10 Multistage severe service control valves are extensively used in various energy systems, such 11 as oil & gas, nuclear etc. The primary purpose of such valves is to control the amount of fluid 12 flow passing through them under extreme pressure changes. As opposed to the conventional 13 valves (butterfly, gate etc.), control valves are often installed in energy systems with 14 geometrically complex trims, comprising of various geometrical features, formed by a 15 complex arrangement of cylindrical arrays. The pressure within the trim varies in controlled 16 steps and hence, cavitation resistance can be embedded in the trim through improved design 17 process for the trim for severe service applications in energy systems. The flow 18 characteristics within a control valve are quite complex, owing to complex geometrical 19 features inherent in such designs, which makes it extremely difficult to isolate and quantify 20 contribution of these features on the flow characteristics. One of the most important design 21 parameters of such trims is the flow coefficient (also known as flow capacity) of the trim 22 which depends on the geometrical features of the trim. The design of valves for particular 23 performance envelop within the energy systems depends on effects of complex trim 24 geometrical features on performance characteristics; hence, the focus of recent research is on 25 quantifying the hydrodynamic behaviour of severe service control valves, including the trims. 26 This includes the estimation of the local flow capacity contributions of the geometrical 27 features of the trim through detailed numerical investigations. In this work, a tool has been 28 developed that can be used to predict the local contribution of geometrical features on the 29 flow coefficient of the trim. It is expected that this work will result in better performance of 30 the energy systems where these valves are used. 31 32 Capacity, Energy Systems 34 35 1.0 Introduction 36 Valves are an integral part of any piping network and are used in a variety of industries for 37 various process control applications. The design of valves is a specialist area and the 38 performance of valves is integral to the performance of the energy systems. The severe 39 service control vales typically have very complex flow paths and it is necessary to have 40 understanding of flow characteristics through the complex pathways to eliminate undesirable 41 effects such as vibrations, noise and cavitation in energy systems. The designs of such valves 42 are carried out with the help of well-known standards but many times undesirable local flow 43 effects cannot be eliminated through such designs. The standards are continuously updated to 44 incorporate state of the art knowledge into the design process through extensive experimental 45 and numerical research work carried out all over the world. Newer designs are continuously 46 * Corresponding Author Tel.: +44 1484 472323 1 only partially applicable. In such cases a thorough fluid dynamic analysis is necessary to 2...