An Introduction To Erosion Types , Causes ,Models And Prevention
Introduction
Erosion is used as a general term to refer to several types of mechanical processes in which a structure or component incurs physical damage. These processes generally remove some of the base material of the affected structure or component, structurally weakening the component.
If the component has a protective coating or an oxide film, erosion processes may remove that coating. Erosion processes can affect many different materials; within the context of nuclear plant systems, they act primarily on metal structures and components. The following types of erosion processes are known to cause mechanical damage to structures and components, and are generally considered when evaluating metallurgical failures:
· Fluid Erosion
· Cavitation
· Solid Particle Erosion
· Impingement
· Flashing Induced Erosion
· Fretting
Fluid Erosion
Fluid Erosion is the mechanical removal of material by a moving fluid. This occurs when the shear stress (𝜏f) for a fluid constrained by a given structure or component exceeds the maximum allowable shear stress (Sv) of the material used for that given structure or component.
The surface of that structure or component will break down and be subject to wear by erosion (figure 1). The surface shear stress for a flowing fluid can be calculated from following relationship for turbulent flows:
where:
𝜌 is the fluid density (lbm/ft³)
f is the Darcy friction factor
V is the average flow velocity (ft/sec)
The material removed by fluid erosion can be either base metal or the protective film formed from corrosion products. Soft metals such as copper and specific copper alloys are very susceptible to erosion damage. Brass, aluminum brass and cupronickel are more susceptible to erosion damage than steel.
Cavitation
Cavitation occurs when the local pressure in a flowing fluid drops below vapor pressure(figure 2) , resulting in the formation of vapor cavities. When the local pressure rises above vapor pressure, the vapor cavities collapse. The collapse of the bubbles results in mechanical damage to the surrounding material due to the impingement of high velocity microjets and shock waves. Estimates of the microjet velocities range from 300 ft/sec to 3,000 ft/sec. For example in the Heater Drains system, cavitation could occur downstream of a level control valve.
Cavitation can occur in a variety of components, including pumps, turbines, valves, orifices and elbows. The four broad categories of cavitation are
bulk, flow curvature, surface roughness and turbulence.
· Bulk cavitation may occur in locations where the flow velocity is increased – such as at the vena contracta (the contracted portion of a liquid jet at or near the orifice from which it issues) of a valve, or in the fluid stream near the impeller in a pump.
· Flow curvature cavitation occurs when a surface curves away from the direction of the flow or the flow curves to attach to a surface.
· Turbulence cavitation is usually associated with low recovery valves where high velocity, low pressure eddies can easily be formed.
· Surface roughness cavitation is caused by low static pressures in the wakes that form downstream of a surface protuberance or obstruction, such as a mound of weld metal or the presence of a backing ring at a weld.
All of these types of cavitation are the result of a reduction in static pressure due to changes in flow conditions. Cavitation can also have dramatic impacts on pump performance
Cavitation levels can be summarized as follows:
1· Incipient cavitation – Occurs intermittently over a restricted area. There is no objectionable noise or vibration and is considered acceptable.
2· Critical cavitation – Continuous light cavitation. Noise and vibration are acceptable and only minor damage is expected after long periods of operation (months to years).
3· Incipient damage – Continuous moderate to heavy cavitation. The onset of pitting will occur after short periods of operation; the noise levels may be objectionable.
4· Choking cavitation – Cavitation is severe enough to cause fluid flow to become choked. Reduction in pressure downstream of cavitation location does not cause a corresponding increase in flow. Noise and vibration levels reach maximum values.
Solid Particle Erosion
Solid particle erosion (figure 7) is caused by solid particles entrained in a fluid stream (usually liquid) impacting on the surface of a structure or component. In nuclear plants, solid particle erosion is most commonly seen in service water systems due to the entrainment of sand or silt.
Impingement
Impingement erosion (figure 8) is caused by liquid droplets entrained in a fluid stream (usually vapor) impacting on the surface of a structure or component. The impact of droplets can produce craters by plastic deformation of the component surface. The surface roughness caused by these deformations can increase the localized shear stress on the material and as a consequence can accelerate the degradation process. This form of material degradation is also referred to as droplet impingement erosion or liquid impact induced erosion.
Impingement damage most commonly occurs in systems that contain wet steam or when water is injected into a steam filled system. Impingement can also occur as the result of partial blockage of a tube, resulting in deflection of the flow stream against the tube wall. Components that are commonly damaged by impingement include condenser tubes, turbine blades, valve seats and valve disks in nuclear plant systems as well as piston rings in engines.
