Destruction of the hydraulic unit shaft: Why it is possible?

Abstract

Hydraulic units produce around 20% from the total world energy and almost 75% from total renewable energy as well as play a vital role in providing energy system stability. The shaft is one of the key lifetime-determining elements for hydraulic units. Its durability, lifetime, fatigue and fracture resistance define reliability and safety of a hydraulic unit in general. Despite the presence of functioning continuous and periodic hydraulic unit technical condition control systems, there are several known cases of shaft destruction leading to their replacement, long downtimes and significant financial losses for energy industries.The goal of this research is determining main reasons for shaft destruction in hydraulic power plant operating conditions. Using computational modeling for a large vertical high-head hydraulic unit as an example shows that a growing crack doesn't significantly influence shaft dynamic properties. As a result, even a long through crack may escape detection by present vibration diagnostics systems. At the same time, non-destructive metal state control methods are also not always effective due to structural design features and large size of hydraulic unit shafts.To prevent serious incidents in the future there is a suggestion to improve vibration diagnostics data-processing algorithms, introducing an additional diagnostic sign – a ratio of registered double rotation frequency runout values to runouts at rotation frequency, which allows to identify a hydraulic unit with a growing shaft circumferential crack even with vibration state meeting compliance norms and requirements. Additionally, it is necessary to estimate residual HU shaft lifetime through calculation taking into account possible initial cracks not revealed previously by non-destructive control.