Risk-based maintenance (RBM) strategy call be used in developing a cost-effective maintenance plan to make financial and safety improvements in a petrochemical system. Most maintenance is imperfect in process industries Previous works oil RBM did not consider imperfect maintenance and thus could hardly meet the engineering requirements of process industries An improved RBM approach based oil the proportional age reduction model is proposed in this paper. The parameters of the failure distribution and maintenance effect of equipment are estimated by maximum likelihood estimation. The financial risk for each subsystem is estimated and compared with the risk criteria. Under the risk constraints, the proper interval of periodic preventive maintenance is determined for the equipment in high-risk subsystems The case of a reforming reaction system in a petrochemical plant IS Studied using the improved RBM approach. The results show most equipment in this system is imperfectly repaired. The imperfect nature of the periodic preventive maintenance means it needs to be carried out more frequently. (C) 2009 Elsevier Ltd. All rights reserved.