By John H. Gummer
Some of the most attractive hydro sites are plagued by silt at certain times of the year. Silt erosion of the hydraulic turbines at these sites is typically controlled by upstream settling chambers and turbine protective coatings; however, further work is needed to better predict and control silt erosion.
The problem of hydro-abrasive erosion of hydraulic machinery is not limited to hydroelectric plants. Similar problems to those encountered in hydroelectric work are also prevalent in the mining industry, dredging work, and waste disposal. The blades of gas turbines are subjected to erosion from high-velocity solid particles and those of steam turbines from liquid droplets. Although experience gained in other industries can and is applied to similar problems in hydraulic turbines, hydro requirements are somewhat unique in requiring high machine efficiencies over relatively long periods between maintenance and in the unpredictability of the concentration, shape, and mineral composition of the particle load to be combated.
The problem of abrasive particles in hydroelectric plants is not new. European and North American plants in alpine areas have combated the problem for years. However, the problem is becoming more universal and more acute with the increasing need for electricity in developing countries and the worldwide drive to promote renewable, non-fossil fuel energy sources. Countries with the most spectacular annual increases in per capita gross domestic product, and subsequently with the most need for economically viable, carbon-free energy, tend to be those with the particle-laden rivers.1 For example, many of India’s potential hydroelectric sites are in the north of the country and are fed by run-off water from melting glaciers. Likewise, many of South America’s viable sites are in the Andes; large rivers in China can carry substantial quantities of particles.
Hydro-abrasive erosion of hydraulic turbines in these and other regions is an ongoing problem and one which needs to be solved or at least mitigated if hydropower in these countries is to reach its full carbon-free potential. The problem is exacerbated by the favorable head and topography of many of the rivers which, at least during parts of the year, carry high particle content. They tend to be in mountainous regions, which imply a high head leading to a relatively low construction cost per kilowatt. They typically have fewer environmental and resettlement problems. However, these economic advantages tend to be somewhat negated if the development requires large expenditures on extensive settling chambers or long periods of diminished generation due to a seasonal high particle load in the river.
Mechanisms of hydro-abrasive erosion
Tests on curved hydraulic conduits show that, on the outside curve of the conduit (equivalent to the pressure side of a Francis turbine blade or a Pelton runner bucket), even at relatively low velocity, particles above 1,000 micrometers (µm) in diameter will not follow the hydraulic contour, and will impact upon and damage the hydraulic surface.2 Particles with diameters between 100 µm and 1,000 µm will tend to be channeled along the outer hydraulic contour, and their propensity for damage will be progressively less. For particle diameters below 100 µm, the surface damage increases considerably. This is because small particles become entrained in the turbulent boundary layer, which encases all hydraulic surfaces, and results in a sand blasting of the surface.
Overall erosion from fine particles, if in sufficient quantity, can be as great as that from large particles. Past research indicates that, due to flow separation, the inside bend surface (equivalent to the suction side of a Francis turbine blade) experiences a steady increase in damage as the particle size decreases. However, this generality has been contested.
The effect of particle density is similar to that of size. A particle of greater density will have greater momentum and thus be more inclined to reach the surface in the case of larger particles and less inclined to be entrained in the boundary layer in the case of smaller particles. A particle can only appreciably damage a softer surface; a particle hardness of 5 Mohs is generally considered the cutoff value for hydraulic turbines. As a general rule, the base materials used for hydraulic components in a hydro-abrasive environment should be as hard as possible.
Theoretically, damage should be proportional to relative free stream velocity cubed. However, probably due to boundary layer effects, this is not entirely reflected in practice. Generally accepted experimentally determined exponents for relative free stream velocity are:
- Pelton bucket, 1.5;
- Pelton nozzles, 2.5;
- Francis runners, 3; and
- Francis wicket gates and cheekplates, 2.5.
