Materials Research Express

PAPER

Water droplet erosion of stainless steel steamturbine bladesTo cite this article: H S Kirols et al 2017 Mater. Res. Express 4 086510

 

View the article online for updates and enhancements.

Related contentTopical ReviewR G Wellman and J R Nicholls

-

Modelling of the micrometric erosionpattern observed on the Tore Supra limitertilesN. Mellet, C. Martin, B. Pégourié et al.

-

Microstructure and fracture mode of amartensitic stainless steel steam turbineblade characterized via scanning augermicroscopy and potentiodynamicpolarizationD Saidi, B Zaid, N Souami et al.

-

This content was downloaded from IP address 132.205.48.51 on 14/09/2017 at 23:53

© 2017 IOP Publishing Ltd

1. Introduction

Water droplet erosion (WDE) of steam turbine blades has been the concern of researchers for decades [1–5]. The complexity of this phenomenon has always been the reason for the continuous research efforts attempting to mitigate its impact on several applications. WDE is encountered in the power generation industry, from steam and gas turbines to elbows and connections of steam lines [6–9]. In steam turbines, the last two low pressure (LP) stages of modern turbine blades are expected to operate in a wet steam medium. Fine mist droplets are usually formed by condensation during steam expansion [10]. As described by Yasugahira et al [8] in figure 1. Water droplets pass through several stages to cause damage to the rotating blade. Firstly, fine mist droplets, 0.1–4 µm [11], deposit on stationary blades. By coalescence, a liquid film is formed, which moves to the leading edges of stationary blades by inertia. Coarse droplets, typically 1 mm in size, are generated at the tip of the leading edges as the water film starts to depart the stationary blades. Finally, these coarse droplets atomize into smaller droplets, 10–400 µm [11], and they move with the steam flow impinging the successive set of rotating blades, which causes erosion. The amount of damage that can happen to the leading edges of a rotating blade is shown clearly in figure 2. In the recent work of Förster [12], it was claimed that the effective droplet size that causes erosion is between 50–200 µm. He added that droplets smaller than 50 µm in size do not cause damage, and those larger than 200 µm atomize into smaller dropletes.

In the literature, most of the presented work analyze the WDE wear of specially prepared coupons [11, 13–15]. Studying erosion wear of actual ex-service turbine blades is also important because it can uncover some unknown parameters that also affect the erosion damage. In general, ex-service steam turbine blades are usually studied by engineers and researchers to analyze the reasons for failure [16–19]. In a survey that studies steam turbine blade failures [16], several failure incidents which occurred in the LP cycle blades were recorded. Erosion was found

H S Kirols et al

086510

MRX

© 2017 IOP Publishing Ltd

4

Mater. Res. Express

MRX

2053-1591

10.1088/2053-1591/aa7c70

8

1

12

Materials Research Express

IOP

17

August

2017

Water droplet erosion of stainless steel steam turbine blades

H S Kirols1, D Kevorkov1, A Uihlein2 and M Medraj1,3

1 Department of Mechanical and Industrial Engineering, Concordia University, 1455 de Maisonneuve Boulevard West, QC, Montreal H3G 1M8, Canada

2 Thermal Power Transverse Technologies, Materials and Component Testing, ALSTOM Power, Brown Boveri Strasse 7, 5401 Baden, Switzerland

3 Department of Mechanical and Materials Engineering, Masdar Institute, Masdar City, PO Box 54224, Abu Dhabi, United Arab Emirates

E-mail: mmedraj@encs.concordia.ca

Keywords: water droplet erosion, stainless steel, steam turbine, turbine blades

AbstractSteam turbine blades are highly subjected to water droplet erosion (WDE) caused by high energy impingement of liquid water droplets. However, most of the published research on this wear phenomenon is performed on laboratory test rigs, instead of addressing WDE of actual steam turbine blades. In this work, the progression of erosion on the surface of ex-service low pressure steam turbine blades was investigated using scanning electron microscopy. The erosion appearance and mechanisms are compared with laboratory test rig results that are carried out using a rotating disk rig according to ASTM G73 standard. Initial and advanced erosion stages could be observed on the steam turbine blades. Similar to the WDE rig coupons, initial pits and cracks were preceded by blade surface roughening through the formation of asperities and depressions. In addition, it was also observed that the twist angle of the turbine blade around its diagonal, is an important parameter that influences its WDE. Twist angle has an effect on: impact angle, erosion appearance, impact speed, and the affected area. Furthermore, according to the current experimental results, multi-ray rig erosion test results are considered the closest simulation to the actual ex-service blade in terms of damage appearance.

