A centrifuge running at the right RPM can still be doing half the job. Speed tells you the force at the bowl wall; it says nothing about how much of the solids you fed it actually left in the cake. The only honest measure of centrifuge efficiency is a mass balance — and it routinely surprises people.
Recovery, not RPM, is the performance number
Two levers set a decanter's duty: the G-force it develops and the time a particle spends in the bowl. RPM sets the first; feed rate sets the second. You can have both nominally right and still recover poorly if the feed is too fast, the pool too shallow, or the differential wrong. Recovery — the fraction of feed solids that reports to the cake — is the number that captures all of it at once.
That is why comparing machines or settings on RPM alone misleads. A mass balance on the three streams — feed, cake (underflow) and centrate (overflow) — tells you what actually happened to the solids.
Building the mass balance
Measure two things on each stream you can reach: a flow rate and a solids concentration. Solids concentration comes from a mud balance or a retort; flow from a bucket-and-stopwatch or the pump curve. Feed solids in must equal cake solids plus centrate solids out — that closure is your check. Recovery is simply the cake's share of the feed solids.
The same balance, run on the barite fraction rather than total solids, gives barite recovery — the number that matters when the centrifuge is there to save weighting material rather than remove drill solids.
The cut hiding inside the number
Recovery and cut point are two sides of one coin. A decanter recovers coarse solids almost completely and fine solids poorly — the partition curve is steep above the cut and shallow below it. So a 60% recovery usually means the machine is catching nearly everything above its cut and losing the fines to the centrate. Raising recovery means moving that cut finer: more G-force, a slower feed, a deeper pool, or a flocculant on a dewatering unit.
This is where recovery ties back to the rest of solids control: the fines the centrifuge misses are exactly the low-gravity solids that then need dilution. A centrifuge running at low recovery quietly raises your dilution bill.
Turning the number into a decision
Once you have recovery, the levers are clear. Low recovery with a wet cake points to feed rate too high — slow it down. Low recovery with a dry cake points to insufficient G for the cut you need — raise RPM (mind the bearings and torque). If recovery is fine but barite recovery is poor on weighted mud, your cut is too coarse and you are discarding weighting material. The mass balance turns a vague sense that the machine is or isn't working into a specific, correctable setting.
The equation
Solids recovery from a stream mass balance:
Recovery (%) = (Qu × Cu) ÷ (Qf × Cf) × 100
Q = volumetric flow rate, C = solids concentration (fraction); subscripts f = feed, u = underflow (cake). Closure check: QfCf = QuCu + QoCo (o = overflow / centrate).
Chase recovery, not speed. If recovery is low, diagnose by the cake: a wet cake means the feed is too fast; a dry cake means you need more G for the cut. And remember the fines you fail to recover don't disappear — they come back as the low-gravity solids that drive your dilution.
Common questions
Why isn't RPM a good measure of centrifuge efficiency?
Because RPM only sets the G-force; it says nothing about residence time or how much of the feed solids actually left in the cake. Two machines at the same RPM can have very different recovery depending on feed rate, pool depth and differential.
What is a typical centrifuge recovery?
It varies with the cut you need, but a decanter recovers coarse solids almost completely and fines poorly, so total-solids recovery on a drill-solids duty commonly lands well short of 100% — the shortfall is the fine fraction lost to the centrate.
How do I raise recovery?
Move the cut finer: increase G-force, slow the feed rate to lengthen residence time, deepen the pool, or add a flocculant on a dewatering unit — while watching torque and cake dryness so you don't overload or plug the bowl.