A particle’s angle of attack to the hydraulic surface gives rise to two distinct erosion mechanisms. A particle approaching nearly normal to the surface will produce impact damage in which the surface is initially cracked, subsequently loosened with further impacts, and finally excavated as the already cracked and loosened particle is removed by another impacting particle. A particle approaching parallel to the surface will scratch and gouge the surface similar to that of mechanical grinding. For angles between the two extremes, the erosion mechanism will be a combination of both.
The resistance to impact or gouging erosion will depend on the characteristics of the hydraulic surface. Very hard base materials tend to be brittle and have limited resistance to impact erosion. On the other hand, extremely hard base materials are very resistant to gouging erosion. Conversely, soft rubbery materials (including soft plastics) exhibit a good resistance to impact erosion but less resistance under the chiseling action of gouging erosion. These generalities do not necessarily apply to protective coatings, for which the effectiveness of the bond between coating and base material and the ability of both the coating and its bond to accommodate flexing of the protected component in operation are additional, equally important, variables.
The shape of the particles is especially relevant for gouging erosion where the particle adopts a cutting action. A sharp-edged, irregular particle can have a far greater deleterious effect on the surface than a well-rounded or spherical particle. Under the gouging mode of erosion, experiments indicate that the erosion rate of a jagged particle can be up to three times that of round particle.
Particle concentration is usually expressed in grams per liter (g/l). However, often parts per million (ppm) by weight is used, with the approximation of 1,000 ppm equal to 1 g/l being normal usage. For a particular facility, particle concentration is essentially a temporal measure (i.e., the greater the concentration, the higher the erosion rate and hence less time to equipment failure). At many facilities, concentration is measured and operation of the units ceases when the particle contraction exceeds a pre-determined value. Erosion rate is essentially proportional to concentration over the practical operating range for a hydropower unit, but there are indications that at higher concentrations at which hydropower stations typically no longer operate, a power law may be more appropriate.
The effect of particles on cavitation in hydraulic reaction turbines is twofold.3 Particles 50 µm in diameter or less provide nuclei for cavitation bubbles, leading to premature commencement of incipient cavitation at Thoma sigma values higher than in pure water. Cavitation bubble development similarly reflects the premature commencement of cavitation. The second, equally serious effect of hydro-abrasive erosion is that it locally changes the hydraulic contour, which, in turn, disrupts the flow and increases both the propensity for and intensity of the cavitation bubble implosion. The impacts from the implosion of the cavitation bubbles fatigue and loosen the hydraulic surface, adding to the erosion damage and making it easier for the impacting particles to remove damaged material. The combination of cavitation and erosion is referred to as a “synergistic” effect, the damage resulting from the combination of the two being far greater than the sum of each acting alone.
Turbine components affected by hydro-abrasive erosion
The relatively low stream velocity in the casing of a Pelton unit means that it is minimally affected by hydro-abrasive erosion. Nozzles, however, suffer greatly; the nozzle spear and trim erode badly, with a resulting decrease in overall turbine efficiency. The inside of Pelton buckets suffers considerable erosion. Splitters tend to suffer impact erosion and the buckets a mixture of impact and gouging. Turbine efficiency is compromised, especially as a result of erosion of the splitter.
As with Pelton units, because of relatively low velocities, the scroll casing of a Francis unit is typically immune from hydro-abrasive erosion. The nose of the stay vanes can be damaged by impact erosion, but usually this a not a major concern. Wicket gates suffer from both impact damage and gouging erosion. Top and bottom cover cheek plates and runner labyrinth seals sustain considerable damage from gouging erosion.
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Francis runners experience major damage at the leading edge due to impact erosion and equally severe loss of material along the length of the blades from a combination of gouging erosion and impact erosion. Trailing edges, because of their initial thinness, are particularly prone to damage.
Erosion of all the above mentioned components eventually means a considerable loss of efficiency. Losses include volumetric efficiency due to increase in labyrinth seal and guide vane clearances, form efficiency due to the change in hydraulic profile of the wicket gates and runner blades, and increase in frictional losses resulting from roughening of the hydraulic surfaces. Paradoxically, the initial erosion of the trailing edges can produce an increase in efficiency due to widening of the flow path; however, this is short-lived as the other detrimental effects of erosion come into play. Likewise, erosion from small particles can grind and hone the surface, leading to an initial reduction in friction. This advantage is soon swamped by a loss in efficiency resulting from deleterious changes in the hydraulic profile.
