40 SHP EQUIPMENT MANUFACTURE41 Turbines A turbine unit consists of a wheel or runner connected to a shaft The shaft spins as a result of the water acting upon the runner and thus drives a generator which converts the rotational power into electricity There are different types of turbines they include Francis Kaplan Propeller Pelton Turgo Cross-flow and Tubular types Choice of turbine in a small hydropower project depends on the head-flow ranges of the site selected

42 Electrical GeneratorsElectrical Generators design depends on the site selection taking into consideration the water head discharge and turbine types Fig 10 shows some of the generator stators under manufacturing

50 MATERIALS FOR SHP EQUIPMENT CONSTRUCTIONMaterials consideration for the turbine blades shows that they can either be fabricated by welding or cast with Aluminum or Stainless steel depending on design consideration and turbine type Fig4 to 9 shows different types of turbines runners For runners it must be resistant to abrasion and cavitation and of good weldability Cast steel is normally used but stainless steel of the following composition is now used

For turbine main shaft sealing polytetrafluorethylene (teflon) is now in use instead of rubber One material problem usually encountered with turbine blades is cavitation due to silt erosion The nature of the silt is quartz rich (65-95) These run over the blades causing damage The silt menace can be effectively tacked by developing a design approach where silt erosion is inherently minimized Effective de-silting of the water entering the penstock could prevent the silt from reaching the turbine Surface treatment of the blades by applying a hard coating on them can also serve as a way out

CORROSION CHALLENGES IN SMALL HYDRO POWER TURBINES

CAVITATIONThe phenomenon of Cavitation and its classificationThe pressure of a liquid at rest or in motion to some extent will form cavities or

bubbles in the liquid When these cavities come to high-pressure area they will collapse and produce huge pressure up to tens of thousands Pa during one millionth second The phenomenon is defined as cavitation whereas the damage of the surface of the materials during the process of cavitation is called pitting of materials In addition the cavitation will cause the following problems

I The rise of temperature in the area where the cavities is collapsed and the partial deformation of the surface of material are observed Usually this partial rise of temperature may reach 200oC ndash 300oC

II The chemical and electrochemical erosion of the metal may occur because of actions of O2 which comes out from water during cavitation

III Electrified phenomenon because of forming thermoelectric couple between the cold surface and partial heat areas

IV Cavitation noise with vibration frequencies of about 100-120 thousand Hz measured in cavitation area

V Vibration caused by water hammer to the surface of the runner passage during the cavities collapse generally is very serious with serious with frequency of several hundreds of thousands Hz

VI The performance of turbine will be abated since the efficiency and other parameters droop caused by development of cavitation

VII Profile cavitation The profile cavitation mainly occurs at the surface of blade which has a several stages of development as shown in figure hellip

a Cavitation appears in the flow field whereas the continuity of flow is destroyed

b The bubble at the tail of the blade back forms a fluctuating areac Cavitation flow begins to destabilize and separate from the blade

VIII Vortex band Cavitation This kind of cavitation as shown in fig 66 will appear at the inlet of the draft tube during the partial and operating conditions especially for francis and fixed blade axial flow turbines and will lead to periodical pressure fluctuation noise and vibration of the turbine

IX Partial Vortex cavitation When high velocity water flows through the clearance the pressure of the liquid will drop to vapour pressure and cavitation will happen in the clearance as shown in Fig

793 Erosion and Erosion Corrosion in Liquid Flow withSolid Particles

Reduction of Cavitation by improved DesignDesilting of Tanks to avoid erosion corosion

Cavitation

Fig Relative Erosion Rate as a Function of Impact Angle for Ductile and Brittle Materials Respectively (air borne Particles)

Figure 739 a) Impingement and b) turbulence corrosion

As mentioned previously erosion corrosion will be particularly intensive when theflowing medium contains solid particles At relatively low velocities we may havethe situation illustrated in Figure 741 The particles hit the surface with a velocity vSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational UseDifferent Forms of Corrosion 141at an angle 1049109 Corrosion products are removed from the surface which is activatedand the corrosion rate on the activated spots may increase strongly If only corrosionproducts are removed all the material loss can (under stable conditions) be measuredas electrochemical corrosion because the electrochemical reaction is necessary for

making new corrosion products to compensate for the removed onesFigure 741 Impacts from solid particles in a liquid flow causing removal of corrosionproducts from the surface (erosion corrosion)With increasing velocity the impact energy increases as well and above a certainlevel not only the corrosion products but also small particles of the material itselfwill be removed This contribution to the material deterioration is a pure mechanicalprocess ie pure erosion Mechanisms and factors that influence pure erosion aredescribed in eg References [734] In strongly erosive environments (high particleconcentration andor high flow velocity) erosion is usually the dominatingdeterioration process while the corrosion contribution forms a smaller proportion(although the absolute corrosion rate may be high) The pure erosion rate can beexpressed by the following formulaW (mmyear) = Kmat 1049109 Kenv 1049109 c 1049109 vn 1049109 f(1049109) (78)where Kmat is a material factor depending in a complex manner on (among otherproperties) hardness and ductility of the substrate Kenv is an environmental factorthat includes the effects of size shape (sharpness) density and hardness of theparticles c is the concentration of particles n is the sondashcalled velocity exponent v isthe particle velocity and 1049109 the impact angle shown in Figure 741For a certain geometrical element it is convenient to replace f(1049109) by a geometricalfunction which eg for pipe bends includes pipe diameter and bend radius Suchfunctions as well as empirical values of the constants in Equation (78) have beendetermined for fluids containing silica sand [735]Equation (78) is very useful in connection with erosion testing in the laboratoryThe proportionality with the particle concentration gives a unique possibility forrealistic acceleration of the tests by using a larger sand concentration than thatexisting under service conditions It also makes it easy to transfer the results to realsand concentrations In this way the testing time can be strongly reducedFor pure particle erosion a value of n of the order of 3 is frequently found Thiscan be explained by a consideration of the kinetic energy of the particles hitting thesurface The total impact energy per area unit and time unit is frac12 mv2 m is the sum ofthe particle mass per area and time unit At constant concentration c m isSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational Use142 Corrosion and Protectionproportional to v and hence the impact energy is proportional to v3 The effect ofthe impact angle 1049109 is very strong but quite different for ductile and brittle basematerials as shown schematically in Figure 742Figure 742 Relative erosion rate as a function of impact angle for ductile and brittlematerials respectively (airborne particles) (After Ives and Ruff [736])The figure is based on experiments with an air jet carrying sharp particles Forliquid flow with sharp particles the top of the curve for ductile materials is lessmarked and usually located at a somewhat higher angle (eg 30ndash40o) Otherwise thedifference between ductile and brittle materials is similar for the latter case also Thecurves can be explained by different erosion mechanisms for ductile and brittlematerials The large difference between materials as to the effect of the impact angleis important for appropriate materials selection under different flow and geometricalconditionsEquation (78) is also of interest when erosion is combined with corrosion It canbe used in modified form for limited ranges of the involved parameters Thelimitation is particularly caused by the fact that the velocity exponent for erosioncorrosion is much less than that for pure erosion (ncorr 1049109 15 usually ncorr 1049109 10)[737] In addition erosion corrosion increases less than proportionally with particleconcentration Together these relationships mean that for combined erosion andcorrosion (corrosivendasherosive wear) W 1049109 cx vy where x lt 1 and 1 lt y lt 3 and bothx and y depend on the relative contribution of erosion and corrosion (they increasewith increasing erosivity and decreasing corrosivity)Because of these relationships we need to be very careful when using Equation(78) on combined erosion and corrosion To make such calculations reasonablyaccurate they must be combined with and based upon several experiments underrelevant conditions

Erosion rate01049109 Impact angle 901049109DuctilematerialBrittlematerialSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational UseDifferent Forms of Corrosion 143More thorough analyses and mechanism studies show that there often is aconsiderable synergy effect of erosion and corrosion Generally the total materialloss rate WT for such material deterioration can be expressed byWT = WE + WC + WEC + WCE (79)WE is the pure erosion rate ie mass loss rate when corrosion is eliminated and Wc

is the corrosion rate in the absence of sand erosion WEC and WCE are both synergyeffects WEC is the increase in erosion rate due to corrosion and WCE is the increasein corrosion rate due to erosionIt is possible to determine the four contributions to the total material loss rate bythe following experimental principles the total material loss rate WT is determinedby weighing the specimen before and after exposure under combined erosive andcorrosive conditions The sum of WC and WCE (the corrosion components) can bemeasured by electrochemical methods during the same exposure (the methodsdescribed in Section 92 can also be used under erosive conditions) WE isdetermined by weighing the specimen before and after exposure in special testswhere corrosion is eliminated by cathodic protection (or possibly by other means)but otherwise under the same conditions as in the former experiments WC can bemeasured electrochemically in tests like the original ones but with all solid particlesexcluded Finally the synergy components WCE and WEC can be derived fromEquation (79) and the mentioned experimentsIn several cases materials for combined erosive and corrosive conditions havebeen evaluated on the basis of separate erosion and corrosion studies and data withthe consequence that the synergistic effects are left out of the evaluation Since oneor the other of these effects may be large the conclusions may be quite wrong Formaterials that usually are passive due to a dense oxide film such as stainless steelsWC is by definition very low But since sand erosion more or less destroys thepassive film the corrosion rate increases strongly and may reach very high valuesie the contribution of WCE may be particularly high for these materials The othersynergy effect WEC is most pronounced for ceramicndashmetallic materials in which themetallic phase has inferior corrosion resistance eg for a cemented carbide with ametallic phase of cobalt (WCndashCo)The deterioration mechanism for WCndashCo (and similar materials) is assumed to beas follows the binder phase of metal around the carbide particles corrodes away sothat the WC particles are more easily removed by erosion The transition from theunexposed state to a corroded and eroded state is schematically illustrated in Figure743 It should be noticed that under extremely erosive conditions or if the metalphase is highly corrosion resistant in the actual environment erosion is dominatingand the effect of the shown mechanism may be insignificantThese relationships and mechanisms of combined erosion and corrosion ondifferent types of material under different conditions are illustrated by experimentalresults available in various publications eg in References [738 739] which alsodescribe a method implying electrochemical determination of corrosion ratessimultaneously on a number of specimensSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational Use144 Corrosion and ProtectionFigure 743 Schematic view of a cemented carbide material a) before testing and b) afterexposure under corrosive and low-erosive conditions

