Cyclone Erosion

This chapter presents information about three diverse topics relevant to cyclone technology thus the title, Some special topics . Two of the topics are related to the gas velocity in the separator cyclone erosion and the critical deposition velocity. The last topic is the working of cyclones or swirl tubes under conditions of high vacuum. 

In many cyclone installations abrasive wear or erosion is of major concern to plant operating and maintenance departments. It is one of the major causes of unscheduled unit shutdowns in units that process abrasive particles such as coal, sand, fly-ash, coke and alumina-based catalyst. Even natural grains, such as the hard tips of rice grains can, and do, erode holes in metal-walled conveying lines and in cyclones which process them. The same is true of certain plastic particles or pellets and of some wood products. 

As shown in Figs. 12.1.1 and 12.1.2 later in this section, several zones within a typical cyclone are especially vulnerable to erosive attack. These include the inlet Target area , the lower cone and the dust hopper and/or upper dipleg (if present). We will discuss each of these areas separately below. 

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This also has ramifications for the effect of solids loading on cyclone erosion we would, based on this, expect wear to be directly proportional to solids loading. This is not incompatible with Eq. (12.1.3), where the variation in the time to failure is qualitatively consistent with this simple model, but a bit stronger (to the power of —1.58 rather than —1.0). As explained above, the wear may become more localized with increasing loading. 

Table 7. Dust Loadings and Cyclone Inlet Velocities above Which Erosion Is Excessive ...

The cyclones are typically designed with diameters of 100—160 cm for ease of maintenance. Cyclone inlet velocities are usually restricted to 18—21 m /s in the first stage and to 20—26 m/s in the second stage to achieve satisfactory pressure drop and erosion characteristics (62). The number of sets of two-stage cyclones thus depends on the total gas flow. Finding room to house all the necessary cyclones within the regenerator frequently requires considerable ingenuity (62). 

The metallurgy of the cyclone equipment has in recent years focused primarily on type 304 H stainless steel. The 304 H material is durable and easy to fabricate and repair, withstands the high regenerator temperatures, and is oxidation- and corrosion-resistant. Essentially all internal surfaces of the cyclone that are subject to erosion are protected with a 2 cm layer of erosion-resistant lining. When installed and cured, most refractory linings are highly resistant to erosion. 

Some PFBC boiler designs incorporate high-temperature, high-pressure (HTHP) filter devices in the flue-gas stream. These are installed primarily to protec t the gas turbine from erosion damage by the fine particles that escape the cyclones, but as the filters remove virtually all the suspended particulates, they also eliminate the need for back-end removal. The commonest HTHP filter elements used are rigid ceramic candles. 

Vortex breakers 1. A device used to straighten out rotary flow in a duct a short distance after a fan. 2. A device found in a cyclone discharge fitted to reduce shell erosion by particulate abrasion. 

Matsen (1985) pointed out a number of additional problem areas in scale-up such as consideration of particle size balances which change over time due to reaction, attrition and agglomeration. Erosion of cyclones, slide valves and other components due to abrasive particles are important design considerations for commercial units which may not be resolved in pilot plants. 

Three patents (MacLean, et al., 1982) assigned to Texaco filed in the latter half of 1980 resulted from an in-house review of all observed instances of erosion of cyclones in Texaco s FCC units. The patents specify that to minimize erosion and maximize collection efficiency, the length of the cyclone relative to its barrel diameter is to conform to the relationship ... 

The vortex of a cyclone will precess (or wobble) about the center axis of the cyclone. This motion can bring the vortex into close proximity to the wall of the cone of the cyclone and pluck off and reentrain the collected solids flowing down along the wall of the cone. The vortex may also cause erosion of the cone if it touches the cone wall. Sometimes an inverted cone or a similar device is added to the bottom of the cyclone in the vicinity of the cone and dipleg to stabilize and fix the vortex. If it is placed correctly, the vortex will attach to the cone and the vortex movement will be stabilized, thus minimizing the efficiency loss due to plucking the solids off the wall and erosion of the cyclone cone. 

There have been several studies in which the flow patterns within the body of the cyclone separator have been modelled using a Computational Fluid Dynamics (CFD) technique. A recent example is that of Slack et a/. 54 in which the computed three-dimensional flow fields have been plotted and compared with the results of experimental studies in which a backscatter laser Doppler anemometry system was used to measure flowfields. Agreement between the computed and experimental results was very good. When using very fine grid meshes, the existence of time-dependent vortices was identified. These had the potentiality of adversely affecting the separation efficiency, as well as leading to increased erosion at the walls. 

Circulating Beds These fluidized beds operate at higher velocities, and virtually all the solids are elutriated from the furnace. The majority of the elutriated sohds, still at combustion temperature, are captured by reverse-flow cyclone(s) and recirculated to the foot of the combustor. The foot of the combustor is a potentially very erosive region, as it contains large particles not elutriated from the bed, and they are being fluidized at high velocity. Consequently the lower reaches of the combustor do not contain heat-transfer tubes and the water walls are protected with refractory. Some combustors have... 

In nnits where a primary regenerator cyclone has failed, it is possible to cut the air back to a superficial velocity of 1.5 ft/sec and limp along nntil eqnipment is available for a shntdown. Feed rate wonld be proportioned to the rednction in air. This techniqne has been employed in sitnations where a nnit dropped a cyclone and when one of the primary diplegs was plngged with refractory. The pressnre drop throngh the air distribntor shonld not be less than 30% of the bed pressnre drop to prevent grid erosion. 

Regenerator cyclones have a typical life of 15-30 years depending upon erosion and mechanical fatigue. The base metal of the cyclones will deteriorate with time leading to graphitization. Once this happens, the metal cannot be welded upon and hence cannot be repaired during a normal unit TAR. This phenomenon is dependent upon time and temperature. COP is used to minimize the temperature to extend the cyclone life. 

P.D. Silva, C. Briens, A. Bemis, Development of a new rapid method to measure erosion rates in laboratory and pilot plant cyclones, Powder Technol. 131 (2003) 111-119. 

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