The HAZOP Study

What if the facility heating, ventilation, and air conditioning (HVAC) system fails What if the main compressor fails  

What if the refrigerant tank relief valve malfunctions  

What if process flow diagrams or drawings are not current  

The questions should continue until all elements are addressed. If a more detailed and complete HAZOP study should be required, the information contained on the What-If analysis worksheets will help facilitate and streamline the pending HAZOP study. 

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, power, and so on. 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. 

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Control actions, e.g., provide alarms to indicate things getting out of control define control actions to get back into control. The HAZOP study is not complete until response to actions has been documented. Initial HAZOP planning should establish the management follow-up procedure that will be used. 

Two members selected from the team act in supportive roles during the PHA. These roles are the leader and the scribe. A team leader is always needed for a PHA and should double as the scribe only for extremely simple hazard evaluations. For more complex analyses or the HAZOP study method, a separate person should always be assigned to scribe. 

The HAZOP study proceeds in a systematic mamier that reduces the possibility of omi ssion. Within a study node, all deviations associated with a given process parameter should be analyzed before the ne.xt proces.s parameter is considered. All deviations for a study node should be analy zed before the team proceeds to the next node. 

Staff requirements for a HAZOP depend on the size and complexity of the process being analyzed. Time and cost are proportional to the size of the process and the experience of the study leader and team members. Table 3.3.4-5 presents estimates of the time needed to perform a PrHA using the HAZOP study method (CCPS, 1992). Study sessions should be limited to 3 consecutive... 

The HAZOP study was developed to identify hazards in process plants and to identify operability problems that, although not hazardous, could compromise a plant s productivity. 

The basic concept behind HAZOP studies is that processes work well when operating under design conditions. When deviations from the process design conditions occur, operability problems and accidents can occur. The HAZOP study method uses guide words to assist the analysis team in considering the causes and consequences of deviations. These guide words are applied at specific points or sections in a process and are combined with specific process parameters to identify potential deviations from intended operation. 

In addition, it provides a list of potential actions that should be evaluated. Table 4.16 is an example of a HAZOP study worksheet. A typical HAZOP study report should include a brief system description, a list of drawings or equipment analyzed, the design intents, the HAZOP study tables, and a list of actions items. 

Table 4.17. Time Estimates for Using the HAZOP Study Method...

Before the HAZOP study is started, detailed information on the process must be available. This includes up-to-date process flow diagrams (PFDs), process and instrumentation diagrams (P IDs), detailed equipment specifications, materials of construction, and mass and energy balances. 

In the forthcoming Hazop standard (IEC 61882, 1999) the Hazop studies are recommended to be carried out throughout the life cycle of a system. But for the concept and definition phase of a system s life cycle other basic methods are suggested (see Fig. 3). 

Note In student design projects, the aim is to produce a competent design and also to communicate to the supervisor what has been done. It is therefore essential to include a summary of the major findings obtained by performing the HAZOP study (this is usually only performed on one... 

The HAZOP study was instrumental in determining the need for an adequate alarm system on each of the specified controllers. If liquid levels within the column are not well controlled, then either flooding (too much liquid) or plate by-passing bythegas (too little liquid) will occur. Both situations lead to a substantial reduction in absorption efficiency with large increases in emission levels. The other important control parameter was shown to be the temperature. If the temperature in the cooling-coil section rises, then there is an appreciable reduction in absorption. Control of temperature is important in the upper sections of the column because it is here that the greatest effect on emission levels occurs. 

The hazard identification methods presented in Sections 1.5.1 to 1.5.6 above are all based on strongly systematic procedures. In the check list method, the systematic is provided by the check list itself. The comprehensiveness can be verified in the matrix (see Figures 1.4 and 1.5). With the FMEA, the systematic is provided by the division of the system into elements and the failure modes considered. In the HAZOP study, the systematic stems from the division of the plant into nodes and lines, then the systematic application of the keywords. With the decision table method, the systematic is inherent to the table. For the FTA and ETA, the systematic is given by the tree and the logical ports. Nevertheless, the work of the team must be traceable, even by persons who did not participate to the analysis. Thus, it is recommended to also document the hazards that were not considered as critical. 

Closing out the HAZOP Study. Although this seems simple and obvious, experience has shown that closing out the study is far... 

The HAZOP Study is a very popular predictive method which was developed in the Mond Division of Imperial Chemical Industries during the 1960s. A HAZOP (Hazard and Operability) study is an analysis method for identifying hazards and problems which prevent efficient operation. Trevor Kletz was an early promoter of the HAZOP Method and in one of his recent books [18], he states ... 

The approach to identifying hazards and operability problems by this technique is to search for deviations from design intents. The first step is to plan the HAZOP study to ensure that there is sufficient time, expertise, and information available. Next, a multidisciplinary team, led by an expert in the technique, is set up. Then, by means of a fixed set of abstract words called guide words (such as more ), the team leader examines each process... 

As an example of the cost effectiveness of the technique, an assessment of the value of HAZOPs on a 38 million project was done. Table III shows the cost effectiveness of the HAZOP study technique for a new project clearly. 

Emphasise should be set on safety and loss prevention issues. The Hazop study could follow the guidelines from Coulson Richardson (1993). More information about this topic may be found in Crowl Lowvar (1990), and other specialised references. 

DEFINING THE PROCESS TO BE STUDIED. This Step identifies the specific vessels, equipment, and instrumentation to be included in the HAZOP study and the conditions under which they are analyzed. Defining the problem involves defining the boundaries of the analysis and establishing an appropriate level of resolution for the study. For most HAZOP studies, the causes of deviations are identified at the component level (i.e., control valve CVlOl fails open). 

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