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Key Questions and Answers in Process Safety

This page contains questions that our process safety practitioners commonly encounter. Answers are provided to help you with your understanding of the topics addressed.

How should human factors be addressed in the PHA?

Human factors are normally addressed in two ways. First, human failures that are causes of hazard scenarios are identified. Second, factors that can influence the likelihood of the human failures are determined.

Are utilities addressed when conducting a PHA?

Utilities of concern for PHA are those that can impact on or affect a release of the regulated chemicals in the process. They should be included in the PHA. Utility failures usually are addressed as causes of hazard scenarios.

Should a PHA be performed for each process change as part of management of change (MOC) reviews?

OSHA’s PSM standard requires that MOC addresses the “impact of change on safety and health”. There is no specific requirement to perform a PHA except for new facilities. Some companies perform PHA for major, extensive, or high risk changes.

During the course of a PHA study, the team members were uncertain if a pressure switch used as a safeguard actually worked. Can credit be taken for this switch as a safeguard?

No. If the functionality or operability of a device is unclear or uncertain, the device should not be claimed as part of a safeguard. The team could have sent an instrument tech to field-verify the operability of the device.

What is meant by Process Safety Culture?

It consists of the attitudes and behaviors of facility personnel towards safe process operations. It is significant because it determines how an organization approaches process safety management. A weak process safety culture has been found to be a factor in serious incidents.

How can failure rates be determined for initiating events in LOPA?

Generic failure data can be used from references such as the book, "Initiating Events and IPLs in LOPA". However, site-specific failure rate data from individual facilities are preferable as they apply to actual operating conditions.

Is it acceptable for LOPA team leaders to pre-populate LOPA worksheets with entries for review by the team?

Usually, scenario information including causes, consequences, and safeguards is already established by the PHA team and that information can be transferred into LOPA worksheets prior to conducting the LOPA study. The initiating event frequency, enabler multipliers, and probability of failure on demand (PFD) values should be developed in conjunction with the LOPA team and not be pre-populated prior to the LOPA.

Should I have to use same PHA method for revalidation of a study that was used in the previous study?

If the earlier PHA is to be replaced, then a different method could be used. If the earlier PHA is being revised, a change from the previous PHA may not be practical. Changing methods will need to be justified but switching to a more sophisticated method would be easier to justify. However, the process safety regulations are performance-based so the choice is whatever is appropriate and justifiable.

Should safeguards be combined in the PHA worksheet when evaluating hazard scenarios during a PHA?

Although there are no specific regulatory requirements for listing safeguards individually in PHA worksheets, the best practice is to keep safeguards separate. A single cause may have multiple different consequences and the safeguards listed may not apply to every consequence. Thus, identifying applicable safeguards for individual consequences is difficult. Also, combining safeguards makes risk ranking and the identification of appropriate recommendations more difficult.

Of the four types of human failure (omission, commission, extraneous acts, and violations), which types are most likely to be overlooked by a PHA team?

Omission errors and commission errors are the easiest human failures to address because they are the most obvious. Procedures spell out specific actions to be taken and those actions either may not be performed or may be performed incorrectly.

Extraneous acts are more difficult to address because of the numerous possibilities outside those actions required. Credible violations of procedures are easier to envision, such as bypassing a nuisance alarm by an operator.