PAPER2017

RECEIVED 25 April 2017

REVISED

22 May 2017

ACCEPTED FOR PUBLICATION

29 June 2017

PUBLISHED 17 August 2017

https://doi.org/10.1088/2053-1591/aa7c70Mater. Res. Express 4 (2017) 086510

2

H S Kirols et al

to be one of the main causes of failure, however, this term resembles all types of erosion, not necessarily WDE. In a rare study that only focused on WDE of in-service steam turbine blades, Staniša et al [17] monitored the WDE damage evolution on low pressure cycle blades of a steam turbine at a power plant in Krsko, Slovenia. They reported an erosion curve using actual in-service measurements. In addition, they developed a model for ero-sion prediction [17]. Wang et al [18] and Mazur et al [19] studied failures of steam turbine blades, they shared a conclusion that blades usually fail due to fatigue cracking, which in turn occurs due to surface wear (i.e. WDE, solid particle erosion and/or corrosion attacks). In their case study for turbine failure at Dresden power plant, Chynoweth et al [20] performed a root cause analysis for observed LP blade failures. They presented some causes and effects leading to the failure. Erosion was found to be one of the reasons for such failure. Furthermore, in his work on assigning the financial liability of turbine blade failures, Missimer [5] categorized the main dam-age mechanisms found in blades as in the following four: (a) fatigue, (b) corrosion, (c) erosion and (d) thermal shock. He added that premature failure usually occurs in the LP blades, and that the first three damage mech-anisms (a–c) and their interactions are the main reasons for such failure. He also claimed that thermal shocks mainly influence high and intermediate pressure cycle blades.

It could be implied from the aforementioned works that turbine blade failure is a result of interacting damage mechanisms. Consequently, most of previous case studies were interested in listing all possible causes of failure found in the turbine blades under investigation. Few of them were concerned particularly with a certain damage phenomenon (i.e. erosion or corrosion), Staniša et al [17] was one of the exceptions, since they were only con-cerned with WDE. Therefore, there is a need to study the different damage marks on the ex-service blade surfaces to further understand the progression of each wear type. In this work, the WDE fracture surfaces found on two different stainless steel ex-service turbine blades are analyzed in order to specifically study their WDE damage. Fracture surface microanalyses for the ex-service steam turbine blades were performed to further understand water droplet erosion.

Besides, there is currently large amount of water droplet erosion data generated using various laboratory rigs studying different materials and surface treatments. However, the link between these results and actual turbine blades erosion is still missing. For instance, it is still not possible to relate time duration of each WDE damage stage observed in the erosion rig experiments to the actual life of the blade [21]. Therefore, the current work aims

Figure 1. Schematic for the water droplet formation, which leads to erosion in steam turbines [8].

Figure 2. Eroded ex-service steam turbine blade.

Mater. Res. Express 4 (2017) 086510

3

H S Kirols et al

at bridging this gap. Moreover, it is difficult to compare results obtained using different rigs due to the lack of complete water droplet impingement erosion standards. The current work will help towards resolving this issue by generating rig results that can be translated to useful information representing actual blades erosion. This is currently needed by steam and gas turbine manufacturers.

2. Experimental procedure

Stainless steel ex-service steam turbine blades were examined in this study. These blades were provided by ALSTOM-Power, Switzerland to identify the WDE mechanisms on different sections of the blades.

The service environment of these blades were considered harsh. Blades may work for several years inside of a steam turbine until maintenance is performed. The received blades are samples of low pressure cycle blades in a typical steam turbine. These blades were rotating at a speed of 3000–3500 RPM with an overall center of rotation to tip length of around 2 meters.

The studied blades (i.e. Blades 1 and 2) are made of martensitic stainless steel, however, they are of different alloys. Figures 2 and 3 show the studied blades. The mechanical properties and composition of these alloys are pro-prietary to ALSTOM power. Blades were initially sectioned by ALSTOM Power in Switzerland. Blade 1 was received in small sections; they represent different parts on the span of its leading edge below the tip. A 1 m-long section of Blade 2 was also received. The first step of this investigation was to qualitatively examine the blades’ sections in order to select some of them for further analysis. The second step was to determine the microstructure of the studied blades. Then, SEM micrographs were taken to study the eroded surfaces at different locations on the blades.

