The desander is the coarse hydrocyclone stage — a bank of large cones that strip sand-sized solids before the desilter and centrifuge see them. This is the deep reference: sizing and feed system, the process failures you read off the discharge (roping vs spraying, feed head), and the mechanical wear of cones, apexes and vortex finders.
Where it sits: downstream of the shakers and ahead of the desilter, on unweighted mud. It develops its separating force from feed head supplied by a dedicated centrifugal pump. Run it on weighted mud and it throws barite away; starve its feed head and it can't separate at all.
A desander is a bank of large hydrocyclones removing sand-sized solids ahead of the desilter. Its faults are mostly about cone size, number, and the feed system that develops the separating force.
Wrong cone size / not enough cones for the flow
Mechanism
Desander cones (typically 6–12 in) are sized and numbered for a target cut and the circulation rate. Too few or too small and the bank is overloaded; the cut shifts coarse.
Shows as
Sand passing to the desilter and centrifuge; cones overloaded; poor sand removal.
Detect / inspect
Compare installed cone size/number and rated capacity against circulation rate and target cut; sample underflow and overflow.
Consequence downstream
Sand loads the desilter and centrifuge, accelerates pump and cone wear, and forces dilution.
Correction
Size the cone bank for the rate and cut; add cones or correct cone size; on unweighted mud run the desander ahead of the desilter.
Run on weighted mud (barite thrown to waste)
Mechanism
Desander cones discard everything coarser than their cut — on weighted mud that includes barite, which is expensive to lose.
Shows as
Barite in the underflow to waste; rising mud cost; unnecessary dilution.
Detect / inspect
Check whether the desander is run on weighted mud; sample underflow for barite.
Consequence downstream
Costly barite loss; the desander shouldn't normally run on weighted mud.
Correction
On weighted mud, bypass the desander and use a mud cleaner (cones over a fine screen) to save barite; run desanders mainly on unweighted mud.
No / undersized dedicated feed (charge) pump
Mechanism
Each hydrocyclone bank needs a dedicated centrifugal pump sized to deliver the design feed head. Sharing or undersizing the pump starves the cones.
Shows as
Low feed head, weak separation; cones spraying weakly or roping.
Detect / inspect
Check for a dedicated, correctly sized feed pump; measure feed pressure vs the head target.
Consequence downstream
Poor separation across the bank; solids carry-over.
Correction
Provide a dedicated feed pump sized for the design head; don't run cyclones off a shared/undersized pump.
Poor suction/discharge arrangement (re-processing or short-circuit)
Mechanism
If the cyclone bank draws from and returns to the same compartment without proper routing, it re-processes clean mud or short-circuits.
Shows as
Re-processing clean mud while dirty mud bypasses; wasted capacity.
Detect / inspect
Trace suction/discharge against the tank arrangement; confirm correct compartment order.
Consequence downstream
The bank does work but solids still get through.
Correction
Route suction from the upstream compartment and discharge downstream; follow correct tank order (see mud system page).
Process & operating failures
Hydrocyclones tell you what they're doing by how they discharge. Reading the underflow — and the feed head behind it — is the whole game.
Roping discharge (overloaded — solids too concentrated)
Mechanism
When a cyclone is overloaded, the underflow discharges as a thick rope instead of a fine spray; separation collapses and solids are dragged to overflow.
Shows as
Rope-like underflow (not a spray); solids in the overflow; poor removal.
Detect / inspect
Observe the underflow: a rope means overloaded/plugging risk. Check feed head, apex size and solids load.
Consequence downstream
Solids carry-over to the active and downstream units; eventual plugging.
Correction
Restore feed head, open/replace the apex, reduce solids load, or add cones; aim for a light spray, not a rope.
Spraying too wide / wet (apex too open, fluid wasted)
Mechanism
An apex too open or too low a solids load gives an overly wet, wide spray that throws good mud to waste.
Shows as
Excess fluid in the underflow; high whole-mud loss; large effluent volume.
Right-size the apex for the load; aim for a fine umbrella spray that's wet enough to flow but not wasteful.
Low feed head / feed pressure
Mechanism
Hydrocyclone separation depends on feed head (commonly targeted around 75 ft of mud, ~ the right inlet pressure for the mud weight). Too low and the cones can't separate.
Shows as
Weak spray, poor cut, solids carry-over across the bank.
Detect / inspect
Measure feed pressure and convert to head for the mud weight (see the feed-head calculator); compare to target.
This reference describes failure modes and engineering principles in general terms. Corrective actions must be matched to your actual equipment, fluid, formation and procedures, and carried out under the relevant rig and safety standards.
Grounded in standard solids-control practice and field references (drilling-fluid solids-control handbooks; hydrocyclone operating practice). SC DrillTech is independent and vendor-neutral.
Take it further
Tools and references built from the same field experience as this page — independent and vendor-neutral.
A cyclone bank that's starved of feed head, run on the wrong mud, or full of worn cones looks like it's working but lets sand through to wear your pumps and centrifuge. An independent evaluation reads every discharge and the feed head behind it.
