PARTIAL - PARALLEL - REDUNDANT AND INTERMEDIATE STORAGE MODULE AVAILABILITY FACTORS IN THE DESIGN OF A CHEMICAL PROCESS PLANT

The consideration of equipment reliability is usually neglected in the design of chemical process plants even though equipment reliability is often responsible for how well a plant performs economically. In this study, the availability (reliability) of two equipment modules was studied and calculated as an availability factor. This availability factor was then used in an economic function to determine the most profitable equipment configuration for a specific requirement. The two equipment configurations were the partial-parallel-redundant (PPR) equipment module, in which two or more identical equipment units of a fraction of the design capacity were specified in place of a single unit capable of handling the entire production; and the intermediate storage module, in which a buffer tank was provided between two equipment units. The availability factors of the modules were calculated according to an analytical procedure based on probability theory and Markov analysis. The results were compared to the results of a simulation program. The module availability factors were a function of the mean time between failures and mean down time of the individual units and of the unknown parameters of the modules. The unknown parameters for the PPR module were the number of equipment units used and the capacity of the units; the unknown parameters for the storage module were the tank size, the tank working volume, and the capacity of the two units surrounding the tank. Comparison of the analytical and simulation results shows that in some cases, use of the analytical procedure may be warranted: the higher the module availability factor, the better the correlation between the analytical procedure and the simulation procedure results. Since the analytical procedure is much less expensive to use than the simulation, further studies should be made to broaden the circumstances under which it can be used.