2005 indiahydro sediment erosion hydraulic turbines (1)

8/10/2019 2005 IndiaHydro Sediment Erosion Hydraulic Turbines (1)

http://slidepdf.com/reader/full/2005-indiahydro-sediment-erosion-hydraulic-turbines-1 1/13

INDIAHYDRO 2005International Workshop and Conference

February 2005

Sediment Erosion in Hydraulic Turbines and Experiences withAdvanced Coating Technologies

Helmut Keck*, Roger Dekumbis, Mirjam Sick

R&D DepartmentVA TECH HYDRO, Hardstrasse 319, CH-8023 Zurich, Switzerland

e-mail: [email protected]

Andreas Lohmberg

Sulzer Innotec Ltd.P.O.Box, 8600 Winterthur, Switzerlande-mail: [email protected]

Key words: CFD, coating, erosion, water turbine

Abstract

The erosion in different types of turbines is discussed. The reasons why Francis turbines areless beneficial with respect to erosion than Pelton or Kaplan turbines are explained. Newtechnologies such as CFD-simulation including numerical prediction of relative erosionintensities and advanced coating applications are shown. The examples out of the experienceof VA TECH HYDRO cover all types of turbines in various continents (Europe, SouthAmerica, Asia). With up-to-date know-how in abrasion research a further exploration of

Hydro power also in highly silt-laden rivers can be promoted.



Helmut Keck, Roger Dekumbis, Mirjam Sick, Andreas Lohmberg

2

1 Introduction

Sediment erosion in hydraulic turbines cannot be avoided completely but can be reduced to aneconomically acceptable level. To achieve this target many aspects from practical experienceup to most recent technologies must be taken into account. The flow behaviour including sand

particles must be followed from the catchment areas and the upper reservoir throughout all theturbine components towards the outlet. Different scales in space and time have to berespected:

Scaling in space:• Large scale:

the dimensions and design of the upper reservoir must be such that water with theminimum possible sand content enters the intake to the power house. The layout of the

power house and the electromechanical equipment must be “erosion friendly”: Selectionof the appropriate type and number of units, low setting level and low specific speed to

provide good safety margin with respect to cavitation and to allow a design with smallrelative flow velocities.

• Medium scale:the turbines shall be designed for an uniform velocity distribution without zones ofvortices or large gradients, and cavitation free in a wide load range. Parts that are subjectto erosion should be easily replaceable and accessible for coatings.

• Small scale:the best protection of surfaces is achieved with hard coatings where tungsten carbides areembedded in thin layers (see below). The erosion resistance of the coatings depends on thesize and hardness of the sand particles, the impact angle of the sand on the surface(strongly influenced by local vortices and turbulence) and eventual implosion of cavitation

bubbles (coatings are rather sensitive to high pressure peaks caused by cavitation or stoneimpacts).

Scaling in time:• Early phase:

the concept of a power plant in silt-laden waters has to be planned to be erosion-friendlyfrom the very beginning, see above comments under large and medium scale. Mistakes in

the early phase can never be compensated later, not even with the best coatings.• Medium phase:

the planner, the designer and the operator must be aware that the units should be run closeto best efficiency point all the time. The variation of head should be small (-> planning ofthe site), the number of units should fit the variation of discharge and the expected standstills for repair (-> planning of the power house), part load operation should be avoided(-> planning of the role of the plant in the grid).

• Late phase:the repair intervals have to be selected carefully: a more frequent repair might beeconomical due to the extreme losses in efficiency with heavily worn components. Acontinuous efficiency monitoring equipment and a planning of stand still in times of



INDIAHYDRO, February 2005, New Delhi, India

3

maximum sand content in the inflowing water can determine the optimal timing of therepairs.

2 Critical components in turbines

In general those components are critical with respect to erosion that have:

• Zones of high flow velocity near the wall• Flows where particles are hitting the surface with high angle of attack (inlet edges, curved

surfaces with strong centrifugal forces on particles, vortices attached to or near the walls)• Implosion of cavitation bubbles close to the walls

The combined effect of cavitation and sand content is a dangerous mutual amplifier [1, 2]:

• Cavitation inception occurs where the tensile strength of water is exceeded in low pressurezones – fine grain particles reduce the tensile strength like nuclei.

