Computer-aided hazard identification of batch operations

Hazard identification is a critical task that needs to be carried out for safe process design and operation. HAZOP studies are widely used for identifying hazard and operability problems. However, HAZOP studies are time consuming, labour intensive and expensive. Automated HAZOP identification systems that emulate the HAZOP technique have been developed to overcome this bottleneck.

This work considers batch processes, where a batch plant moves through a number of different stages during operation, rather than each equipment item remaining in a “steady state”, as is normal for continuously operating plants. In batch plants deviations which can lead to hazards can arise both from deviations from operating procedures and process variable deviations. Therefore, the effect of operator actions needs to be considered.

Much of the research on automated HAZOP identification, based on signed-directed graphs, has concentrated on continuous plants. However, this technique is inappropriate for batch processes. The signed-directed graph approach used in continuous plant HAZOP emulators is found to be unsuitable for batch plants. The main problem is that it does not keep state related information as the HAZOP analysis moves from one operating instruction to the next.

Preliminary research at Loughborough University has demonstrated the feasibility of applying stated-based qualitative simulation in this domain. The research described in this thesis builds on an existing proof of concept system. The project develops a prototype automated batch HAZOP identification system, called CHECKOP. CHECKOP takes a plant description and a set of operating instructions as input and produces a HAZOP report automatically. Object- oriented technologies are utilised to model the state changes required for automated batch HAZOP analysis.

To produce a product in a batch process, a plant operator follows a series of operating instructions. For an operating procedure to be analysed by a computerised system, such as an automated HAZOP system, it must be formally represented. CHECKOP verifies if an operating procedure achieves its desired results and does not also lead to any additional, unexpected effects. Deviations may be applied to the operating instructions to simulate batch HAZOP. CHECKOP simulates the change of plant state as each operating instruction is executed. Potential hazards which result from the operating instructions are tested for. CHECKOP infers the consequences if a certain instruction in the procedure is not executed, or if the instruction is carried out too early or too late, etc.

The original, existing proof of concept system was unstructured. CHECKOP defines a formal template representation for the operating instructions. A modelling structure is derived to describe the state and connectivity of plant items. A state-based simulation engine simulates the effect of the operating instructions upon the plant model. CHECKOP contains a rule- based system to test for potential hazards or operability issues.