Flashing Induced Erosion
Flashing induced erosion (figure 9) is the result of spontaneous vapor formation caused by sudden pressure changes. This commonly occurs in drain and vent lines downstream of valves in liquid systems where the fluid is near saturation pressure. As some of the liquid flashes to vapor, it undergoes a rapid expansion of volume that increases the fluid velocity and accelerates the remaining liquid phase, liquid droplets and/or liquid film, leading to erosion. Typical examples are found downstream of Feed water Heater shell level control valves.
Fretting
Fretting occurs when tight fitting metal surfaces experience cyclic relative motion that causes them to impact on or rub against each other. The relative motion abrades one or both surfaces, producing debris from base metal or corrosion products. The debris may remain in contact with the original components, further increasing the abrasive effects of fretting. The debris can also prevent precision devices from operating due to fouling. Fretting has been a concern in steam generator tube bundles and control rods.
Mitigation and Prevention of Erosion
Erosion damage can generally be mitigated by using more resistant materials. Reduction in flow velocity will also reduce the amount of damage caused by most erosion mechanisms. The following sections discuss methods that may be used to prevent or mitigate some of the erosion type damage mechanisms discussed in this training guide.
Fluid Erosion Damage
is generally combated by using more resistant material and reducing flow velocities.
Cavitation
Cavitation Damage may be mitigated by using more resistant materials. Design alteration, including use of cascading orifices or low recovery valves, is a method for preventing cavitation.
Solid Particle Erosion
Eliminating or reducing the solid particles is the preferred method of preventing or mitigating solid particle erosion. This could be done by installing strainers, or preventing unintended ingress. Use of a more resistant material and velocity reduction will also mitigate solid particle erosion.
Impingement
Eliminating or reducing moisture will combat impingement. Use of a more resistant material and velocity reduction will also mitigate impingement. Design solutions such as installing turning vanes, waste plates and improving flow conditions (such as using long radius elbows or bends) should be considered.
Flashing Induced Erosion
Use of resistant material will combat flashing induced erosion
Fretting
Use of a more resistant material may help mitigate fretting. Elimination of the source of the relative motion will eliminate fretting. Improved mechanical fitup or use of lubricants may also mitigate or prevent fretting. Varying the location of metal to metal contact will mitigate the consequences of fretting by distributing the damage more widely.
THEORIES OF EROSION
Erosion is commonly measured in terms of a parameter W which is equal to the mass of material removed from the surface divided by the mass of the eroding material. Occasionally it is more convenient to refer the parameter to the volume loss divided by the volume of eroding material. In either case the parameter is dimensionless.
In most cases W > 0, a condition which indicates that material is removed during erosion but under certain circumstances W < 0.
DUCTILE MATERIAL MODELS
The trajectory of a particle cutting and removing material was calculated, and the eroded volume, V, was determined to be given by the expression (Finnie 1960):
where
𝜎f is flow stress,
m is the particle mass,
v𝗈 is the impact velocity,
K is the ratio of vertical force to horizontal force on the particle,
d is the depth of cut.
g(𝛼) is a function describing the effect of attack angle 𝛼.
Sheldon and Kanhere formula
where
d is the (spherical) particle diameter,
𝜌p is the particle density,
H is the Vickers hardness value of the material.
This theory leads to a greater velocity dependence than expected from energy arguments (proportional to v² )
BRITTLE MATERIAL MODELS
The load at which crack propagation occurs is related to the distribution of surface flaws through the Weibull statistics. The approximate area. A, of cracked material is calculated for a particle penetration depth of h, and the volume removed per impact is set proportional to Ah. The final equation for the erosion rate , W, is expressed in terms of the particle size, r, the particle velocity, vo and Weibull constants, m and 𝜎o Sheldon and Finnie (1966):
where the exponents a and b are given by:
a = 3(m-0. 67) / (m-2) for round particles
a = 3 . 6 (m-0. 67) / (m-2) for angular particles
b = 2. 4 (m-0. 67) /(m-2) for either shape
For particles much stiff er than the target, the constant k1 , is given
where E is the modulus of elasticity of the target and 𝜌 is the density of the particle.
