**4. Steam turbine blade erosion**

In the last stages of low-pressure steam turbines, the steam expands to well below saturation conditions, and a portion of the vapour condenses into liquid. Although the condensation droplets are very small (5–10 μm), some of them are deposited on the surfaces of the stationary blades (guide vanes), where they coalesce into films and migrate to the trailing edge. Here they are torn off by the steam flow, in the form of much larger droplets. In the wake between stator and rotor, these large droplets slowly accelerate under the aerodynamic forces of the steam. However, when these large droplets enter the plane of rotation of upcoming rotating blades, they have gained only a fraction of the steam velocity. As a result of the difference in steam and droplet absolute velocities, the droplets collide with the rotating blades with a velocity that is almost equal to the circumferential velocity of the blades. In a modern 3600 rpm turbine, the impact speed is estimated to be as high as 700 m/s [15, 19].

#### **4.1. Quantification of droplet impact erosion**

**3.10. Dependence on liquid properties**

**3.11. Correlation with mechanical properties**

**3.12. Effects of alloying elements and microstructure**

cold-worked materials [14].

cobalt base alloys [4, 15].

**4. Steam turbine blade erosion**

flow [4].

102 Cavitation - Selected Issues

Most liquid impact erosion tests have been performed with water at normal atmospheric conditions. However, some tests have been performed with different liquid properties, which show that the erosion varies with approximately the 2nd to 2.5th power of liquid density and the 1/2–3/4 power of the inverse of viscosity. Moreover, a slight increase in erosion is found by an increase in the impacting liquid temperature. This mechanism is explained by the increased shear damage of the target surface, which is caused by the resulting lateral jet

It is always desirable to correlate erosion resistance of materials with a single mechanical property. It is found that hardness, resilience, toughness, tensile strength, ductility and the strain energy can affect erosion resistance greatly, but comprehensive knowledge is not yet available in this field [4, 14, 15, 17, 18]. Moreover, a ductile material with more toughness is more erosion resistant. Also annealed materials show greater erosion resistance than the

Improved erosion resistance has been associated with alloying elements such as chromium, manganese and cobalt. The effect of nickel is inconsistent. Fine microstructure is advantageous and so is the ability of the surface layer to become work-hardened as a result of impact-induced deformation. The extremely high erosion resistance of Stellite (cobaltchromium-tungsten alloy) has been explained by a microstructure consisting of small hard carbide particles in a strong but more ductile matrix. Very high erosion resistance has been reported for chromium-manganese steels (about 10% Cr and 12% Mn) that undergo austeniticmartensitic phase transformation under impingement. Simoneau et al. found that low stacking-fault energy is the key to high erosion resistance in austenitic stainless steel as well as

In the last stages of low-pressure steam turbines, the steam expands to well below saturation conditions, and a portion of the vapour condenses into liquid. Although the condensation droplets are very small (5–10 μm), some of them are deposited on the surfaces of the stationary blades (guide vanes), where they coalesce into films and migrate to the trailing edge. Here they are torn off by the steam flow, in the form of much larger droplets. In the wake between stator and rotor, these large droplets slowly accelerate under the aerodynamic forces of the steam. However, when these large droplets enter the plane of rotation of upcoming rotating blades, they have gained only a fraction of the steam velocity. As a result of the difference in steam and droplet absolute velocities, the droplets collide with the rotating blades Since the recognition of the erosion phenomenon in the low-pressure stages of steam turbines, many experiments have been designed to study the erosion phenomenon on laboratory scale by simulating the conditions as observed in a real steam turbine. Worthington (1908) was the first who studied liquid-solid impact experimentally by using high-speed photography. The first study about erosion was carried out in a Parsons steam turbine plant in 1925. Honneger [2], Gardner (1932) and de Haller [9] were the pioneers in the experimental study of steam turbine blade erosion. The basic methodology to simulate the droplet impact erosion on a laboratory scale is to arrange a high-speed droplet impact on a target surface. This high-speed droplet impact is, typically, achieved in a rotating test rig where the specimen alone or the specimen and nozzle both rotate in a controlled environment to achieve a predesigned impact speed and number impacts on the target surface [15, 20]. Some scientists achieved this high-speed droplet impact by using some special arrangements where they used a chamber that is closed at the one end by a piston and at the other end by the specimen surface. By operating on the piston, a highspeed jet or a shock wave is produced, which subsequently impacts on the target surface [14, 21].

