Maintaining the scrubber at the proper concentration proved difficult. It sat idle, unfilled, and became a haven for spiders.

Another pitfall is equipment pressure limits. Use the ASME-defined temperatures not the limits for alarms and trips. The code fixes maximum allowable working pressure (MAWP); trips prescribe maximum allowable operating pressure (MAOP). Trip settings change; avoid ones above 80% of relief settings because that’s the point when valves start to open.

Once the meeting gets started the facilitator needs to get everyone involved. One idea is to go around the table for the first hour asking for opinions from all. However, maintain discipline once the discussion is started. When a sidebar conversation goes too long, ask those involved to provide an opinion on the question at hand or for a contribution to the main discussion. Sometimes, sidebars produce enlightening ideas.

With a design approach that is methodical and with thoughtful planning in preparation for the PHA or HAZOP, this painful process can effectively serve its purpose.


This story origionally appeared on our sister site ChemicalProcessing.com

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