Process Hazard Analysis (PHA) has become routine for chemical processes at every step of its lifecycle. The “Bow-Tie” method offers a graphic alternative to the traditional Hazard and Operability (HAZOP) method and the “What-If” method. One recognized hazard in the chemical industry is the runaway exothermic reaction. This article illustrates application of the  Bow-Tie methodology for conducting an initial PHA study for a semi-batch chemical reactor system that has potential for a runaway exotherm event. The PHA application described presents general concepts and key issues and is not intended to present a complete case study. 

Design review
The accepted practice in the chemical industry is to conduct a progressive series of increasingly detailed safety reviews for a new or significantly modified chemical process. Initial PHA studies are most often conducted during the conceptual design stage, with the objective being to identify and evaluate:

• major hazards associated with the chemicals involved in the process
•  major hazards associated with the process and its equipment
•  changes to operating procedure
•  fundamental process safety and control philosophy and criteria
•  significant hazard control safeguards (hardware, software, and administrative)
•  aspects of the process/project that are anticipated to require additional safety review and evaluation in more detail

Initial PHA studies can be conducted using several methodologies; the most common are the “What-if” and the Hazard and Operability (HAZOP). The  Bow-Tie analysis offers a cost-effective alternative approach for processes that are well understood. During the early stages of a project, it is premature to apply an extensive and rigorous review method such as Quantitative Risk Analysis (QRA) or Fault Tree Analysis (FTA), because a large portion of the information needed for these studies is not yet available. The Bow-Tie methodology is a qualitative PHA approach ideally suited for initial analysis of an existing process, or for application during the middle stages of project design.

The scope of a PHA can vary significantly, depending on numerous factors; including:

• nature of the materials involved (reactivity, toxicity, flammability, stability, physical properties and others)
• nature and complexity of the process chemistry and reaction kinetics
• maturity of the process (new process, old process, existing process being modified)
• degree of understanding related to the process, the chemicals, the equipment (operating history)
•  regulatory and code compliance issues
•  facility siting studies
•  proposed modifications (to the process, the equipment, control systems, organizational structure, and administrative management systems)

Batch and semi-batch synthesis chemical reaction processes that have the potential for runaway exotherms require a thorough and systematic identification and evaluation of process hazards and risks. This is best accomplished in progressive stages, as the design becomes more developed. Successful design reviews represent an optimum trade-off of several factors: the amount of information available to the PHA team, scope of the PHA study, cost to make modifications and the degree of remaining unknowns in the design. Timing is important. If the analysis is conducted too late, the cost for modifications can be significantly increased. If the analysis is conducted too early, items may be identified for which no action can be taken until the design progresses further to conclusion.

A screening PHA can be viewed as an optimization technique that allows identification of significant risk exposures with a minimum investment of resources. Screening studies can identify aspects of the process and project that can be excluded from additional or detailed review. In addition, a screening study may identify specific scenarios that provide input to formal QRA risk study.

The Bow-Tie technique
The Bow-Tie PHA methodology represents a synergistic adaptation of three powerful conventional system safety techniques (Fault Tree Analysis, Causal Factors Charting and Event Tree Analysis)¹. The Bow-Tie approach is highly effective for initial PHAs to ensure high probability-high consequence events have been identified and addressed. It can be described as a combined application of a high-level fault tree and a high level event tree. It provides a representation of the causes of a hazardous scenario event, likely outcomes, and the measures in place to prevent, mitigate, or control hazards. Existing safeguards (barriers) are identified and evaluated for adequacy. Additional protections are recommended where appropriate. Typical cause scenarios are identified and depicted on the pre-event side (left side) of the bow-tie diagram (Figure 1). Credible consequences and scenario outcomes are depicted on the post-event side (right side) of the diagram, and associated barrier safeguards are included. One attribute of the Bow-Tie method is that in its visual form, it depicts the risks in ways that are readily understandable to all levels of operations and management.

Generic Bow-Tie diagram

Figure 1: Click to enlarge

The conventional Fault Tree Analysis (FTA) methodology focuses on a designated "top event" and looks backwards in time to identify those specific factors, conditions and events that could, in combination, result in that specified top event².  From a time progression perspective, the fault tree ends with the top event. Conversely, the conventional Event Tree begins with a specified event and looks forward in time. It is an inductive approach identifying and evaluating potential outcomes from a designated set of conditions and options³. Both techniques have the capability to identify and evaluate existing, or proposed, safeguard measures that prevent, or mitigate, adverse consequences. The Causal Factors Chart (CFC)⁴, sometimes referred to as the Events and Causal Factors (ECF) chart, is a formal, and systematic, incident investigation and root cause analysis technique. It combines critical thinking, logical analysis, and graphic representations to analyze and depict an accident event scenario. CFC also has been applied to root cause analysis. The ECF chart depicts the necessary and sufficient events and causal factors associated with a specific accident scenario.