Guidewords

A HAZOP is used to question every part of a process to discover what deviations from the intention of the design can occur and what their causes and consequences may be. This is done systematically by applying suitable guidewords. This is a systematic detailed review technique, for both batch and continuous plants, which can be applied to new or existing processes to identify hazards. 

A reladvely simple example of a HAZOP study using guidewords is shown for a boiler drum in Fig. 15.7.1 and Table 15.7.2. The intent of die operation is to maintain die water level in die horizontal drum between 30 and 40% of die volume. 

Mosley et al. (2000) describe a "chemistry hazard analysis" approach, similar to a hazard and operability (HAZOP) study method applied at the early development stages of a new process. Deviations from an intended chemical reaction are identified using typical HAZOP guidewords. Examples of deviations and consequences developed using this approach are shown in Table 4.10. Analyzing the basic chemistry of a process, where chemical reactions are intended to occur, can help ensure the consequences of deviating from the intended reaction are understood. 

Use of a protocol to identify reactive hazards (e.g., checklist or specific guidewords). 

Knowlton, E. R. An Introduction to Hazard and Operability Studies—A Guideword Approach, 7th Printinff. Vancouver, British Columbia Chemetics International Co., 1989. 

Hazard and Operability Study (HAZOP)— A systematic qualitative technique to identify and evaluate process hazards and potential operating problems, using a series of guidewords to examine deviations from normal process conditions.ti)... 

Table 1.7 HAZOP guidewords with definitions and examples.

HAZOP reviews follow a definitive guideword approach, step by step. A What-If analysis is usually combined with a checklist in the petrochemical industry to provide a "road map" for the review. 

In order to perform a HAZOP review a specialized team leader is used to guide the review team through the process. The team leader follows a standard format with special guidewords and deviations which need to be addressed. Because a standardized listing is used for all systems, some unnecessary and unimportant issues may be addressed in some portions of the system under review. 

It has a specific guideword listing and the process under review is subdivided into small sections for analysis. 

A HAZOP study is undertaken by the application of formal, systematic, and critical examination of the process and engineering intentions of the process design. The potential for hazards or operability problems are thus assessed, and malfunction of individual items of equipment and associated consequences for the whole system are identified. This examination of the design is structured around a specific set of parameters and guidewords, which ensures complete coverage of all major possible problems. 

The review meeting follows a structured format. The complete process to be studied is divided into discrete nodes. For each node, every parameter or guideword deviation is considered. For each deviation, causes are identified. For each cause, consequences are identified. For each consequence, existing protection is identified. After considering existing protection, recommendation for action are made if the remaining level of risk is consider unacceptable. Clarifying remarks are included as appropriate. 

A What-If review is generally very similar in organization except What-If questions,usually referred to from a checklist are substituted for guidewords and parameters. 

Guideword (GW) Simple word or phrase used to generate deviations by application on a system or process activities (e.g. pressure, level, level, temp, etc.). 

The following are typical guideword parameter, deviations, and possible causes that are used in HAZOP reviews. This listing is by no means exhaustive and each review should be supplemented or tailored to meet the needs of a particular facility. 

Guideword (GW) A simple word or phrase used to generate deviations by operations on parameters. 

All guidewords to be used by the CHAZOP review team should be defined and documented. Care must be taken to ensure that all CHAZOP participants understand their precise meaning and... 

The guideword process can be supplemented by additional topics/questions based on an analysis of previously experienced design deficiencies and operational incidents. For instance, ICI has collated a database of over 350 operational incidents that it uses to refine its CHAZOP Study process. Some example questions for the CHAZOP Study are given in Appendix 8E at the end of this chapter. Of particular interest to the study is the effect of partial or catastrophic failures, recovery mechanisms (e.g., rollback and roll-forward), and the general usability of the system (e.g., the need for multiple screens to access data, screen refresh times, meaningful information displays). The list of questions can be expanded with operational and regulatory experience. 

A CHAZOP report should be prepared outlining the process undertaken, the definition of guidewords used, and any resultant recommended actions. The CHAZOP report will normally include the completed CHAZOP table. An example of part of a CHAZOP table is shown in Table 8.2. The report should be reviewed for its accuracy, and signed and approved accordingly. Actions should be reviewed for satisfactory closure as part of the Validation Report. Actions may affect specification and design documentation, and/or testing. 

Boimdary Values (Guidewords Minimum, Zero, Maximum)... 

Invalid Arguments (Guidewords Alphanumeric, Integer, Decimal)... 

Special Values (Guidewords Null-Entry, Function Keys)... 

Performance (Guidewords Sequence, Timing, Volume of Data)... 

Security and Access (Guidewords User Categories, Passwords)... 

Error Handling and Recovery (Guidewords Messages, Alarms)... 

Figure 2. Simplified reflneiy SC flow diagram Table 1. Sample guidewords and parameters for HAZOP...

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