• Wavy surface patterns generated by erosion can easily generate local cavitation in zoneswhich were cavitation free in the original smooth condition.

• Implosion of cavitation bubbles might produce micro jets where particles are shot againstthe wall and create high pressure peaks in shock waves that damage the metallurgicsurface (and also the hard coatings). Such damaged surfaces are again more vulnerable tosand erosion due to the distortion of flow.

Based on above basics it is quite easy to predict the zones of high erosion in the various typesof turbines:

Pelton:• Nozzles, i.e. needles and seat rings (high flow velocity)• Bucket inlet edges (high impact angle)• Bucket surface (high flow velocity, high force perpendicular to the wall, eventual also

cavitation partly caused by damaged inlet edges)

Fig. 1 shows a typical example of erosion on a needle of a 2-jet Pelton. Please note that theundamaged coated needle had the identical impact by sediments.

Fig. 1 Needles of a 2-jet Pelton unit subjectto sand erosion. The coated anduncoated needle have been inoperation simultaneously (thecoating is SXH 48 by VA TECHHYDRO).




4

Francis:• Guide vanes:

at the outlet of the guide vanes the flow velocity is much larger than the relative flowvelocity at runner inlet (esp. for high head units). The highest erosion occurs in the gap

between guide vanes and facing plates triggered by the vortices of the gap flow, see Fig. 2.At small guide vane openings (part load) the centrifugal forces acting on sand particlescan be larger than the drag force of the water entering the runner [3]: the particles arecentrifuged on an orbit between guide vanes and runner causing severe erosion on theinner surface of the guide vanes.

Fig. 2: Typical erosion pattern on thefacing plate of a Francis turbine, incomparison with a numerical simulation.

• Labyrinth rings:strong vortices in the chambers of stepped labyrinths, high impact angle. The erosion inlabyrinth rings and the increase of leakage losses is a major origin of the efficiency dropof high head Francis units in silt-laden water, see chapter 6.2.

• Runner inlet edgesThe high impact angle causes erosion despite the rather low relative velocity. The highesterosion occurs usually in the zone of the horse shoe vortex in the junction between inletedges and crown and band (Fig 4.2 above)

• Runner outlet:although the flow velocities are larger on the suction side, the highest erosion takes placeon the pressure side due to the forces acting on the particles. The vortices (in the fillets)and downstream of the trailing edge create nice erosion patterns on crown and band, Fig

4.2 middle and below.Pumps / Pump Turbines:For pump turbines the situation is analogous to Francis units, except that the dynamic loadingof the guide vanes and hence the non-uniformity in pump mode is even larger than for Francisunits. For pumps with unregulated diffuser vanes the erosion in the tip clearance gap of guidevanes is eliminated. A major concern for pumps is the interaction between erosion andcavitation at the runner inlet edge. Therefore coating of the blade inlet is very beneficial, seeFig. 3.




5

Fig. 3Pump impeller where 1 blade has

been coated for test purposes with a

VA TECH HYDRO coating.The uncoated blade shows severeerosion at blade inlet near shroud.

Kaplan:• Blades:

The flow velocities on the blades are larger than in the rest of the machine, especially onthe suction side near the blade tip. Typically the tip vortex is a major origin of erosion,

partly due to it’s’ combination with tip cavitation, Fig. 10.

3 Impact on efficiency and selection of the right type of turbine

For Francis turbines already small erosion intensities in the zone of the guide vane tip

clearance and the labyrinth rings can cause a dramatic loss in efficiency, see chapter 6.2.Besides that also the axial thrust is strongly increased with worn out labyrinths.

For Pelton units the impact of erosion on efficiency is much less. A wavy bucket surfaceincreases the friction losses but these are relatively small. Severe damages on the nozzles andinlet edges of the buckets will create significant drop in efficiency.

For Kaplan units the impact of erosion on efficiency depends on the sensitivity of the designon an increased tip clearance of the blades.

