Technical Blog
Bridging the Gap Between Cause & Effect-Based Trips and IPLs in PHAs
At Cognascents, we’re always curious about how process safety practices play out in the real world. In a recent Question of the Week (QOTW), we asked our network a timely question:
What percentage of cause & effect-based trips show up as safeguards or IPLs in your Process Hazard Analysis (PHA)?
Here’s how the response broke down:
These results suggest that for most respondents, only 25 – 50% of their cause & effect-based trips are formally documented as safeguards or independent protection layers (IPLs) within their PHA.
Why Does This Matter?
Cause & effect (C&E) matrices are foundational tools in defining interlocks and trips intended to prevent or mitigate hazardous events. If a significant portion of these trips are not credited or documented as safeguards in the PHA, it raises several questions:
- Are these trips fully validated and maintained?
- Do PHA teams have access to current C&E documentation during the review?
- Are these trips considered too complex or conditional to qualify as IPLs?
- Or are we simply missing opportunities to credit them where appropriate?
If the intent behind a trip is to protect equipment, personnel, or the environment—it should at least be evaluated for safeguard or IPL credit. Even if the interlock in question is intended to prevent or mitigate escalation, ignoring the interlock in PHA and subsequently in the safety critical list/PM Program can lead to those instruments not being functional if they are called upon.
Cognascents posits that if we are seeing this trend for interlocks, what else could our PHAs be missing in their analysis? Are there gaps between what’s captured in the Pressure Safety Valve (PSV) overpressure scenarios and what’s addressed in the PHA? Proper scenario identification is the foundation of any PHA. Without that, sites cannot make the correct strategic decision in risk maintenance, management, and reduction.
Now, Let’s Ask:
What percentage of cause & effect-based trips should show up as safeguards or IPLs in PHAs?
Here’s how the response broke down:
That means 100% of respondents believe at least half – or more – of these trips should be credited. This stark contrast highlights a persistent gap between perceived best practices and actual implementation.
Where the Disconnect Happens
The greatest challenge lies in how PHAs are structured. Specifically, the balance between identifying causes and consequences.
Some teams start with a list of potential causes that lead to certain deviations (e.g., “high pressure”). The challenge occurs when there is not one specific cause or final element that could lead to the deviation in question. Those teams focused solely on cause-based development do not always take a step back and consider if the deviation and associated consequence itself is credible, which can result in overlooking valid initiating events that stem from more complex operational conditions (e.g. abnormal feed conditions due to upstream process upset or seasonal changes). Even when the cause is properly documented, some PHA teams have challenges identifying alternate consequences that are still worth risk assessing when the consequences in question are not the primary consequence or stem from a partial failure in the cause. Think of a firing valve for a fired heater or boiler. If that valves fails fully closed and does not allow fuel gas to pass it is just an unintended trip of the equipment, but if it fails too far closed to maintain a stable flame while still remaining partly open and thus allowing fuel gas to pass, you have a much more dangerous flame out condition.
Other PHAs begin with consequences, which can lead to missing underlying causes entirely. In both cases, important scenarios—or associated safeguards/IPLs—can be unintentionally missed.
So, How Do We Resolve the Gap?
To close this gap, it’s beneficial to take a blended approach—developing scenarios by evaluating causes and consequences together, while also cross-checking against your PSV documentation, C&E matrix, and other important protection based PSI (e.g. Control narratives or Safety Requirement Specifications [SRS]). This ensures that both initiating events and protection measures are thoroughly considered, and that no critical safeguards are left out of the discussion.
When most HAZOP workflow look something like the figure below, Cognascents recognizes that the majority of the hazardous scenarios can be captured by following the normal process. However, if one additional step were added near the end of the workflow before going to the next node – guiding the PHA team to review the C&E matrix and PSV sizing documentation for instruments and relief devices covered in that node – the likelihood of missing pertinent protections and their intended functions would be greatly reduced.