The erosion model developed by Evans . assumes that the erosion rate is proportional to the amount of material removed by each impact event. The volume, V, lost per impact is calculated from the depth, h, of penetration and the maximum size of the lateral cracks formed during impact. Since the lateral crack size is proportional to the radial crack size cr , V is given by the following equation:
cr is crack formed during impact
you can follow that it is wide range
Erosion Wear Models
Finnie Model’s
A sharp edged particle moves into the specimen surface causing deformation and removal of
surface material
where,
Q is the volume of material removed,
M is total mass of the abrasives,
V is velocity of the abrasive particles,
p is constant horizontal component of the contact stress on the surface,
𝜑 is the ratio of depth of contact (l) to the depth of cut (yt),
K is the ratio of vertical to horizontal force components of the total force acting on the particle (considered constant and equal to 2)
α is the angle of approach (impact angle) for the particles.
Bitter’s model:
In Bitter’s model represented, deformation wear by WD and cutting wear by WC. Cutting wear has two component depends on the impact angle. Deformation and Cutting wear equation as follows:
where
WC1 = cutting wear loss if impinging particle stays in the target after collision,
WC2 = cutting wear loss if particle leaves the target surface after collision,
𝛜 & Q = wear factor of deformation and cutting wear, respectively,
V = incident velocity of erosive particle,
M = total mass of impinging particles,
K & K1 = constants determined by the elastic properties of target,
C = constant and
𝛼 = impact angle
Hutching’s Model:
Stack studied the hutching’s model of erosion and reviewed that, when spherical particle strikes on
the ductile material at normal, erosion wear occurs by the formation and subsequent detachment of
platelets of metal lying parallel to the eroded surface. Model gives information the about
detachment of platelets is only possible when the accumulated plastic strain within the fragment,
after many cycles of plastic deformation, reaches a critical value.
Where
Hd - Dynamic hardness
Dt - Density of the target material
Dp - Density of erodent particles
Ui - Erodent particle velocity.
The term 𝛼r/𝜖c ² cannot be measured independently. Hutchings assumed the value of 𝛼r/𝜖c ² is equal to 0.7
Sundararajan and Shewmon erosion model
The basic idea of the model is that erosion loss is processed under high-strain-rate and hence adiabatic deformation conditions, and therefore, the mechanical response of the target is dynamic.
The criteria for the removal of such lip are assumed to be based on a critical strain which the lips attain after a number of particle impacts.
Where
Ui - velocity of impacting particles.
Dp – density
Cp - specific heat
Tm - melting temperature
Hs - static hardness of the target material
Introduction
Erosion is used as a general term to refer to several types of mechanical processes in which a structure or component incurs physical damage. These processes generally remove some of the base material of the affected structure or component, structurally weakening the component.
If the component has a protective coating or an oxide film, erosion processes may remove that coating. Erosion processes can affect many different materials; within the context of nuclear plant systems, they act primarily on metal structures and components. The following types of erosion processes are known to cause mechanical damage to structures and components, and are generally considered when evaluating metallurgical failures:
· Fluid Erosion
· Cavitation
· Solid Particle Erosion
· Impingement
· Flashing Induced Erosion
· Fretting
Fluid Erosion
Fluid Erosion is the mechanical removal of material by a moving fluid. This occurs when the shear stress (𝜏f) for a fluid constrained by a given structure or component exceeds the maximum allowable shear stress (Sv) of the material used for that given structure or component.
 |
| FIG 1 |
The surface of that structure or component will break down and be subject to wear by erosion (figure 1). The surface shear stress for a flowing fluid can be calculated from following relationship for turbulent flows:
𝜌 is the fluid density (lbm/ft³)
f is the Darcy friction factor
V is the average flow velocity (ft/sec)
The material removed by fluid erosion can be either base metal or the protective film formed from corrosion products. Soft metals such as copper and specific copper alloys are very susceptible to erosion damage. Brass, aluminum brass and cupronickel are more susceptible to erosion damage than steel.
Cavitation
Cavitation occurs when the local pressure in a flowing fluid drops below vapor pressure(figure 2) , resulting in the formation of vapor cavities. When the local pressure rises above vapor pressure, the vapor cavities collapse. The collapse of the bubbles results in mechanical damage to the surrounding material due to the impingement of high velocity microjets and shock waves. Estimates of the microjet velocities range from 300 ft/sec to 3,000 ft/sec. For example in the Heater Drains system, cavitation could occur downstream of a level control valve.
 |
| FIG 2 |
Cavitation can occur in a variety of components, including pumps, turbines, valves, orifices and elbows. The four broad categories of cavitation are
bulk, flow curvature, surface roughness and turbulence.