794 Influencing Factors and Conditions in Liquids andLiquidndashGas Mixtures

Some of the factors discussed in Section 793 are also important in cases withoutsolid particles present Local geometry and irregularities on the surface affect thelocal flow pattern and are therefore of great significance for erosion corrosion Flowvelocity affects corrosion in different ways eg as shown in Table 75 (see furtherdiscussion below) In some cases increased flow velocities may reduce the corrosionrate by removing deposits and improving transport of oxygen thereby promotingpassivation or by improving transport of inhibitors to the metal surface But in mostcases increased flow velocity will increase the corrosion rate as described inSections 624 791 792 and 793The materials properties most important in relation to erosion corrosion arethermodynamic nobleness the ability to form mechanically stable and protectingsurface films and the liability to rapid passivation at the start of exposure as well asrapid repassivation after removal of surface films The materials listed in Table 75can roughly be divided into three groups based on their behaviour at the threevelocities represented in the tablea) Carbon steel and iron are active in the whole actual range of flow rates Thecorrosion rate increases steadily with increasing flow velocity mainly due tomore efficient supply of oxygen There are some corrosion products (rust mixedwith calcium carbonate) intact on the steel surface at all three velocities whichlimit the corrosion rate to some extent The critical velocity for completeremoval of deposits on steel depends for one thing on the relative amount ofrust as shown in some experiments in connection with cathodic protectionb) All the copper alloys are protected reasonably well by surface oxides

DE-SILTING OPERATION IN SMALL HYDROPOWER

Selection of Materials for Seals

Selection of Materials For Bearings

Materials for electrical Components

General Materials Selection Procedure

12 Selection of MaterialsA material is that out of which anything is or may be made Much number of factors are affecting for the material selection They are properties of materials performance requirements materialrsquos reliability safetyPhysical attributes environmental conditions availability disposability and recyclability and finally economic factors In these properties

1) One of the most important factors affecting selection of materials for engineering design is the properties of the materials The important properties of the materials are mechanical thermal chemical properties etc2) The material of which a part is composed must be capable of performing a partrsquos function (always it must be possible or not) without failure3) A material in a given application must also be reliable4) A material must safely perform its function5) Physical attributes such as configuration size weight and appearance sometimes also serve functional requirements can be used6) The environment in which a product operates strongly influences service performance7) A material must be readily available and available in large enough quantity for the intended application8) The cost of the materials and the cost of processing the materials into the product or part The development and manufacture of satisfactory products at minimum cost is to make a sound economic choice of materials

The material selection process involves the following major operationsbull Analysis of the materials application problembull Translation of the materials application requirements to materials property valuesbull Selection of candidate materialsbull Evaluation of the candidate materials

And in any material selection the following requirements are focused They are1) High material stiffness is needed to maintain optimal shape of performance2) Low density is needed to reduce gravity forces3) Long-fatigue life is needed to reduce material degradationThe optimal design of the rotor blades is today

to optimize its material selection guidelines especially in terms of the friction coefficient on the bearings in hydroelectric turbines

BearingsBearings locations inside dams must withstand the pressure of rushing water The bearings must be designed so that they can handle even the peak loads that are produced during earthquakes Afterall safety has to be the top priority These low friction bearings combines high load carrying capacity with long maintenance and lubrication intervals

Material Selection for Penstockhttphydropowerinelgovtechtransferpdfsido-10107-vol1-pt2pdfThe turbine manufacturer may have recommended a certain materias for the penstock You may want to consider other material that might be less expensive The most common penstock materials include

i Polyvinyl Chloride (PVC)ii Steel

iii Polyethyleneiv Fiber Reinforced epoxy)v Transite (asbestos cement)

For each of these materials you must consider a number of factors

Cost Availability Physical Properties (Friction Strength chemistry) Joining Methods and installation limitations

This factors are greatly influenced by a number of local and special conditions

Materials availability relates to manufacturing marketing and local demend Certain materials are available only in specific size ranges The head and flow conditions encountered will dictate the physical properties required in the materials to be used and thus influence the cost

Each material alternative is governed by specific joining and installation requirements Joining methods that require special skills or equipment will tend to increase construction costs In addition certain materials are not recccommended for above ground installation In most instances and especially for above ground installations some of the restrained joints will be required Restrained joints include welding concreting or flanging of pipe to prevent joint pullout when the penstock is pressurized

bubbles in the liquid When these cavities come to high-pressure area they will collapse and produce huge pressure up to tens of thousands Pa during one millionth second The phenomenon is defined as cavitation whereas the damage of the surface of the materials during the process of cavitation is called pitting of materials In addition the cavitation will cause the following problems

I The rise of temperature in the area where the cavities is collapsed and the partial deformation of the surface of material are observed Usually this partial rise of temperature may reach 200oC ndash 300oC

II The chemical and electrochemical erosion of the metal may occur because of actions of O2 which comes out from water during cavitation

III Electrified phenomenon because of forming thermoelectric couple between the cold surface and partial heat areas

IV Cavitation noise with vibration frequencies of about 100-120 thousand Hz measured in cavitation area

V Vibration caused by water hammer to the surface of the runner passage during the cavities collapse generally is very serious with serious with frequency of several hundreds of thousands Hz

VI The performance of turbine will be abated since the efficiency and other parameters droop caused by development of cavitation

VII Profile cavitation The profile cavitation mainly occurs at the surface of blade which has a several stages of development as shown in figure hellip

a Cavitation appears in the flow field whereas the continuity of flow is destroyed

b The bubble at the tail of the blade back forms a fluctuating areac Cavitation flow begins to destabilize and separate from the blade

VIII Vortex band Cavitation This kind of cavitation as shown in fig 66 will appear at the inlet of the draft tube during the partial and operating conditions especially for francis and fixed blade axial flow turbines and will lead to periodical pressure fluctuation noise and vibration of the turbine

IX Partial Vortex cavitation When high velocity water flows through the clearance the pressure of the liquid will drop to vapour pressure and cavitation will happen in the clearance as shown in Fig

793 Erosion and Erosion Corrosion in Liquid Flow withSolid Particles

Reduction of Cavitation by improved DesignDesilting of Tanks to avoid erosion corosion

Cavitation

Fig Relative Erosion Rate as a Function of Impact Angle for Ductile and Brittle Materials Respectively (air borne Particles)

Figure 739 a) Impingement and b) turbulence corrosion

As mentioned previously erosion corrosion will be particularly intensive when theflowing medium contains solid particles At relatively low velocities we may havethe situation illustrated in Figure 741 The particles hit the surface with a velocity vSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational UseDifferent Forms of Corrosion 141at an angle 1049109 Corrosion products are removed from the surface which is activatedand the corrosion rate on the activated spots may increase strongly If only corrosionproducts are removed all the material loss can (under stable conditions) be measuredas electrochemical corrosion because the electrochemical reaction is necessary for