Although the above potential change to the PHA workflow may minimize the modifications to the PHA process, in some instances it is not the most efficient way to achieve the desired results. Conducting this review with the entire PHA team can extend PHA session durations and place additional burdens on resources that are already being tasked with doing more in today’s facilities. Cognascents’ perspective is that if an individual or small group can do more work during preparation to save the time of five to six people during the workshop, then that is an approach worth considering. Particularly in PHA revalidations, where strong analysis and content already exists, shifting the cross-check steps into the preparation phase can provide the needed efficiencies to improve scenario identification while minimizing impacts on the larger PHA team and process.
Key considerations for the Process Safety Engineer or 3rd party facilitator when performing advanced preparation work to identify potentially missed scenarios in previous PHA efforts include:
- Integrate C&E & PSV scenario reviews into your PHA prep.
Review C&Es and PSV documentation prior to the PHA to identify and credit applicable trips and safeguards. As written, the above guidance is no different than what the vast majority of PHAs are already doing by collecting C&Es and PSV documentation prior to the PHA. The suggested change is to use these documents proactively to ensure that all valid scenarios are considered in the analysis, not just to confirm IPL will work. - Close the loop with your SIS or I&E teams.
Trips often exist but are not communicated clearly across disciplines. Clarify the intent of each trip and what scenario(s) the instrument is intended to prevent. Additional documentation, like Safety Requirement Specification (SRS) or control narratives may assist if an automation / instrumentation representative is unavailable. - Leverage alarm rationalization.
Trips may be associated with alarms that should be evaluated for their role in protection. Since each alarm within a unit is rationalized, regardless of its documentation within a PHA, good content on consequence of deviation, corrective actions, and possible causes can be found in this document. - Standardize IPL criteria.
Ensure your team has a consistent understanding of what qualifies as an IPL—including functionality, specificity, independence, and testability. This will improve the quality and consistency of the preparation work to ensure alignment during the PHA. - Document clearly.
Any modifications made during advanced preparation should be clearly flagged for increased scrutiny by the full PHA team, so it’s evident which analysis is drafted, and which has already been previously reviewed and approved. If a trip or PSV is purposefully not credited as a safeguard/IPL for a scenario identified in other PSI documents, the PHA team should document the rationale for traceability and future revalidation.
Final Thoughts
The gap between what instrumented trips are designed to protect in the field and what’s documented in the PHA can lead to either missed opportunities for risk reduction credits—or false assumptions about protection. Unfortunately, that’s the best case scenario, where increased review of the C&Es and PSV documentation reduce existing risk ranking by providing previously unidentified protection layers; however, the challenge goes beyond risk reduction measures. If initiating events and consequences that place a demand on the associated protections are not developed and risk assessed, it likely indicates that the PHA team is overlooking credible hazardous scenarios, leaving the facility exposed to potential incidents that could have otherwise been prevented.
Closing that gap requires more than just better safety critical lists and PM programs. It calls for structured collaboration across engineering, operations, instrumentation, and safety teams, with shared ownership of C&E diagrams, PSV evaluations, and IPL criteria. A safety critical list cannot be fully populated without the risk placing a demand on the critical protections being properly identified. Suitable PM programs cannot be created from those safety critical lists if all the demands and size of the cumulative risk gap is unknown.
When these elements align, PHAs deliver on their full potential: producing credible, actionable insights that strengthen decision-making and create safer facilities for the future.
Have a different perspective? We’d love to hear how your team approaches this topic.
Meet the Author
Jeffrey Miller, P.E. – Technical Manager – PHA & PSM
Jeffrey is an experienced chemical engineer specialized in the area of process hazard analysis, pressure relief analysis, alarm rationalization, operating procedures generation, process simulation, and MOC implementation. He has experience facilitating facility-wide PHAs, leading HAZOP and LOPA revalidation efforts, and assisting in the development of PHA guidance documents.
Related Cognascents Solutions
At Cognascents, we have the expertise to meet your needs. Explore the variety of related services we offer and discover how our tailored approach can support your business.
Our mission is to empower our clients with innovative solutions that enhance process safety, reliability, asset integrity, and technical business acumen. To elevate your engineering solutions, please contact us today.