Ease of repair:For Pelton units exchanging the needles and seat rings and repairing the buckets orexchanging the runner is quick and easy. Also for Kaplan units the possibilities to repair the

blade tips are good. However, repairing an eroded Francis unit is time consuming andexpensive (dismantling runner, guide vanes, exchange labyrinth rings, facing plates, repair therunner with narrow spacing at blade outlet).

The disadvantages of a Francis unit for erosive water are manifold:• Many components suffer from erosion and require a complete dismantling for repair.• Runners of small and medium size cannot be fully coated due to limited access.• The eroded runner bears the risk of dangerous cracks.•

The design for exchangeable labyrinth rings and facing plates increases the costs.




6

• Eroded labyrinth rings strongly increase the axial thrust and reduce the efficiency.• At part load: erosion on guide vanes due to centrifugal effect on sand particles and erosion

inside the runner due to part load vortices.• The drop in efficiency for a certain amount of silt passing through the unit is larger than

for Pelton or Kaplan.

Conclusion: in the high head range (above 300 m) the Pelton concept is superior to theFrancis concept, and for the low head range (below 50 m) Kaplan units should be preferredversus Francis units. It shall not be forgotten that the long term economics of a plant witherosive waters does not primarily depend on the efficiency in the as-new condition but on theintegral efficiency between two repairs, the costs for each repair, the frequency of repair andthe loss of production due to standstill.

4 Benefits of CFD to reduce erosion

CFD (Computational Fluid Dynamics) has been used as tool for the hydraulic design andoptimisation of water turbines for more than 15 years. Especially the step from 3D-Euleranalysis in the runner alone to 3D-Navier Stokes computations in all turbine componentsenhanced the numerical capabilities tremendously [4]. But what can CFD contribute to reduceerosion?

There are 2 main targets:A. Enable cavitation free conditions in a wide operating range.

B. Improve the flow uniformity and identify zones where coating should be applied.Leakage flow, high turbulence, wakes and vortices lead to irregular movements of the

particles causing strong erosion damage on the surfaces. Thus, optimisation of the hydraulic profile with respect to erosion damage aims at uniformity of the flow and the elimination ofvortices. CFD supports the design engineer in evaluating alternative designs by identifyingzones of high risk of erosion. It is evident that a Francis runner can be designed for improvederosion behaviour at the expense of a somewhat lower efficiency [5]. Still, in cases of highlysilt-laden water erosion takes place even in the best–designed turbine. Therefore surfaces haveto be protected by coating. Here, CFD simulation helps to identify those zones which areaffected most by erosion and to decide, which surfaces have to be coated.

This section presents the CFD simulation of the flow field and the predicted erosion damagein a Francis runner whereby the results of the CFD simulation are compared to theobservations made at the plant.

4.1 The Francis runner case

The power plant under consideration, situated in the Swiss Alps, was subject to strong erosiondamage and had to be repaired regularly. In order to reduce repair costs guide vanes andrunner were coated which then led to a significantly longer lifetime of the machine. At thesame time CFD simulations were carried out in order to identify the underlying flow patternsand in order to validate the CFD method by comparing the simulation results with the




7

observations at site. In a first step the erosion damage on the guide vanes was predictednumerically, ref. [6]. Fig. 2 shows the typical erosion on the facing plates with goodcorrespondence to the CFD-erosion simulation. In a second step erosion of the labyrinth seal

was investigated and described in ref. [7]. This paper presents the results of the CFDsimulation of the runner at full load operation.

