An under-coned desilter ropes and bypasses solids; an over-coned one wastes fluid and horsepower. The number of cones is not a guess — it falls straight out of the flow the bank has to process and the rated capacity of each cone. Get it right and the bank runs in a clean spray at its design cut.
Size to the suction, not the hole
The rule that trips people up: a hydrocyclone bank must be sized to process 100–125% of the flow entering its own suction tank — not 100% of the flow going down the hole. Solids control runs on the surface at a rate set by the centrifugal feed pumps, which are themselves sized at about 125% of the rig-pump rate so the cones can process every barrel and still keep the tanks turning over. Size the cone count to that suction flow and you guarantee the whole active stream is presented to the cones.
Cone capacities are known numbers
Each cone has a rated throughput at its design feed head. A 4-inch desilter cone passes on the order of 60 gallons per minute (up to ~80 gpm at full head), removing about half of the solids in the 15–20 micron range. A 10-inch desander cone passes on the order of 500 gallons per minute, removing about half of the solids in the 40–50 micron range. Divide the flow the bank must process by the per-cone capacity and you have the cone count — then round up so no cone is overloaded into a rope.
Feed head sets the cut
Cones are designed for a specific feed head — commonly around 75 feet (70–80 ft), which for typical mud weights works out to a feed pressure of roughly four times the mud weight in ppg. Too little head and the cut coarsens and the underflow ropes; too much and you waste energy and erode the apex. Because head is pressure divided by density, feed pressure rises with mud weight to hold the same head, so the manifold pressure must be checked and matched as weight changes.
The check that proves it
Once installed, the bank tells you whether it is sized right: every cone should run a light, umbrella-shaped spray with a hollow air core. A rope on any cone means that cone is overloaded — the bank is under-coned, the apex is pinched, or the upstream shaker is passing too much load. Never run cones deliberately in a rope; a rope cuts cone efficiency in half or worse and accelerates wear.
The equation
Number of cones required:
Ncones = (1.0 to 1.25 × Qsuction) ÷ qcone
Qsuction = flow into the bank’s suction tank (gpm); qcone ≈ 500 gpm for a 10″ desander cone, ~60–80 gpm for a 4″ desilter cone. Round up. Feed pressure to hold ~75 ft head:
Feed pressure (psi) ≈ 4 × mud weight (ppg) [= 0.052 × MW × 75]
Size the bank to the suction flow and round the cone count up, never down — an over-coned bank simply idles a few cones, but an under-coned one ropes and dumps solids to overflow. Then set the manifold pressure to the cones’ design head and watch for the spray on every apex.
Common questions
How many desilter cones do I need?
Divide 100–125% of the flow into the desilter suction tank by the per-cone capacity (about 60 gpm for a 4-inch cone) and round up. Size to the surface suction flow, not the downhole circulation rate.
What feed pressure should a desilter run?
Enough to hold roughly 75 feet of head, which for typical mud works out to about four times the mud weight in ppg. Check the manifold pressure and raise it as mud weight increases to keep the same head.
Why should cones never run in a rope?
A rope underflow means the cone is overloaded — solids build up, the air core collapses, and coarse solids bypass to the overflow. It cuts separation efficiency in half or worse and accelerates apex wear. The correct discharge is a light spray.
