Search academic texts

Information × Registration Number 0216U006891, 0116U006349 , R & D reports Title Determining the causes of working blades failure of the 4-th stage of the low-pressure cylinder of turbine generator based on the scientific and technical fractography features examination of the fracture surfaces and estimation of the technical state of operated metal using its mechanical properties popup.stage_title Head Nykyforchyn Hryhoriy, Registration Date 29-07-2016 Organization Karpenko Physico-Mechanical Institute of the National Academy of Science of Ukraine popup.description2 On the base of analyzing all the possible factors affected on the blades workability of the 4th stage of the low-pressure rotor and could lead to their destruction was concluded that first of all the blade number 91 broken due to the influence of different factors. In particular such factors usually considered: additional stress concentration at the end of hardened layers (caused by the high roughness of the hardened surface due to its insufficient melting), insufficient thickness and insufficiently high micro hardness and strength of the hardened layers (its promote the hardening layers cracking under the influence of working load, change of blades oscillations frequency, as well as facilitate its resonance and, as a result, increase the amplitude loading). The high stress concentration and the possibility of contact of the base metal with the condensate (with aggressive components dissolved in its) create favorable conditions for corrosion-fatigue subcritical crack growth in a cross section of the blades up to its destruction. The fracture of the blade number 56 is associated with significant porosity of the hardened layer, which promoted to it cracking (crack growth have fixed from almost all the pores in the coating cross-section) and change the natural frequency of oscillation of the blade. Moreover thought that this blade most likely destroyed after the fracture of segment-like bandage bar due to subcritical corrosion-fatigue crack growth in it. The destruction of the bandage (it strength was lower than stated in the quality certificate if estimated for hardness), has violated vibrational state of the blade, increase the amplitude of its pulsations and facilitate crack initiation from the large pores in the hardened layer as a stress concentrators. It created the preconditions for corrosion-fatigue cracks generation in the blade and their development until its destruction. In general, the both blades fractured due to corrosion-fatigue crack growth in its cross-section during so short operation time. It would be realized as a combined effect of the favorable stress-strain state (in particular, through the cracking of the coating on the blades leading edges obtained by electric spark alloying (ESA) method, the change of the blades oscillations frequency, insufficient packing density of blades in the groove of disc) and the presence of condensate in a phase transition zone. To avoid such damages in the future, in addition to compliance by the CHP staff of the traditional regulatory requirements for water chemistry, temperature, force, vibration, mechanical operating parameters of the blades recommended: 1.To prevent the possibility of it high roughness of the surface of hardened layers at least a visual inspection of the steam turbine rotors blades must put into the practice of the acceptance of blades from the supplier. 2.To neutralize the negative impact of the residual tensile stress (near 500 MPa at the transition from the parent blades metal and hardened layer deposited by ESA technique), is recommended to treat the hardened surface by the sandblasting (better the wet type) to create on the surface of coating the compressive stresses up to 200 MPa. 3.To ensure the stability properties of the coating, deposited by ESA technique, from the blade to blade the input control for micro-hardness of the hardened layer must entered. 4.To avoid the harmful effects on the stress state of bandage bar the control of the compliance with the requirements on its installation (distance between the blade surface and the bend in the bandage bars and the angles of its bending) must entered. The input control of the bandage strength (at least by control of their hardness) must entered too. 5.To eliminate the possibility of blades moving under the action of centrifugal forces (especially during shut-downs or starts of the turbine generator) the independent control of the blades density packing during their assembly in the groove disc of a steam turbine rotor must be entered. 6.To reduce the harmful effects of aggressive components of the technological environment concentrated in the condensate the requirements for water chemistry of steam and feed water, condensate quality in the turbine must respected.5481 Product Description popup.authors Бабій Леонтій Омелянович Кречковська Галина Василівна Курнат Іван Миколайович Рабик Богдан Іванович Студент Олександра Зиновіївна Цирульник Олександр Тимофійович popup.nrat_date 2020-04-02 Close