PSI - Issue 60

A.H.V. Pavan et al. / Procedia Structural Integrity 60 (2024) 277–285 A.H.V.Pavan/ StructuralIntegrity Procedia 00 (2024) 000 – 000

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1. Introduction Thermal power plants contribute to ~56.8 % of the total power production in India as per Power Sector at a glance (2023). A typical thermal power plant works as follows: water is converted to steam in a boiler from which work is extracted in a turbine and the same is converted to electrical energy in a generator. There are a series of turbines in large thermal power plants namely, High Pressure (HP) turbine, Intermediate Pressure (IP) turbine and Low Pressure (LP) turbine. HP turbine takes steam input from boiler at high temperature and pressure for converting it to mechanical energy and the low temperature, intermediate pressure steam output is sent back to the boiler. After heating the intermediate pressure steam to high temperature, it is directed to IP turbine where further mechanical energy is extracted and the output consisting of intermediate temperature and low pressure steam is directed to LP turbine which extracts further mechanical energy and pumps the outlet steam to condenser. The condenser condenses the steam and this water is subsequently heated in a series of heaters before sending it to boiler to convert it to high temperature and high pressure steam which is again fed into the HP turbine. Pathak et. al. (2016) summarized that LP turbine contributes to ~37.6 % of total power output of a turbine. LP turbine blades (as shown in Fig. 1(a)) are usually larger than HP and IP turbine blades as the steam expands at lower pressures and larger blades are required to capture the energy of the lower pressure steam. This leads to increase in the tensile radial stresses, bending stresses due to steam impingement on the blades and centrifugal stresses increase as indicated by Zachary et. al (2013), thus, calling for stringent mechanical requirements. X5CrNiCuNb16-4 is a martensitic stainless steel with copper precipitates that can meet the aforementioned requirements. There are various material degradation mechanisms that can occur in LP turbine operating conditions. One of the degradation mechanisms is water droplet erosion as seen in Fig. 1(b). As the steam expands, Lee et. al. (2003) presented that condensation occurs leading to the formation of very fine water droplets suspended in steam (in mist form). These droplets when they impinge over the leading edge of the turbine blades at high velocities, tend to cause water droplet erosion. Kirols et. al. (2017) mentioned that water droplet erosion is known to occur typically on the last two stages of the LP turbine blades This degradation is deleterious to the mechanical integrity of the turbine blades. In order to prevent the LP turbine blades from water droplet erosion, the leading edge of LP turbine blades are usually hardened by processes such as flame hardening, induction hardening and laser hardening, the latter being a recent development. During flame hardening, a weld torch or a burner is directed at the leading edge of the blade for a given duration and the part is then allowed to cool creating a hardened surface as studied by Swift et. al. (2013). In the case of induction hardening process, the leading edge of the blades are placed in between the water cooled copper coils which creates eddy currents when AC current passes through the coils, thus, heating it instantly as deliberated by Swift et. al. (2013), after which the part is allowed to cool, creating a hardened surface. In the case of laser hardening, a laser is directed over the leading edge of the LP turbine blade and is scanned over the region of interest. This laser beam locally heats the location of impingement to higher temperatures which is followed by instant cooling by absorption of this heat by the bulk material, thus, creating a hardened layer which is a few mm thick as inferred from studies of Swift et. al. (2013) and Yao et. al. (2010).

Fig. 1. LP turbine blade showing (a) various features, (b) typical water droplet erosion on blade leading edge