Desander, a metal or polyurethane vortex cone (cyclone) used on a drilling rig to centrifuge returning drilling mud and remove a high percentage API sand (paritcles less than 74 micron in diameter). The desander can be operated mechanically or by fluid flowing in a hydrocyclone, a metal cone-shaped device having no moving parts. A vortex pipe extends from the top of the cone into the desander and causes a whirling motion in the mud that is pumped tangentially into the desander. The heavier, coarse sand particles are separated from the mud by centrifugal force and exit the bottom of the cone, and the mud with finer particles exits the top. A desander is described by its maximum inner diameter, which ranges from 6 to 12 inch. (15 – 30 cm). The cones are oriented at any angle from verticcal to 20 degree from horizontal and normally monted on the shale shaker and reserve tanks. The desander inlet is located downstream from the shale shaker and discharges into the desilter, which has a smaller cone. Sand left in drilling mud can cause excessive excessive abrasion in the mud pumps when the mud is recirculated.
The spoil laden bentonite slurry is pumped from the hydrofraise via 6 inch and 8 inch pipelines back to the cleaning /desanding plant. Pumping distances were up to 600 metres. The plant consisted of 2 stores units having a combined throughput capacity of 1000 cubic meters per hour.
The desanding plant basically comprised two desander screening and cycloning units each linked to a hydrofraise machine. One of the units was of 450 metres per hour nominal capacity while the other, which also had to handle the return bentonite slurry flow from the diaphragm wall concreting operation, was of 550 metre per hour.
Material excavated by each of the two hydrofraises and slurry from the trench was pumped by pipeline to its respective element of the stores desanding plant. In each of these the flow passed to a box feeding a 1.2m multiply 2m scalping screen. This was equipped with an wedge screening surface sloping downwards in the direction of the flow and having 5mm openings.
The undersize was collected in the primary tank beneath the screen from which twin pumps – powered at 150Kw for the two – handled a flow of 600m per hour to a pair of 675mm diameter cyclones. The under-flow from these ( which made a cut at 80 microns size ) fell into a collector which fed avibrating dewatering screen. The surface of this was inclined at an angle upwards with the flow, the oversize material going to the waste marterial chute and heap. Screen undersize fell into the secondary tank from which it was pumped ,at 100 cubic metres per hour by a 4 inch, 30kw pump to a futher two cyclone, these being of 230mm diametre. Undersize from these , at 40 microns, again fell to the bank of ten 80mm diameter cyclones arranged around the 230mm ones. Again, underflow from these units ( at 20 microns size ) went to the collector ahead of the dewatering screen.
All the cyclone overflows were directed to a stabilising tank that fed the desanding plant’s main slurry storage tank. This stored the cleaned bentonite which was prepared in an adjacent high shear mixer. From the main slurry storage tank material was fed by pump back to the trench under excavation.