If someone asks what a desilter actually takes out of the mud, the answer is refreshingly specific: the silt-sized fraction, roughly 15 to 44 microns. That single band is why the desilter exists, where it sits in the train, and why it can’t do the shaker’s job or the centrifuge’s.
The number and where it comes from
A desilter is a bank of small hydrocyclones — typically 4-inch or 5-inch cones — and that small cone diameter is what sets the cut. Fed at the right pressure, the cone spins the fluid so heavier, larger particles are thrown to the wall and drop out of the apex as underflow, while the cleaned fluid leaves the top. The smaller the cone, the finer the particle it can fling out, and a 4-inch desilter cone reliably targets solids in the 15 to 44 micron range.
That band is the silt fraction — finer than sand, coarser than the ultra-fines. It sits precisely in the gap that the shaker screen can no longer catch (screens on a working deck cut coarser) and that the centrifuge is reserved for below. Remove it here, mechanically and cheaply, or it stays in the mud and drives viscosity and abrasion.
Why the desilter, and not the shaker or centrifuge
The solids-control train is a cascade of progressively finer cuts, and each stage exists because the one before it cannot reach that size economically. The shaker takes the coarse cuttings; the desander (larger 6–12-inch cones) takes the sand-sized solids and runs first; the desilter then takes the finer silt the desander passes. Only what the desilter still misses — the true fines — is left for the centrifuge, which is slower and more expensive to run.
So the desilter’s 15–44 micron band is not arbitrary; it is the rung on the ladder between sand and fines. Skip it or run it poorly and that silt loads the centrifuge, drives up dilution, and abrades the very pumps that feed the whole system. There is one important limit: colloidal solids below about 2 microns cannot be removed by any hydrocyclone — that ultra-fine fraction is chemistry’s problem, not the desilter’s.
Reading the underflow
The desilter tells you whether it is making its cut by how the apex discharges. The correct underflow is a light, fanned spray — a fine umbrella of concentrated solids and a little liquid. A steady rope of thick slurry means the apex is overloaded or plugged and the cone is dumping whole mud, not making its silt cut; a wide watery cone means it is barely loaded. The spray is the sign the 15–44 micron work is actually happening.
Feed head matters as much as cone size: starve the cones of pressure and even a perfect 4-inch cone will not develop the swirl needed to fling out silt. That is why desilter performance is read as a pair — the right cones and the right feed head — not cone size alone.
The cut point
A desilter’s separation band, set by its small cones:
Desilter (4–5″ cones) → removes ≈ 15–44 micron solids
Coarser than this: shaker & desander (6–12″ cones, sand). Finer than this: decanter centrifuge. Below ~2 micron (colloidal): no hydrocyclone can remove it.
Judge a desilter by two things: cone size and underflow. The 15–44 micron cut only happens with small (4–5″) cones fed at proper head, and the proof is a light spray at the apex — not a rope. If it ropes, it is dumping mud, not making its silt cut.
Common questions
What micron does a desilter remove?
A desilter removes drilled solids in roughly the 15 to 44 micron range, using small 4- to 5-inch hydrocyclone cones. That is the silt fraction — finer than a desander removes, coarser than a centrifuge's cut.
What is the difference between a desander and a desilter?
Both are hydrocyclones; the difference is cone size and cut. A desander uses larger 6–12-inch cones to remove sand-sized solids and runs first; a desilter uses small 4-inch (or smaller) cones to remove finer 15–44 micron silt and runs after the desander.
Can a desilter remove very fine (colloidal) solids?
No. Colloidal solids below about 2 microns cannot be removed by any hydrocyclone, including a desilter. That ultra-fine fraction drives plastic viscosity and is managed by the centrifuge's finest duty and by chemistry, not by cyclones.