2.1. Visual inspectionsEarly erosion stages were observed on some sections of Blade 1, but not on Blade 2. Therefore, Blade 1 was mainly studied for the erosion progression, from initiation till severe erosion. The 1 m-long section of Blade 2 provided an opportunity to see different erosion damage appearances on the same blade; especially, those occurring during late erosion stages.

In order to study how erosion develops, sections of Blade 1 were examined to determine which of them had all erosion stages on its surface. Section 4 from figure 3 was chosen as it had a wide eroded area, which included all erosion stages.

The span length of an ideal LP blade is in the range of 2 meters [22]. The received portion of Blade 2 is nearly half of its whole span, as illustrated by the sectioning line on a typical turbine blade in figure 4. Although the impact speed increases with distance from the blade’s root, the damage severity did not follow this trend. Differ-ent erosion appearances were recognized at different locations of Blade 2, as seen in figure 5. It is known that for aerodynamic reasons, the blade has an angle of twist along its diagonal [23]. It is proposed that the main reason for the different erosion appearances on the blade is related to the change in impact angle, which is expected to change with the change in the angle of twist, as also illustrated in figure 5. This idea will be further elaborated later in the following sections of this paper. Another effect of the angle of twist is the size of the eroded surface area, as also shown on figure 5. It seems that the affected area decreases as the angle of twist decreases towards the tip of the blade. This point will also be addressed in later sections.

2.2. MetallographyFor the analysis of fracture surfaces, it was important to determine the grain sizes of the analyzed blade specimens. This was done in order to understand the types of failure observed. Figure 6 shows the microstructure of Blade 1. Grain size measurement was done after surface preparation and etching using Kalling II solution, according to ASM handbook for metallography and microstructure [24]. This microstructure was found to be similar to what was reported by Barlow and Toit [25], in their report on the effect of austenitizing temperature on the final

Figure 3. Received sections of the leading edge of Blade 1. Red rectangle represents the region of section 4 that is analyzed by SEM.

Mater. Res. Express 4 (2017) 086510

4

H S Kirols et al

properties of AISI 420 martensitic stainless steel. The grain boundaries were easily identified and the grain size was measured and found to be in the range of 50–100 µm.

For Blade 2, the same etchant as Blade 1 was not effective. Therefore, based on a recommendation from ALSTOM, a mixture of distilled water, HCL and HNO3 was used. The grain boundaries were identified, the grain size of this blade material was measured to be in the range of 30–100 µm.

3. Results and discussion

3.1. Fracture surface micro-analyses

Figure 4. Typical turbine blade.

Figure 5. Eroded surface area of Blade 2. Red rectangles represent the regions that are analyzed by SEM.

Figure 6. Microstructure of Blade 1.

Mater. Res. Express 4 (2017) 086510

5

H S Kirols et al

3.1.1. Erosion progressionThe erosion appearance on three sections of blades 1 and 2 were studied and analyzed under the SEM. Studied sections are indicated by red rectangles on figures 3 and 5. Figures 7–9 show a combinations of SEM micrographs for the studied sections. Erosion severity increases closer to the leading edge, as indicated by the arrow in figure 7. It is clear from the SEM micrographs that the erosion progression on blade 1 has an overall similar appearance to section (i) of blade 2. However, section (ii) of blade 2 shows a different overall appearance. As discussed earlier, section 4 of blade 1 showed all stages of erosion on its surface, which gives a full picture of the erosion damage progression. Parts of figures 7(a)–(d) are studied separately in more details in the following parts of the paper. In addition, similarity between section 4 of blade 1 and section (i) of blade 2 will be highlighted.

3.1.1.1. Surface roughening, and the formation of surface depressions and asperitiesThis section discusses the erosion appearance found in figure 7(a). For ductile materials such as stainless steel, Haag [11] described the erosion progression to be in the following sequence: (a) roughening of the surface; (b) formation of micro-cracks and the propagation of these cracks; (c) the detachment of material and the formation of pitting. Field et al [26] also presented a model for the water droplet erosion damage process. They explained that during the incubation stage, the hydraulic pressure caused by the droplets’ impacts produce what is called as surface depressions, and upon repetitive impacts the depth of these depressions increases. Due to the presence of these surface irregularities, the surface becomes more influenced by the lateral outflow after impact, which causes the extrusion of the surface layers forming asperities. Such depressions were clearly identified on the surface of the studied section, as seen in figure 10. Their sizes were measured and found in the range of 200 µm, which can be related to the effective droplet sizes range impacting the surface, 50–200 µm [12]. It is also clear from figure 10 that adjacent depressions are sharing the same asperities. According to the grain size measurement and the size of the depressions, it can be assumed that each depression affects from 2 to 4 grains in the case of Blade 1. These results agree with results of our previous article [13], where first step in the erosion process was the roughening of the surface by the formation of asperities and irregularities.

