3.6 Qualitative Evaluation of Inherently Safer Design (QEISD)
3.6.1 Stage I: Identification of inherent hazards in a process unit
It is believed that through the right and effective approach of identifying intrinsic hazards would lead to the best process of generating ISD options if suitable mechanism is used to find the hazards. Therefore, the purpose of this stage is to detect potential inherent hazards within a process unit through the application of Register, Investigate, and Prioritise (RIP) as the sub-tool for this stage. This is to allow a simple and systematic generation of potential source of hazards in the process in a single tool.
RIP represents three simple steps: i) Register, ii) Investigate and iii) Prioritise. For illustration, Figure 3.10 demonstrates the RIP tool in a single diagram for a reactor to identify the inherent hazards. For the first step, Register, is developed based on process heuristics which is supported by three criteria; design factor, process attribute and hazard indicator. Design factor represents the common design elements in a process unit such as chemical substances, process routes, process conditions, type of process unit etc. which significantly need to be explored because the existence of intrinsic hazards mostly contributed by the above design elements. Process attribute captures the characteristics available in the design factor, for example, reactant, solvent, and catalyst are the characteristics for chemical substances. Hazard indicator denotes the unsafe behaviour of the process attribute. For instance, flammable,
Stage Sub-Tool Technique
Stage I:
Identification of Inherent Hazards
Register, Investigate,
Prioritise (RIP)
Process Heuristics TRIZ-Predictive Failure Analysis
Process Safety Databases Stage II:
Generation of ISD options
Inherent Design Heuristics
(IDH)
Heuristic of ISD concept IS guidewords Stage III:
Ranking of ISD options
Inherently Feasible Matrix
(IFM)
Conventional Process Design Stages
Stage IV:
Evaluation of ISD options
Inherently Safer Matrix
(ISM)
ISD Heuristic IS Guidewords Interaction Matrix
Figure 3.10: Process flow
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to identify hazaards using the RRIP tool for a reaactor
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Figure 3.11: Process heuristics for a reactor
Table 3.4: Predictive Inherent Hazard Analysis (PIHA) for chemical reaction Step i: Ideal State We want no thermal runaway reaction in the nitration of
toluene
Step ii: Inverse Ideal State We want a thermal runaway reaction to happen in the nitration of toluene
Step iii: Exaggerate We want to generate the reaction heat and release in the process and cause severe injury, fatality and damage Step iv: Find resources How to accomplish this?
What intrinsic resources are required?
The third step, Prioritise, is to identify the dominant hazards because not all predicted hazards are necessarily hazardous. In addition, not all the listed hazards would cause accidents when the chosen materials or process conditions are not credible as hazardous or well below the safety threshold limit values. Therefore, some common process safety databases such as Bretherick’s Handbook, National Fire Protection Association (NFPA) ranking, Incompatible Chemicals Database, Material Safety Data Sheets (MSDS), flammability limits, TCPA and any experimental results are used to prioritise the potential hazards in the studied process. For example, Table 3.5 shows the threshold quantities based on the ranges of heat reaction obtained from Toxic Catastrophe Prevention Act (TCPA, 2004) to provide guidance related to the limit of quantity of chemicals in terms of inventory. Appendix I listed several threshold limits of process safety criteria based on common references as a guideline to prioritise the predicted hazards.
Table 3.5: TCPA (2004) guidelines to show the threshold quantities based on heat of reaction
Heat of Reaction (cal/g) Threshold Quantity (lb)
100 ≤ −∆H ≤ 200 13,100
200 ≤ −∆H ≤ 300 8,700
300 ≤ −∆H ≤ 400 6,500
400 ≤ −∆H ≤ 500 5,200
500 ≤ −∆H ≤ 600 4,400
600 ≤ −∆H ≤ 700 3,700
700 ≤ −∆H ≤ 800 3,300
800 ≤ −∆H ≤ 900 2,900
900 ≤ −∆H ≤ 1000 2,600
−∆H ≥ 1000 2,400
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The outcomes from RIP analysis must be documented by the analyst to ensure all identified hazards are communicated properly throughout the process lifecycle. The results shall be referred in every safety review or during management of change session. Table 3.6 shows the proposed RIP form for recording the inputs and findings for Stage I with brief descriptions on the functions of each column.