The materials, which are to be tested in the test rig to find out their erosion resistance, are milled in a cylindrical, rectangular or a button-like shape [15]. Ahmad et al. performed the experiments with blade-like specimens. These specimens experience similar impact angles and impact speeds on their surfaces in the erosion rig as those observed for the corresponding blade spans in the real steam turbine, whereby the erosion process is accelerated by increasing the droplet impact number [22].

The test duration is normally motivated by the test intent specification to greatly accelerate the erosion process in such a way that monotonic saturating material loss gradients can be established within a feasible time frame. To quantify the erosion, the weight loss is interpreted with the help of the material density to reflect the volumetric erosion of the material. As a second key figure, the area-specific first-time derivative, i.e. the erosion rate, is derived and evaluated. As the erosion rate reflects the tendency of a material to erode at a given erosive environment, its reciprocal value will reflect the resistivity of the material to erode in terms of time taken by the erosive environment for a given degree of material degradation [15].

#### **4.2. Protection against droplet impact erosion**

As soon as the phenomenon causing the droplet impact erosion has been well understood and agreed amongst the scientific community, different remedial measures have been proposed and implemented to avoid the erosion of steam turbine blades. In the first step, the axial spacing between stator and rotor has been increased. In an increased path between stator and rotor, the droplets are further accelerated and broken up into smaller droplets under the action of steam aerodynamic forces. Moreover, trailing edges of the stator guide vanes are deliberately made thinner. This leads to smaller initial secondary droplets produced from 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 remedial cure [23, 24].

materials have been used as reference materials [4]. This idea could gain popularity if the same reference material was tested worldwide in a similar test rig under the same testing conditions. Similarly some theoretical parameters have been proposed to define the erosion resistance of materials [32]. However, these parameters cannot be used to predict the erosion resistance of materials mainly due to the difficulty to evaluate these parameters. Sometimes these param-

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

http://dx.doi.org/10.5772/intechopen.80768

105

In another approach, droplet impact erosion phenomenon has been correlated with material fatigue mechanism; see, e.g. [7, 14]. According to this theory, as both erosion and fatigue are triggered by the repeated stress pulses, therefore both processes can be similarly analysed and tackled. However, the idea to correlate droplet impact erosion with fatigue is not exclusively agreed and understood within the scientific community. On the other hand, some authors tried to correlate the erosion resistance of a material on its surface microstructure. Similarly, interatomic bond strength and the size and distribution of surface flaws have been tried to define the erosion resistance of the concerned material [7]. Although hardness is used universally to assess the erosion resistance of a material, resilience and toughness are the parameters, which also gain consideration and importance in the scientific community [15, 33–35].

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 drop-

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

let absolute velocity, its size and initial boundary as well as the flow conditions.

eters even failed to verify the empirically observed results and dependencies.

**5. Summary**

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 as that of Stellite 6B [27].

### **4.3. Prediction of droplet impact erosion**

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

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 materials have been used as reference materials [4]. This idea could gain popularity if the same reference material was tested worldwide in a similar test rig under the same testing conditions.

Similarly some theoretical parameters have been proposed to define the erosion resistance of materials [32]. However, these parameters cannot be used to predict the erosion resistance of materials mainly due to the difficulty to evaluate these parameters. Sometimes these parameters even failed to verify the empirically observed results and dependencies.

In another approach, droplet impact erosion phenomenon has been correlated with material fatigue mechanism; see, e.g. [7, 14]. According to this theory, as both erosion and fatigue are triggered by the repeated stress pulses, therefore both processes can be similarly analysed and tackled. However, the idea to correlate droplet impact erosion with fatigue is not exclusively agreed and understood within the scientific community. On the other hand, some authors tried to correlate the erosion resistance of a material on its surface microstructure. Similarly, interatomic bond strength and the size and distribution of surface flaws have been tried to define the erosion resistance of the concerned material [7]. Although hardness is used universally to assess the erosion resistance of a material, resilience and toughness are the parameters, which also gain consideration and importance in the scientific community [15, 33–35].
