Hydrocyclones are major components in mud desander, desilter and drilling mud cleaner and assist most of their works. A hydrocyclone has a upper casing, a lower casing, a underflow mouth as well as auxiliary parts. The upper casing is an inlet chamber in cylindric with an inlet pipe on one side and a vortex tube in top middle as the overflow. The lower casing is a cone whose angle ranges from 15°~20° at the tip. The underflow mouth is located at the bottom as the discharge port for solids.

The work principle of hydrocyclones is to separate two or several components with different densities with the aid of centrifugal force. After the mixture is sent into hydrocyclones by certain pressure, it gradually flow into a high-speed rotation where the heavy phase form an outer swirl by moving downwards along the axis, outwards along radial direction, downwards again along the cone till they flow out of underflow; while the light phase form upward inner swirl by moving towards axis and upwards till they go out from overflow outlet. Thus, the separation is done.
 

Sand fill is the commonest anormaly to hydrocyclone and is categorized as:

Sand fill at the underflow: it happens as a consequence of improper adjustment that results in dry bottom or of excessive silt content which brings about overloaded hydrocyclone. It should be removed immediately as it affects the purification ability and accelerates abrasion of lining and vortex tube. Usually the best ways are to enlarge underflow outlet or to have more solids taken away by preceding equipment.

Sand fill at the inlet: it can be partial or overall. Partial sand fill decelerates fluid flow inside inlet, leaks enormous drilling fluids from underflow and sometimes clean mud which may flow backwards from the overflow as, at that time, hydrocyclone is not more functional than a hopper. Overall sand fill is prone to back flow drilling mud from the overflow. Inlet sand fill generally stems from poor drilling fluids management, and most likely from going round the shale shaker or abrading shaker screens.

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