A Functional approach to HAZOP studies

This paper presents a functional approach to HAZOP studies in which the plant is divided along strictly functional lines starting from the purpose of the plant and continuing down to the function of a single node, e.g. a pipe section. The approach leads to nodes with simple functions such as liquid transport, gas transport, liquid storage, gas-liquid contacting etc. From the functions of the nodes the selection of relevant process parameters and deviation variables follow directly. The knowledge required to perform the pre-meeting task of dividing the plant along functional lines is that of chemical unit operations and transport processes plus a little familiarity with the plant a hand. Thus the preparatory work may be performed by a chemical engineer with just an introductory course in risk assessment. In any given plant there will be several nodes with the same function, e.g. gas transport. These nodes are analysed together, which removes some of the repetitive nature of the HAZOP meeting. The functional approach lends itself directly for implementation into a computer aided tool for HAZOP studies. Such a tool will facilitate finding causes far away from the site of the deviation. The approach is exemplified by a HAZOP study of an industrial scale heat integrated distillation pilot plant HIDiP at the Dept. of Chemical Engineering. The purpose of the plant is to separate a binary mixture into pure products using the least amount of energy. This plant is divided into 8 sections, which are further divided into 20 nodes. Typically in this pilot plant a section is divided into 1-5 nodes. Several of the nodes have a similar function, e.g. transport of liquid. Two HAZOP studies of a section of the plant are compared, i.e. one traditional with one one based on the functional approch. Introduction Since the development of hazard and operability (HAZOP) studies by ICI in the mid 60's they have been a cornerstone in risk assessment of process plants. The purpose of the HAZOP study is to investigate how the facility responds to deviations from design intent or normal operation, e.g. to find out if the plant has sufficient control and safety features to ensure, that it can cope with expected deviations encountered during normal operations. The HAZOP study is traditionally performed as a form of structured brainstorming exercise facilitated by a HAZOP study leader and the experience of the participants. A traditional HAZOP study has the following phases (Skelton, 1997):