making new corrosion products to compensate for the removed onesFigure 741 Impacts from solid particles in a liquid flow causing removal of corrosionproducts from the surface (erosion corrosion)With increasing velocity the impact energy increases as well and above a certainlevel not only the corrosion products but also small particles of the material itselfwill be removed This contribution to the material deterioration is a pure mechanicalprocess ie pure erosion Mechanisms and factors that influence pure erosion aredescribed in eg References [734] In strongly erosive environments (high particleconcentration andor high flow velocity) erosion is usually the dominatingdeterioration process while the corrosion contribution forms a smaller proportion(although the absolute corrosion rate may be high) The pure erosion rate can beexpressed by the following formulaW (mmyear) = Kmat 1049109 Kenv 1049109 c 1049109 vn 1049109 f(1049109) (78)where Kmat is a material factor depending in a complex manner on (among otherproperties) hardness and ductility of the substrate Kenv is an environmental factorthat includes the effects of size shape (sharpness) density and hardness of theparticles c is the concentration of particles n is the sondashcalled velocity exponent v isthe particle velocity and 1049109 the impact angle shown in Figure 741For a certain geometrical element it is convenient to replace f(1049109) by a geometricalfunction which eg for pipe bends includes pipe diameter and bend radius Suchfunctions as well as empirical values of the constants in Equation (78) have beendetermined for fluids containing silica sand [735]Equation (78) is very useful in connection with erosion testing in the laboratoryThe proportionality with the particle concentration gives a unique possibility forrealistic acceleration of the tests by using a larger sand concentration than thatexisting under service conditions It also makes it easy to transfer the results to realsand concentrations In this way the testing time can be strongly reducedFor pure particle erosion a value of n of the order of 3 is frequently found Thiscan be explained by a consideration of the kinetic energy of the particles hitting thesurface The total impact energy per area unit and time unit is frac12 mv2 m is the sum ofthe particle mass per area and time unit At constant concentration c m isSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational Use142 Corrosion and Protectionproportional to v and hence the impact energy is proportional to v3 The effect ofthe impact angle 1049109 is very strong but quite different for ductile and brittle basematerials as shown schematically in Figure 742Figure 742 Relative erosion rate as a function of impact angle for ductile and brittlematerials respectively (airborne particles) (After Ives and Ruff [736])The figure is based on experiments with an air jet carrying sharp particles Forliquid flow with sharp particles the top of the curve for ductile materials is lessmarked and usually located at a somewhat higher angle (eg 30ndash40o) Otherwise thedifference between ductile and brittle materials is similar for the latter case also Thecurves can be explained by different erosion mechanisms for ductile and brittlematerials The large difference between materials as to the effect of the impact angleis important for appropriate materials selection under different flow and geometricalconditionsEquation (78) is also of interest when erosion is combined with corrosion It canbe used in modified form for limited ranges of the involved parameters Thelimitation is particularly caused by the fact that the velocity exponent for erosioncorrosion is much less than that for pure erosion (ncorr 1049109 15 usually ncorr 1049109 10)[737] In addition erosion corrosion increases less than proportionally with particleconcentration Together these relationships mean that for combined erosion andcorrosion (corrosivendasherosive wear) W 1049109 cx vy where x lt 1 and 1 lt y lt 3 and bothx and y depend on the relative contribution of erosion and corrosion (they increasewith increasing erosivity and decreasing corrosivity)Because of these relationships we need to be very careful when using Equation(78) on combined erosion and corrosion To make such calculations reasonablyaccurate they must be combined with and based upon several experiments underrelevant conditions

Erosion rate01049109 Impact angle 901049109DuctilematerialBrittlematerialSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational UseDifferent Forms of Corrosion 143More thorough analyses and mechanism studies show that there often is aconsiderable synergy effect of erosion and corrosion Generally the total materialloss rate WT for such material deterioration can be expressed byWT = WE + WC + WEC + WCE (79)WE is the pure erosion rate ie mass loss rate when corrosion is eliminated and Wc

is the corrosion rate in the absence of sand erosion WEC and WCE are both synergyeffects WEC is the increase in erosion rate due to corrosion and WCE is the increasein corrosion rate due to erosionIt is possible to determine the four contributions to the total material loss rate bythe following experimental principles the total material loss rate WT is determinedby weighing the specimen before and after exposure under combined erosive andcorrosive conditions The sum of WC and WCE (the corrosion components) can bemeasured by electrochemical methods during the same exposure (the methodsdescribed in Section 92 can also be used under erosive conditions) WE isdetermined by weighing the specimen before and after exposure in special testswhere corrosion is eliminated by cathodic protection (or possibly by other means)but otherwise under the same conditions as in the former experiments WC can bemeasured electrochemically in tests like the original ones but with all solid particlesexcluded Finally the synergy components WCE and WEC can be derived fromEquation (79) and the mentioned experimentsIn several cases materials for combined erosive and corrosive conditions havebeen evaluated on the basis of separate erosion and corrosion studies and data withthe consequence that the synergistic effects are left out of the evaluation Since oneor the other of these effects may be large the conclusions may be quite wrong Formaterials that usually are passive due to a dense oxide film such as stainless steelsWC is by definition very low But since sand erosion more or less destroys thepassive film the corrosion rate increases strongly and may reach very high valuesie the contribution of WCE may be particularly high for these materials The othersynergy effect WEC is most pronounced for ceramicndashmetallic materials in which themetallic phase has inferior corrosion resistance eg for a cemented carbide with ametallic phase of cobalt (WCndashCo)The deterioration mechanism for WCndashCo (and similar materials) is assumed to beas follows the binder phase of metal around the carbide particles corrodes away sothat the WC particles are more easily removed by erosion The transition from theunexposed state to a corroded and eroded state is schematically illustrated in Figure743 It should be noticed that under extremely erosive conditions or if the metalphase is highly corrosion resistant in the actual environment erosion is dominatingand the effect of the shown mechanism may be insignificantThese relationships and mechanisms of combined erosion and corrosion ondifferent types of material under different conditions are illustrated by experimentalresults available in various publications eg in References [738 739] which alsodescribe a method implying electrochemical determination of corrosion ratessimultaneously on a number of specimensSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational Use144 Corrosion and ProtectionFigure 743 Schematic view of a cemented carbide material a) before testing and b) afterexposure under corrosive and low-erosive conditions

794 Influencing Factors and Conditions in Liquids andLiquidndashGas Mixtures

Some of the factors discussed in Section 793 are also important in cases withoutsolid particles present Local geometry and irregularities on the surface affect thelocal flow pattern and are therefore of great significance for erosion corrosion Flowvelocity affects corrosion in different ways eg as shown in Table 75 (see furtherdiscussion below) In some cases increased flow velocities may reduce the corrosionrate by removing deposits and improving transport of oxygen thereby promotingpassivation or by improving transport of inhibitors to the metal surface But in mostcases increased flow velocity will increase the corrosion rate as described inSections 624 791 792 and 793The materials properties most important in relation to erosion corrosion arethermodynamic nobleness the ability to form mechanically stable and protectingsurface films and the liability to rapid passivation at the start of exposure as well asrapid repassivation after removal of surface films The materials listed in Table 75can roughly be divided into three groups based on their behaviour at the threevelocities represented in the tablea) Carbon steel and iron are active in the whole actual range of flow rates Thecorrosion rate increases steadily with increasing flow velocity mainly due tomore efficient supply of oxygen There are some corrosion products (rust mixedwith calcium carbonate) intact on the steel surface at all three velocities whichlimit the corrosion rate to some extent The critical velocity for completeremoval of deposits on steel depends for one thing on the relative amount ofrust as shown in some experiments in connection with cathodic protectionb) All the copper alloys are protected reasonably well by surface oxides

DE-SILTING OPERATION IN SMALL HYDROPOWER

Selection of Materials for Seals

Selection of Materials For Bearings

Materials for electrical Components

General Materials Selection Procedure

12 Selection of MaterialsA material is that out of which anything is or may be made Much number of factors are affecting for the material selection They are properties of materials performance requirements materialrsquos reliability safetyPhysical attributes environmental conditions availability disposability and recyclability and finally economic factors In these properties

1) One of the most important factors affecting selection of materials for engineering design is the properties of the materials The important properties of the materials are mechanical thermal chemical properties etc2) The material of which a part is composed must be capable of performing a partrsquos function (always it must be possible or not) without failure3) A material in a given application must also be reliable4) A material must safely perform its function5) Physical attributes such as configuration size weight and appearance sometimes also serve functional requirements can be used6) The environment in which a product operates strongly influences service performance7) A material must be readily available and available in large enough quantity for the intended application8) The cost of the materials and the cost of processing the materials into the product or part The development and manufacture of satisfactory products at minimum cost is to make a sound economic choice of materials

The material selection process involves the following major operationsbull Analysis of the materials application problembull Translation of the materials application requirements to materials property valuesbull Selection of candidate materialsbull Evaluation of the candidate materials

And in any material selection the following requirements are focused They are1) High material stiffness is needed to maintain optimal shape of performance2) Low density is needed to reduce gravity forces3) Long-fatigue life is needed to reduce material degradationThe optimal design of the rotor blades is today

to optimize its material selection guidelines especially in terms of the friction coefficient on the bearings in hydroelectric turbines

BearingsBearings locations inside dams must withstand the pressure of rushing water The bearings must be designed so that they can handle even the peak loads that are produced during earthquakes Afterall safety has to be the top priority These low friction bearings combines high load carrying capacity with long maintenance and lubrication intervals

Material Selection for Penstockhttphydropowerinelgovtechtransferpdfsido-10107-vol1-pt2pdfThe turbine manufacturer may have recommended a certain materias for the penstock You may want to consider other material that might be less expensive The most common penstock materials include

i Polyvinyl Chloride (PVC)ii Steel

iii Polyethyleneiv Fiber Reinforced epoxy)v Transite (asbestos cement)

For each of these materials you must consider a number of factors

Cost Availability Physical Properties (Friction Strength chemistry) Joining Methods and installation limitations

This factors are greatly influenced by a number of local and special conditions

Materials availability relates to manufacturing marketing and local demend Certain materials are available only in specific size ranges The head and flow conditions encountered will dictate the physical properties required in the materials to be used and thus influence the cost

Each material alternative is governed by specific joining and installation requirements Joining methods that require special skills or equipment will tend to increase construction costs In addition certain materials are not recccommended for above ground installation In most instances and especially for above ground installations some of the restrained joints will be required Restrained joints include welding concreting or flanging of pipe to prevent joint pullout when the penstock is pressurized

793 Erosion and Erosion Corrosion in Liquid Flow withSolid Particles

Reduction of Cavitation by improved DesignDesilting of Tanks to avoid erosion corosion

Cavitation

Fig Relative Erosion Rate as a Function of Impact Angle for Ductile and Brittle Materials Respectively (air borne Particles)

