Solid particle erosion

General Description. Solid particle erosion (SPE) is the loss of surface material that results from repeated impacts of small, solid particles. In some cases SPE is a useful phenomenon, as in sandblasting and highspeed abrasive waterjet cutting, but it is a serious problem in many engineering systems, including steam and jet turbines, pipelines and valves carrying particulate matter, and fluidized bed combustion systems. 

Solid particle erosion can occur in a gaseous or liquid medium containing solid particles. In both cases, particles can be accelerated or decelerated, and their directions of motion can be changed by the fluid. This is more significant in liquid media, and slurry erosion is generally treated as 

Erosion versus Abrasion. The distinction between erosion and abrasion should be clarified, because the term erosion has often been used in connection with situations that might be better classed as abrasion. Solid particle erosion refers to a series of particles striking and rebounding from the surface, while abrasion results from the sliding of abrasive particles across a surface under the action of an externally applied force. The clearest distinction is that, in erosion, the force exerted by the particles on the material is due to their deceleration, while in abrasion it is externally applied and approximately constant. This serves as a good working definition of the difference between the two phenomena. A clear-cut distinction between erosion and abrasion is difficult in some cases, particularly for very dense particle distributions in liquid or gas media, in which a pack of particles can develop and slide across the surface, which would be classed as abrasion. 

In the erosion literature, materials are broadly classified as ductile or brittle, based on the dependence of their erosion rate on a. Ductile materials, such as pure metals, have a maximum erosion rate, E, at low angles of incidence (typically 15 to 30°), while for brittle materials, such as ceramics, the maximum is at or near 90°. These two classical extremes are illustrated in Fig. 9. A variety of curves intermediate between these classical extremes exist and in some cases the same material exhibits behavior that shifts from one extreme to the other, depending on erosion conditions. 

The erosion rate, E, is conunonly given in terms of mass or volume of material removed per unit mass of erodent impacted, volume being preferred because it permits thickness loss comparisons between materials of different density. Implicit is the assumption that the dimensions of the eroded area and the particle concentration are unimportant, which is a good approximation for dilute flows. Metals and ceramics differ in the dependence of E on a, as mentioned above, and also in their response to velocity and particle size and shape. E generally shows a power-law velocity dependence  

---

In support of the power recovery expander market for fluid catalytic cracking units in refineries, some turboexpander manufacturers have an ongoing program to improve the solid particle erosion characteristics of the machine. Improved erosion characteristics will result in longer blade life, less downtime, and consequently greater profits for the users. 

A.V. Levy, Solid Particle Erosion and Erosion-Corrosion of Materials, ASM International, 1995. 

In order to elucidate the effect of the whisker orientation on the erosion behavior of material SN-C, erosion tests were carried out in directions both parallel and perpendicular to the whisker orientation. It is apparent that in the highly directional whisker-reinforced silicon nitride material, solid particle erosion in the direction parallel to the whisker orientation resulted in a faster rate of material removal compared to that in the perpendicular direction (Fig. 20.4). 

Marrero, M. et al. (1993), Solid-particle erosion of in situ reinforced Si3N4 , Wear, 162-164, 280 1. 

Srinivasan, S., and Scattergood, R.O. (1991), Rcurve effects in solid particle erosion of ceramics , Wear, 142, 115-33. 

Li, J. and I.M. Hutchings. "Resistance of Cast Polyurethane Elastomers to Solid Particle Erosion." Wear 135 (1990) 293-303. 

A good summary of the behavior of steels in high temperature steam is available (45). Calculated scale thickness for 10 years of exposure of ferritic steels in 593°C and 13.8 MPa (2000 psi) superheated steam is about 0.64 mm for 5 Cr—0.5 Mo steels, and 1 mm for 2.25 Cr—1 Mo steels. Steam pressure does not seem to have much influence. The steels form duplex layer scales of a uniform thickness. Scales on austenitic steels in the same test also form two layers but were irregular. Generally, the higher the alloy content, the thinner the oxide scale. Excessively thick oxide scale can exfoliate and be prone to under-the-scale concentration of corrodents and corrosion. Exfoliated scale can cause solid particle erosion of the downstream equipment and dogging. Thick scale on boiler tubes impairs heat transfer and causes an increase in metal temperature. 

This wear consists of (i) erosive wear and (ii) percussive wear. Erosion can occur by jets, liquid droplets, and implosion of bubbles formed in the fluid and streams of solid particles. Solid particle erosion occurs by the impingement when discrete solid particles strike the surface and the contact stress arises from the kinetic energy of the particles flowing in an air or liquid stream as it encounters the surface. Wear debris formed in erosion occurs as a result of repeated impacts (60). Neighboring particles may exert contact forces, and flowing fluid when present will cause drag. Under some conditions, gravitational force may be important. 