3.1.1.2. Detachment of asperities and the formation of pits on the surfaceThis section discusses the second erosion stage where the pits start initiating on the surface, as shown at location (b) indicated on figure 7. According to several researchers [3, 11, 13, 26–28], asperities and irregularities are the main reason for pits formation. Figure 11 shows that pits form around depressions, mainly due to the loss of asperities. According to Heymann [3, 28], asperities act as stress raisers and help to initiate fatigue cracks due to the radial outflow of droplets. This is expected to be the case, and explains why initial pits are seen in the vicinity of asperities. Therefore, based on this observation a mechanism is proposed in figure 12 for the erosion of this section of Blade 1. As illustrated in this figure, the impact pressure and the lateral outflow of the water droplets extrude asperities on the surface. Several depressions share the same asperity, since the material flow is expected to be in several directions as illustrated by the red arrows in figure 12. Later on, micro-cracks start forming around the roots of these strained asperities due to high stress concentrations. At the end, asperities are detached forming pits due to further water droplet impacts and outflows.

It was also observed that pits are oriented parallel to the direction of rotation, as illustrated in figures 11 and 13. The reason for such damage appearance could be attributed to the fact that during rotation the incident water droplets flow in a certain direction. In his work on determining the financial liabilities due to turbine failure, Mis-simer [5] indicated that pits on eroded blades, usually have a certain shape or pattern. This observation will be further discussed in the following sections.

3.1.1.3. Pits coalescencePart (c) of figure 7 shows that the erosion damage is in a more advanced erosion stage. Pits formed in earlier erosion stages increase in size and number. Later on, they start merging, creating regions consisting of eroded and un-eroded surfaces. It could be deduced from this that erosion progression is non-uniform on the surface. Most

Figure 7. Combined images for an erosion region on the surface of section 4 of the ex-service Blade 1 as illustrated in figure 3. Part (a) is the location where erosion marks start to be detected, parts (b-d) show different erosion stages.

Mater. Res. Express 4 (2017) 086510

6

H S Kirols et al

likely the un-eroded surfaces are of depression, as shown in figure 13. In addition, an interesting observation is that pits merge parallel to the direction of rotation. This can be seen in the alignment of the large pits parallel to the direction of rotation in figure 13. These observations indicate that erosion appearance is highly dependent on the direction of the droplet impact.

3.1.1.4. Severe erosion marks formation on the surfaceAs the erosion advances, the blade’s surface starts losing more material leading to the creation of more stress raisers; therefore, more prospective locations for further pitting. Figure 14 shows the increase of erosion severity till it reaches its maximum at the leading edge. The effect of the rotation direction of the blade, and how the damage is oriented parallel to it, is more obvious now. The erosion appearance in figure 15 indicates that different parts of the surface of the blade encountered different erosion rates.

In order to explain such erosion appearance the following erosion mechanism is proposed. Erosion appear-ance depends on the angle of impact, which on the other hand depends on the direction of the water droplets and steam flow. This could be proved by observing that the direction of formed pits are skewed parallel to the direction of rotation as shown in figures 13 and 14. In addition, asperities and surface irregularities, are probable locations for crack formation and pits initiation. Figures 10 and 11 show the formation of such irregularities due to the detachment of asperities, which means that some locations on the surface erode faster than other. This indicates that erosion of the same surface is a non-uniform damage process. Subsequently, after a certain period of time, this difference in the erosion rates causes the formation of different erosion levels on the surface, leading eventually to certain damaged surface appearance. Usually, this appearance is in the form of skewed features ori-ented in the direction of rotation. However, as the angle of twist increases, the impact angle increases towards 90°, and the erosion appearance changes. This change was observed in the erosion appearance of section (ii) of blade 2 shown in figure 9. The influence of the angle of twist on the WDE of turbine blades is discussed in the following part of the paper.

3.1.2. Influence of the angle of twist on the WDE of turbine bladesThe angle of twist is expected to affect the following four main WDE parameters: (a) angle of impact, (b) erosion appearance (c) affected area, and (d) speed of impact. These aspects are addressed in the following sub sections.

