Strip solids control down to one sentence and it is this: sorting particles by size, and removing each one as early as the size allows. Every screen label, every cone cut, every centrifuge setting is a statement about microns. Get comfortable with the size ladder and the whole train suddenly reads as one coherent system instead of a row of unrelated machines.
The scale: how small is a micron
A micron (µm) is one-thousandth of a millimetre. For a sense of scale, a human hair is roughly 70 µm across — which, not by coincidence, is about where “sand” begins. Below that, the particles you are fighting are invisible to the eye and behave less like grit and more like a thickening agent.
The API solids classification
The industry sorts drilled solids into bands by size, and each band is somebody’s job to remove:
| Class | Size | Reference |
|---|---|---|
| Sand | > 74 µm | Coarser than a 200-mesh screen (the sand-test cut) |
| Silt | 2–74 µm | Fine drilled solids; desilter / centrifuge territory |
| Colloidal / ultra-fine | < 2 µm | Reactive clay; only the centrifuge or dilution touches it |
Mesh vs micron — and why the label changed
“Mesh” counts the openings per linear inch of screen, so a higher mesh number is a finer screen. But mesh alone never told you the true opening, because the thickness of the wire changes how much space is left between the strands — two 200-mesh screens can have different actual cut points. That is exactly why API RP 13C labels screens by a measured cut point in microns rather than raw mesh: microns describe the hole, mesh only counts the wires.
Each stage owns a size band
Lay the equipment against the ladder and the sequence explains itself — coarse to fine, surface to centrifuge:
- Shale shaker — the coarsest cut, removing everything from cuttings down to roughly 74–100 µm depending on the screen.
- Desander — sand, ~40–74 µm.
- Desilter — silt, ~15–40 µm.
- Decanting centrifuge — the finest cut, reaching down into the few-micron range and below, and the only stage that can recover barite or strip colloidal fines.
Size isn’t the whole story — density matters too
Two particles of the same size don’t necessarily behave the same, because settling depends on both size and density. Drilled solids sit around 2.6 SG; barite is ~4.2 SG. A separator’s pull rises sharply with particle size and with the density difference between solid and fluid:
That single relationship is why a centrifuge can split heavy barite from same-sized drilled solids — it is separating on density once size alone runs out — and why the biggest particles are always the cheapest and easiest to remove.
The principle that falls out of the ladder
Read the size ladder and the golden rule of solids control becomes obvious: remove a solid at the coarsest stage that can catch it. A particle caught big and early, at the shaker, is cheap to remove. Miss it, and every circulation grinds it finer — down the ladder, rung by rung — until it disperses into colloidal material that no screen or cone can touch and only the centrifuge or fresh dilution can chase. One missed solid becomes an expensive one.
Key takeaways
Microns are the language the whole system speaks. Learn the bands — sand above 74, silt down to 2, colloidal below — trust the API RP 13C micron cut over raw mesh, and remember that density rides alongside size in deciding what settles. Then the order of the train, the choice of screen and the centrifuge’s knack for barite all stop being separate facts and become one idea: catch every particle at the largest size it will ever be.