PSI - Issue 13

Evgeniia Georgievskaia / Procedia Structural Integrity 13 (2018) 971–975 E. Georgievskaia / Structural Integrity Procedia 00 (2018) 000 – 000

972

1. Introduction

Hydraulic turbines have proven themselves as reliable and stably operating equipment, thus within many years they have not been considered as a potential source of serious hazard. As a result, a little attention was paid to issues of reliability, safety and lifetime of the hydraulic turbine, the essential component of which is evaluation of durability. However periodically incidents and accidents occur, information about which can be found, for example, in overview papers of Liu X. et al. (2016), Trivedi C. et al. (2013), Georgievskaia E. (2018), Ming Zhang et al. (2016). For Russia accident at Sayano-Shushenskaya Hydroelectric Plant occurred in 2009 became a decisive point. After it a great number of new industry regulatory documents controlling different aspects of hydraulic turbine life cycle and directed for increase of hydraulic equipment operation safety was developed. However, until now there is no normative technique for assessment of hydraulic turbine durability at the operation stage considering operating conditions and individual features. Under current conditions this is a highly challenging issue especially for equipment having been used for a long time.

Nomenclature σ a

dynamic component of stresses static component of stresses

σ s

ℓ

crack length

N K

number of cycles

stress intensity factor (SIF) threshold value of SIF

K th K С HF LF

fracture toughness high-frequency low-frequency

NDT

non-destructive testing

2. Features of operating conditions

Hydraulic units including powerful ones as highly-maneuverable facility are often used to cover energy peak demand and maintain stability for electric power system that results in often change of their operating modes. Operation factors as noted by Georgievskaia E. V. (2017. Influence of tr ansition…), Zouhar J. et al. (2016) and many others are prevailing during assessment of hydraulic turbine durability and service life determining frequency of influence and level of element loading. Level of dynamic stresses σ a can significantly differ in various modes changing from several percents (in relation to static component σ s ) in the mode being optimum by efficiency to tens of percents in off-design modes far from nominal power. Fig. 1 shows specific relative level of dynamic stresses σ a /σ s in design and off-design modes according to the results on processing of experimental researches performed by specialists of NPO CKTI JSC for a number of high-head powerful Francis units. Traditionally off-design modes include nonstationary (low and part load, forced by power, speed-no-load), in which at the design stage long-term operation was not supposed, and transient modes, particularly, hydraulic unit startups-shutdowns. As the provided data show (Fig. 1), in part load (35 – 50 %) modes clear dynamic stress surge is observed. Increased dynamic stresses results in quick accumulation of damages in material, previous crack formation in load-bearing elements and finally destruction of hydraulic turbine due to degradation failure. However, not all cracks are similarly hazard in terms of equipment operation reliability assurance. Some foreign specialists, for example D. Frunzăverdel et al. (2010) consider that even extended through cracks (Fig. 2а) in the area of blade adjoining with runner crown of Francis turbine due to high structure stiffness don’t significantly influence safety of hydraulic turbine operation and affect only cost and duration of repair for their elimination. Such cracks are often detected only during scheduled shutdown of the hydraulic unit, as they don’t almost influence turbine parameters controlled during operation.