3.1.2.1. The effect of the blade’s angle of twist on the impact angle and the erosion appearanceFigure 9 shows the studied location on section (ii) of Blade 2, it provides a full overview for the erosion stages found at this location. Figures 15(a)–(c) show SEM micrographs for the different locations illustrated on figure 9. The erosion appearance on the surface showed less or no skewed features. It is believed that the water droplet impact angle on section (ii) is near 90°.

To further prove that the erosion appearance in figures 9 and 15 is due to an impact angle close to 90°, a WDE lab experiment was carried out using a multi-ray nozzle. This nozzle produces an average water droplet size of 271 µm, which is close to the effective droplet size found in the LP cycles of steam turbines [12]. This experiment was carried out at 400 m s−1, and at an angle of impact of 90°. WDE study using Multi-ray nozzle is more repre-sentative of the actual erosion encountered in steam and gas turbines than single-ray tests, for example. Single-ray water droplets test does not address the interaction between droplets with the solid surface in the vicinity of the damage features that have already been initiated. This is because in single-ray tests, impingement is mostly

Figure 8. Combined images for an erosion region on the surface of section (i) of figure 5 of ex-service Blade 2.

Figure 9. Combined images for an erosion region on the surface of section (ii) of figure 5 of ex-service Blade 2.

Mater. Res. Express 4 (2017) 086510

7

H S Kirols et al

confined to the crater region. Hence, the multi-ray test is closer to what is encountered in real turbines. Also, sin-gle-ray tests do not address the interaction between droplets during the erosion process. For example, in actual turbines, droplets impinge the blades at different time. The droplet that reaches the surface first spreads causing the next droplet to impact a water layer (film) instead of the metal surface directly. This kind of interaction is encountered in the multi-ray experiments. It is worth mentioning that both the WDE rig test sample and the ex-service blade 2 are of the same material, 12% Cr stainless steel. Figure 15(d) shows the surface of the sample at

Figure 10. The formation of asperities between depressions on the surface of section 4 of Blade 1 as illustrated in section (a) of figure 7.

Figure 11. Pits oriented in the direction of rotation (magnified part of section (b) of figure 7).

Figure 12. The formation of surface pitting due to the detachment of asperities.

Mater. Res. Express 4 (2017) 086510

8

H S Kirols et al

the end of this erosion experiment. There are similarities between the erosion appearances of this WDE rig test and figure 15(c), especially the formation of small equally spaced features (pits) on the surfaces of both samples. These results support the hypothesis that the erosion appearance in figure 15(c) is due to an impact angle close to 90°. Furthermore, this method of simulating the erosion of service blades in controlled laboratory conditions is discussed in a later section.

Moreover, the effect of angle of twist on the size of the affected area should also be investigated. In his work, Ahmad [29] claimed that areas away from the leading edge towards the trailing edge are protected from erosion due to the decrease in the impact angle. Ahmad [29] was only discussing the change of angle on the cross section of the blade, at a certain location on its span, where no change in the angle of twist is taking place. According to the visual observations in figure 5, it is noticed that as the angle of twist increases towards the root of the blade, there is less protections for areas away from the leading edge causing larger eroded surfaces. An imaginary line indicating the change in the affected area on the surface of the blade is used as an illustration in figure 16.

3.1.2.2. The effect of the blade’s angle of twist on the speed of impactThe speed of impact is a function of three parameters: (a) linear speed of the rotating blades, (b) the speed of the water droplets in the steam flow, (c) and the impact angle. Since, the angle of twist influences the impact angle as discussed in the previous section, it is expected that it will also affect the speed of impact.

3.1.3. WDE failure modes steam turbine bladesFurther SEM analysis was carried out to study the mode of failure of the examined blades. The aim of this investigation was to determine the material loss mechanism. Figure 17 is taken for section (i) of Blade 2, as

Figure 13. Coalescence of formed pitting around depressions (from section (c) of figure 7).

Figure 14. (a) and (b) Erosion at different levels of severity of blade 1 taken from section (d) of figure 7. (c) and (d) Are for section (i) of blade 2 (i.e. enlarged parts of figure 8).

Mater. Res. Express 4 (2017) 086510

9

H S Kirols et al

illustrated in figure 5. The inset of figure 17 is generated by tilting the SEM stage at an angle of 50° to see in between the skewed features.