Methods to predict erosion in hydraulic turbines
State-of-the-art computational fluid dynamics (CFD) methods are employed to further understand the mechanics of hydro-abrasive erosion and, in particular, to design erosion-resistant hydraulic profiles. This typically implies lower specific speeds than normal for the given head (hence lower relative velocities), coupled with fewer jets in the case of Pelton turbines and longer, less sharply contoured blades for Francis turbines. Where the probability of severe hydro-abrasive erosion is high, it is sometimes economically expedient to sacrifice “as new” turbine efficiency for a greater resistance to hydro-abrasive erosion. The setting of a Francis turbine relative to minimum tailwater level should be greater than normally accepted to ensure minimal cavitation damage.
The computerized methods successfully predict the region of maximum wear and can somewhat mitigate the problem by refining hydraulic design and predicting the exact type and position of protective coatings needed. However, they are incapable of accommodating all the independent variables involved in hydro-abrasive erosion of any particular turbine. For this, one has to resort to semi-empirical methods based upon the myriad of data from operating turbines and from accelerated wear laboratory tests. The semi-empirical methods combine the various factors that influence the rate of erosion under particular and definable particle conditions but, because of the deeper setting of an erosion-prone Francis unit, typically do not account for the synergistic effects of cavitation and hydro-abrasive erosion.
The simplest of the various erosion criteria employs the factor H x C, where H is the net head of the turbine in meters and C is the average annual particle concentration in g/l of all particles with a diameter of > 50 µm.4 The proposed ranges for hydro-abrasive erosion damage risk are:
- H x C = > 7: severe;
- H x C = >0.7 and < 7: moderate; and
- H x C = < 0.7: negligible.
Proposed by Nozaki as an extension of the Zu Yan approach is the modified particle concentration factor, which is the product of the annual average particle concentration in g/l and modifying coefficients related to the variables of particle size, hardness, shape, and runner material.5 The factor is shown in Equation 1:
- PE = P x a x k1 x k2 x k3
where:
- PE is the modified suspended concentration in g/l;
- P is the measured suspended concentration in g/l; and
- Factors a, k1, k2, and k3 depend on the type and geometry of the particles and type of runner material (see Table 1).
The final value is then used in curves of PE against turbine net head to predict times between maintenance. Figure 1 gives an example of the curves for Francis runners. Similar charts are available for Francis wicket gates, Pelton runners, and Pelton spears and trim.5
A more sophisticated approach, which takes into consideration the component makeup of the particle load and accommodates variable particle concentrations in respect to time (as opposed to average annual concentrations), is that given by the Abrasion Index. This index considers the contribution in terms of hardness and size of each component mineral in the particle load and then incorporates this in a formula linking the hardness of the base material and relative free stream velocity to obtain the erosion rate.6
The main use of these empirical measures is in initial feasibility work to determine whether hydro-abrasive erosion is going to be a problem at a site. If the answer is in the affirmative, the project designer will want to consider incorporating particle exclusion facilities, a hydro-abrasive erosion friendly turbine and powerhouse design, and possibly turbine protective coatings.
The various approaches to the problems of hydro-abrasive erosion typically take an overall view and do not consider the individual effects of turbine operational efficiency at and away from the “whirl-free” region in the case of reaction turbines and off-design operating conditions for impulse units. Considering the increase in turbulence and cavitation at operation away from the point of best efficiency, hydro-abrasive erosion will be greater the further the unit operates away from its optimum region – especially if synergistic effects are dominant.