Figure 739 a) Impingement and b) turbulence corrosion

As mentioned previously erosion corrosion will be particularly intensive when theflowing medium contains solid particles At relatively low velocities we may havethe situation illustrated in Figure 741 The particles hit the surface with a velocity vSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational UseDifferent Forms of Corrosion 141at an angle 1049109 Corrosion products are removed from the surface which is activatedand the corrosion rate on the activated spots may increase strongly If only corrosionproducts are removed all the material loss can (under stable conditions) be measuredas electrochemical corrosion because the electrochemical reaction is necessary for

making new corrosion products to compensate for the removed onesFigure 741 Impacts from solid particles in a liquid flow causing removal of corrosionproducts from the surface (erosion corrosion)With increasing velocity the impact energy increases as well and above a certainlevel not only the corrosion products but also small particles of the material itselfwill be removed This contribution to the material deterioration is a pure mechanicalprocess ie pure erosion Mechanisms and factors that influence pure erosion aredescribed in eg References [734] In strongly erosive environments (high particleconcentration andor high flow velocity) erosion is usually the dominatingdeterioration process while the corrosion contribution forms a smaller proportion(although the absolute corrosion rate may be high) The pure erosion rate can beexpressed by the following formulaW (mmyear) = Kmat 1049109 Kenv 1049109 c 1049109 vn 1049109 f(1049109) (78)where Kmat is a material factor depending in a complex manner on (among otherproperties) hardness and ductility of the substrate Kenv is an environmental factorthat includes the effects of size shape (sharpness) density and hardness of theparticles c is the concentration of particles n is the sondashcalled velocity exponent v isthe particle velocity and 1049109 the impact angle shown in Figure 741For a certain geometrical element it is convenient to replace f(1049109) by a geometricalfunction which eg for pipe bends includes pipe diameter and bend radius Suchfunctions as well as empirical values of the constants in Equation (78) have beendetermined for fluids containing silica sand [735]Equation (78) is very useful in connection with erosion testing in the laboratoryThe proportionality with the particle concentration gives a unique possibility forrealistic acceleration of the tests by using a larger sand concentration than thatexisting under service conditions It also makes it easy to transfer the results to realsand concentrations In this way the testing time can be strongly reducedFor pure particle erosion a value of n of the order of 3 is frequently found Thiscan be explained by a consideration of the kinetic energy of the particles hitting thesurface The total impact energy per area unit and time unit is frac12 mv2 m is the sum ofthe particle mass per area and time unit At constant concentration c m isSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational Use142 Corrosion and Protectionproportional to v and hence the impact energy is proportional to v3 The effect ofthe impact angle 1049109 is very strong but quite different for ductile and brittle basematerials as shown schematically in Figure 742Figure 742 Relative erosion rate as a function of impact angle for ductile and brittlematerials respectively (airborne particles) (After Ives and Ruff [736])The figure is based on experiments with an air jet carrying sharp particles Forliquid flow with sharp particles the top of the curve for ductile materials is lessmarked and usually located at a somewhat higher angle (eg 30ndash40o) Otherwise thedifference between ductile and brittle materials is similar for the latter case also Thecurves can be explained by different erosion mechanisms for ductile and brittlematerials The large difference between materials as to the effect of the impact angleis important for appropriate materials selection under different flow and geometricalconditionsEquation (78) is also of interest when erosion is combined with corrosion It canbe used in modified form for limited ranges of the involved parameters Thelimitation is particularly caused by the fact that the velocity exponent for erosioncorrosion is much less than that for pure erosion (ncorr 1049109 15 usually ncorr 1049109 10)[737] In addition erosion corrosion increases less than proportionally with particleconcentration Together these relationships mean that for combined erosion andcorrosion (corrosivendasherosive wear) W 1049109 cx vy where x lt 1 and 1 lt y lt 3 and bothx and y depend on the relative contribution of erosion and corrosion (they increasewith increasing erosivity and decreasing corrosivity)Because of these relationships we need to be very careful when using Equation(78) on combined erosion and corrosion To make such calculations reasonablyaccurate they must be combined with and based upon several experiments underrelevant conditions

Erosion rate01049109 Impact angle 901049109DuctilematerialBrittlematerialSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational UseDifferent Forms of Corrosion 143More thorough analyses and mechanism studies show that there often is aconsiderable synergy effect of erosion and corrosion Generally the total materialloss rate WT for such material deterioration can be expressed byWT = WE + WC + WEC + WCE (79)WE is the pure erosion rate ie mass loss rate when corrosion is eliminated and Wc

is the corrosion rate in the absence of sand erosion WEC and WCE are both synergyeffects WEC is the increase in erosion rate due to corrosion and WCE is the increasein corrosion rate due to erosionIt is possible to determine the four contributions to the total material loss rate bythe following experimental principles the total material loss rate WT is determinedby weighing the specimen before and after exposure under combined erosive andcorrosive conditions The sum of WC and WCE (the corrosion components) can bemeasured by electrochemical methods during the same exposure (the methodsdescribed in Section 92 can also be used under erosive conditions) WE isdetermined by weighing the specimen before and after exposure in special testswhere corrosion is eliminated by cathodic protection (or possibly by other means)but otherwise under the same conditions as in the former experiments WC can bemeasured electrochemically in tests like the original ones but with all solid particlesexcluded Finally the synergy components WCE and WEC can be derived fromEquation (79) and the mentioned experimentsIn several cases materials for combined erosive and corrosive conditions havebeen evaluated on the basis of separate erosion and corrosion studies and data withthe consequence that the synergistic effects are left out of the evaluation Since oneor the other of these effects may be large the conclusions may be quite wrong Formaterials that usually are passive due to a dense oxide film such as stainless steelsWC is by definition very low But since sand erosion more or less destroys thepassive film the corrosion rate increases strongly and may reach very high valuesie the contribution of WCE may be particularly high for these materials The othersynergy effect WEC is most pronounced for ceramicndashmetallic materials in which themetallic phase has inferior corrosion resistance eg for a cemented carbide with ametallic phase of cobalt (WCndashCo)The deterioration mechanism for WCndashCo (and similar materials) is assumed to beas follows the binder phase of metal around the carbide particles corrodes away sothat the WC particles are more easily removed by erosion The transition from theunexposed state to a corroded and eroded state is schematically illustrated in Figure743 It should be noticed that under extremely erosive conditions or if the metalphase is highly corrosion resistant in the actual environment erosion is dominatingand the effect of the shown mechanism may be insignificantThese relationships and mechanisms of combined erosion and corrosion ondifferent types of material under different conditions are illustrated by experimentalresults available in various publications eg in References [738 739] which alsodescribe a method implying electrochemical determination of corrosion ratessimultaneously on a number of specimensSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational Use144 Corrosion and ProtectionFigure 743 Schematic view of a cemented carbide material a) before testing and b) afterexposure under corrosive and low-erosive conditions

794 Influencing Factors and Conditions in Liquids andLiquidndashGas Mixtures

Some of the factors discussed in Section 793 are also important in cases withoutsolid particles present Local geometry and irregularities on the surface affect thelocal flow pattern and are therefore of great significance for erosion corrosion Flowvelocity affects corrosion in different ways eg as shown in Table 75 (see furtherdiscussion below) In some cases increased flow velocities may reduce the corrosionrate by removing deposits and improving transport of oxygen thereby promotingpassivation or by improving transport of inhibitors to the metal surface But in mostcases increased flow velocity will increase the corrosion rate as described inSections 624 791 792 and 793The materials properties most important in relation to erosion corrosion arethermodynamic nobleness the ability to form mechanically stable and protectingsurface films and the liability to rapid passivation at the start of exposure as well asrapid repassivation after removal of surface films The materials listed in Table 75can roughly be divided into three groups based on their behaviour at the threevelocities represented in the tablea) Carbon steel and iron are active in the whole actual range of flow rates Thecorrosion rate increases steadily with increasing flow velocity mainly due tomore efficient supply of oxygen There are some corrosion products (rust mixedwith calcium carbonate) intact on the steel surface at all three velocities whichlimit the corrosion rate to some extent The critical velocity for completeremoval of deposits on steel depends for one thing on the relative amount ofrust as shown in some experiments in connection with cathodic protectionb) All the copper alloys are protected reasonably well by surface oxides

DE-SILTING OPERATION IN SMALL HYDROPOWER

Selection of Materials for Seals

Selection of Materials For Bearings

Materials for electrical Components

General Materials Selection Procedure

12 Selection of MaterialsA material is that out of which anything is or may be made Much number of factors are affecting for the material selection They are properties of materials performance requirements materialrsquos reliability safetyPhysical attributes environmental conditions availability disposability and recyclability and finally economic factors In these properties

1) One of the most important factors affecting selection of materials for engineering design is the properties of the materials The important properties of the materials are mechanical thermal chemical properties etc2) The material of which a part is composed must be capable of performing a partrsquos function (always it must be possible or not) without failure3) A material in a given application must also be reliable4) A material must safely perform its function5) Physical attributes such as configuration size weight and appearance sometimes also serve functional requirements can be used6) The environment in which a product operates strongly influences service performance7) A material must be readily available and available in large enough quantity for the intended application8) The cost of the materials and the cost of processing the materials into the product or part The development and manufacture of satisfactory products at minimum cost is to make a sound economic choice of materials

The material selection process involves the following major operationsbull Analysis of the materials application problembull Translation of the materials application requirements to materials property valuesbull Selection of candidate materialsbull Evaluation of the candidate materials

And in any material selection the following requirements are focused They are1) High material stiffness is needed to maintain optimal shape of performance2) Low density is needed to reduce gravity forces3) Long-fatigue life is needed to reduce material degradationThe optimal design of the rotor blades is today

to optimize its material selection guidelines especially in terms of the friction coefficient on the bearings in hydroelectric turbines

