Brief Solids Control Drilling Fluid
Solids Removal equipment: Hydrocyclone related Device
The hydrocyclone has a conical-shaped portion inwhich most of the settling takes place. Usually, a cyclindrical feed chamber is located at the large end of the conical section. At he apex of the conical section is the under-flow opening for the solids discharge. In operation, the under-flow opening is usually facing downward for convenience. Near the top end of the feed chamber, the inlet nozzle is situated tangential to the inside circumference andon a plane perpendicular, or nearly so, to the top-to-bottom central axis of the hydrocyclone. A hollow cyclinder,called the "vortex finder", extends axially from the top into the barrel of the hydrocyclone past the inlet. The inside of the vortex finder forms the over-flow outlet for the liquid discharge or effuent. The "size" of a hydrocylone is determined by the largest inside diameter of the conical portion. The hydrocyclone owes its centrifugal force to the tangential velocity of the slurry entering the feed chamber. An axial velocity component is creat by the axial thrust of the feed stream leaving the blind annular space of the feed chamber.
In a hydrocyclone , the incoming stream spirals down along the wall of the conical section to the under-flow opening, reverses axial direction, and then spirals upward to exit at the over-flow opening. A properly-designed hydrocyclone can be adjusted in such manner that when a clean liquid is fed into it. Special fine shale shaker screens for secondary recovery from primary separation hydrocyclones can be used for unweighted water-base muds. The secondary-separation screening process diminishes the volume of discard by returning the silt with associated free liquid and clays to the system. In a situation where the increased mud weight, total solids content ,and abrasiveness of the returned solids do not harm the drilling operation, the secondary-separation screens can be used to advantage. The under-flow of the primary hydrocyclone removes only a small fraction of the total solids in the cyclone feed. This under-flow contains, selectively, the coarsest fraction in the mud. In order to reduce drilling cost it is better to remove the coarser fraction rather than to attempt to distinguish closely between barite and light solids.
Hydrocylone-fine-screen conbinations are used normally to weighted muds. Application of the secondary centrifuge will probably remain rare in the case of weighted muds. Occasionally reference is made to "double-stage" centrifuging solids weighted muds is quite different from that for unweighted muds.
For example, an ordinary unweighted fresh-water mud will usually contain approximately 50 lb of solids per bbl. If clay constitutes a very high per centage of these solids, viscosity can be easily controlled with a small amount of thinner. On the other hand, a 15 lb/gal mud will contain approximately 380lb of solids per bbl. Obviously, thesolids content can not be signigficantly reduced if specific weight has to be maintained. Higher solids removal capacities are needed in the case of faster drilling rates. The potential drilling benefits justify installation of the best equipment available.
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