Technical Paper small and more or less covered) are supposed.

'Softer' steel erodes quicker than the surrounding refractory; hence, 'roughness' or unevenness occurs. If the catalyst-jet is in the same direction as the strip, erosion occurs by impacting particles which knock the present particles (of both catalyst and substrate) like billiard-balls away; hence, eroded depth increases further [A].

The top of the curve of the punched tab can be rather steep and sharp. Most times the refractory remains intact, damages occur rather because of other mechanical abuse.

Typical Sliding wear can be observed in the inlet horns of cyclones, etc.

To 'show' such an erosive behaviour, which gives at same time an indication about the 'erosion-resistance' of the materials themselves, reference is made to the erosion (=cavitation) rate-time curves e.g. as obtained in Westinghouse Steam Division spray impingement facility, i.e. perpendicular water droplet impact on a number of steels (1956-1959). See figure below.

If the jet is perpendicular with the strip, erosion occurs conform above; hence, if the depth attains a certain degree where catalyst/substrate particles start to absorb the impact of fresh catalyst particles. The billiard- effect is strongly reduced because the hit particles cannot get enough speed and/or cannot be uplifted over the opposite 'edge' again [B].

When the jet angle is almost parallel with the surface and more or less perpendicular to the strip, abrasive eddy's will be caused in the cavity due to the creation of a vacuum by which (re)circulation of the catalyst occurs [C].

At an angle of around 25 - 30° with the surface (depending on type, size and velocity of the abrasive) the prevailing sliding action [C] changes into an impact erosive action [A & B].

Hence, the smaller the length (= joint thickness), the faster a particle is stopped and/or 'held' in place, the slower erosion can proceed.

In practice evidence can be found e.g. at slide-valve discs: at the front edge the punched tabs erode away by catalyst impingement, almost without damaging the refractory, in a particular pattern.

Influence of impinging angle at varying particles sizes

For Impact wear the same pattern of dependence to the relative hardness between the abrasive and the base material exists as of Sliding wear, i.e. a lower erosion in the Low-state [Tieflage], when the particle hardness is less or equal to the base material, and high erosion when the hardness of the particles is larger or very large is than the base material (High-state [Hochlage]). In-between there is the sharp increase

Typical erosion pattern of end-punched tabs of a disc slide & cyclone cone

Cavitation wear is often observed at the vicinity of spray nozzles, where jets can hit the lined walls or in swirl tube separators, where catalyst will

Typical erosion pattern in Cyclones entries 18 ENGINEER THE REFRACTORIES May 2019 Issue

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