BearingsBearings locations inside dams must withstand the pressure of rushing water The bearings must be designed so that they can handle even the peak loads that are produced during earthquakes Afterall safety has to be the top priority These low friction bearings combines high load carrying capacity with long maintenance and lubrication intervals

Material Selection for Penstockhttphydropowerinelgovtechtransferpdfsido-10107-vol1-pt2pdfThe turbine manufacturer may have recommended a certain materias for the penstock You may want to consider other material that might be less expensive The most common penstock materials include

i Polyvinyl Chloride (PVC)ii Steel

iii Polyethyleneiv Fiber Reinforced epoxy)v Transite (asbestos cement)

For each of these materials you must consider a number of factors

Cost Availability Physical Properties (Friction Strength chemistry) Joining Methods and installation limitations

This factors are greatly influenced by a number of local and special conditions

Materials availability relates to manufacturing marketing and local demend Certain materials are available only in specific size ranges The head and flow conditions encountered will dictate the physical properties required in the materials to be used and thus influence the cost

Each material alternative is governed by specific joining and installation requirements Joining methods that require special skills or equipment will tend to increase construction costs In addition certain materials are not recccommended for above ground installation In most instances and especially for above ground installations some of the restrained joints will be required Restrained joints include welding concreting or flanging of pipe to prevent joint pullout when the penstock is pressurized

Fig Relative Erosion Rate as a Function of Impact Angle for Ductile and Brittle Materials Respectively (air borne Particles)

Figure 739 a) Impingement and b) turbulence corrosion

As mentioned previously erosion corrosion will be particularly intensive when theflowing medium contains solid particles At relatively low velocities we may havethe situation illustrated in Figure 741 The particles hit the surface with a velocity vSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational UseDifferent Forms of Corrosion 141at an angle 1049109 Corrosion products are removed from the surface which is activatedand the corrosion rate on the activated spots may increase strongly If only corrosionproducts are removed all the material loss can (under stable conditions) be measuredas electrochemical corrosion because the electrochemical reaction is necessary for

making new corrosion products to compensate for the removed onesFigure 741 Impacts from solid particles in a liquid flow causing removal of corrosionproducts from the surface (erosion corrosion)With increasing velocity the impact energy increases as well and above a certainlevel not only the corrosion products but also small particles of the material itselfwill be removed This contribution to the material deterioration is a pure mechanicalprocess ie pure erosion Mechanisms and factors that influence pure erosion aredescribed in eg References [734] In strongly erosive environments (high particleconcentration andor high flow velocity) erosion is usually the dominatingdeterioration process while the corrosion contribution forms a smaller proportion(although the absolute corrosion rate may be high) The pure erosion rate can beexpressed by the following formulaW (mmyear) = Kmat 1049109 Kenv 1049109 c 1049109 vn 1049109 f(1049109) (78)where Kmat is a material factor depending in a complex manner on (among otherproperties) hardness and ductility of the substrate Kenv is an environmental factorthat includes the effects of size shape (sharpness) density and hardness of theparticles c is the concentration of particles n is the sondashcalled velocity exponent v isthe particle velocity and 1049109 the impact angle shown in Figure 741For a certain geometrical element it is convenient to replace f(1049109) by a geometricalfunction which eg for pipe bends includes pipe diameter and bend radius Suchfunctions as well as empirical values of the constants in Equation (78) have beendetermined for fluids containing silica sand [735]Equation (78) is very useful in connection with erosion testing in the laboratoryThe proportionality with the particle concentration gives a unique possibility forrealistic acceleration of the tests by using a larger sand concentration than thatexisting under service conditions It also makes it easy to transfer the results to realsand concentrations In this way the testing time can be strongly reducedFor pure particle erosion a value of n of the order of 3 is frequently found Thiscan be explained by a consideration of the kinetic energy of the particles hitting thesurface The total impact energy per area unit and time unit is frac12 mv2 m is the sum ofthe particle mass per area and time unit At constant concentration c m isSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational Use142 Corrosion and Protectionproportional to v and hence the impact energy is proportional to v3 The effect ofthe impact angle 1049109 is very strong but quite different for ductile and brittle basematerials as shown schematically in Figure 742Figure 742 Relative erosion rate as a function of impact angle for ductile and brittlematerials respectively (airborne particles) (After Ives and Ruff [736])The figure is based on experiments with an air jet carrying sharp particles Forliquid flow with sharp particles the top of the curve for ductile materials is lessmarked and usually located at a somewhat higher angle (eg 30ndash40o) Otherwise thedifference between ductile and brittle materials is similar for the latter case also Thecurves can be explained by different erosion mechanisms for ductile and brittlematerials The large difference between materials as to the effect of the impact angleis important for appropriate materials selection under different flow and geometricalconditionsEquation (78) is also of interest when erosion is combined with corrosion It canbe used in modified form for limited ranges of the involved parameters Thelimitation is particularly caused by the fact that the velocity exponent for erosioncorrosion is much less than that for pure erosion (ncorr 1049109 15 usually ncorr 1049109 10)[737] In addition erosion corrosion increases less than proportionally with particleconcentration Together these relationships mean that for combined erosion andcorrosion (corrosivendasherosive wear) W 1049109 cx vy where x lt 1 and 1 lt y lt 3 and bothx and y depend on the relative contribution of erosion and corrosion (they increasewith increasing erosivity and decreasing corrosivity)Because of these relationships we need to be very careful when using Equation(78) on combined erosion and corrosion To make such calculations reasonablyaccurate they must be combined with and based upon several experiments underrelevant conditions

Erosion rate01049109 Impact angle 901049109DuctilematerialBrittlematerialSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational UseDifferent Forms of Corrosion 143More thorough analyses and mechanism studies show that there often is aconsiderable synergy effect of erosion and corrosion Generally the total materialloss rate WT for such material deterioration can be expressed byWT = WE + WC + WEC + WCE (79)WE is the pure erosion rate ie mass loss rate when corrosion is eliminated and Wc

is the corrosion rate in the absence of sand erosion WEC and WCE are both synergyeffects WEC is the increase in erosion rate due to corrosion and WCE is the increasein corrosion rate due to erosionIt is possible to determine the four contributions to the total material loss rate bythe following experimental principles the total material loss rate WT is determinedby weighing the specimen before and after exposure under combined erosive andcorrosive conditions The sum of WC and WCE (the corrosion components) can bemeasured by electrochemical methods during the same exposure (the methodsdescribed in Section 92 can also be used under erosive conditions) WE isdetermined by weighing the specimen before and after exposure in special testswhere corrosion is eliminated by cathodic protection (or possibly by other means)but otherwise under the same conditions as in the former experiments WC can bemeasured electrochemically in tests like the original ones but with all solid particlesexcluded Finally the synergy components WCE and WEC can be derived fromEquation (79) and the mentioned experimentsIn several cases materials for combined erosive and corrosive conditions havebeen evaluated on the basis of separate erosion and corrosion studies and data withthe consequence that the synergistic effects are left out of the evaluation Since oneor the other of these effects may be large the conclusions may be quite wrong Formaterials that usually are passive due to a dense oxide film such as stainless steelsWC is by definition very low But since sand erosion more or less destroys thepassive film the corrosion rate increases strongly and may reach very high valuesie the contribution of WCE may be particularly high for these materials The othersynergy effect WEC is most pronounced for ceramicndashmetallic materials in which themetallic phase has inferior corrosion resistance eg for a cemented carbide with ametallic phase of cobalt (WCndashCo)The deterioration mechanism for WCndashCo (and similar materials) is assumed to beas follows the binder phase of metal around the carbide particles corrodes away sothat the WC particles are more easily removed by erosion The transition from theunexposed state to a corroded and eroded state is schematically illustrated in Figure743 It should be noticed that under extremely erosive conditions or if the metalphase is highly corrosion resistant in the actual environment erosion is dominatingand the effect of the shown mechanism may be insignificantThese relationships and mechanisms of combined erosion and corrosion ondifferent types of material under different conditions are illustrated by experimentalresults available in various publications eg in References [738 739] which alsodescribe a method implying electrochemical determination of corrosion ratessimultaneously on a number of specimensSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational Use144 Corrosion and ProtectionFigure 743 Schematic view of a cemented carbide material a) before testing and b) afterexposure under corrosive and low-erosive conditions

794 Influencing Factors and Conditions in Liquids andLiquidndashGas Mixtures

Some of the factors discussed in Section 793 are also important in cases withoutsolid particles present Local geometry and irregularities on the surface affect thelocal flow pattern and are therefore of great significance for erosion corrosion Flowvelocity affects corrosion in different ways eg as shown in Table 75 (see furtherdiscussion below) In some cases increased flow velocities may reduce the corrosionrate by removing deposits and improving transport of oxygen thereby promotingpassivation or by improving transport of inhibitors to the metal surface But in mostcases increased flow velocity will increase the corrosion rate as described inSections 624 791 792 and 793The materials properties most important in relation to erosion corrosion arethermodynamic nobleness the ability to form mechanically stable and protectingsurface films and the liability to rapid passivation at the start of exposure as well asrapid repassivation after removal of surface films The materials listed in Table 75can roughly be divided into three groups based on their behaviour at the threevelocities represented in the tablea) Carbon steel and iron are active in the whole actual range of flow rates Thecorrosion rate increases steadily with increasing flow velocity mainly due tomore efficient supply of oxygen There are some corrosion products (rust mixedwith calcium carbonate) intact on the steel surface at all three velocities whichlimit the corrosion rate to some extent The critical velocity for completeremoval of deposits on steel depends for one thing on the relative amount ofrust as shown in some experiments in connection with cathodic protectionb) All the copper alloys are protected reasonably well by surface oxides