The SEM micrographs indicate two main points. First, the dimensions of the pits found on the surface of the blade are in the size range of the grains of this material, 30–100 µm, or in some cases multiples of these values. The possible explanation for the formation of such pits is due to inter-granular crack propagation. However, it is also expected that at such level of erosion there should be a mixed failure mode of intergranular and transgranular that causes such damage.

Furthermore, figures 15(c) and 17 illustrate the similarity between the damage mechanisms of the two sections of Blade 2 (i.e. i and ii), despite the difference in the overall erosion appearance. It suggests that the impact angle did not affect the mode of failure, however, mainly the degree of damage and the erosion appear-ance. This hypothesis agrees with describing the effect of the impact angle, as it was reported to influence both the relative speed of impact [29] and the appearance of the eroded surfaces [5].

3.2. Comparison between WDE rig results and damage found on ex-service turbine bladesLaboratory experimentation of WDE is important to better understand this wear phenomenon. However, due to the large amount of interacting parameters in the erosion process, a lot of work is still needed to fully

Figure 15. The progression of erosion in figure 9 from sections (a)–(c), and the erosion appearance of a WDE rig results done using the multi-ray nozzle producing 271 µm droplets at 400 m s−1 (d).

Figure 16. Illustration for the proposed theory for the effect of the angle of twist.

Mater. Res. Express 4 (2017) 086510

10

H S Kirols et al

simulate actual damage. At our lab we are trying several test procedures to increase the complexity of the erosion experiment incrementally; our work was reported in several articles [9, 13, 14, 21, 30, 31]. Figure 18 shows the erosion appearance of two types of erosion experiments, single water droplet ray and multi-ray erosion. These macrographs show similarity in the erosion appearance, however, multi-ray nozzle provides the opportunity of studying a wider eroded area. Also multi-ray experiments allow the interaction between water droplets rays, making the rig experimental results closer to what is observed for the steam turbine blades.

Figure 15 showed an SEM comparison in the erosion appearance of a test done using multi-ray nozzle and that of ex-service turbine Blade 2. The micrographs show a great similarity in the erosion appearance, which indicates that multi-ray WDE experiments is a good approach to simulate the actual erosion damage. However, in order to extrapolate the WDE rig test results to actual damage taking place in-service, attention should be given to the difference in results between tests done using single-ray experiments and those done using multi-ray experiments. It would be interesting to study how to extrapolate test results of different nozzle designs. This would be a first step towards extrapolating experimental results to actual damage observed in actual steam or gas turbines.

3.3. Prediction of actual erosion damage using laboratory rig resultsSimulating the WDE conditions using laboratory instruments proved to be a very effective tool. Experimental results can be used to build computational models to represent the damage. However, in some occasions it is not possible to simulate the actual in-service conditions by laboratory equipment, due to technical difficulties. One of these difficulties is related to the test speeds used by most modern WDE erosion rigs [4, 32]. For instance, the WDE rig used in this work is considered one of the advanced WDE rigs, as it can reach a rotational speed of 20 000 RPM (linear speed of 500 m s−1). Although, this speed is considered one of the highest compared to other laboratory rigs, it is still below what can be reached by actual turbines blades [33]. In some cases, the leading edge at the tip of the blade may reach a linear speed of 900 m s−1 [33]. Therefore, there should be a method to extrapolate results for low speed tests to those done at higher speeds. In our recent work [21], an energy approach was discussed as an effective method of representation for the WDE process. Comparisons showed that testing a sample with the highest impact speed of 475 m s−1 using 220 µm droplets showed less erosion compared to

Figure 17. SEM micrographs of the damage on section (i) of figure 5 of Blade 2.

Figure 18. WDE of 12% Cr stainless steel samples tested using different nozzles.

Mater. Res. Express 4 (2017) 086510

11

H S Kirols et al

testing the same material at 350 m s−1 using 603 µm droplets [21]. The erosion rate was 4 times greater for the sample tested with larger droplet size and less speed [21]. Although, in real WDE of steam turbines, the effective water droplet size that causes most of the damage is in the range of 50–200 µm droplets [12], these results indicate that testing with larger droplet is a feasible tool to predict WDE at higher speeds, while taking into account the impact energy. Therefore, it could be used as an estimation for WDE at service conditions that are not achievable using experimental setups.

