Process Hazard Studies

Safety legislation in the EU and the USA. A brief review of the range of Hazard Study (HS) that may be considered in both major and minor projects, outlining the principal features of HS 0–7. Illustration of the relevance of each study to the main phases of a project. The place of HAZOP study within the series is shown. The choice of HS methods for projects of different hazard potential is listed including alternatives to HAZOP study. Finally, an illustrative checklist is provided for the very important HS2 (project definition stage).

Keywords

Safety legislation; hazard studies 0–7; project life cycle; HS selection; HAZOP alternatives; HS2 checklist

Within the process industries, significant attention has been given to the development of comprehensive safety management systems (SMSs) or SHE management systems with the objective of protecting workers, the public, and the environment. There are also requirements within legislation such as the Seveso II Directive, European Union (EU) Directive 96/82/EC,9 and subsequent country-specific legislation, requiring those companies handling hazardous materials to have in place an adequate SMS and to fulfill specified obligations. These requirements range from the preparation of Major Accident Prevention Policies to submission of detailed safety reports to a competent authority. Other non-EU countries have similar legislation—for example, the Office of Safety and Health Administration (USA) (OSHA) regulation 29CFR, Part 1910.119 (1992), Process Safety Management (PSM)10 in the USA. An integral element of such systems is the use of systematic techniques for the identification of hazards. In addition to meeting legal requirements, there are considerable business benefits to be gained from the use of a systematic and thorough approach to hazard identification. These benefits include improvement of quality, faster start-up, and a reduction of subsequent operability problems.

For a new project, the greatest benefit is obtained by carrying out a number of studies throughout the design process. One such sequence is the Hazard Study (HS) methodology developed by ICI which used six stages.4,11 Each study verifies that the actions of previous studies have been carried out and signed off, and that the hazard and environmental issues have been identified and are being addressed in a timely and detailed manner.

2.1 HS 1—Concept Stage Hazard Review

In this first study, the basic hazards of the materials and the operation are identified and SHE criteria set. It identifies what information is needed and the program of studies required to ensure that all SHE issues are adequately addressed. The aspects covered may include reaction kinetics, toxicity data, environmental impact, and any special process features that need further evaluation. In addition, any constraints due to relevant legislation are identified. A decision may be taken on which of the remaining hazard studies (two to six) should also be undertaken. It is also important at this early stage to apply the principles of inherent SHE12,13 within the design. This aims to eliminate, avoid, or reduce potential hazards in the process.

2.2 HS 2—HAZID at Front-End Engineering Design (FEED) or Project Definition Stage

This study typically covers hazard identification and risk assessment, operability and control features that must be built into the detailed design, and any special environmental features to be covered.

It is important that the safety integrity levels (SILs)14,15 of any safety instrumented systems (SISs) are addressed during this study as the design will still be flexible and simple design changes may be applied which will reduce the SILs and so simplify the design.

At the end of HS 2, the level of development of design and piping and instrumentation diagrams (P&IDs) would be “approved for design” (AFD). All the main features should have been added but the finer ones will not. It is helpful to examine the AFD diagrams for the more blatant errors using a form of checklist. An example is given in the final section of this chapter.

2.3 HS 3—Detailed Design Hazard Study

This normally involves a detailed review of a firm design aimed at the identification of hazard and operability problems. Relief and blowdown studies, area classification, personal protection, and manual handling may, if appropriate, be included at this stage. HAZOP studies are normally carried out at this stage.

2.4 HS 4—Construction/Design Verification

This review is performed at the end of the construction stage. The hardware is checked to ensure it has been built as intended and that there are no violations of the designer’s intent. It also confirms that the actions from the detailed design hazard study are incorporated, and operating and emergency procedures are checked.

2.5 HS 5—Pre-Commissioning Safety Review

This examines the preparedness of the operations group for start-up and typically covers training, the final operating procedures, preparation procedures, and readiness for start-up including function testing, cleanliness, and purging. Confirmation of compliance with company and legislative standards is done at this stage, for example, under the Pre Start-up Safety Review (PSSR) required under the OSHA PSM legislation in the USA.

2.6 HS 6—Project Close-Out/Post Start-Up Review

This study, carried out a few months into the production phase, confirms that all outstanding issues from the previous five studies are complete and seeks any lessons that might give useful feedback to future design work.

In addition to these six studies, two more may be included. These are usually referred to as study zero and study seven, to fit with the numbering scheme used above.

2.7 HS 0—Consideration of Inherently Safer or Less Polluting Systems

Study zero takes place between the Research and Technical Departments before the concept stage. It attempts to identify and to incorporate the inherently safer and greener ideas as early as possible so that they will be part of the final design.

2.8 HS 7—Demolition/Abandonment Reviews

This study can take place before the final shut down but the objective of the study is to identify those issues which should be dealt with during the demolition process. It should address issues such as cleaning methods and standards, size reduction, recovery and recycle of working inventories, recycle of equipment, safe disposal of nonrecyclable materials/equipment, and location of potentially harmful/toxic materials in the equipment or soil. In addition it should address the integrity of lifting devices/brackets, access routes, and the sequence of removal bearing in mind that some equipment may be supporting other equipment.

2.9 Overview of Hazard Studies

The relationship of HS 1–6 to the project life cycle is shown in Figure 2.1. Experience shows that the use of HS 1 and 2 ensures key conceptual issues are dealt with early in the life of the project and not left to the HAZOP study. Use of HS 1 and 2 makes the HAZOP study easier and faster.

While HAZOP is one of the most flexible techniques for hazard identification, there are other identification and assessment techniques which can be used at the detailed design stage to supplement HAZOP (Figure 2.2).

Whenever HAZOP study is the chosen technique, its use should be justified on the basis of complexity, inherent hazards, or the costs of the operation. While HAZOP study is ideally suited to novel processes, hazardous process or complex processes, it can equally be used in simple and repeat designs although there may be fewer benefits. Considerable benefits can also be found from its use for modifications or for change of use of plant.

More information on the techniques used for hazard identification and the way in which these relate to an overall process for risk assessment is given by Pitblado16 and by Crawley and Tyler.2

2.10 Illustrative Checklist for HS 2

An example of a checklist for use at conclusion of HS 2 to ensure common problems have been considered and are covered before the detailed design work begins.