**5. Summary**

the water film at the trailing edges. To reduce the number of droplets impacting the rotating blades, the moisture in the last stages of steam turbines was extracted. This was achieved by providing suction slots on the stator surface. Moreover, the stationary guide vanes were heated up to evaporate the water film developed on the guide vane surface. However, amongst all, heating up the stationary guide vanes is proved to be the most efficient erosion

If materials are tested at a laboratory scale where erosive environment can be kept as constant as possible, then materials classification can be established on the basis of measured surface degradation. If tested for a constant time period, this process leads to the determination of relative material erosion resistance. Using this criterion, titanium is found to have more erosion resistance than the steel alloys of the same or even greater hardness. As the erosion starts and intensifies on the blade leading edges, it is ensured that the blade leading edges are particularly more resistant against droplet impact erosion. The erosion resistance of blade leading edges has been improved by different case-hardening methods, which include laser treatments, induction or flame-hardening as well as shot peening of the blade materials. Moreover, blade leading edges have been shielded with Stellite and tool steel to protect the base metal against erosion [4, 15, 24]. Laser gas nitriding can effectively be used to increase the erosion resistance of titanium materials [20, 25, 26]. On the other hand, shot peening as well as the laser shock peening is found to be ineffective. Stellite 6B is found to be the best choice for steel blade shielding, whereas beta titanium alloy has more or less the same erosion resistance

After understanding the mechanism leading to droplet impact erosion, quantifying the erosion on laboratory scale and proposing different erosion mitigation measures, the scientists, then, tried to predict droplet impact erosion in steam turbine blades. This idea gained popularity as the prediction of the erosion of low-pressure steam turbine blades eventually helps in the prediction of the service life of turbines. Different empirical and theoretical theories have been suggested and proposed to predict the droplet impact erosion. However, these prediction theories only worked for some specific materials and turbine environments. If the blade materials and/or impacting droplet parameters were changed, these theories completely failed to predict the droplet impact erosion. It should be noted that the erosion of steam turbine blading is a function of different steam, liquid and material properties and these properties may also depend upon each other. These dependencies make the prediction of steam turbine blade erosion more and more complicated. In real life, the operating conditions of the individual steam turbines are not constant, which makes the prediction of erosion

The prediction of steam turbine blade erosion is a direct measure of the erosion resistance and behaviour of the concerned material in response to a given erosive environment. Taking into account the above fact, a normalised erosion resistance has been proposed. This normalised erosion resistance is defined as "the volume loss rate of a test material, divided by the volume loss rate of a specified reference material similarly tested and similarly analysed". In different occasions, austenitic stainless steel (170 HV hardness), stainless steel (type 308) and some other

remedial cure [23, 24].

104 Cavitation - Selected Issues

as that of Stellite 6B [27].

**4.3. Prediction of droplet impact erosion**

a challenge for the researchers working on this topic [28–31].

The erosion of last-stage steam turbine blades is a subproblem within the domain of steam condensation in the low-pressure stages of steam turbines. The slowly accelerating secondary droplets, detached from the trailing edges of the guide vanes, eventually hit the following rotating blades with an impact speed, which is mostly determined by the blade circumferential speed. The droplets impact on the suction side of the blade mainly due to the droplet impact velocity relative to the moving blades. The position of droplet impact on the blade, axial penetration of the droplets and their angle of impact are largely influenced by the droplet absolute velocity, its size and initial boundary as well as the flow conditions.

Upon a droplet impact, the material degradation is mainly triggered by the impact shock pressure and subsequent lateral jetting. The impact shock pressure is largely influenced by the compressive nature of the flow and subsequent shock wave generation in the compressed liquid. The shock wave speed is not invariant and depends upon the droplet impact velocity and impacting medium. Using water as an impacting medium, a droplet with an impact speed of 500 m/s may produce a shock pressure on the target surface, which is up to 10 times greater than the corresponding stagnation pressure and exceeds the yield strength of many steel alloys. Although the intensity of impact shock pressure seems to be independent of impacting droplet size, its duration and the impacted area depend upon the droplet size. The other deteriorating agent, i.e. lateral jetting, having the speed of several times the impact velocity, becomes important when the surface is composed of several discontinuities. These discontinuities might already be pre-existent on the surface or are the results of erosion itself.

The erosion can be related to the impact velocity by a power law equation of the type f(x) = kx<sup>n</sup>. The value of n basically depends upon the type of material. It is found that for ductile materials, n varies from 3 to 5, whereas for brittle materials, values as high as 7 are observed. Erosion is found to be an angle-dependent process where the perpendicular impact plays the dominant role. The dependence of erosion on impact angle can be explained by the dependence of erosion on impact speed, if it is assumed that the erosion proceeds with the normal component of impact velocity only. Among others, impacting droplet size is a key parameter contributing to the erosion of low-pressure steam turbine blades. It is also found that volume loss per droplet impact increases with droplet size with a simple power law relation Erosion~D<sup>n</sup> droplet, where value of n is found to be 3.2 up to 3.5 for common blade materials.

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An Overview of Droplet Impact Erosion, Related Theory and Protection Measures in Steam…

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Erosion is found to be a time-dependent process, which eventually leads to a saturation stage. An eroded blade sustains most of its life under this saturation stage. The intensity of saturating period and the initiation of saturation depend upon the erosive environment harshness as well as the material properties. Most important is that the eroded surface undergoes several changes during the erosion process and eventually tends to enter a stable regime, which is then characterised by the saturation of erosion as well as the eroded surface structure.

The erosion resistance of a material can be presented by its physical as well as mechanical properties. Within materials having the same metallurgical structure, the erosion resistance increases with the surface hardness. When comparing materials having different metallurgical structures, the other material properties are required to explain the relative material erosion resistance. Hardness-induced elastic resilience and toughness are found to be effective parameters when materials from different groups are compared.

Prediction of erosion is interesting in many aspects as it may serve for the prediction of next blade repair work as well as helping to understand the basic erosion phenomenon. The prediction of erosion may be accomplished once the erosion strength of the target surface is explicitly defined and understood. Being a function of several independent variables, the problem of erosion prediction can be examined by introducing appropriate dimensionless numbers.
