Study nodes

Item Study node Process parameters Deviations (Guide words) Possible causes Possible consequences Action required Assigned to i i... 

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. 

Choose a study node (vessel, line, operating instruction). 

Describe the design intent of the study node. For example, vessel V-l is designed to store the benzene feedstock and provide it on demand to the reactor. 

Repeat steps 4 through 10 until all applicable process parameters have been considered for the given study node. 

Repeat steps 2 through 11 until all study nodes have been considered for the given section and proceed to the next section on the flow sheet. 

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). 

The guide words are applied to the study node of the cooling coils and the stirrer with the designated process parameters. 

In Figure 10-8, identify the study nodes of the reactor process, as shown. 

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. 

In this approach, each guide word is combined with relevant process parameters and applied at each point (study node, process section, or operating step) in the process that is being examined (see table 5.7). 

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). 

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. 

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). 

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