PHASUITE: AN AUTOMATED HAZOP ANALYSIS TOOL FOR CHEMICAL PROCESSES Part I: Knowledge Engineering Framework

ion of process: functional representation The device or physical systems can be abstracted into different levels of abstraction. For example, an electronic amplifier is composed of different components including transistors, resistors, capacitors and so on. Each component behaviour can be described in terms of their currents and voltages. Using the techniques described above, a description of device behaviour can be generated in terms of currents and voltages. However, the behaviour of the device as a whole, which is an amplifier, is also of interest. And it is a higher level description. Sembugamoorthy and Chandrasekaran (1986) proposed the functional representation (FR) framework, for representing goal-directed, flexible reasoning that bridges the device-level behaviour with the descriptions at the component level. In FR, the function of the overall device is described first and the behaviour of each component is then described in terms of its contribution to the function. In the FR framework, the structure is represented by listing the component names and their functions and indicates how the components are put together to make the device, i.e., describe the relations between the components. The basic idea in describing how a device achieves its function is that of a causal process description (CPD). A CPD can be thought of as a directed graph whose nodes are predicates about the states of the device, and the links are the causal transitions. The explanatory annotations include By-CPD, ByFunction-Of and By-Domain-Law, and so on. The FR framework has been applied to solve different problems. Sticklen and his group (Pegah et al., 1993) applied FR to simulation. They describe a simulation of the fuel transport system of an F-18 aircraft using this technique. They also (Sticklen et al., 1991) describe integrating qualitative and quantitative simulations in a FR framework. The FR scheme helps the simulation systems focus on those specific quantitative equations which are to be used in the actual computation. Goel and Stroulia (1996) describe a functional model called structure-behaviour-function (SBF) to describe a device and applied it for adaptive design. FR is also applied in representing problem solvers, and representation of scientific theories. A summary of FR application can be found in Chandrasekaran (1994). The work on creating generic device libraries, where device classes at different levels of system description are represented along with parameterized structural representation and the corresponding CPD, can be found in Chandrasekaran et al. (1998). Current HAZOP analysis can be divided into two types, operation-centred analysis and equipment-centered analysis, based on the emphasis of the analysis and basic analysis node. Equipment-centred analysis is mainly applied to continuous processes, where the analysis progresses from upstream to downstream, starting from a deviation in a piece of equipment. However there is no need to consider each line and every single minor item of equipment, instead, smaller items are grouped into logical units according to their functionality. Operation-centred analysis is mainly applied to batch processes, since operational sequence of steps is important for batch processes. Deviation starts from one operation and is propagated along the operational sequence. In fact, from functional representation point of view, operation and equipment are two different abstraction levels of the process. Compared to the device and its components, operation can be analogous to device and the equipments used to carry out the operation can be analogous to components. The terminology of operation is used to describe a function carried out by one or more equipments. Equipments are real entities, while operations are not. For example, the function or the goal of the operation ‘TransferWithPump’ is to transfer materials (substances) from one place (source) to another place (sink) using a pump through several pipelines. From this description, the equipments used in this operation can be identified to include a pump, a source, a sink, pipes between those equipments, valves on pipe, controllers and so on. Since operation emphasizes on functionality, it is possible that the functionality is carried out by different kinds of equipments. As an example, a centrifuge pump may be used in the operation of ‘TransferWithPump’ to transfer normal materials, but cryogenic pump is needed when transferring cryogenic materials. If abstraction of the different level of the process is not explicitly presented, different operations must be defined when any of its components are different. For example, we will have operations for TransferWithCentrifugePump and TransferWithCryogenicPump. The complexity of the combinations in that case will be exponential. So the operation and equipment represent the different levels of the process. Functional representation is adopted as the methodology to bridge the gap between these two levels. Operation-centred analysis emphasizes on overall functionality, and overall analysis for the equipments carrying out the operation as a group. Equipment-centred analysis emphasizes on hazard analysis at equipment level. Decomposing the process representation to two abstraction levels based on functional representation gives us the capability to describe the process in more detail, and thus produces better results. Operation-centred analysis supplies the hazard results as well as operability results, which are hard to achieve without the abstraction. By connecting the operation and equipment level, more detailed results can be obtained, which are hard to achieve through only the operation level analysis. Basic design Operation model emphasizes on functionality, and is created according to functional representation. Equipment model emphasizes on relationships between the local process variables. For example, the functional representation for TransferWithPump consists of: . Intended function: transfer materials fm1, m2, m3g from Source A to Sink B, using pump; pump can be centrifuge pump, or cryogenic pump and so on. Trans IChemE, Part B, Process Safety and Environmental Protection, 2005, 83(B6): 509–532 PHASUITE: PART I 527