On the other hand, the relation between the speed of the rotating blade and the angle of twist should be taken into consideration, when defining the impact speeds inside actual steam turbines. Although, the tip of the blade may reach speeds up to 900 m s−1, the angle of twist at the tip of the blade is less than 90°. The effect of the angle of twist on the angle and speed of impact was discussed in section 3.1.2. According to this discus-sion, it can be deduced that at the tip of the blade the angle of impact is minimum. This may reduce the relative speed of impact lower than the linear speed of the rotating blade. However, usually the blade shows more dam-age appearance at the tip’s leading edge than that of locations away from it towards the root of the blade. This could be explained by noting that although this impact speed is still relatively lower than the linear speed of the blade, the relative speed of impact at the blade’s tip is still high enough to cause this severe erosion marks. This is because the linear speed of the blade increases with distance from the root of the blade but the impact speed is lower than the linear speed due to the twist angel. Therefore, when choosing speed for erosion experiments that simulates erosion in steam blades, this point should be taken into consideration.

4. Concluding remarks

In this work, WDE of ex-service turbine blades were comprehensibly discussed. Comparison of the diffe- rent erosion appearances was presented. In addition, important points are discussed to help prediction of actual erosion damage using experimental results. The main findings of this work can be summarized as follows:

1. Erosion progression on the surface of ex-service steam turbine blades, from initiation till severe stages was studied with the aid of SEM micrographs.

2. Asperities and depressions precede initial cracks and pits formation in the ex-service blades similar to the WDE rig samples.

3. The angle of twist is an important parameter that influences the WDE of turbine blades. It has an effect on: (a) erosion appearance, (b) impact angle, (c) impact speed, (d) affected area.

4. The relation between the speed of the rotating blade and the angle of twist should be taken into consideration, when defining the impact speeds inside actual steam turbines.

5. Multi-ray WDE rig experiments showed great similarities with real erosion wear appearance. These experiments showed the closest simulation to the actual ex-service blade in terms of damage appearance. Hence, in order to extrapolate the WDE rig results to actual damage of in-service blades, attention should be given to the differences between tests done using single-ray and those done using multi-ray nozzles.

6. The prediction of actual erosion damage using experimental results is possible, if a technique is developed to solve the issue of the large difference between impact speeds encountered in steam turbines and those obtainable in test rigs. This can be achieved using larger water droplets and smaller impact speeds.

Acknowledgments

Authors of this work would like to acknowledge the support of ALSTOM Power, Switzerland for funding this work (Grant no. TTT PR 2014-1285). In addition, they would also like to thank colleagues at the Thermodynamics of Materials Group (TMG) of Concordia University for their help in carrying out the experiments.

References

[1] Honegger E 1927 Tests on erosion caused by jets Brown Boveri Rev. 14 95–104[2] Cook S S 1929 Water-hammer erosion in turbines Proc. Univ. Durham Phil. Soc. 8 88–100[3] Heymann F J 1967 On the time dependence of the rate of erosion due to impingement or cavitation STP 408 Erosion by Cavitation or

Impingement (ASTM International) pp 70–110[4] Elliott D E, Marriott J B and Smith A 1970 Comparison of erosion resistance of standard steam turbine blade and shield materials on

four test rigs Characterization and Determination of Erosion Resistance (STP 474) (ASTM International) pp 127–61[5] Missimer J R 2004 Turbine blade failures, who pays? Proc. of the Failure Analysis Program and Related Papers presented at the Int.

Conference and Exposition on Fatigue, Corrosion Cracking, Fracture Mechanics and Failure Analysis, ASM[6] Li R, Mori M and Ninokata H 2012 A calculation methodology proposed for liquid droplet impingement erosion Nucl. Eng. Des.

242 157–63

Mater. Res. Express 4 (2017) 086510

12

H S Kirols et al

[7] Khan J R and Wang T 2008 Simulation of inlet fogging and wet-compression in a single stage compressor including erosion analysis 2008 ASME Turbo Expo (9 June 2008–13 June 2008) (Berlin: American Society of Mechanical Engineers)

[8] Yasugahira N, Namura K, Kaneko R and Satoh T 1990 Erosion resistance of titanium alloys for steam turbine blades as measured by water droplet impingement Titanium Steam Turbine Blading, Workshop Proc. (Palo Alto, California, 9–10 November 1988) vol 383 pp 385–402

[9] Gujba A K, Hackel L, Kevorkov D and Medraj M 2016 Water droplet erosion behaviour of Ti–6Al–4V and mechanisms of material damage at the early and advanced stages Wear 358–9 109–22

[10]Byeong-Eun L, Riu K-J, Shin S-H and Kwon S-B 2003 Development of a water droplet erosion model for large steam turbine blades KSME Int. J. 17 114–21

