Keywords

Essentials of HAZOP; engineering standards; design envelope; timing; purpose of HAZOP; limitations of HAZOP

3.1 Essential Features

A HAZOP study is a structured and systematic examination of a planned or existing process or operation. At the outset of the study, the team creates a conceptual model (design representation) of the system or operation. This uses all available, relevant material such as a firm, detailed design, an outline of operating procedures, material data sheets, and the reports of earlier hazard studies. Hazards and potential operating problems are then sought by considering possible deviations from the design intention of the section or stage under review. The design intention is a word picture of what should be happening and should contain all of the key parameters that will be explored during the study. It should also include a statement of the intended operating range (envelope). This is usually more limiting than the physical design conditions. For those deviations where the team can suggest a cause, the consequences are estimated using the team’s experience and existing safeguards are taken into account. Where the team considers the risk to be nontrivial or where an aspect requires further investigation, a formal record is generated to allow the problem to be followed up outside the meeting. The team then moves on with the analysis.

The validity of the analysis obviously depends upon having the right people in the team, the accuracy of the information used, and the quality of the design. It is normally assumed that the design work has been done in a competent manner so that operations within the design envelope are safe. Even where this is the case, the later stages of the project must also be carried out correctly—that is, engineering standards are followed and there are proper standards of construction, commissioning, operation, maintenance, and management. A good HAZOP study tries to take account of these aspects and of the changes that can reasonably be expected during the lifetime of the operation. A study will sometimes identify problems that are within the design limits as well as problems which develop as the plant ages or are caused by human error.

A key feature of timing of a HAZOP study is that the design must be firm and the P&IDs must be frozen—a situation that requires management commitment and forward planning.

3.2 The Purpose

One purpose of a HAZOP study is to identify and evaluate any remaining hazards within a planned process or operation that were not identified or designed out in earlier stages. The hazards may be several types, including those to people and property, both on- and off-site. It is also important to consider the potential effects to the environment. Regardless of the type of hazard, many have directly related financial consequences.

HAZOP studies are also normally used to identify significant operability or quality problems and this will be included as a defined objective of a study. A survey of EPSC members carried out in 1999 as part of the preparation for the first edition of this Guide found that over 90% of the respondents included significant operability problems in the scope of the search. Operability problems arise through the reliability as well as the manner of the plant operation, with consequences such as downtime, damaged equipment, and the expense of lost, spoilt, or out-of-specification product leading to expensive re-run or disposal costs. The need to consider quality issues varies greatly with the details of the operation but in some industries it is a crucial area. Of course, many operability problems also lead to hazards, giving a dual reason for identifying and controlling them. A HAZOP study may also consider quality issues in the proposed design.

It is advisable to cover aspects of maintenance operations, including isolation, preparation, and removal for maintenance since these often create hazards as well as an operability problem. Where there are manual operations or activities, it may be necessary to analyze the ergonomics of the whole operation or activity in detail.

3.3 Limitations

Difficulties may be caused by inadequate terms of reference or poor definition of the study scope. The intention of a HAZOP study is not to become a re-design meeting. Nevertheless, some actions may result in changes to the design and potential problems may be found within the intended range of operation.

The analysis of problems within a HAZOP study is normally qualitative although, increasingly, simple risk assessment is used to help the team to decide on the need for action and the action itself. Some of the problems may need a fuller quantitative analysis, including quantitative risk assessment (QRA). This would be done outside the HAZOP meeting.

A HAZOP study is not an infallible method of identifying every possible hazard or operability problem that could arise during the actual operations. Expertise and experience within the team is crucial to the quality and completeness of a study. The accuracy and extent of the information available to the team, the scope of the study, and the manner of the study all influence its success. Only a systematic, creative, and imaginative examination can yield a high-quality report but even then, not every potential problem will necessarily be found. Additionally, the study will only be effective if the issues identified during the study are resolved and put into practice. Some important factors for success are listed in Chapter 12.