Liquid erosion

EROSION, LIQUID - Removal of films or metal by mechanical action and corrosion of active metal. 

Long-term trending of maintenance data is very useful to enable an understanding of equipment reliability issues. Plant simulations will not identify some of the critical concerns such as corrosion, erosion, liquid and gas maldistribution, and potential slug formation in piping. A detailed review of historical equipment inspection and problem/repair history is required to understand various maintenance, metallurgy and reliability issues. 

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

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. 

It is general practice, at the design stage, to assume an inhibitor efficiency of 0.9 (more precisely this figure should be 0.85 for condensate, 0.9 for gas, and 0.95 for oil streams), thus Fj = 0.1. However, lower figures should be considered where high flow velocities are expected to produce erosion-corrosion attacks in the presence of disturbances. In fact, it is possible to have erosive liquid flow at local flow disturbances such as weld beads, pin ends in connections, bends, size reduction. 

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. 

Pump Materials of Construction In the (memical industry, the selection of pump materials of construction is dictated by considerations of corrosion, erosion, personnel safety, and liquid contamination. The experience of pump manufacturers is often valuable in selecting materi s. See section on materials. 

Pressure. Flow 0t(AP/pd - Only source of energy is from fluid being atomized. Simplicity and low cost. Limited tolerance for solids uncertain spray with high-viscosity liquids susceptible to erosion. Need for special designs (e.g., hypass) to achieve turndown. 

In liquid ejectors or aspirators, the hquid is the motive fluid, so the gas pressure drop is low. Flow of slurries in the nozzle may be erosive. Otherwise, the design is as simple as that of the Venturi. 

Side entering mixers are used for blending pui-poses. The side entering propeller type mixer is economical and establishes an effective flow pattern in almost any size tank. Because the shaft seal is below the liquid level, its use in fluids without corrosive and erosive properties is usually ideal. 

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. 

Where erosion by a liquid-borne abrasive is involved, the behavior of a corrosion-resistant alloy will depend largely on the rate at which erosion removes the passive film and the rate at which it reforms. 

Note Liquid dump valves are normally fail closed to prevent giis blowby. That means that in ca.se of loss of instrument gas or air pressure, the spring will drive the valve to the closed position. However, the valve can mechanically fail because the level controller malfunctions or the seat cuts out due to solids erosion, which would cause it to fail in an open configuration. 

Leak Deterioration Erosion Corrosion Impact damage Vibration Low pressure Low liquid level... 

Forced circulation Viscous and solid-containing liquids can be circulated. Enables an erosion-fouling balance. Circulation rate can be controlled. Relatively expensive due to extra shell volume. Cost of pump and pumping. Leakage of material at stuffing box. 

However, movement at appreciable rates can result in another form of attack that is brought about by the conjoint action of erosion and corrosion hence the term erosion-corrosion that includes all forms of accelerated attack in which protective films, and even the metal surface itself, are removed by the abrasive action of movement of a fluid (gas or liquid) at high velocity. In general, the higher the velocity the more abrasive the solution. 

Table 1.29 tabulates most known examples of erosion corrosion problems occuring in aqueous systems. Historically, erosion corrosion first became a problem with the copper alloy (70%Cu 29%Zn l%Sn) condensers of naval shipsErosion corrosion of copper alloys has been an ongoing problem since then. The other major problem areas are (a) power plants where steels are exposed to water or water/steam mixtures in the temperature range 90°-280°C (b) the oil and gas industry where steels are exposed to various liquid, gas, and sometimes solids combinations containing carbon dioxide. 

Very rapid and highly localised pitting is sometimes observed on components exposed to very turbulent flow conditions leading to cavitation in the stream. In general, these conditions appear to induce corrosion rather than erosion on cast iron surfaces, in contradistinction to what usually happens with other metals, apparently because the erosive component of the liquid flow scours away corrosion-stifling films and allows the development of very active electrochemical cells on the exposed metal surfaces . 

Sulphuric acid is frequently made, stored and conveyed in lead. The corrosion resistance is excellent (see Figure 4.15) provided that the sulphate film is not broken in non-passivating conditions. Rupture of the film may be caused by erosion by high velocity liquids and gases containing acid spray. 

Morch, K. A., Dynamics of Cavitation Bubbles and Cavitating Liquids, Treatise on Materials Science and Technology, Vol 16, Erosion, Academic Press, London, p. 309-353 (1979)... 

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