Safety and reliability in the design of plant initially relies upon the application of various codes of practice and standards, which represent the accumulation of knowledge and experience of individual experts and the industry as a whole.  Such application is usually backed up by the experience of the engineers involved, who might have been previously concerned with the design, commissioning or operation of similar plant. While codes of practice create uniformity in the levels of protection across similar facilities, they do not infer ‘low risk' because they do not explicitly take account of the effects of equipment malfunction, operator error or external factors in a given situation.

During the design of a new plant, design personnel are under pressure to keep the project on schedule.  This pressure frequently results in errors and oversights.  HAZOP study is an opportunity to correct these before such changes become too expensive, or impossible to accomplish.

Besides safety hazards, the HAZOP technique is very effective for identifying plant operability problems, threats to the environment, product quality, plant throughput and for highlighting critical maintenance requirements.

HAZOP is a powerful technique but the extent to which it can uncover all foreseeable hazards is limited by the knowledge, experience and deductive skills of the HAZOP team. For these reasons, it is difficult to assess the ‘quality' of a given HAZOP in any objective or auditable way. Audits can be carried out to establish that the process has been followed, but they cannot verify the competence of the team.

HAZOP assumes that the plant is designed to appropriate codes and standards, built using correct materials of construction, installed correctly, and that safety systems are properly maintained. In other words, it is safe when operated as intended. HAZOP will find reasons why the plant may operate differently to the way it was designed to operate but it is not the tool to rectify a defective plant.

HAZOP addresses single failures, or multiple failures with a common cause leading to a deviation, but does not consider multiple, independent, coincident failures. These lower frequency events are outside the direct experience of most people and are better addressed by a risk analysis study, if warranted by the extent of the potential consequences.