As the name implies, the purpose of Safe Operating Limits (SOL) is to define the limits beyond which a process will not intentionally be operated and at which troubleshooting ceases, replaced by pre-determined actions to bring the process to a safe state. Pretty important information. I am sure this is why exceedance of Safe Operating Limits is recommended as a key performance indicator (metric) for understanding and managing process safety performance in the refining and chemical industries. According to CCPS, API RP 754 and IOGP Report 456, SOL exceedances represent “Challenges to Safety Systems” and are considered a Tier 3 Leading Indicator. Tier 3 indicators can be considered “early warning signs” of potential process safety management performance problems. SOLs must be determined and documented for compliance with OSHA Process Safety Management (PSM) regulation 1910.119.


Despite their importance and broad applicability across process industries, a recent survey conducted by exida demonstrates that the determination and application of SOLs can vary significantly from company to company. For example, when asked what single piece of information was mostimportant to establishing safe operating limits, 51% of the respondents answered “SIS trip points and relief valve settings” or “Normal operating limits” instead of “Design constraints”, “Mechanical Integrity Limits”, or “Integrity Operating Windows”.  Results such as these raise thought provoking questions (e.g., how can one compare performance metrics from site to site if SOLs are calculated differently?) and highlight where industry can improve.

The survey of 150 functional safety practitioners from around the world, included the following additional topics:

  • What is the procedure for defining SOLs?
  • Where is SOL information stored?
  • Is process history reviewed to evaluate / validate safe operating limits?
  • What action occurs with SOL exceedance?
  • Do SOL exceedances count as a Process Safety Leading Indicator?
  • Are alarms defined to capture SOL exceedances?
  • Are operators trained / aware of the safe operating limits for the process units that they monitor and control?
  • Use of safe operating limit information when performing a PHA, a LOPA, or an alarm rationalization? 
  • Are SOL Exceedances stored in a database for performance evaluation?

The results of the survey were published and presented at the AICHE 15thGlobal Congress on Process Safety.

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Tagged as:     Todd Stauffer     Alarm Rationalization     Alarm Management  

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