Droplet erosion

Four distinct forms of erosive wear have been identified soHd-particle erosion, Hquid-droplet erosion, cavitation erosion, and slurry erosion. 

Fig. 1.91 Variation in possible erosion corrosion surface morphology (a) single phase attack of mild steel in water (b) droplet erosion corrosion of 12% Cr steel...

Different mixing models such as the lEM (interexchange with the mean) model [67], the droplet erosion and diffusion model [68], the engulfment deformation diffusion model and the engulfment model [69, 70], and the incorporation model [71] have been proposed. 

By virtue of these crystallographic features, the cobalt-based alloys are inherently resistant to those forms of wear that involve a microfatigue constituent, namely sliding wear, fretting, cavitation erosion, and liquid droplet erosion [7],... 

The ASTM droplet erosion test involves putting specimens on the tip of a propeller that can achieve hypersonic tip velocities and the propeller is exposed to a rain field of controlled water droplets (Figure 10). Plastic materials are used for windshields and radomes on aircraft. Needless to say, it is necessary for the plastics used for these applications to have resistance to damage fiom high-velocity water droplets, fo fact, many plastics are readily destroyed by mach 2 water plets. Reinforced composites are often used for these types of... 

Figure 10. Schematic of the ASTM G 73 water droplet erosion test...

The best thermal efficiency at part load can be achieved at maximum core outiet temperature. A constant temperature imphes that the coolant mass flow increases proportionally with load. However, because the reactor core requires a minimal coolant mass flow, in particular for downward flow regions of the two-pass or three-pass concept (Fig. 8.2), the reactor must be operated with a colder core outlet temperature in the lower load range. Once the core outlet temperature and thus the turbine inlet temperature becomes so small that condensation and droplet erosion must be expected in the HP turbine, the steam has to bypass the turbine. Likewise, the reheat temperature is colder in the lower load range because it cannot exceed the core outiet temperature, and the steam also has to bypass the IP and LP turbines. 

In certain mass-transfer operations, such as spray columns, the Sauter mean diameter is a useful quantity to know. However, in studies of droplet erosion or droplet separation, the volume or mass median diameter is more physically meaningful. This volume-average (median) drop diameter is related to the Sauter-mean diameter through the following approximation (AIChE, 1978). 

Erosion is the deterioration of a surface by the abrasive action of solid particles in a liquid or gas, gas bubbles in a liquid, liquid droplets in a gas or due to (local) high-flow velocities. This type of attack is often accompanied by corrosion (erosion-corrosion). The most significant effect of a joint action of erosion and corrosion is the constant removal of protective films from a metal s surface. This can also be caused by liquid movement at high velocities, and will be particularly prone to occur if the solution contains solid particles that have an abrasive action. 

The abrasion resistance of cobalt-base alloys generally depends on the hardness of the carbide phases and/or the metal matrix. For the complex mechanisms of soHd-particle and slurry erosion, however, generalizations cannot be made, although for the soHd-particle erosion, ductihty may be a factor. For hquid-droplet or cavitation erosion the performance of a material is largely dependent on abiUty to absorb the shock (stress) waves without microscopic fracture occurring. In cobalt-base wear alloys, it has been found that carbide volume fraction, hence, bulk hardness, has Httie effect on resistance to Hquid-droplet and cavitation erosion (32). Much more important are the properties of the matrix. 

Another advantage of the radial reacrion turbine is that it can be designed to accept condensation in any amount without efficiency deterioration or erosion. This is possible because there are two forces acring on suspended fog particles, the deceleration force and the centrifugal force, and these two forces can be balanced against each other to prevent the droplets from impinging on specially shaped blades. The process is expl ned as follows ... 

The metal loss and resulting split were caused by erosion from impingement of high-velocity steam in which droplets of water were entrained. The conical pinnacles in the affected region are typical of deterioration by this mechanism. 

Close examination of each circular spot revealed a small forest of erosion cones (Fig. 11.35). Erosion was caused by the impingement of high-velocity steam probably mixed with droplets of condensate. The circular, equally spaced erosion sites apparently reproduced corresponding, equally spaced circular openings in the exhaust inlet. Compare Case History 11.3. 

Dust-laden streams can also cause operational problems. A turboexpander that can efficiently process condensing streams (gas with fog droplets suspended) can usually handle a stream with suspended solid particles, as long as the particle size does not exceed 2-3 p. The newer designs reduce erosion of expander back rotor seals by disposing of... 

Water droplets may eause erosion, espeeially in axial eompressors... 

Erosion. The abrasive is likely to be gas borne (as in catalytic cracking units), liquid borne (as in abrasive slurries), or gravity pulled (as in catalyst transfer lines). Because of the association of velocity and kinetic energy, the severity of erosion may increase as some power (usually up to the 3d) of the velocity. The angle of impingement also influences severity. At supersonic speeds, even water droplets can be seriously erosive. There is some evidence that the response of resisting metals is influenced by whether they are ductile or brittle. Probably most abrasion involved with hydrocarbon processing is of the erosive type. 

COMMENTS The Carnot vapor cycle as illustrated by Example 2.1 is not practical. Difficulties arise in the isentropic processes of the cycle. One difficulty is that the isentropic turbine will have to handle steam of low quality. The impingement of liquid droplets on the turbine blade causes erosion and wear. Another difficulty is the isentropic compression of a liquid-vapor mixture. The two-phase mixture of the steam causes serious cavitation problems during the compression process. Also, since the specific volume of the saturated mixture is high, the pump power required is also very high. Thus, the Carnot vapor cycle is not a realistic model for vapor power cycles. 

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