Particle exclusion methods
The first weapon against hydro-abrasive erosion is to try to remove it before it reaches the hydro facility. The most efficient method of removing particles is to provide a large head water reservoir which, if of great enough volume and length, will settle out all harmful particles. However, although very effective, it must be remembered that unless provided with effective bottom outlets, a reservoir is only a delaying device. Eventually it will fill and, given adverse topography, deposited particles will reach the turbine inlets.
Many mountainous hydroelectric facilities do not lend themselves to large upstream reservoirs. In these cases, the only viable solution is settling chambers with flushing facilities. The design of such chambers is well established, and their efficacy in removing particles greater than 1 millimeter (mm) is universally recognized. For convenience, incoming load can be divided into three categories: coarse (>200 µm); medium (75 µm to 200 µm); and fine (<75 µm). Reservoirs are efficient in removing all particles down to the upper fine size but settling chambers have their limitations. Although they remove virtually all coarse particles, depending on incoming concentration, they can pass about 80 percent of fine particles. For medium particles, removal effectiveness is a function of particle size, falling between the two extremes. As hard, sharp particles as small as 50 µm in large quantities can cause damage to hydraulic turbines, it is important for designers to acknowledge that, even with settling chambers, in heavily particle-laden rivers (up to a 20 g/l peak concentration in some Himalayan rivers7) a significant particle content will have to be accommodated in the turbines and auxiliaries at certain times of the year.
Protective coatings
As settling chambers inevitably pass substantial quantities of aggressive particles, in many practical instances the turbine still has to be designed for, and its surfaces protected against, the maximum expected particle load downstream of the chambers at which operation of the units is allowed.
Two criteria for judging the effectiveness of a coating are: particle levels at which the turbine is shut down are as high as possible; and the time between repairs is as long as possible. A third criterion is the ease and speed by which repairs to the coating can be made.
Protective coatings fall into two categories: “hard” coatings such as welded Stellite and thermally applied ceramic and tungsten carbide; and “soft” coatings, which are typically a brush, trowel, or spray-on polymer. Variants of the pure hard coating are the thermally applied systems of hard particles in a softer matrix. These hybrid systems bridge the gap between hard and soft coatings while maintaining the potentially superior bonding strength of the thermal application process when compared with the brush or spray-on application of soft coatings. Conversely, the resistance of soft coatings against particle erosion depends on the type of polymer, the surface quality, and the bond efficiency. Given the correct composition and bond for the particular application, a soft coating can be every bit as effective against particle erosion as a hard coating.
Hard ceramic coatings have a poorer resistance to cavitation compared with soft coatings. On the other hand, soft coatings are particularly susceptible to damage from water-borne stones and hard debris. Soft coatings, being inherently more flexible than hard ones, can better accommodate any movement of a protected component in service. In the case of narrow hydraulic channels, soft coatings may be the only option owing to insufficient clearance to accommodate the equipment for hard coating application. Hard and hybrid coatings are typically thinner than soft ones and have more stringent thickness tolerances.
If a coating is worn in service, the old coating must be removed, the component weld repaired if the base metal is damaged, and the coating reapplied. Being brush applied, soft coatings are easier to repair than hard coatings. Repair of soft coatings on larger units can be done in-situ. Hard coatings are far more difficult to repair; typically, the remaining hard coating must be removed by grinding and a new coating applied in the supplier’s factory. This is very time-consuming but can be accelerated to a certain extent in multi-unit stations experiencing substantial particle erosion by installing hard coating repair equipment at the project site.
The choice of a hard or soft coating depends on the characteristics of the particular component to be protected, the hydraulic forces to which it is subjected in service, and the ease of application and repair of the coating. In choosing the type of coating, it is prudent to remember there are very few components that suffer only pure impact or pure gouging erosion. Typically, it is a combination of both.
Accordingly, hard thermally applied coatings are more suited to components requiring tight dimensional clearances such as Pelton nozzles, Pelton buckets, Francis runner wearing rings, and Francis cheek plates. Because of their ease of repair, the prevalent use of soft coatings is on the water passages of stay vanes (if necessary), wicket gates, and runners; however, depending on the prevailing site conditions, hard and hybrid coatings are also used for the hydraulic surfaces of wicket gates and runner blades.
