Desanders and desilters are the same machine at two sizes. Both are banks of hydrocyclones — cones with no moving parts that separate purely on geometry and pressure — and the only fundamental difference between them is cone diameter. That single dimension sets the cut point, the volume each cone handles, and where the bank sits in the train. Understand the cone-size trade-off and you can size, sequence and read a cyclone bank without guesswork.
Why diameter decides the cut
Inside any cone, feed enters tangentially, spins, and centrifugal force throws the denser solids to the wall and down to the apex while clean fluid spirals up and out the top. The tighter the spin, the finer the particle it can fling out — and a smaller cone spins the slurry tighter. So a small-diameter cone makes a finer cut but passes less volume; a large cone makes a coarser cut but handles far more flow per cone.
| Unit | Cone size | Cut point | Removes |
|---|---|---|---|
| Desander | 6–12″ cones | ~40–74 µm | Sand-sized solids; high volume per cone |
| Desilter | 4″ (and smaller) cones | ~15–40 µm | Silt-sized solids; many cones banked in parallel |
Because each small desilter cone passes little fluid, a desilter is a bank of many cones working in parallel; a desander needs only a few large ones to move the same volume.
Order: coarse before fine
The desander always sits ahead of the desilter, for the same reason the shaker sits ahead of both: take out the big stuff first so the finer stage isn’t flooded. Feed sand-laden mud straight to a desilter and its small cones overload, rope and pass solids they should have caught. Remove the sand first and the desilter sees only the silt it was sized for.
Sizing: process all the flow, with margin
A cyclone bank only works if it can process the entire circulating rate — ideally with margin to spare. Bank too few cones for the flow and the surplus mud simply bypasses unprocessed, so the bank looks busy while solids sail past. The rule is to install cones for at least 100–125% of the maximum circulating rate, and to feed them at their design head.
The weighted-mud problem
On a weighted mud, a desilter becomes a liability. Its 15–40 µm cut is fine enough to throw away barite (sized to pass 200 mesh, ~74 µm and finer) along with the drilled silt — you would be discarding the weight material you paid for. The fix is the mud cleaner: a desilter bank mounted over a fine screen, so the cone underflow drops onto a screen that returns the valuable barite to the active system and discards only the coarser drilled solids. On weighted mud, run a mud cleaner, not bare desilters.
Reading the bank
The apex spray tells you the truth a gauge can’t: a fine, umbrella-shaped spray means the cone is developing full spin and rejecting solids with little fluid; a wet, rope-like stream means it is overloaded or starved of head and you are losing fluid, not solids; a bone-dry apex that keeps plugging means the cut is too aggressive for the feed. Walk the bank and look at every cone.
Key takeaways
Cone diameter sets the cut: big cones for sand at high volume, small cones for silt in parallel banks. Run the desander before the desilter, bank enough cones to process all the flow at proper head, and switch to a mud cleaner the moment the mud is weighted so you stop discarding barite. Then read the apex spray — half the diagnosis is visible before you touch an instrument.