One of the elements of the PSM standard that most people are familiar with is Management of Change (MOC). The purpose of the MOC process is to provide a systematic way to manage changes to the process unit that are not “replacements in kind”. This process is managed differently by most companies, and sometimes, managed differently across sites within the same company. One of the requirements of the OSHA 1910.119 PSM standard is that if the proposed change to the process results in a change to the applicable Process Safety Information (PSI), the Process Safety Information shall be updated. As a result, the P&IDs for the process unit get updated, safety data sheets (SDS) get updated and/or added, and operating procedures are updated to reflect the changes to the process. However, one piece of PSI that often gets overlooked during the MOC process is the relief system design basis.
A process unit’s relief system design basis is the backbone of the unit’s safety systems and serves as a last line of defense against overpressure. Changes to the process unit must be reflected in the relief system design basis to show that the relief systems are adequate and will safely prevent overpressure. Failure to capture changes to the process in the unit’s relief system design basis can leave gaps in the overall integrity of a plant’s protection layers. Thinking back to the “swiss cheese” model of process safety, when this gap aligns with holes in other protection layers, the right combination of initiating event and timing can slip through the holes and result in a process safety incident.
A common approach to evaluating a relief system design basis during the MOC process is to verify whether or not a particular overpressure scenario is covered in the existing relief valve sizing. Simply verifying whether the scenario was addressed is not enough. Significant changes to the process can affect the relief system design basis in major ways. Things to consider when evaluating a unit’s relief system design basis during the MOC process include but are not limited to:
- Composition- Is the composition of the feed to the unit changing as a result of the proposed change? More volatile compounds typically require more relieving area than less volatile compounds. Running a feed with a different composition into the unit without assessing whether the relief systems can handle the new feed could essentially leave you without an up-to-date design basis altogether.
- Operating Conditions– Are the process conditions changing as a result of the proposed change? Will the process be run at higher pressures? Higher operating pressures (especially at HP/LP interfaces) may result in new credible overpressure scenarios for downstream equipment or in higher required relief rates for already existing scenarios. Another point to consider is operating temperature. Higher operating temperatures or larger heat duties in exchangers could result in higher relief rates for vaporization overpressure scenarios or even create new ones.
- Pressure Sources– Is the proposed change introducing new sources of pressure that could cause overpressure? A new compressor where one did not exist before or a new pressure letdown station could pose a new means by which overpressure could occur.
- Fire Circle– Does the new layout or addition of new equipment introduce flammable materials to an area that was previously considered not to be in a fire zone? If so, then all of the relief valve calculations for the protected equipment in the area should be evaluated for inclusion of a fire case, not just the new equipment being added.
These are just some of the things to consider when evaluating the relief system design basis for a process unit during the MOC process. Even if a thorough analysis shows that the current relief system design is adequate and that no changes to the design are needed, it is better to have put in the work and know for sure than to take a chance that an existing relief valve will be adequate for any new or previously identified overpressure scenarios.