The effect of ultra-high molecular weight poly-(ethylene) (UHMWPE) the on mechanical and solid particle erosive wear behavior of aramid fabric reinforced-epoxy composites has been investigated [64]. A siUca sand of a size of 150-280 fim was used as an erodent. The erosive wear rate of UHMWPE in aramid-epoxy composite exhibits a lower value in comparison to neat composites. A maximum erosion rate was observed at an impingement angle 60 , and the material behaves in a semiductile manner. 

Erosion is one of several wear modes involved in tribocorrosion. Solid particle erosion is a process by which discrete small solid particles, with inertia, strike the surface of a material, causing damage or material loss to its surface. This is often accompanied by corrosion due to the environment. A major environmental factor with significant influence on erosion-corrosion rates is that of flow velocity, but this should be set in the context of the overall flow field as other parameters such as wall shear stress, wall surface roughness, turbulent flow intensity and mass transport coefficient (this determines the rate of movement of reactant species to reaction sites and thus can relate to corrosion wall wastage rates). For example, a single value of flow velocity, referred to as the critical velocity, is often quoted to represent a transition from flow-induced corrosion to enhanced mechanical-corrosion interactive erosion-corrosion processes. It is also used to indicate the resistance of the passive and protective films to mechanical breakdown [5]. 

The appearance of the eroded surface impacted by liquid drops is different from solid particle erosion. Liquid drop... 

Under other circumstances Quid Qow can cause erosion of the surface through the mechanical force of the Quid itself. When solids are present in the liquid they can cause wear or solid particle erosion. In either case, the rate of attack can be accelerated by the combined effects of erosion and corrosion. Erosion corrosion results when the passive Blms that form on alloys are removed and the underlying metal is attacked [25]. The rate can be very rapid. 

Solid Particle Erosion of Utility Steam Turbines 1985 Workshop, Electric Power Research Institute, Palo Alto, CA, Proceedings CS-4683, August 1986. 

In Section 1.4.4, the erosion test was outlined in the list of hardness determination methods and it was concluded that it was of minimal practical importance. However, solid particle erosion is a serious problem in gas turbine operations and in plants where powders are handled and it is of course used as a secondary shaping method in ceramic technology. Therefore it is more useful to consider how a knowledge of ceramic hardness contributes to an assessment of erosion. Figure 5.19 outlines how a knowledge of the process has developed through models taken from the types of indentation test damage already discussed in this chapter. 

Plastics are frequently used for applications requiring erosion resistance, but there does not seem to be much activity or interest in the tribology community of the 1990s. However, there are a number of tests that are applied and have been used to rate erosion resistance of plastics. Erosion, by definition, is progressive loss of material fiom a solid surface due to mechanical interaction between that surfitce and a fluid, a multicomponent fluid, or impinging liquid or solid particles (3). The field of erosion is usually separated into a number of forms of erosion liquid erosion, either continuous stream or droplet, solid particle erosion, slurry erosion, and cavitation erosion. Each have separate laboratory tests. 

Figure 11. Solid particle erosion rate of several plastics compared to metals (100 m/s, 12S-1S0 pm quarts abrasive, 90 degree impact angle).

Hardness versus Wear Resistance. The wear processes that are usually mitigated by the use of hard surfaces are low-stress abrasion, wear in systems involving relative sliding of conforming solids, fretting wear, galling, and to some extent, solid-particle erosion (Ref 3). Unfortunately there are many caveats to this statement, and substrate/coating selection should be... 

Barkoula N-M and Karger-Kocsis J (2002) Processes and influencing parameters of the solid particle erosion of polymers and their composites, J Mater Sci 37 3807-3820. 

Over the last few decades or so, a substantial amount of work on elevated temperature erosion of metals and alloys has been carried out. This work is reviewed, providing details of materials subjected to elevated temperature erosion and the test conditions, in ref. [36]. The tests were conducted over a wide range of test conditions and test materials. Based on these investigations, the important factors that govern the solid particle erosive wear of metallic materials at elevated temperature are described below. 

Solid particle erosion is an area that has received increased scientific attention over the last few decades. Technological advancements in several high temperature applications such as gas turbine engines, power plants, etc., have led to increased interest in elevated temperature erosion. However, investigations into the elevated temperature erosion of metals and alloys have been very limited. One reason for the scarcity of published work in this discipline is the non-availability of suitable test facilities. 

---