Semi-Markov processes with semi-regenerative states for the availability analysis of chemical process plants with storage units

Abstract

Design to capacity is an engineering principle that is increasingly applied in chemical industry, among others owing to increasing plant sizes and associated investments. This principle aims to reduce over-capacity, over-sized buffers and excessive redundancy. Concurrently, a high level of availability is targeted over the entire production chain. The consequences of unavailability of highly interconnected chemical process plants can be significant because a technical disruption in one plant is able to spread over the entire production network. In chemical process plants not only technical equipment determines the availability, but also storage units, which are able to bridge times of planned or unplanned interruptions of production. To find a balance between the principle of design to capacity and high production availability, the influence of different design parameters, such as capacity of production units, redundancy concept and the size of storage units, have to be evaluated and integrated in the design process. In this article, we present an analytical method for availability evaluation based on extending Semi-Markov processes integrating storage units and multiple production lines. Semi-regenerative states are used to capture the characteristics of storage units, and an approach is proposed in this work to assign distributions for the remaining holding times in these states. The proposed modelling and analysis are demonstrated on two case studies.