Process units erosive velocities

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. 

The inlet velocity in commercial cyclone installations is typically between 15 to 18 m/s (50 to 60 ft/s) for heavily loaded cyclones. This increases to 23 to 26 m/s (75 to 85 ft/s) for most lightly-loaded or second-stage units. Even higher velocities may be found in some tertiary installations. Even so, at inlet velocities of more than about 30 m/s, the danger of erosion, especially in the lower part of the cone, or cyclone body, rises sharply when processing abrasive solids. 

Obviously, if pressure drops on the order of 6 to 8 kPa are acceptable from a process point of view, an 8 or 13-unit multicyclone (or some number in between) could prove to be a viable option for the task at hand. If not, other types of separation equipment may have to be considered such as a baghouse, an electrostatic precipitator, or a wet scrubber. Nevertheless, even if the pressure drop across the cyclones were acceptable, one would still need to consider the long-term wear implications associated with operating the cyclones at velocities in the range of 35 to 40 m/s. If the solids being processed are not especially abrasive and/or if they are sufficiently flne in size, it may be possible to operate at these velocities. If not, erosion-protective liners may need to be installed. Some bare-metal multicyclone systems have been observed to operate for many years at velocities of 70 to 85 m/s while processing several tons per day of rather abrasive sand-like particles that were under about 25 fim in size. 

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