[11]Haag M 2009 Untersuchungen zur schädigungsentwicklung an dampfturbinenwerkstoffen infolge von wassertropfenerosion Masters Thesis, Technische Universität Kaiserslautern

[12]Förster S 2010 Untersuchungen des strömungsmechanischen Einflusses verschiedener Erosionsstufen an Dampfturbinen- Niederdruck- Beschaufelungen Masterarbeit, Hochschule für Technik und Wirtschaft Berlin

[13]Kirols H S, Kevorkov D, Uihlein A and Medraj M 2015 The effect of surface roughness on the initiation of water droplet erosion Wear 342–3 198–209

[14]Mahdipoor M S, Kirols H S, Kevorkov D, Jedrzejowski P and Medraj M 2015 Influence of impact speed on water droplet erosion of TiAl compared to Ti6Al4V Sci. Rep. 5 14182

[15]Luiset B, Sanchette F, Billard A and Schuster D 2013 Mechanisms of stainless steels erosion by water droplets Wear 303 459–64[16]Dewey R P and Rieger N F 1985 Survey of steam turbine blade failures EPRI Report CS-3891 (Palo Alto, CA: EPRI) [17]Staniša B, Schauperl Z and Grilec K 2003 Erosion behaviour of turbine rotor blades installed in the Krsko nuclear power plant Wear

254 735–41[18]Wang W-Z et al 2007 Failure analysis of the final stage blade in steam turbine Eng. Fail. Anal. 14 632–41[19]Mazur Z, Hernández-Rossette A and García-Illescas R 2006 Investigation of the failure of the L-0 blades Eng. Fail. Anal. 13 1338–50[20]Chynoweth J M, Gerzen G S and Tomala R W 1996 Root cause analysis of low pressure turbine blade failure Proc. of the American Power

Conf., 58th Annual Meeting (Chicago: Illinois Institute of Technology) vol 58 pp 1230–6[21]Kirols H S, Mahdipoor M S, Kevorkov D, Uihlein A and Medraj M 2016 Energy based approach for understanding water droplet

erosion Mater. Des. 104 76–86[22]Uihlein A, Maggi C M and Keisker I 2012 Water droplet erosion at steam turbines: testing method and validation Electrical Power

Research Institute (EPRI), Technology innovation: Proc.: Int. Conf. on Solid Partical and Liquid Droplet Erosion (Milan) Technical Report[23]Wood N B and Morton V M 1984 Inlet angle distribution of last stage moving blades for large steam turbines Int. J. Heat Fluid Flow

5 101–11[24]Vander-Voort G F, Lucas G M and Manilova E 2004 Metallography and microstructures of stainless steels and maraging steels

Metallography and Microstructures (ASM Handbook vol 9) (ASM International) pp 670–700[25]Barlow L D and Du Toit M 2012 Effect of austenitizing heat treatment on the microstructure and hardness of martensitic stainless steel

AISI 420 J. Mater. Eng. Perform. 21 1327–36[26]Field J E, Lesser M B and Dear J P 1985 Studies of two-dimensional liquid-wedge impact and their relevance to liquid-drop impact

problems Proc. R. Soc. A 401 225–49[27]Bowden F P and Brunton J H 1961 The deformation of solids by liquid impact at supersonic speeds Proc. R. Soc. A 263 433–50[28]Heymann F J 1970 Erosion by liquids Mach. Des. 118–24[29]Ahmad M 2009 Experimental assessment of droplet impact erosion of low-pressure steam turbine blades PhD Thesis Von der Fakultät

Energie, Verfahrens und Biotechnik der Universität Stuttgart, Shaker Verlag, Achen, Germany[30]Kirols H S, Mahdipoor M S, Kevorkov D and Medraj M 2016 Water Droplet Impingement Erosion: Testing, Mechanisms And Improved

Representation XXIV ICTAM (Montreal, Canada, August 2016)[31]Gujba A K, Hackel L and Medraj M 2016 Water droplet erosion performance of laser shock peened Ti–6Al–4V Metals 6 262[32]Ahmad M, Casey M and Sürken M 2009 Experimental assessment of droplet impact erosion resistance of steam turbine blade materials

Wear 267 1605–18[33]Ryzhenkov V A, Lebedeva A I and Mednikov A F 2011 Erosion wear of the blades of wet-steam turbine stages: Present state of the

problem and methods for solving it Therm. Eng. 58 713–8–

Mater. Res. Express 4 (2017) 086510