Assessment of Dynamic Instability in Once Through Steam Generator

Hydrodynamic flow instability in Once Through Steam Generators (OTSG) is one of the important problems in the design and operation of Liquid Metal Fast Breeder Reactors (LMFBRs). Under certain operating conditions, water flow in OTSG is susceptible to instability due to the close coupling between the thermal and hydraulic processes. Sustained flow oscillations due to instability are undesirable since they result in flow mal-distribution among the tubes resulting in thermal stress, mechanical vibrations and system control problems. It is therefore, necessary to assess the operating conditions, under which instability occurs so that the system may be designed to operate always under stable conditions. The cause of the main type of instability, important for the design of SGs is the propagation of density waves. This type of low frequency instability is referred to in literature as parallel-channel, density wave, time delay or mass flow-void feedback instability. Dynamic instability (density wave oscillation DWO) occurs because of the phase mismatch between the primary perturbation (water flow) and the response to this perturbation (pressure drop). As many tubes are operating under essentially constant pressure heads, this mismatch can lead to sustained/diverging oscillations. Water flow oscillation in tubes manifests as oscillations in the steam temperature at the tube outlet/pressure fluctuations. However it is difficult to instrument individual tubes in SG for such measurement in an operating plant. If the flow oscillation in the tube manifests itself in the overall module flow, then fluctuation in the overall flow/flow noise could be utilized for on-line stability measurements. Towards this, experiments were conducted in the sodium heated once through steam generator in an OTSG model. To confirm the extent of oscillation in the steam temperature and in inlet water flow, 3 tubes out of 19, were monitored besides overall module flow. Main objective of the present study was to assess the occurrence of dynamic instability in SG through module inlet flow perturbations, measured by ΔP measurements across the orifice at entry to the tubes and steam temperature fluctuation measurement at the outlet of tube by bare thermocouples. This paper discusses the experiments carried out in the Steam Generator model of Prototype Fast Breeder Reactor to investigate the instability phenomenon, the instrumentation details, the results and its discussion.