A scenario recently came up in one of our HAZOP / LOPA studies that made me question how well our industry risk assesses utility systems. Our clients’ approaches to utility systems in Process Hazard Analyses (PHAs) are quite diverse. Some of the approaches include:
- Considering utility systems outside of the scope of a PHA and ignoring them altogether;
- Reviewing some utilities but not others;
- Performing a risk assessment on utility systems by way of a check-the-box effort; and,
- Performing a true risk assessment on all of their utility systems.
The scenario we reviewed was a tower system with a flow control valve on the inlet (fail closed), a bottoms level control valve (fail closed), three level control valves (LCV) on separate pump-around systems (all fail closed), and a pressure control valve on the overhead accumulator (fail open).
During the PHA, our team evaluated a liquid overfill with subsequent overpressure scenario. First, the team discussed failure of the bottoms control valve causing the valve to close. The team determined the other level control valves (on the pump-arounds) were more than adequately sized to handle this failure. Failure of all three of the valves would cause an overfilling scenario resulting in escalation. Next, we considered a loss of instrument air as all the LCVs were fail closed. However, the inlet valve was also fail closed, so this scenario did not lead to an overfilling event. The team agreed the tower would not overfill due to a single point failure and the HAZOP continued.
When the study got to the instrument air header, the team identified that the LCVs previously mentioned were all on a sub-header and could be isolated via a manual valve. In this case, if that manual valve were to be closed, all LCVs would close and the inlet FCV would remain open. This led the team to finally play out an overfill scenario resulting in a LOPA and subsequent gap, which required a PHA recommendation.
By applying this lesson to all utilities, Cognascents challenges you to assess your PHA approach to utility systems. Take a step back and ask yourself, “Are the utility P&IDs at my facility detailed enough to evaluate the case detailed in this write-up?” Like the varying levels of scrutiny given to utility systems in PHAs, there are different levels of quality across utility P&IDs.
What hazards are hidden in your utility systems? This case is not uncommon and is not exclusive to instrument air.
Are you wondering where to start? Start with your PSI and update your P&IDs. Once your P&IDs are current and accurate, your PHAs can do a deep dive into the risks presented by your utility systems.