DE-SILTING OPERATION IN SMALL HYDROPOWER

Selection of Materials for Seals

Selection of Materials For Bearings

Materials for electrical Components

General Materials Selection Procedure

12 Selection of MaterialsA material is that out of which anything is or may be made Much number of factors are affecting for the material selection They are properties of materials performance requirements materialrsquos reliability safetyPhysical attributes environmental conditions availability disposability and recyclability and finally economic factors In these properties

1) One of the most important factors affecting selection of materials for engineering design is the properties of the materials The important properties of the materials are mechanical thermal chemical properties etc2) The material of which a part is composed must be capable of performing a partrsquos function (always it must be possible or not) without failure3) A material in a given application must also be reliable4) A material must safely perform its function5) Physical attributes such as configuration size weight and appearance sometimes also serve functional requirements can be used6) The environment in which a product operates strongly influences service performance7) A material must be readily available and available in large enough quantity for the intended application8) The cost of the materials and the cost of processing the materials into the product or part The development and manufacture of satisfactory products at minimum cost is to make a sound economic choice of materials

The material selection process involves the following major operationsbull Analysis of the materials application problembull Translation of the materials application requirements to materials property valuesbull Selection of candidate materialsbull Evaluation of the candidate materials

And in any material selection the following requirements are focused They are1) High material stiffness is needed to maintain optimal shape of performance2) Low density is needed to reduce gravity forces3) Long-fatigue life is needed to reduce material degradationThe optimal design of the rotor blades is today

to optimize its material selection guidelines especially in terms of the friction coefficient on the bearings in hydroelectric turbines

BearingsBearings locations inside dams must withstand the pressure of rushing water The bearings must be designed so that they can handle even the peak loads that are produced during earthquakes Afterall safety has to be the top priority These low friction bearings combines high load carrying capacity with long maintenance and lubrication intervals

Material Selection for Penstockhttphydropowerinelgovtechtransferpdfsido-10107-vol1-pt2pdfThe turbine manufacturer may have recommended a certain materias for the penstock You may want to consider other material that might be less expensive The most common penstock materials include

i Polyvinyl Chloride (PVC)ii Steel

iii Polyethyleneiv Fiber Reinforced epoxy)v Transite (asbestos cement)

For each of these materials you must consider a number of factors

Cost Availability Physical Properties (Friction Strength chemistry) Joining Methods and installation limitations

This factors are greatly influenced by a number of local and special conditions

Materials availability relates to manufacturing marketing and local demend Certain materials are available only in specific size ranges The head and flow conditions encountered will dictate the physical properties required in the materials to be used and thus influence the cost

Each material alternative is governed by specific joining and installation requirements Joining methods that require special skills or equipment will tend to increase construction costs In addition certain materials are not recccommended for above ground installation In most instances and especially for above ground installations some of the restrained joints will be required Restrained joints include welding concreting or flanging of pipe to prevent joint pullout when the penstock is pressurized

making new corrosion products to compensate for the removed onesFigure 741 Impacts from solid particles in a liquid flow causing removal of corrosionproducts from the surface (erosion corrosion)With increasing velocity the impact energy increases as well and above a certainlevel not only the corrosion products but also small particles of the material itselfwill be removed This contribution to the material deterioration is a pure mechanicalprocess ie pure erosion Mechanisms and factors that influence pure erosion aredescribed in eg References [734] In strongly erosive environments (high particleconcentration andor high flow velocity) erosion is usually the dominatingdeterioration process while the corrosion contribution forms a smaller proportion(although the absolute corrosion rate may be high) The pure erosion rate can beexpressed by the following formulaW (mmyear) = Kmat 1049109 Kenv 1049109 c 1049109 vn 1049109 f(1049109) (78)where Kmat is a material factor depending in a complex manner on (among otherproperties) hardness and ductility of the substrate Kenv is an environmental factorthat includes the effects of size shape (sharpness) density and hardness of theparticles c is the concentration of particles n is the sondashcalled velocity exponent v isthe particle velocity and 1049109 the impact angle shown in Figure 741For a certain geometrical element it is convenient to replace f(1049109) by a geometricalfunction which eg for pipe bends includes pipe diameter and bend radius Suchfunctions as well as empirical values of the constants in Equation (78) have beendetermined for fluids containing silica sand [735]Equation (78) is very useful in connection with erosion testing in the laboratoryThe proportionality with the particle concentration gives a unique possibility forrealistic acceleration of the tests by using a larger sand concentration than thatexisting under service conditions It also makes it easy to transfer the results to realsand concentrations In this way the testing time can be strongly reducedFor pure particle erosion a value of n of the order of 3 is frequently found Thiscan be explained by a consideration of the kinetic energy of the particles hitting thesurface The total impact energy per area unit and time unit is frac12 mv2 m is the sum ofthe particle mass per area and time unit At constant concentration c m isSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational Use142 Corrosion and Protectionproportional to v and hence the impact energy is proportional to v3 The effect ofthe impact angle 1049109 is very strong but quite different for ductile and brittle basematerials as shown schematically in Figure 742Figure 742 Relative erosion rate as a function of impact angle for ductile and brittlematerials respectively (airborne particles) (After Ives and Ruff [736])The figure is based on experiments with an air jet carrying sharp particles Forliquid flow with sharp particles the top of the curve for ductile materials is lessmarked and usually located at a somewhat higher angle (eg 30ndash40o) Otherwise thedifference between ductile and brittle materials is similar for the latter case also Thecurves can be explained by different erosion mechanisms for ductile and brittlematerials The large difference between materials as to the effect of the impact angleis important for appropriate materials selection under different flow and geometricalconditionsEquation (78) is also of interest when erosion is combined with corrosion It canbe used in modified form for limited ranges of the involved parameters Thelimitation is particularly caused by the fact that the velocity exponent for erosioncorrosion is much less than that for pure erosion (ncorr 1049109 15 usually ncorr 1049109 10)[737] In addition erosion corrosion increases less than proportionally with particleconcentration Together these relationships mean that for combined erosion andcorrosion (corrosivendasherosive wear) W 1049109 cx vy where x lt 1 and 1 lt y lt 3 and bothx and y depend on the relative contribution of erosion and corrosion (they increasewith increasing erosivity and decreasing corrosivity)Because of these relationships we need to be very careful when using Equation(78) on combined erosion and corrosion To make such calculations reasonablyaccurate they must be combined with and based upon several experiments underrelevant conditions

Erosion rate01049109 Impact angle 901049109DuctilematerialBrittlematerialSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational UseDifferent Forms of Corrosion 143More thorough analyses and mechanism studies show that there often is aconsiderable synergy effect of erosion and corrosion Generally the total materialloss rate WT for such material deterioration can be expressed byWT = WE + WC + WEC + WCE (79)WE is the pure erosion rate ie mass loss rate when corrosion is eliminated and Wc

is the corrosion rate in the absence of sand erosion WEC and WCE are both synergyeffects WEC is the increase in erosion rate due to corrosion and WCE is the increasein corrosion rate due to erosionIt is possible to determine the four contributions to the total material loss rate bythe following experimental principles the total material loss rate WT is determinedby weighing the specimen before and after exposure under combined erosive andcorrosive conditions The sum of WC and WCE (the corrosion components) can bemeasured by electrochemical methods during the same exposure (the methodsdescribed in Section 92 can also be used under erosive conditions) WE isdetermined by weighing the specimen before and after exposure in special testswhere corrosion is eliminated by cathodic protection (or possibly by other means)but otherwise under the same conditions as in the former experiments WC can bemeasured electrochemically in tests like the original ones but with all solid particlesexcluded Finally the synergy components WCE and WEC can be derived fromEquation (79) and the mentioned experimentsIn several cases materials for combined erosive and corrosive conditions havebeen evaluated on the basis of separate erosion and corrosion studies and data withthe consequence that the synergistic effects are left out of the evaluation Since oneor the other of these effects may be large the conclusions may be quite wrong Formaterials that usually are passive due to a dense oxide film such as stainless steelsWC is by definition very low But since sand erosion more or less destroys thepassive film the corrosion rate increases strongly and may reach very high valuesie the contribution of WCE may be particularly high for these materials The othersynergy effect WEC is most pronounced for ceramicndashmetallic materials in which themetallic phase has inferior corrosion resistance eg for a cemented carbide with ametallic phase of cobalt (WCndashCo)The deterioration mechanism for WCndashCo (and similar materials) is assumed to beas follows the binder phase of metal around the carbide particles corrodes away sothat the WC particles are more easily removed by erosion The transition from theunexposed state to a corroded and eroded state is schematically illustrated in Figure743 It should be noticed that under extremely erosive conditions or if the metalphase is highly corrosion resistant in the actual environment erosion is dominatingand the effect of the shown mechanism may be insignificantThese relationships and mechanisms of combined erosion and corrosion ondifferent types of material under different conditions are illustrated by experimentalresults available in various publications eg in References [738 739] which alsodescribe a method implying electrochemical determination of corrosion ratessimultaneously on a number of specimensSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational Use144 Corrosion and ProtectionFigure 743 Schematic view of a cemented carbide material a) before testing and b) afterexposure under corrosive and low-erosive conditions

