Systems safety analysis, HAZOP study

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. 

Also, the design practice includes P ID documentation, database specification and verification of purchased equipment, control design and performance analysis, software configuration, real-time simulation for DCS system checkout and operator training, reliability studies, interlock classification and risk assessment of safety instrumented systems (SIS), and hazard and operability (HAZOP) studies. 

The first task is crucial in process system safety analysis, because the effectiveness of the other two tasks depends on it. The technique of hazard and operability (HAZOP) study is a systematic approach to identifying most potential hazards and operating problems. The technique in contrast to the traditional methods is simple, creative, and flexible. 

The HazOp study differs from the FMEA and ETBA in that some suggest that the best time to conduct a HazOp is when the design is fairly firm (Goldwaite 1987). Conventional system safety wisdom dictates that the system safety effort be as far upstream as practical, with a facility preliminary hazard analysis developed as part of the initial design effort and completed by the 35% stage. Also, a HazOp study tends to include human factors and operator errors whereas a traditional FMEA or ETBA normally examines hardware failures only. 

Once all what-if analysis questions have been asked and answered along with all completed HAZOP studies of system components, a final report should be written to document all findings and recommendations. In the chemical industry (in the United States), this report is normally referred to as a process hazard analysis. This report is required under both OSHA and EPA regulations for facilities that handle or contain certain chemical commodities at certain defined quantity thresholds. However, when HAZOP studies and what-if analyses are used in general industry application, documentation of the results can be included in a written report along with any other system safety analyses that may have been performed (as described in previous chapters). If the HAZOP and what-if exercises were conducted as standalone analyses, then a final written report should be... 

This chapter briefly described the use and application of both the HAZOP study and what-if analysis, when each can and should be performed during the various phases of the product or project life cycle, and how each can be used to evaluate the viability of a new or (more commonly) an existing system. The occupational safety and health practitioner should seriously consider using these techniques to ensure a more comprehensive analysis and overall understanding of the inherent safety of any system or process. 

This analysis is done early in the design stage, and would be a part of a Hazop (hazard and operability) study. It is the initial step in the system safety analysis, and it considers the total system. 

For this paper we treat hazard assessment as a combination of two interrelated concepts hazard identification, in which the possible hazardous events at the system boundary are discovered, and hazard analysis, in which the likelihood, consequences and severity of the events are determined. The hazard identification process is based on a model of the way in which parts of a system may deviate fi om their intended behaviour. Examples of such analysis include Hazard and Operability Studies (HAZOP, Kletz 1992), Fault Propagation and Transformation Calculus (Wallace 2005), Function Failure Analysis (SAE 1996) and Failure Modes and Effects Analysis (Villemeur 1992). Some analysis approaches start with possible deviations and determine likely undesired outcomes (so-called inductive approaches) while others start with a particular unwanted event and try to determine possible causes (so-called deductive approaches). The overall goal may be safety analysis, to assess the safety of a proposed system (a design, a model or an actual product) or accident analysis, to determine the likely causes of an incident that has occurred. 

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. 

After consideration of these questions, detailed mechanical design drawings will be made of all of the BOP items that need to be sourced or fabricated. A basic electrical line diagram will be drawn up that shows how the various electrical and control items link together, with safety features included. A hazard identification (hazid) and/or hazard and operability study (hazop) or similar safety analysis will also be carried out. These considerations are outside the scope of this book but it is hoped that the reader would have gained enough understanding of the systems to be able to inquire further. 

FMEA was originally developed by the US department of defence for military purposes today it is one of the first choices for performing a system safety analysis. It shall be mentioned that, apart from this System FMEA, there are also variants for process improvement called Process FMEA. According to the system structure, all components are analysed with respect to possible failures, and - similar to the HAZOP study - causes, consequences, and detection as well as risk mitigation measures are analysed and improvements proposed. Unlike the HAZOP, however, FMEA uses neither parameters nor guidewords but relies on the expertise and creativity of the analysis team to discover all potential failure modes of the system components. 

Using the simulated biodiesel production process as case study, basically Jeerawongsuntom and team (2011) integrated the human-machine interface with HAZOP analysis for safety protection. Safety instmmented system was developed to create interlocks such as automatic shutdown to prevent the system from being exposed to extreme or... 

New systems or processes may also need to be qualified from an operational safety perspective. This is particularly relevant in the case of chemical synthesis involving exothermic reactions. Critical safety aspects are usually identified using hazard operability or HAZOP assessments and studies. For example, a HAZOP analysis of an exothermic reaction vessel would involve consideration of the consequence of failure of the motors for mixers or circulation pumps for cooling water. Thus, the qualification of such a system would involve checks and assessment to ensure that the system/process can be operated safely and that pressure relief valves or other emergency measures are adequate and functional. 

Sometimes referred to as energy flow analysis, HazOps is used primarily in the petrochemical industry. It uses a multidisciplinary team similar to a system safety working group for the systematic review of the flow of materials through a process. It concentrates on key locations in the process known as study nodes and uses a series of guide words and parameters to examine possible deviations and the causes and consequences of deviations (Goidwaite 1985). 

As identified in a safety review (e.g., process hazard analysis [PHA], What-lf Analysis, Hazard and Operability Study [HAZOP]), a defined part (section or subsystem or item of equipment) of a process that has a design intention that is specific and distinct from the design intention of other process parts, which allows the study team to analyze the specific equipment or system in an organized fashion. 

In addition to updated content of the first edition, the revised second edition of the Basic Guide to System Safety has a more expanded and useful glossary of terms it also contains a new chapter describing the basic concept, utility, and function of the hazard and operability study (HAZOP) and what-if analysis. Both of these analytical techniques have been used quite routinely and successfully in the petrochemical industry for decades. As with all analytical methods and techniques presented in this text, it is suggested that the HAZOP smdy and what-if analysis have definite application to general industry operations as well. 

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