· Bulk cavitation may occur in locations where the flow velocity is increased – such as at the vena contracta (the contracted portion of a liquid jet at or near the orifice from which it issues) of a valve, or in the fluid stream near the impeller in a pump.
 |
| FIG 3 |
· Flow curvature cavitation occurs when a surface curves away from the direction of the flow or the flow curves to attach to a surface.
 |
| FIG 4 |
· Turbulence cavitation is usually associated with low recovery valves where high velocity, low pressure eddies can easily be formed.
 |
| FIG 5 |
· Surface roughness cavitation is caused by low static pressures in the wakes that form downstream of a surface protuberance or obstruction, such as a mound of weld metal or the presence of a backing ring at a weld.
 |
| FIG 6 |
Cavitation levels can be summarized as follows:
1· Incipient cavitation – Occurs intermittently over a restricted area. There is no objectionable noise or vibration and is considered acceptable.
2· Critical cavitation – Continuous light cavitation. Noise and vibration are acceptable and only minor damage is expected after long periods of operation (months to years).
3· Incipient damage – Continuous moderate to heavy cavitation. The onset of pitting will occur after short periods of operation; the noise levels may be objectionable.
4· Choking cavitation – Cavitation is severe enough to cause fluid flow to become choked. Reduction in pressure downstream of cavitation location does not cause a corresponding increase in flow. Noise and vibration levels reach maximum values.
Solid Particle Erosion
Solid particle erosion (figure 7) is caused by solid particles entrained in a fluid stream (usually liquid) impacting on the surface of a structure or component. In nuclear plants, solid particle erosion is most commonly seen in service water systems due to the entrainment of sand or silt.
 |
| FIG 7 |
Impingement erosion (figure 8) is caused by liquid droplets entrained in a fluid stream (usually vapor) impacting on the surface of a structure or component. The impact of droplets can produce craters by plastic deformation of the component surface. The surface roughness caused by these deformations can increase the localized shear stress on the material and as a consequence can accelerate the degradation process. This form of material degradation is also referred to as droplet impingement erosion or liquid impact induced erosion.
Impingement damage most commonly occurs in systems that contain wet steam or when water is injected into a steam filled system. Impingement can also occur as the result of partial blockage of a tube, resulting in deflection of the flow stream against the tube wall. Components that are commonly damaged by impingement include condenser tubes, turbine blades, valve seats and valve disks in nuclear plant systems as well as piston rings in engines.
 |
| FIG 8 |
Flashing Induced Erosion
Flashing induced erosion (figure 9) is the result of spontaneous vapor formation caused by sudden pressure changes. This commonly occurs in drain and vent lines downstream of valves in liquid systems where the fluid is near saturation pressure. As some of the liquid flashes to vapor, it undergoes a rapid expansion of volume that increases the fluid velocity and accelerates the remaining liquid phase, liquid droplets and/or liquid film, leading to erosion. Typical examples are found downstream of Feed water Heater shell level control valves.
 |
| FIG 9 |
Fretting
Fretting occurs when tight fitting metal surfaces experience cyclic relative motion that causes them to impact on or rub against each other. The relative motion abrades one or both surfaces, producing debris from base metal or corrosion products. The debris may remain in contact with the original components, further increasing the abrasive effects of fretting. The debris can also prevent precision devices from operating due to fouling. Fretting has been a concern in steam generator tube bundles and control rods.
 |
| FIG 10 |
Erosion damage can generally be mitigated by using more resistant materials. Reduction in flow velocity will also reduce the amount of damage caused by most erosion mechanisms. The following sections discuss methods that may be used to prevent or mitigate some of the erosion type damage mechanisms discussed in this training guide.
Fluid Erosion Damage
is generally combated by using more resistant material and reducing flow velocities.
Cavitation
Cavitation Damage may be mitigated by using more resistant materials. Design alteration, including use of cascading orifices or low recovery valves, is a method for preventing cavitation.
Solid Particle Erosion
Eliminating or reducing the solid particles is the preferred method of preventing or mitigating solid particle erosion. This could be done by installing strainers, or preventing unintended ingress. Use of a more resistant material and velocity reduction will also mitigate solid particle erosion.
Impingement
Eliminating or reducing moisture will combat impingement. Use of a more resistant material and velocity reduction will also mitigate impingement. Design solutions such as installing turning vanes, waste plates and improving flow conditions (such as using long radius elbows or bends) should be considered.
