Flow induced vibrations of multi-span tube bundles of large condensers: experimental studies on full scale models in steam cross-flow

Large condensers, as found in 900 MW PWR power plants, are characterized by the existence at the turbine exhaust of very high velocities which are assumed to decrease rapidly and cause no damage to the tubes by erosion or vibration phenomena. Nevertheless, in some cases important vibrational motions of condenser tubes were observed: tubes impacted and rubbed one against the another, which led to failures by longitudinal thinning and fretting, or by transverse fatigue fracture. Depending on the tube nest arrangement, these failures could be located in small areas, or be widespread through the whole condenser, affecting nearly all the tubes exposed directly to steam flow. As far as vibration phenomena are concerned, the large amplitude motion of the tubes occurred generally, but not necessarily, when turbines were heavily loaded and/or when circulating water inlet was very cold. In order to define new design rules, tests have been carried out to study the flow induced vibration phenomena on such tube bundles, that is to say multi-span tube bundles submitted to low pressure steam flow, with simulation of the condensation phenomenon on the tubes. The full scale models are described and the main results analysed. (U.K.)