Commercially available products can be applied to a damaged component as a mastic filler. This application repairs the component surface and provides a resistant surface to subsequent hydro-abrasive erosion. A caution: the mastic has no strength in tension; if the erosion of the base material is substantial, the structural integrity of the component is inevitably compromised.
In general, all commercial coatings show a markedly improved hydro-abrasive erosion resistance in service when compared with commonly used base materials such as soft Martensitic stainless steel. In addition to actual in-service performance, coating materials are often evaluated under laboratory conditions with the ensuing published results showing substantial lower erosion rates than the typical base material and proprietary coatings of other suppliers. These results and those from previous hydro projects may or may not be relevant to a particular facility. Experience has shown that coating performance depends very much on actual site conditions. A coating may perform well at one site or in general laboratory tests but may not be suitable for the site under study.8 However, both laboratory testing and past experience at similar sites are good starting points for the development of a tailor-made coating for a new development.
Contractual considerations
There is no known hydraulic machine code that specifically applies to hydro-abrasive erosion guarantees. The International Electrotechnical Commission (IEC) Technical Committee #4’s Working Group 29 is working on this problem, and the draft code “Guide for Dealing with Abrasive Erosion in Water” may be available in the future. IEC Code 60609 addresses wear of specific turbine parts; it covers the guarantee and evaluation for cavitation damage in reaction turbines and pumps (Part 1) and impulse turbines (Part 2). However, Clause 1.3 of this code specifically excludes damage due to abrasive solids in the water, stating that “if relevant, the types of minerals and size of solid (sand) particles in the water analysis and, if it reaches significant proportion, shall be the subject of a special agreement.”
This statement is reflected in most equipment contracts, which give a loss of material criterion for cavitation damage, but typically exclude erosion damage due to particles in the water. The recommendation of IEC 60609 for the purchaser to enter into a mutually acceptable agreement with the proposed contractor for combined cavitation and particle damage guarantee sounds simple in principle but is extremely difficult to apply in practice. Contractors are understandably loath to guarantee their products against particle erosion when the incoming particle conditions cannot be entirely controlled. The possibility of synergistic cavitation and subsequent erosion only exacerbates the situation. A cavitation guarantee is meaningless if the hydraulic surfaces are aggressively attacked by particles.
Contractors’ acceptance of guarantees for hydro-abrasive erosion would be beneficial to both purchaser (in order for him to assure unit output) and the contractor, in order for him to receive commensurate consideration during bid evaluation of his research and development expenditure on particle damage.
A major obstacle to obtaining guarantees for particle damage from a manufacturer is the problem of exactly defining the particle load and monitoring it during the guarantee period to ensure compliance with the limits stipulated in the contract. Historical records are obviously a guide to establishing the contractual particle load, but, because of the unknowns of the efficacy of settlement in reservoirs, power channels, and settling chambers, these may not be adequate for guarantee purposes. In addition, a change to the historical particle load is possible because of upstream landslides, an event which is almost impossible to design for.
The current methods for monitoring particle load are sample taking and off-line analysis, which is slow and labor intensive. On-line optical or acoustic diffusion techniques are being developed.9 Coupled with the problems of defining and monitoring the actual particle load is the possibility of gross damage to any protective surface by large debris (stones, rocks, and hard foreign objects); however, this is unlikely (but not impossible) if settling facilities are installed and the inlet water channels are lined.
Further research and development into all aspects of combating hydro-abrasive erosion is necessary if the protection of hydraulic turbines against this form of damage is to be put onto a contractual basis as covered by an appropriate international code acceptable to all parties.
Notes
- Krishnamachar, P., and S. Rangnekar, “Correlation of Hydropower Potential of Silt Load of Rivers – Means to Access Damage by Silt Due To Not Harnessing Hydropower,” HydroVision 2008 Conference Papers CD-Rom, HCI Publications, Kansas City, Mo., USA, 2008.
