Guidewords, HAZOP

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. 

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. 

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 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. 

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

One effective systematic method, a variation on which will be outlined further in this book, is the SWIFT or Structured What-If Technique. SWIFT is a systems-based risk identification technique that employs structured brainstorming, using pre-developed guidewords or headings in combination with prompts elicited from participants (which often begin with the phrases What if... or How could... ), to examine risks and hazards at a systems or subsystems level [1]. The technique was originally developed as a simpler alternative to HAZOP (see Sect. 13.6.2). 

The use of HAZOP to assess healthcare and HIT risks is less well documented than with FMEA. This may reflect the technique s orientation towards the analysis of ordered and predictable processes which are characteristics that cannot be assumed in a healthcare setting. However, if one were able to construct a useful and relevant set of guidewords and parameters, careful application of the technique along with other systematic methods could prove fruitful. 

Like HAZOP, STPA works on a model of the system and has guidewords to assist in the analysis, but because in STAMP accidents are seen as resulting from inadequate control, the model used is a functional control diagram rather than a physical component diagram. In addition, the set of guidewoids is based on lack of control rather than physical parameter deviations. While engineering expertise is still required, guidance is provided for the STPA process to provide some assurance of completeness in the analysis. 

The discussions to do with hazards analyses that have been provided up to this point in this chapter have been predicated on an assumption that the unit being analyzed is a processing operation— typically a section of a refinery, chemical plant, or oil/gas production facility. However, the techniques that have been discussed can be used, when adapted appropriately, to other types of industrial operation. For example, the deviation guidewords of a HAZOP study (the technique is discussed in the next chapter) can be modified to address transportation issues, as illustrated in Table 5.7. Reverse Flow, e.g., becomes Vehicle, Train, or Ship Reverses. ... 

Having completed the discussion to do with a process guideword, the team moves on to the next guideword, or to the next node if aU of the guidewords have been discussed until the HAZOP is concluded. 

The first way of applying the concepts of inherent safety is work through the guidewords during each formal hazards analysis (HAZIDs and HAZOPs). Maher et al. (2011) discuss the use of HAZIDs to promote inherently safer designs. 

The more complex the system or process to be evaluated, the more essential is the need for a HAZOP study. The HAZOP study is conducted in much the same way as the what-if analysis, usually by the same review team. There are minor differences, however, in terminology and approach. In the HAZOP study, certain guidewords are normally used to aid the review team and help identify specific areas where deviations from design intent can occur. Guidewords can include pressure, flow, level, temperature, and power. HAZOP also attempts to identify the severity of the outcome if such deviations from the norm occur as well as the probability or likelihood of occurrence. The hazard risk matrix established and explained in Chapter 2 (Table 2.3) can be used for this purpose since it provides both severity and probability rankings for a given hazardous situation. 

HAZOP is the most widely used technique by ICI developed several decades ago and involves vessel-to-vessel and pipe-to-pipe review. The entire flow sheet is systematically reviewed by design, safety, and process engineers using a set of guidewords to identify... 

Step 2 Identify how the failure could occur using a set of human failure guideword prompts. In this case, the Human HAZOP (Hazard and Operability) (Kirwan and Ainsworth, 1994) tool was applied. 

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