Failure Analysis and Design Considerations: Fluid Velocity Effects

Pump impellers and ship propellers often show cavitation damage effects.

Metals generally owe their corrosion resistance to a tightly adherent, protective film on the metal surface. This film may consist of reaction products, adsorbed gases, or both. Any mechanical disturbance of the protective film can stimulate attack of the underlying metals until either the protective film is reestablished or the metal has been corroded away. The mechanical factor may result from abrasion, impingement, turbulence, or cavitation.

Erosion-corrosion is encountered most frequently in pumps, valves, centrifuges, elbows, impellers, inlet ends of heat exchanger tubes, agitated tanks, and so forth. Experience with lead valves is an illustration. Hard lead specimens exposed to hot, stagnant dilute sulfuric acid (H2SO4) have suffered negligible attack. However, throttled valves of the same material exposed to high-velocity effects have failed in less than a week.

Locations in flowing systems where there are sudden changes in direction or flow cross section, as in heat exchangers where water flows from the water boxes into the tubes, are likely places for erosion-corrosion. Under these conditions, which stimulate some corrosion of the metal surface, the effects of flow velocity may be to displace the corrosion product, thereby exposing fresh metal to the corrosion action of the solution. This action leads to a much increased corrosion rate. The corrosion products themselves can subsequently precipitate downstream and cause the plugging of tube, screens, and valves.

When the fluid velocity is higher than a certain threshold value, cavitation or the formation and collapse of voids in fluids (e.g., water) can occur because of the momentary occurrence of low pressure. It occurs whenever the absolute pressure at a point in the liquid stream is reduced to the vapor pressure of the fluid. Cavitation damage is the wearing away of metal by mechanical damage resulting from the repeated impact blows produced by collapse of voids within a fluid. Pump impellers and ship propellers often show cavitation damage effects.

Wet steam traveling at high velocities (900 to 1,200 m/s) can cause considerable damage to power plant condenser tubes, turbine blades, valve seats and disks, piston rings in engines, etc. Removal of moisture droplets from the steam is helpful but not always possible. Consequently, the practical solution of steam impingement problems usually involves a combination of several procedures: improved flow design, reduction of water droplet content of the steam, and the use of materials having higher resistance to erosion-corrosion damage (e.g., stainless steels) in areas where impingement is anticipated.

This article is adapted from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006),

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