In modern chemical plants, there is a class of continuous processes operated under varying loads. In these processes, the standby mechanisms are needed to ensure that the downstream demand is always satisfied during the operations. Although a few related studies have been reported in literature, a comprehensive analysis of multilayer standby mechanisms in processes under varying loads has not been carried out. In this paper, a generalized mathematical model is developed to automatically generate the optimal standby mechanisms for any given processes by minimizing the total expected lifecycle expenditure. A Matlab code has been developed to perform these optimization tasks via genetic algorithm. The feasibility and effectiveness of the proposed model are demonstrated with the case study concerning a fan system for providing instrument air in a typical chemical plant. From the optimization results, one can determine the optimum configurations of the standby mechanisms, which include: (1) the number of protection layers, (2) the number of both online and spare sensors in the measurement channels, (3) the corresponding voting-gatelogic in each channel, (4) the inspection intervals of switch and (5) the number of spares for switch. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.