Comprehensive Modeling and Optimization of Standby Mechanisms in Continuous Chemical Plants

In a continuously operated chemical plant, not only the critical unit may fail after a long period of operation, but also its process load may vary from time to time. Thus, it is necessary to incorporate standby mechanisms to maintain uninterrupted production and also satisfy the unexpected increases/decreases in external demands throughout the operation horizon. Although a few related studies were reported in the literature, a comprehensive analysis of multilayer standby mechanisms in continuous processes still has not been carried out. To address this need, a generalized mathematical programming model has been constructed in this work to automatically synthesize the optimal designs and maintenance policies of the standbys for any continuous process by minimizing the total expected lifecycle expenditure. A MATLAB code has been developed to execute the optimization runs via genetic algorithm. The feasibility and effectiveness of the proposed model and solution method are demonstrated in this paper with case studies concerning the pump systems in a typical chemical plant. From the optimization results, one can obtain proper design specifications of the standby mechanisms, which include: (1) the number of protection layers (or online warm standbys) and the number of cold standbys stored offline; (2) the numbers of installed sensors in each measurement channel, the corresponding voting-gate logic, and their spares; and (3) the inspection interval of switch and the number of its spares.