Plant hazard analysis HAZOP

In the previous chapter, it was established that in industry, plant hazards can cause harm to property (plant—machinery, asset), people, or the environment. So, it is important to develop some means of analyzing these and come up with a solution. Unfortunately, it is not as straightforward as it sounds. There are plenty of plant hazard analysis (PHA) techniques and each of them has certain strengths and weaknesses. Also each specific plant and associated hazard has specific requirements to be matched so that hazard analysis will be effective. In this chapter, various hazards (in generic terms) will be examined to judge their importance, conditions, quality, etc. so that out of so many techniques available for PHA it is possible to select which one is better (not the best because that needs to be done by experts specifically for the concerned plant) suited for the type of plant. So, discussion will be more toward evaluation of PHA techniques. Some PHA is more suited for process safety management (PSM) and is sometimes more applicable for internal fault effects [e.g., hazard and operability study (HAZOP)]. In contrast, hazard identification (HAZID) is applicable for other plants, especially for the identification of external effects and maj or incidents. HAZID is also covered in this chapter. As a continuation of the same discussion, it will be better to look at various aspects of risk analysis with preliminary ideas already developed in the previous chapter. In risk analysis risk assessment, control measures for safety management systems (SMSs) will be discussed to complete the topic. 

Step 0 In Fig. V/4-1 -4 1, it has been shown that LOPA starts from plant hazard analysis (PHA)/HAZOP results. This is the normal case, but it could be started afresh also. In both the cases, it is necessary to fix up the scope and boundary as... 

When considering release scenarios, the most hazardous unit in a plant should be chosen, based on inventoiy and process conditions. The idea is to imagine the release of material in the fastest way that is reasonably possible. The worst realistic scenario should be considered. This can be based on the outcome of a review, from a HAZOP study or a hazard analysis. The time a scenario will take is almost always considered to be continuous, because after a few minutes a stable dispersion distance exists. Making the time longer will not necessarily change the hazard distance. 

Hazard analysis (HAZAN) is a quantitative way of assessing the likelihood of failure. Other names associated with this technique are risk analysis, quantitative risk assessment (QRA), and probability risk assessment (PRA). Keltz [44] expressed the view that HAZAN is a selective technique while HAZOP can be readily applied to new design and major modification. Some limitations of HAZOP are its inability to detect every weakness in design such as in plant layout, or miss hazards due to leaks on lines that pass through or close to a unit but cany material that is not used on that unit. In any case, hazards should... 

Hazard and Operability Analysis (Hazop) (Kletz, 1992) is one of the most used safety analysis methods in the process industry. It is one of the simplest approaches to hazard identification. Hazop involves a vessel to vessel and a pipe to pipe review of a plant. For each vessel and pipe the possible disturbances and their potential consequences are identified. Hazop is based on guide words such as no, more, less, reverse, other than, which should be asked for every pipe and vessel (Table 1). The intention of the quide words is to stimulate the imagination, and the method relies very much on the expertise of the persons performing the analysis. The idea behind the questions is that any disturbance in a chemical plant can be described in terms of physical state variables. Hazop can be used in different stages of process design but in restricted mode. A complete Hazop study requires final process plannings with flow sheets and PID s. 

Process hazard analysis (PHA) Any of a number of techniques for understanding and managing the risk of a chemical process or plant. Examples of PHA techniques include HAZOP, checklists, what-if methods, fault tree analysis, event tree analysis, and others. 

Hazard and risk analysis is a vast subject by itself and is extensively covered in the literature [22]. In order to plan to avoid accidental hazards, the hazard potential must be evaluated. Many new methods and techniques have been developed to assess and evaluate potential hazards, employing chemical technology and reliability engineering. These can be deduced from Fault Tree Analysis or Failure Mode Analysis [23], In these techniques, the plant and process hazard potentials are foreseen and rectified as far as possible. Some techniques such as Hazards and operability (HAZOP) studies and Hazard Analysis (HAZAN) have recently been developed to deal with the assessment of hazard potentials [24]. It must be borne in mind that HAZOP and HAZAN studies should be properly viewed not as ends in themselves but as valuable contributors to the overall task of risk management... 

The process hazards analysis is conducted by an experienced, multidisciplinary team that examines the process design, plant equipment, operating procedures, and so on, using techniques such as hazard and operability studies (HAZOP), failure mode and effect analysis (FMEA), and others. The process hazards analysis recommends appropriate measures to reduce the risk, including (but not limited to) the safety interlocks to be implemented in the safety interlock system. 

According to OSHA, Phillips had not conducted a process hazard analysis or equivalent (such as HAZOP) in its polyethylene plants. 

All employed dryers must be equipped with clear and comprehensive operating manuals prepared jointly by the equipment supplier and the plant operating management. The manual should be learned by the operators, who are to be trained in recognition of hazardous situations and risk assessment with the material being dried. Very useful techniques, known as Hazop (hazard and operability studies) and Kazan (hazard analysis), may be applied successfully in both new and existing processing plants [39,40]. 

References 77 and 78 review some of the procedures developed for identifying plant hazards systematically. Among the better known are the hazard and operability study (Hazop), a qualitative technique based on the piping and instrumentation diagram, and hazard analysis (Hazan), used to quantify... 

The hazard and operability analysis (HAZOP) technique was developed to identify both hazards and operability problems in chemical process plants. An interdisciplinary team and an experienced team leader are required. In a HAZOP application, a process or operation is systematically reviewed to identify deviations from desired practices that could lead to adverse consequences. HAZOPs can be used at any stage in the life of a process. 

Now, efforts have been made to develop suitable software for various methods of plant (process) hazard analysis (PHA). There are a number of papers available where through suitable software it is possible not only to automate one PHA method but to integrate several automated PHA methods such as event tree/fault tree (Chapter V) with HAZOP or HAZOP with FMEA, which will be discussed in the next clause in this chapter. 

Overall, then, the only additional RMP requirement for plants already covered by the OSHA process safety management regulation is the hazard assessment (including offsite consequence analyses of worst-case and non-worst-case accidental release scenarios). This hazard assessment must not be confused with the process hazard analysis (PHA). The hazard assessment is a study of what will happen in the event of an accidental release and usually includes, for exanple, air dispersion simulations. The PHA (e.g., HAZOP) studies the hazards present in the process and seeks to minimize them through redesign or modifications to operating procedures. 

A What-if analysis is used to assess consequences of deviations from normal operating conditions by asking "what if... " questions. This approach is often used for reviewing plant or process modifications. Because the procedure is less structured than some other methods, e.g, HAZOP, care should be taken to identify also the less obvious hazards [2,3,255]. 

The Dow and Mond Indices and Hazop presented in Chapter 4 are widely used for the safety evaluations of process plants. They cover well those risks and hazards existing on a chemical plant. However a lot of detailed information is needed to complete those analysis. In the early stage of process design many of the required process details are still unknown. Therefore the presented safety analysis methods are not directly applicable in their full mode. 

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