794 Influencing Factors and Conditions in Liquids andLiquidndashGas Mixtures

Some of the factors discussed in Section 793 are also important in cases withoutsolid particles present Local geometry and irregularities on the surface affect thelocal flow pattern and are therefore of great significance for erosion corrosion Flowvelocity affects corrosion in different ways eg as shown in Table 75 (see furtherdiscussion below) In some cases increased flow velocities may reduce the corrosionrate by removing deposits and improving transport of oxygen thereby promotingpassivation or by improving transport of inhibitors to the metal surface But in mostcases increased flow velocity will increase the corrosion rate as described inSections 624 791 792 and 793The materials properties most important in relation to erosion corrosion arethermodynamic nobleness the ability to form mechanically stable and protectingsurface films and the liability to rapid passivation at the start of exposure as well asrapid repassivation after removal of surface films The materials listed in Table 75can roughly be divided into three groups based on their behaviour at the threevelocities represented in the tablea) Carbon steel and iron are active in the whole actual range of flow rates Thecorrosion rate increases steadily with increasing flow velocity mainly due tomore efficient supply of oxygen There are some corrosion products (rust mixedwith calcium carbonate) intact on the steel surface at all three velocities whichlimit the corrosion rate to some extent The critical velocity for completeremoval of deposits on steel depends for one thing on the relative amount ofrust as shown in some experiments in connection with cathodic protectionb) All the copper alloys are protected reasonably well by surface oxides

DE-SILTING OPERATION IN SMALL HYDROPOWER

Selection of Materials for Seals

Selection of Materials For Bearings

Materials for electrical Components

General Materials Selection Procedure

12 Selection of MaterialsA material is that out of which anything is or may be made Much number of factors are affecting for the material selection They are properties of materials performance requirements materialrsquos reliability safetyPhysical attributes environmental conditions availability disposability and recyclability and finally economic factors In these properties

1) One of the most important factors affecting selection of materials for engineering design is the properties of the materials The important properties of the materials are mechanical thermal chemical properties etc2) The material of which a part is composed must be capable of performing a partrsquos function (always it must be possible or not) without failure3) A material in a given application must also be reliable4) A material must safely perform its function5) Physical attributes such as configuration size weight and appearance sometimes also serve functional requirements can be used6) The environment in which a product operates strongly influences service performance7) A material must be readily available and available in large enough quantity for the intended application8) The cost of the materials and the cost of processing the materials into the product or part The development and manufacture of satisfactory products at minimum cost is to make a sound economic choice of materials

The material selection process involves the following major operationsbull Analysis of the materials application problembull Translation of the materials application requirements to materials property valuesbull Selection of candidate materialsbull Evaluation of the candidate materials

And in any material selection the following requirements are focused They are1) High material stiffness is needed to maintain optimal shape of performance2) Low density is needed to reduce gravity forces3) Long-fatigue life is needed to reduce material degradationThe optimal design of the rotor blades is today

to optimize its material selection guidelines especially in terms of the friction coefficient on the bearings in hydroelectric turbines

BearingsBearings locations inside dams must withstand the pressure of rushing water The bearings must be designed so that they can handle even the peak loads that are produced during earthquakes Afterall safety has to be the top priority These low friction bearings combines high load carrying capacity with long maintenance and lubrication intervals

Material Selection for Penstockhttphydropowerinelgovtechtransferpdfsido-10107-vol1-pt2pdfThe turbine manufacturer may have recommended a certain materias for the penstock You may want to consider other material that might be less expensive The most common penstock materials include

i Polyvinyl Chloride (PVC)ii Steel

iii Polyethyleneiv Fiber Reinforced epoxy)v Transite (asbestos cement)

For each of these materials you must consider a number of factors

Cost Availability Physical Properties (Friction Strength chemistry) Joining Methods and installation limitations

This factors are greatly influenced by a number of local and special conditions

Materials availability relates to manufacturing marketing and local demend Certain materials are available only in specific size ranges The head and flow conditions encountered will dictate the physical properties required in the materials to be used and thus influence the cost

Each material alternative is governed by specific joining and installation requirements Joining methods that require special skills or equipment will tend to increase construction costs In addition certain materials are not recccommended for above ground installation In most instances and especially for above ground installations some of the restrained joints will be required Restrained joints include welding concreting or flanging of pipe to prevent joint pullout when the penstock is pressurized

Erosion rate01049109 Impact angle 901049109DuctilematerialBrittlematerialSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational UseDifferent Forms of Corrosion 143More thorough analyses and mechanism studies show that there often is aconsiderable synergy effect of erosion and corrosion Generally the total materialloss rate WT for such material deterioration can be expressed byWT = WE + WC + WEC + WCE (79)WE is the pure erosion rate ie mass loss rate when corrosion is eliminated and Wc

is the corrosion rate in the absence of sand erosion WEC and WCE are both synergyeffects WEC is the increase in erosion rate due to corrosion and WCE is the increasein corrosion rate due to erosionIt is possible to determine the four contributions to the total material loss rate bythe following experimental principles the total material loss rate WT is determinedby weighing the specimen before and after exposure under combined erosive andcorrosive conditions The sum of WC and WCE (the corrosion components) can bemeasured by electrochemical methods during the same exposure (the methodsdescribed in Section 92 can also be used under erosive conditions) WE isdetermined by weighing the specimen before and after exposure in special testswhere corrosion is eliminated by cathodic protection (or possibly by other means)but otherwise under the same conditions as in the former experiments WC can bemeasured electrochemically in tests like the original ones but with all solid particlesexcluded Finally the synergy components WCE and WEC can be derived fromEquation (79) and the mentioned experimentsIn several cases materials for combined erosive and corrosive conditions havebeen evaluated on the basis of separate erosion and corrosion studies and data withthe consequence that the synergistic effects are left out of the evaluation Since oneor the other of these effects may be large the conclusions may be quite wrong Formaterials that usually are passive due to a dense oxide film such as stainless steelsWC is by definition very low But since sand erosion more or less destroys thepassive film the corrosion rate increases strongly and may reach very high valuesie the contribution of WCE may be particularly high for these materials The othersynergy effect WEC is most pronounced for ceramicndashmetallic materials in which themetallic phase has inferior corrosion resistance eg for a cemented carbide with ametallic phase of cobalt (WCndashCo)The deterioration mechanism for WCndashCo (and similar materials) is assumed to beas follows the binder phase of metal around the carbide particles corrodes away sothat the WC particles are more easily removed by erosion The transition from theunexposed state to a corroded and eroded state is schematically illustrated in Figure743 It should be noticed that under extremely erosive conditions or if the metalphase is highly corrosion resistant in the actual environment erosion is dominatingand the effect of the shown mechanism may be insignificantThese relationships and mechanisms of combined erosion and corrosion ondifferent types of material under different conditions are illustrated by experimentalresults available in various publications eg in References [738 739] which alsodescribe a method implying electrochemical determination of corrosion ratessimultaneously on a number of specimensSOFTbank E-Book Center Tehran Phone 6640387966493070 For Educational Use144 Corrosion and ProtectionFigure 743 Schematic view of a cemented carbide material a) before testing and b) afterexposure under corrosive and low-erosive conditions

794 Influencing Factors and Conditions in Liquids andLiquidndashGas Mixtures

Some of the factors discussed in Section 793 are also important in cases withoutsolid particles present Local geometry and irregularities on the surface affect thelocal flow pattern and are therefore of great significance for erosion corrosion Flowvelocity affects corrosion in different ways eg as shown in Table 75 (see furtherdiscussion below) In some cases increased flow velocities may reduce the corrosionrate by removing deposits and improving transport of oxygen thereby promotingpassivation or by improving transport of inhibitors to the metal surface But in mostcases increased flow velocity will increase the corrosion rate as described inSections 624 791 792 and 793The materials properties most important in relation to erosion corrosion arethermodynamic nobleness the ability to form mechanically stable and protectingsurface films and the liability to rapid passivation at the start of exposure as well asrapid repassivation after removal of surface films The materials listed in Table 75can roughly be divided into three groups based on their behaviour at the threevelocities represented in the tablea) Carbon steel and iron are active in the whole actual range of flow rates Thecorrosion rate increases steadily with increasing flow velocity mainly due tomore efficient supply of oxygen There are some corrosion products (rust mixedwith calcium carbonate) intact on the steel surface at all three velocities whichlimit the corrosion rate to some extent The critical velocity for completeremoval of deposits on steel depends for one thing on the relative amount ofrust as shown in some experiments in connection with cathodic protectionb) All the copper alloys are protected reasonably well by surface oxides

DE-SILTING OPERATION IN SMALL HYDROPOWER

Selection of Materials for Seals

Selection of Materials For Bearings

Materials for electrical Components

General Materials Selection Procedure

12 Selection of MaterialsA material is that out of which anything is or may be made Much number of factors are affecting for the material selection They are properties of materials performance requirements materialrsquos reliability safetyPhysical attributes environmental conditions availability disposability and recyclability and finally economic factors In these properties

1) One of the most important factors affecting selection of materials for engineering design is the properties of the materials The important properties of the materials are mechanical thermal chemical properties etc2) The material of which a part is composed must be capable of performing a partrsquos function (always it must be possible or not) without failure3) A material in a given application must also be reliable4) A material must safely perform its function5) Physical attributes such as configuration size weight and appearance sometimes also serve functional requirements can be used6) The environment in which a product operates strongly influences service performance7) A material must be readily available and available in large enough quantity for the intended application8) The cost of the materials and the cost of processing the materials into the product or part The development and manufacture of satisfactory products at minimum cost is to make a sound economic choice of materials