Flashing Induced Erosion
Use of resistant material will combat flashing induced erosion
Fretting
Use of a more resistant material may help mitigate fretting. Elimination of the source of the relative motion will eliminate fretting. Improved mechanical fitup or use of lubricants may also mitigate or prevent fretting. Varying the location of metal to metal contact will mitigate the consequences of fretting by distributing the damage more widely.
THEORIES OF EROSION
Erosion is commonly measured in terms of a parameter W which is equal to the mass of material removed from the surface divided by the mass of the eroding material. Occasionally it is more convenient to refer the parameter to the volume loss divided by the volume of eroding material. In either case the parameter is dimensionless.
In most cases W > 0, a condition which indicates that material is removed during erosion but under certain circumstances W < 0.
DUCTILE MATERIAL MODELS
The trajectory of a particle cutting and removing material was calculated, and the eroded volume, V, was determined to be given by the expression (Finnie 1960):
where
𝜎f is flow stress,
m is the particle mass,
v𝗈 is the impact velocity,
K is the ratio of vertical force to horizontal force on the particle,
d is the depth of cut.
g(𝛼) is a function describing the effect of attack angle 𝛼.
Sheldon and Kanhere formula
d is the (spherical) particle diameter,
𝜌p is the particle density,
H is the Vickers hardness value of the material.
This theory leads to a greater velocity dependence than expected from energy arguments (proportional to v² )
BRITTLE MATERIAL MODELS
The load at which crack propagation occurs is related to the distribution of surface flaws through the Weibull statistics. The approximate area. A, of cracked material is calculated for a particle penetration depth of h, and the volume removed per impact is set proportional to Ah. The final equation for the erosion rate , W, is expressed in terms of the particle size, r, the particle velocity, vo and Weibull constants, m and 𝜎o Sheldon and Finnie (1966):
a = 3(m-0. 67) / (m-2) for round particles
a = 3 . 6 (m-0. 67) / (m-2) for angular particles
b = 2. 4 (m-0. 67) /(m-2) for either shape
For particles much stiff er than the target, the constant k1 , is given
The erosion model developed by Evans . assumes that the erosion rate is proportional to the amount of material removed by each impact event. The volume, V, lost per impact is calculated from the depth, h, of penetration and the maximum size of the lateral cracks formed during impact. Since the lateral crack size is proportional to the radial crack size cr , V is given by the following equation:
cr is crack formed during impact
you can follow that it is wide range
Erosion Wear Models
Finnie Model’s
A sharp edged particle moves into the specimen surface causing deformation and removal of
surface material
where,
Q is the volume of material removed,
M is total mass of the abrasives,
V is velocity of the abrasive particles,
p is constant horizontal component of the contact stress on the surface,
𝜑 is the ratio of depth of contact (l) to the depth of cut (yt),
K is the ratio of vertical to horizontal force components of the total force acting on the particle (considered constant and equal to 2)
α is the angle of approach (impact angle) for the particles.
Bitter’s model:
In Bitter’s model represented, deformation wear by WD and cutting wear by WC. Cutting wear has two component depends on the impact angle. Deformation and Cutting wear equation as follows:
WC1 = cutting wear loss if impinging particle stays in the target after collision,
WC2 = cutting wear loss if particle leaves the target surface after collision,
𝛜 & Q = wear factor of deformation and cutting wear, respectively,
V = incident velocity of erosive particle,
M = total mass of impinging particles,
K & K1 = constants determined by the elastic properties of target,
C = constant and
𝛼 = impact angle
Hutching’s Model:
Stack studied the hutching’s model of erosion and reviewed that, when spherical particle strikes on
the ductile material at normal, erosion wear occurs by the formation and subsequent detachment of
platelets of metal lying parallel to the eroded surface. Model gives information the about
detachment of platelets is only possible when the accumulated plastic strain within the fragment,
after many cycles of plastic deformation, reaches a critical value.
Hd - Dynamic hardness
Dt - Density of the target material
Dp - Density of erodent particles
Ui - Erodent particle velocity.
The term 𝛼r/𝜖c ² cannot be measured independently. Hutchings assumed the value of 𝛼r/𝜖c ² is equal to 0.7
Sundararajan and Shewmon erosion model
The basic idea of the model is that erosion loss is processed under high-strain-rate and hence adiabatic deformation conditions, and therefore, the mechanical response of the target is dynamic.
The criteria for the removal of such lip are assumed to be based on a critical strain which the lips attain after a number of particle impacts.
Ui - velocity of impacting particles.
Dp – density
Cp - specific heat
Tm - melting temperature
Hs - static hardness of the target material
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