4.2 CFD method for erosion prediction

Sand erosion is modelled by applying the Lagrange method i.e. tracking a large number ofindividual particles in the flow field. Each particle represents a sample of particles that followan identical path. The motion of particles is described by the Basset-Boussinesq-Oseen-equation whereby experimentally based correlations are used for the drag and the influence ofturbulent fluid motion. During the Lagrangian tracking the number of particles impinging on asurface is recorded. Out of these data the removal of the wall is calculated using either theFinnie model or the Tabakoff model (see ref. [8]). A comparison of the two models in the

present case shows more realistic results with the Tabakoff model.In a one way coupling the flow field is simulated without particles and the Lagrangiantracking is done as post processing. If the volume fraction of particles is very high, one waycoupling is no more sufficient but a two way coupling has to be applied which takes intoaccount the interaction between the fluid and the particles in one combined simulation. In the

present case both models show the same results which means that one way coupling issufficient.All simulations presented here are carried out with the Reynolds-averaged Navier-Stokes(RANS) solver CFX5 (ref. [8]). The computational domain includes one runner blade only

with periodic boundary conditions and with a prolonged inlet and outlet region in order to prevent strong effects due to inlet and outlet boundary conditions. It is discretised using a180’000 nodes hexahedral computational grid. In order to check grid dependency of theresults one simulation was carried out on a 670’000 nodes grid showing no remarkablediscrepancies to the simulation on the coarse grid. Spatial discretisation is of second orderaccuracy with a blend of first order accuracy, turbulence is modelled with the standard k- ε model. One simulation was carried out using the Reynolds stress model but the change ofturbulence model showed no significant effect on the results.

Calculations have been performed for different particle sizes but also for particle samples accordingto the field measurement, see Fig. 4.

Fig. 4 Particle samples: Field test and Rosin-Rammler approximation




8

4.3 Results

Most severe erosion damage is experienced at site• at the leading edge of the runner, especially on the suction side at shroud• on the pressure side of the blade• downstream of the trailing edge on the shroud and on the hub.

CFD prediction Observation at site

Leading edge, suction side

Trailing edge, hub

Trailing edge, shroud

Fig. 5 Comparison between CFD prediction and erosion damage observed at site




9

Accordingly, the CFD simulation shows most damage at the leading edge, the pressure side ofthe blade and downstream of the runner blade as to be seen in Fig. 5. At the junction betweeninlet edge and the 2 lateral shrouds (outside bands) the classical horse shoe vortex causes high

local erosion. This vortex is caused by the fact that the inlet boundary layer from thedistributor walls is hitting the blunt inlet edge of the runner. In the central runner disc in the

plane of symmetry of the 2-flooded runner such a horse shoe vortex does not exist as the inletof the disc is hit by the main flow without shear from side wall boundary layers. The CFD-simulation indicates this difference in erosion at the inlet edge between central disc and lateralshroud quite nicely.At blade outlet (trailing edge) the pattern of erosion damage is well predicted by the CFDsimulation both on hub and shroud.However, it shall be mentioned that the CFD simulation is not yet able to provide accurateabsolute erosion predictions. It is a valuable tool to obtain relative erosion intensities and toevaluate different designs relative to each other .

5 Coating technologies

There are different coating technologies actually used for fighting wear due to silt oncomponents in hydro machinery, for example thermal treatment under reactive atmosphere,thermal spraying and painting. The created or deposited materials range from nitrides,chromium oxide and tungsten carbide to soft coatings.We have tested different coatings under laboratory conditions using for example Plasmaspraying and HVOF-Spraying (HVOF: high velocity oxygen fuel) [9, 10]. Best results wereobtained by using the HVOF process in combination with tungsten carbide based powder. The

powder is injected in a rocket engine-like equipment and then propelled at extremely highspeed to the surface, where a dense hard-metal like coating is produced.Field tests at hydropower stations from pilot customers in Switzerland (Pelton and Francis),Chile (Pelton) as well as from China (Kaplan) confirmed the excellent results obtained underlaboratory conditions (see examples below).Today, tungsten carbide based HVOF coatings are a standard in combating wear. The basisfor this success is the high resistance of such coatings against silt abrasion combined with thehigh flexibility of the method to coat different components at reasonable cost. Since 1983 VATECH HYDRO coated over 2300 components for hydroelectric machinery: for example forPelton turbines 90 runners with over 500 pairs of nozzle tips and -rings, for Francis- or Pump

turbines 96 runners, 600 wicket gates and over 100 labyrinth- and facing rings each.

