HazOp study nodes

A HazOp study resembles a FMEA in that the guide words and parameters tend to describe failure modes and the consequences examined in HazOp studies parallel the effects described in an FMEA. The HazOp study nodes may have the characteristics of the critical items identified by an FMEA. 

HAZOP focuses on study nodes, process sections, and operating steps. The number of nodes depending on the team leader and study objectives. Conservative studies consider e er line and vessel. An experienced HAZOP leader may combine nodes. For example, the cooling looser . .ater chlorination system may be divided into a) chlorine supply to venturi, b) recirculation loop, and e) to .er water basin. Alternatively, two study nodes may be used a) recirculation loop and tower water basin, and b) chlorine supply to venturi. Or one study node for the entire process. 

If the process uses a single large study node, deviations may be missed. If study nodes are small, many are needed and the HAZOP may be tedious, moreover the root cause of deviations and their potential consequences may be lost because part of the cause may be in a different nude. 

I iach study node is examined for potentially hazardous process deviations. First, i he design inte-iit of the equipment and the process parameters is determined and recorded. Process de iatiuns from the design are determined by associating guide words with important process parameters. (iiiidt words for a HAZOP analysis are shown in Table 3.3.4--1 process parameters and dt. i itions are shown in 1 able, T3.4-2. 

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. 

If too much of a process is included in a single study node, deviations may be missed. If too little of a process is included, the study can become tedious. In addition, root causes of deviations and their potential consequences can become separated. Too many study nodes is common for novice HAZOP study leaders. On the positive side, a study with too many nodes is less likely to miss scenarios than one with too few nodes. 

The HAZOP team examines each study node for potentially hazardous process deviations. First, the design intent is defined to delineate the purpose of the equipment and the process parameters. Process deviations are determined by combining guide words with the important process parameters. The established set of guide words is shown in Table 4.14. 

Table 10-6 presents one type of basic HAZOP form. The first column, denoted Item, is used to provide a unique identifier for each case considered. The numbering system used is a number-letter combination. Thus the designation 1 A would designate the first study node and the first guide word. The second column lists the study node considered. The third column lists the process parameter, and the fourth column lists the deviations or guide words. The next three columns are the most important results of the analysis. The first column lists the possible... 

Perform a HAZOP study on this unit to improve the safety of the process. Use as study nodes the cooling coil (process parameters flow and temperature) and the stirrer (process parameter agitation). 

These reviews can be either in addition to or combined with periodic process hazard analyses (PHAs) by using methods such as what-if analysis and HAZOP studies. The latter should consciously focus on identifying scenarios in which intended reactions could get out of control and unintended reactions could be initiated. One means of accomplishing this as part of a HAZOP study has been to include chemical reaction as one of the parameters to be investigated for each study node. Johnson and Unwin (2003) describe other PHA-related approaches for studying chemical reactivity hazards. 

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. 

PERFORMING THE STUDY. A HAZOP Study focuses on specific points of a process called "study nodes," process sections, or operating steps. Depending on the experience of the study leader, the portion of a process included in a single study node can vary. In the most conservative studies, every line and vessel are considered separately. If the HAZOP study leader is experienced, he or she may elect to combine two or more lines into a single study node. For example, the cooling water chlorination system (Example Process 2) could be separated into three study nodes (chlorine supply to venturi, recirculation loop, and tower water basin), two study nodes (recirculation loop and tower water basin combined as a single study node), or one study node (the entire process). 

As defined in the CCPS Book "Guidelines for Hazard Evaluation Procedures (CCPS, 2008), the nodes can be the operating steps in a batch process analyzed by a HAZOP study. Following this approach, the nodes were based on the coke drums steps hsted on Table 1. Therefore, for each step of the coke drum cycle, a node was defined considering the drum and aU the ahgmnents needed to perform that phase. Preferably, the nodes boimdaries included the valves that should be opened to allow the operating step to occur. 

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). 

In HAZOP studies and what-if analyses, the review team must first identify the areas or components of the system that will each be analyzed during the review process. In the chemical industry, these individual components are typically referred to as nodes. There are three basic criteria for identifying the nodes to be reviewed (Nolan 1994) ... 

Once the individual nodes have been identified, the HAZOP smdy and what-if analysis can be initiated. As stated earlier, the information developed through the what-if analysis can then be used as input data for the more complete and detailed HAZOP study of the vapor degreasing system. 

Node Term used to identify individual system components when conducting a HAZOP study. 

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