Numerical Simulation of Cold Helium Safety Discharges into a Long Relief Line

Abstract All existing and currently constructed large superconducting particle accelerators use liquid or supercritical helium for transferring cooling power from the cryogenic plant to the accelerator magnets and cavities. These accelerators have extremely elongated structures and therefore require widespread cryogenic distribution systems as well as advanced gas management systems. The design and operation of their cryogenic system are strongly affected by the requirements of high reliability and operating cost minimization. This strongly influences pressure equipment safety strategies. Becauseaccidental helium discharges from the accelerator cryostats and cryomodules cannot be excluded, possibilities of recovering helium releases from safety devices are taken into consideration. Collecting discharged helium and transferring it back to the cryoplant via a long recovery line is not only an option, but also a must. Usually the baseline design choice for the helium recovery system is a set of safety valves connected to a bare relief line that ends in a gas bag. However, rapid and fast discharges of cold helium into warm relief lines can result in significantly unsteady, compressible and thermal flows. Therefore the proper designing and sizing of the recovery system have to be supported by detailed analyses of all expected fluid dynamics and thermodynamics phenomena. This paper describes the numerical simulations of cold helium discharges into a long, warm safety relief line. The simulations have been done for the helium recovery system of the superconducting proton accelerator that is under construction at ESS in Lund, Sweden. The paper discusses the model assumptions and presents some example results.