6 Examples

From the very large experience of VA TECH HYDRO in coating all kinds of turbinecomponents in different regions of the world the following selection shall be presented:

6.1 Pelton runner in Chile

The Pelton plant Alfalfal/Chile has 2 vertical 6-jets Pelton units (H=690m, P=85, 4 MW,

bucket width: 545 mm). The erosion for uncoated runners was severe, Fig. 6, the loss of




10

material due to erosion was 1’200 kg after 105’000 t sand. The sand concentration is between1,2 and 1,8 kg/m 3 and about 70 % of the particles have a hardness above 5,5 Mohs. Thecoating by VA TECH HYDRO gave excellent results, Fig. 7.

Fig. 6: Uncoated Pelton runner(Alfalfal/Chile) after 105’000 t sand passageor approx. 1 year of operation.

Fig. 7: Runner Alfalfal with VA TECHHYDRO coating SXH 70 after 120’000 tsand passage.

The two coated runners had a loss of material of 100 and 150 kg after an operational period

with 105’000 and 120’000 t sand, respectively. This means a factor of 10 in economy ofmaterial lost!In addition, the achievable turbine output has been recorded as function of sediment load

passed through the turbine, Fig. 8.

Fig. 8: Output of Pelton unitAlfalfal with coated anduncoated runners .

The first coating SXH 48 (Plasma) gave already a nice improvement. However, the newestgeneration of VA TECH HYDRO coatings SXH 70 (HVOF) has proven to be an outstandingsuccess.




11

It shall be emphasised that large differences between various coating technologies exist. Thequality of coating strongly depends on the applied material (e.g. tungsten carbide), theapplication method (e.g. HVOF) and the practical know-how in spraying (e.g. by robotics).

6.2 Francis Turbine in Switzerland

The main parameters of the plant Pradella with 4 units are:Head range: 430 – 500 mOutput per units: 80 MWOut runner diameter: 1’800 mmSand content: 0,1 – 0,8 kg / m 3 Minerals with Mohs > 5,5: 56 % (mostly quartz from Swiss glaciers)The original units built in the 1960’s did not have any coating and suffered from severe dropin efficiency due to erosion, mainly in the labyrinths and gaps between guide vanes and facing

plates. With the coating applied by VA TECH HYDRO after a complete overhaul in the1990’s showed that the efficiency drop over time has been significantly reduced, Fig. 9. Thetime span between repairs has been increased by a factor of 2.5.

Fig. 9: Benefit of coatingfor the high head Francisunit Pradella. After 25’000hours of operation the gainin production due to higherefficiency of the coated

turbines corresponds to 6times the costs of 1complete coating.

6.3 Kaplan turbine in China

The plant Qing Tong Xia on the Yellow River (with sand concentrations up to 20 kg/m 3) has been upgraded by new VA TECH HYDRO Kaplan runners whereby both an erosion-friendly blade design as well as a coating of the new generation has been applied. The comparison between the old and new runner after approx. 8’000 hours of operation is as follows:

R a

t i o p r o

d u c

t i o n

b e n e

f i t v s . c o s

t s o

f 1 c o a

t i n g

e f f i c i e n c y

[ % ]

85

86

87

88

89

90

91

92

0 5000 10000 15000 20000 25000 300000

1

2

3

4

5

6

7

8

9

100 2 4 6 8

hours of operation

years of operation

e f f i c i e n c y d r o p

w i t h o u t c o a t i n g

e f f i c i e n c y d e v e l o p m e n t w i t h S X H c o a t i n g

coated components :• guide vanes• facing rings• labyrinths• runner

p r o d

u c t i o n

b e n e

f i t f o r

o n e

t u r b i n

e u n i t

efficiencygain

R a

t i o p r o

d u c

t i o n

b e n e

f i t v s . c o s

t s o

f 1 c o a

t i n g

e f f i c i e n c y

[ % ]