- Ortmanns, C., and S. Prigent, “Turbine Abrasion and Desilting Chamber Design,” Hydro 2006 Conference Papers CD-Rom, Aqua-Media International. Ltd., Sutton, Surrey, United Kingdom, 2006.
- Shengcai, L.L., “Cavitation Enhancement in Silt Erosion: Obstacles and Way Forward,” 5th International Symposium of Cavitation, Osaka, Japan, 2003.
- Zu-Yan, M., “Review of Research on Abrasion and Cavitation of Silt Laden Flows through Hydraulic Turbines in China,” 18th IAHR Symposium on Hydraulic Machinery and Cavitation,” International Association of Hydraulic Engineering and Research, Valencia, Spain, 1996.
- Nozaki, T., “Technical Report: Estimation of Repair Cycle of Turbine Due To Abrasion Caused By Suspended Sediment and Determination of Desilting Basin Capacity,” Japan International Cooperation Agency, 1990.
- Sharma, S.K., “Sediment Management in Himalayan Rivers,” HydroVision 2006 Conference Papers CD-Rom, HCI Publications, Kansas City, Mo., USA, 2006.
- Dhar, D.K., and P. Dul, “Silting Problems in Hydropower Plants,” Waterpower XV Conference Papers CD-Rom, HCI Publications, Kansas City, Mo., USA, 2007.
- Sharma, M.K., G.S. Grewal, and A.K. Singh, “Silt Erosion in Indian Hydroelectric Projects – Laboratory Studies of Thermal Spray Coatings over Hydro Turbine Components,” HydroVision 2008 Conference Papers CD-Rom, HCI Publications, Kansas City, Mo., USA, 2008.
- Bishakarma, M.B., “Online Monitoring of Sediments in Hydropower Plants: A System for Assessing the Turbine Exposure and Sediment-Induced Effects,” Waterpower XIV Conference Papers CD-Rom, HCI Publications, Kansas City, Mo., USA, 2005.
Mr. Gummer is a director of Hydro-Consult Pty Ltd., a company specializing in feasibility studies, specification, and contract administration of mechanical and electrical equipment for hydroelectric projects. John has worked for 45 years on equipment specification and equipment contractual problems with the World Bank and about 20 government organizations and private consulting companies on numerous hydro projects, many of which have been in hydro-abrasive prone environments.
Maintenance-friendly designs
A unit operating with particle-laden water should be designed for ease of maintenance and repair. This makes the standard barrel-type vertical design for Francis turbines, where the generator must be removed in order to access the turbine parts, unsuitable – other than for very large units where repair in situ is possible.
Small- to medium-sized vertical Francis units – typical in high to medium head plants – should be designed for bottom dismantling of the runner, bottom ring, discharge ring, and wicket gates. This design provides for access to these components and the head cover and check plates for repair. Because of demand for this feature, the size of Francis turbines with this arrangement is increasing. Runner diameters of up to 5.1 meters have been constructed with bottom dismantling.
The question of whether to choose Pelton or Francis turbines has been ongoing for many years. There is no clearcut answer, as both suffer from hydro-abrasive erosion. A Pelton unit is easier to maintain, but civil costs will be higher in order to accommodate the larger machine. This may be compensated, in part, in a surface facility by the cost of the additional excavation required for Francis unit. Each project has to be considered separately and a reasoned judgment made based on economics, ease of maintenance, site topography, and geology.
Particles can erode heat exchanger tubes if velocities are too high and block them if velocities are too low. If particles are known to be a problem, then closed loop cooling water systems are installed with duplicate raw water/closed loop water heat exchangers. Raw water piping should be stainless steel. The turbine shaft seal should be designed for hydro-abrasive conditions and, if required by the design, provided with a supply of well-filtered water to reduce wear.
Availability of spare runners, nozzles, wicket gates, and cheek plates enable damaged parts to be quickly replaced and subsequently repaired and/or recoated for re-installation during the next maintenance outage.