The material selection process involves the following major operationsbull Analysis of the materials application problembull Translation of the materials application requirements to materials property valuesbull Selection of candidate materialsbull Evaluation of the candidate materials

And in any material selection the following requirements are focused They are1) High material stiffness is needed to maintain optimal shape of performance2) Low density is needed to reduce gravity forces3) Long-fatigue life is needed to reduce material degradationThe optimal design of the rotor blades is today

to optimize its material selection guidelines especially in terms of the friction coefficient on the bearings in hydroelectric turbines

BearingsBearings locations inside dams must withstand the pressure of rushing water The bearings must be designed so that they can handle even the peak loads that are produced during earthquakes Afterall safety has to be the top priority These low friction bearings combines high load carrying capacity with long maintenance and lubrication intervals

Material Selection for Penstockhttphydropowerinelgovtechtransferpdfsido-10107-vol1-pt2pdfThe turbine manufacturer may have recommended a certain materias for the penstock You may want to consider other material that might be less expensive The most common penstock materials include

i Polyvinyl Chloride (PVC)ii Steel

iii Polyethyleneiv Fiber Reinforced epoxy)v Transite (asbestos cement)

For each of these materials you must consider a number of factors

Cost Availability Physical Properties (Friction Strength chemistry) Joining Methods and installation limitations

This factors are greatly influenced by a number of local and special conditions

Materials availability relates to manufacturing marketing and local demend Certain materials are available only in specific size ranges The head and flow conditions encountered will dictate the physical properties required in the materials to be used and thus influence the cost

Each material alternative is governed by specific joining and installation requirements Joining methods that require special skills or equipment will tend to increase construction costs In addition certain materials are not recccommended for above ground installation In most instances and especially for above ground installations some of the restrained joints will be required Restrained joints include welding concreting or flanging of pipe to prevent joint pullout when the penstock is pressurized

Some of the factors discussed in Section 793 are also important in cases withoutsolid particles present Local geometry and irregularities on the surface affect thelocal flow pattern and are therefore of great significance for erosion corrosion Flowvelocity affects corrosion in different ways eg as shown in Table 75 (see furtherdiscussion below) In some cases increased flow velocities may reduce the corrosionrate by removing deposits and improving transport of oxygen thereby promotingpassivation or by improving transport of inhibitors to the metal surface But in mostcases increased flow velocity will increase the corrosion rate as described inSections 624 791 792 and 793The materials properties most important in relation to erosion corrosion arethermodynamic nobleness the ability to form mechanically stable and protectingsurface films and the liability to rapid passivation at the start of exposure as well asrapid repassivation after removal of surface films The materials listed in Table 75can roughly be divided into three groups based on their behaviour at the threevelocities represented in the tablea) Carbon steel and iron are active in the whole actual range of flow rates Thecorrosion rate increases steadily with increasing flow velocity mainly due tomore efficient supply of oxygen There are some corrosion products (rust mixedwith calcium carbonate) intact on the steel surface at all three velocities whichlimit the corrosion rate to some extent The critical velocity for completeremoval of deposits on steel depends for one thing on the relative amount ofrust as shown in some experiments in connection with cathodic protectionb) All the copper alloys are protected reasonably well by surface oxides

DE-SILTING OPERATION IN SMALL HYDROPOWER

Selection of Materials for Seals

Selection of Materials For Bearings

Materials for electrical Components

General Materials Selection Procedure

12 Selection of MaterialsA material is that out of which anything is or may be made Much number of factors are affecting for the material selection They are properties of materials performance requirements materialrsquos reliability safetyPhysical attributes environmental conditions availability disposability and recyclability and finally economic factors In these properties

1) One of the most important factors affecting selection of materials for engineering design is the properties of the materials The important properties of the materials are mechanical thermal chemical properties etc2) The material of which a part is composed must be capable of performing a partrsquos function (always it must be possible or not) without failure3) A material in a given application must also be reliable4) A material must safely perform its function5) Physical attributes such as configuration size weight and appearance sometimes also serve functional requirements can be used6) The environment in which a product operates strongly influences service performance7) A material must be readily available and available in large enough quantity for the intended application8) The cost of the materials and the cost of processing the materials into the product or part The development and manufacture of satisfactory products at minimum cost is to make a sound economic choice of materials

The material selection process involves the following major operationsbull Analysis of the materials application problembull Translation of the materials application requirements to materials property valuesbull Selection of candidate materialsbull Evaluation of the candidate materials

And in any material selection the following requirements are focused They are1) High material stiffness is needed to maintain optimal shape of performance2) Low density is needed to reduce gravity forces3) Long-fatigue life is needed to reduce material degradationThe optimal design of the rotor blades is today

to optimize its material selection guidelines especially in terms of the friction coefficient on the bearings in hydroelectric turbines

BearingsBearings locations inside dams must withstand the pressure of rushing water The bearings must be designed so that they can handle even the peak loads that are produced during earthquakes Afterall safety has to be the top priority These low friction bearings combines high load carrying capacity with long maintenance and lubrication intervals

Material Selection for Penstockhttphydropowerinelgovtechtransferpdfsido-10107-vol1-pt2pdfThe turbine manufacturer may have recommended a certain materias for the penstock You may want to consider other material that might be less expensive The most common penstock materials include

i Polyvinyl Chloride (PVC)ii Steel

iii Polyethyleneiv Fiber Reinforced epoxy)v Transite (asbestos cement)

For each of these materials you must consider a number of factors

Cost Availability Physical Properties (Friction Strength chemistry) Joining Methods and installation limitations

This factors are greatly influenced by a number of local and special conditions

Materials availability relates to manufacturing marketing and local demend Certain materials are available only in specific size ranges The head and flow conditions encountered will dictate the physical properties required in the materials to be used and thus influence the cost

Each material alternative is governed by specific joining and installation requirements Joining methods that require special skills or equipment will tend to increase construction costs In addition certain materials are not recccommended for above ground installation In most instances and especially for above ground installations some of the restrained joints will be required Restrained joints include welding concreting or flanging of pipe to prevent joint pullout when the penstock is pressurized

1) One of the most important factors affecting selection of materials for engineering design is the properties of the materials The important properties of the materials are mechanical thermal chemical properties etc2) The material of which a part is composed must be capable of performing a partrsquos function (always it must be possible or not) without failure3) A material in a given application must also be reliable4) A material must safely perform its function5) Physical attributes such as configuration size weight and appearance sometimes also serve functional requirements can be used6) The environment in which a product operates strongly influences service performance7) A material must be readily available and available in large enough quantity for the intended application8) The cost of the materials and the cost of processing the materials into the product or part The development and manufacture of satisfactory products at minimum cost is to make a sound economic choice of materials

The material selection process involves the following major operationsbull Analysis of the materials application problembull Translation of the materials application requirements to materials property valuesbull Selection of candidate materialsbull Evaluation of the candidate materials

And in any material selection the following requirements are focused They are1) High material stiffness is needed to maintain optimal shape of performance2) Low density is needed to reduce gravity forces3) Long-fatigue life is needed to reduce material degradationThe optimal design of the rotor blades is today

to optimize its material selection guidelines especially in terms of the friction coefficient on the bearings in hydroelectric turbines

BearingsBearings locations inside dams must withstand the pressure of rushing water The bearings must be designed so that they can handle even the peak loads that are produced during earthquakes Afterall safety has to be the top priority These low friction bearings combines high load carrying capacity with long maintenance and lubrication intervals

Material Selection for Penstockhttphydropowerinelgovtechtransferpdfsido-10107-vol1-pt2pdfThe turbine manufacturer may have recommended a certain materias for the penstock You may want to consider other material that might be less expensive The most common penstock materials include

i Polyvinyl Chloride (PVC)ii Steel

iii Polyethyleneiv Fiber Reinforced epoxy)v Transite (asbestos cement)

For each of these materials you must consider a number of factors

Cost Availability Physical Properties (Friction Strength chemistry) Joining Methods and installation limitations

This factors are greatly influenced by a number of local and special conditions

Materials availability relates to manufacturing marketing and local demend Certain materials are available only in specific size ranges The head and flow conditions encountered will dictate the physical properties required in the materials to be used and thus influence the cost

Each material alternative is governed by specific joining and installation requirements Joining methods that require special skills or equipment will tend to increase construction costs In addition certain materials are not recccommended for above ground installation In most instances and especially for above ground installations some of the restrained joints will be required Restrained joints include welding concreting or flanging of pipe to prevent joint pullout when the penstock is pressurized

Material Selection for Penstockhttphydropowerinelgovtechtransferpdfsido-10107-vol1-pt2pdfThe turbine manufacturer may have recommended a certain materias for the penstock You may want to consider other material that might be less expensive The most common penstock materials include

i Polyvinyl Chloride (PVC)ii Steel

iii Polyethyleneiv Fiber Reinforced epoxy)v Transite (asbestos cement)

For each of these materials you must consider a number of factors

Cost Availability Physical Properties (Friction Strength chemistry) Joining Methods and installation limitations

This factors are greatly influenced by a number of local and special conditions

Materials availability relates to manufacturing marketing and local demend Certain materials are available only in specific size ranges The head and flow conditions encountered will dictate the physical properties required in the materials to be used and thus influence the cost

Each material alternative is governed by specific joining and installation requirements Joining methods that require special skills or equipment will tend to increase construction costs In addition certain materials are not recccommended for above ground installation In most instances and especially for above ground installations some of the restrained joints will be required Restrained joints include welding concreting or flanging of pipe to prevent joint pullout when the penstock is pressurized