PT Notes
Addressing Inherent Weaknesses in PHA
PT Notes is a series of topical technical notes on process safety provided periodically by Primatech for your benefit. Please feel free to provide feedback.
Current process hazard analysis (PHA) methods suffer from a variety of weaknesses which almost certainly results in incomplete studies with hazard scenarios being missed. Knowledge of the weaknesses allows PHA teams to compensate for them to the extent possible when performing studies. Here are some weaknesses with some suggestions on how to address them:
Role of subjective judgment
PHA studies depend on people to use their imagination and creativity to identify hazard scenarios. In those parts of PHA where subjective judgment is critical, team members should practice using scenarios that have already been assessed by experts so that they can make a comparison with their own assessments and calibrate their judgment against that of experts.
Only departures from design intent are addressed
PHA focuses on looking for ways the process may deviate from the design intent and does not evaluate the adequacy of the design intent itself. A separate design review must address matters such as the operability of the process, application of inherently safer design principles, value engineering, and compliance with codes, standards and regulations.
Ability to address all aspects of design intent
There are many aspects of design intent for a process and it is a significant challenge to identify those aspects for which deviations will result in hazard scenarios of concern. The more aspects of design intent that are included in a study, the more design representations must be consulted by the team and the study becomes more onerous and frustrating for team members. However, not all aspects of design intent need to be considered for each part of a process and, indeed, the important aspects likely will vary for each part. Team leaders should encourage PHA teams to identify relevant aspects of design intent.
Scenario detail
Traditional PHA methods, such as the HAZOP study, do not capture all details of hazard scenarios that may be important for assessing their importance and that may be needed for more detailed studies, such as layers of protection analysis (LOPA). For example, aspects of intermediate events and enablers may be omitted. Enablers play key roles in real-world accidents and their inclusion in PHA studies is desirable.
An Events column can be added to the PHA worksheet to provide important scenario details, such as the specific location of a release. Similarly, an Enablers column provides a location to capture important enabling events and conditions.
Identification of human failures
Human failures involving errors of omission are relatively straightforward to identify and to a lesser extent so are errors of commission and violations. However, credible extraneous acts that may result in significant consequences are more difficult to identify owing to the multitude of possibilities and the imponderable nature of many of them. PHA teams should consider carefully the range of possibilities to avoid omitting such errors.
Root causes of hazard scenarios
Usually, PHA does not address the fundamental root causes of scenarios, such as human and organizational factors. Typically, practitioners identify immediate or basic causes. Currently, there are no consistent practices on the level of causality that should be used in the hierarchy of causes. The key issue is how deep do teams need to go in order to identify needed risk reduction measures.
Ability to identify multiple failures
Multiple failures involve two or more events occurring together, either at the same time or separated in time. Multiple failures may occur independently of one another or dependently. If multiple failures are not addressed in PHA, important scenarios may be missed and the risks of scenarios may be underestimated. PHA teams tend to focus on single failures which are more easily addressed than multiple failures using current PHA methods. Team leaders should encourage teams to consider multiple failures.
Consideration of dependent failures
Some apparently independent multiple failures may be dependent and the likelihood of the multiple failure will then be higher than otherwise would be estimated. Multiple failures can be as likely as single failures since effectively they reduce to a single failure. Current inductive PHA methods do not incorporate a formal consideration of dependent failures and must rely on the team to recognize their potential occurrence and importance.
Consideration of domino effects
Domino effects produce hazard scenarios that involve escalating consequences as a chain of linked events propagates throughout and beyond the process where they originate. Historically, they have been addressed in PHA as external events. This approach is limited. PHA teams must recognize that domino effects are part of a chain and that further domino effects may be possible.
Identification of worst-consequence rather than worst-risk scenarios
Usually, scenario severity values are estimated assuming the worst-case consequence in which all safeguards fail. However, the worst-case consequence scenario may not be the worst-case risk scenario for the same initiating event, although some practitioners implicitly assume the two are the same. Ideally, both the worst-consequence and worst-risk scenarios should be addressed in the PHA worksheet.
Focus on individual parts of a process
In conducting PHA studies, usually the process is divided into sections, called nodes, or systems and subsystems, in order to focus the analysis and make it manageable. Unfortunately, such process subdivision may result in missing scenarios that involve multiple or all parts of the process. Process subdivision leads to a de-emphasis on the identification of system incidents. They should be addressed in a global node or system.
Uniqueness of process subdivision
There is no unique way of subdividing a process into nodes, or systems and subsystems. Process subdivision may influence the results of a study. Certainly, if super-nodes are used that combine lines and vessels, it is likely that some scenarios may be missed owing to the difficulty of their identification in a more complex section of the process. Guidelines should be followed for subdividing a process with the goal of choosing the optimal number of nodes, or systems and subsystems, in order to perform the study efficiently and effectively.
Utility and support system failures
PHA studies must consider the failure of utility and support systems. Their failure can play an important role in the safety of a process.
Treatment of modes of operation
PHA studies should be performed for all modes of process operation before the process experiences those modes. Existing PHA approaches do not require any particular form of consideration of modes of operation, nor, indeed, do they encourage their treatment. Some practitioners focus on normal operation and conduct a single PHA. However, any claims that such a PHA adequately addresses other modes of operation are highly questionable. Other modes of process operation should be studied at a comparable level of detail to normal operation.
Treatment of non-steady-state processes
Traditional PHA methods have been adapted to address non-steady-state processes, such as batch processes, by performing PHA for each step or stage in the process. However, deviations in the timing, duration and sequence of steps and stages, interactions between steps and stages, the operation of interlocks and permissives, and the occurrence of simultaneous operations within the process can be challenging to address. PHA studies on non-steady-state processes should be performed in a systematic manner to help address these issues.
Addressing human factors issues
Human failures as a cause of hazard scenarios and the factors that impact them must be addressed. The former is a natural part of PHA and the latter can be included by elaborating on the reasons for the identified human failures. However, the performance of a separate human factors study that precedes the PHA is beneficial as it permits a complete focus on this critical aspect of process safety and helps to ensure that human factors issues are identified that might not arise if human factors are addressed only within PHA.
Addressing facility siting issues
A key concern in processes is with the spatial relationship between the hazards present and the locations of people in the process and facility, particularly in occupied buildings, such as control rooms. As for human factors, some facility siting issues can be addressed readily within a PHA, but others are better examined separately in a study that precedes the PHA to provide the opportunity to address issues that might not arise if facility siting were addressed only within PHA.
Interactions between processes
Causes of failures in one process may originate within other processes and the consequences of an incident in one process may extend to other processes. This is an issue whenever a facility is divided into individual linked processes for the purposes of PHA. Interactions between processes must be considered when conducting PHA studies.
Conservative assumptions
Owing to the subjective nature of PHA and the uncertainties involved, conservative assumptions often are made throughout the performance of PHA. Considerably conservative results can be produced that may lead to the inappropriate expenditure of resources that would be better invested elsewhere or in a different way. PHA team leaders should be alert to the possibility of overly conservative judgments by team members.
Prediction of real-world accidents
Often, accidents that occur in processes involve combinations of many events and it is highly unlikely that a PHA team would ever have suggested the particular sequence of events in a PHA study. At best, a PHA team can identify the most critical initial events of the sequence so that suitable risk reduction measures can be determined.
These weaknesses are discussed in greater detail in the article:
Requirements for improved process hazard analysis (PHA) methods, Journal of Loss Prevention in the Process Industries, Volume 32, November 2014, Pages 182–191.
The article also provides criteria to guide the development of improved PHA methods.
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