85

86

87

88

89

90

91

92

0 5000 10000 15000 20000 25000 300000

1

2

3

4

5

6

7

8

9

100 2 4 6 8

hours of operation

years of operation

e f f i c i e n c y d r o p

w i t h o u t c o a t i n g

e f f i c i e n c y d e v e l o p m e n t w i t h S X H c o a t i n g

coated components :• guide vanes• facing rings• labyrinths• runner

p r o d

u c t i o n

b e n e

f i t f o r

o n e

t u r b i n

e u n i t

efficiencygain




12

Uncoated old runner Coated new runnerEroded blade area 100 % of surface 1,2 % of surfaceMaterial loss 700 – 1300 kg not measurableAverage erosion depth 1 -2 mm 0,017 mm

(5 % of coating thickness)Estimated loss in efficiency above 2 % below 0,2 %

Fig. 10a and 10b show the uncoated old runner and the coated new runner, respectively after8’250 hours of operation the amount of sand having passed through the turbine was 11 Mio. t.Only a small zone on the suction side near the tip vortex showed some erosion, Fig. 11. In thisarea, the tip vortex contains a small amount of cavitation making the erosion intensity moreaggressive and the coating less resistant. On the remaining part of the blade the coating wasstill in very good shape providing a perfect protection of the steel surface of the blades.

Fig. 10a: uncoated Kaplan runner Fig. 10b: SXH-coated Kaplan runner(after similar erosive conditions asuncoated runner in Fig. 9a.

Fig. 11: after 8250 h: the only part of thecoated blade where erosion wasnoticeable is a small zone at blade tip dueto local cavitation.




13

7 Conclusion

Today the know-how about flow phenomena with impact on erosion and appropriate coatingtechnologies is such that an economic operation of turbines in silt-laden waters is possible.There is no reason not to utilize the renewable Hydro energy in regions with high sedimentcontent in rivers. However, sustainable solutions from the project planning to turbine designup to proper application of coatings must be put together. In case of existing reservoirs withhigh sediment content it might even be worthwhile to continuously release a controlledamount of sediment through coated turbines [12].

Acknowledgements

The authors would like to thank their employers for permission to publish this paper.

References

[1] Duan, C.G., Karelin, V.Y., “Abrasive Erosion and Corrosion of Hydraulic Machinery” ,Imperial College Press, 2002

[2] Brekke H., “Is it possible to avoid cavitation pitting and reduce sand erosion?” , Conf.HYDRO 2004, Porto

[3] Thapa, B., Brekke H., “Effect of sand particle size and surface curvature in erosion ofhydraulic turbine” , 22 nd IAHR Symposium, Stockholm, 2004

[4] Keck H., Drtina, P., Sick, M., “A breakthrough – CFD Flow Simulation for a CompleteTurbine” , Sulzer Technical Review 1/97

[5] Pande, V. K., Shrinivas Rao, V., A new design approach for combating silt erosion ,Proceedings of Hydro 2003, 3-6 November 2003, Dubrovnik, Croatia.

[6] Drtina, P., Krause, M., Abrasion on a Francis turbine guide vane – Numerical simulationand field tests, Proceedings of 14th IAHR Symposium on Hydraulic Machinery andCavitation, 1994.

[7] Mack, R., Drtina, P., Lang, E., Numerical prediction of erosion on guide vanes inlabyrinth seals in hydraulic turbines, WEAR 233-235 (1999), 685-691.

[8] ANSYS CFX, CFX5.6 Theory Manual , Harwell, UK, 2004.[9] Grein, H., Schachenmann, A., “Abrasion in hydroelectric machinery” , Sulzer Technical

Review 1 (1992)

[10] Krause, M., Grein, H., “Abrasion research and prevention” , Sulzer Technical Review 2(1993)

[11] Grein, H., Kalberer, A., Müller, E., “Surface protection against Hydroabrasive Wear” ,Sulzer Hydro Report, e20.07.34-ZC97-10.

[12] Huwyler, P, “Möglichkeiten zur Abführung von Feststoffen aus Stauseen über dieTriebwasserleitung” , Fachtagung „Entlandung von Stauräumen“,Uni der Bundeswehr,

München, (2002)

2005 indiahydro sediment erosion hydraulic turbines (1)

Documents