API Plan 31 routes the seal flush through a cyclone separator: clean overflow to the seal chamber, dirty underflow back to pump suction. It works inside a narrow four-parameter envelope — when the envelope fits, it buys years of seal life at no utility cost.
How the API Plan 31 Cyclone Separator Produces the Clean-Fraction Split
Pump discharge pressure drives a tangential flow into the cyclone, fluid spins down the conical wall, and the centrifugal-vs-drag balance on each particle decides which way it leaves. Plan 31 sits in the recirculation family alongside Plan 11 discharge recirculation, with the cyclone inserted between discharge tap and seal chamber.
- Centrifugal split: heavier particles migrate to the wall and exit through the apex; lighter and density-matched fines rise through the vortex finder to the seal chamber.
- D50 cut point: per the AMIT 145 mining curriculum at the University of Alaska Fairbanks, d50 is the particle size with a 50 percent probability of going to overflow vs underflow. A cyclone with d50 around 25 microns cannot separate a slurry whose fines are 5 to 10 microns. Geometry sets the cut point, not pressure.
- 50/50 flow split: the clean and dirty legs each carry approximately half the recirculation flow per sealfaqs.com. Sizing the recirculation orifice on total flow without accounting for this halving is a common spec error.
The Four-Parameter API Plan 31 Cyclone Separator Operating Window
The 2:1 specific gravity rule is the headline criterion but only one of four parameters that govern fit. Vendor pages stop at SG; the cyclone fails on the other three.
| Parameter | Window | What it controls |
|---|---|---|
| Specific gravity ratio | Solids SG ≥ 2× flush fluid SG | Centrifugal-vs-drag balance |
| Particle size | ~90% removal of solids ≥10 µm per sealfaqs.com; max size < 1/4 smallest orifice diameter | Whether fines fall above d50 |
| Solids loading | < 10% by volume | Whether the apex can pass the underflow mass |
| Separator differential | 15-200 psi across separator; ≥25 psi pump discharge to seal chamber | Flow drive across the circuit |
The pressure window is the one most often misread. The 15 to 200 psi range drives flow through the separator; the 25 psi minimum drives flow across the circuit. Neither sharpens separation — AMIT 145 and McLanahan both note feed pressure barely moves d50; geometry does. If fines sit below the cyclone’s d50, you need a smaller cyclone, not more pressure. These breaches drive mechanical seal selection on slurry pumps.
Four API Plan 31 Cyclone Separator Failure Modes in Field Service
Recognizing the failure mode by name shortens the diagnostic loop from weeks to hours.
- Apex roping on over-loaded underflow — McLanahan’s engineers named this the dominant cyclone failure: when the apex cannot pass the solids load, the underflow ropes and the cyclone stops separating. The field signature of a slurry loaded beyond ~10 percent solids by volume.
- Low-SG and sub-d50 pass-through — the most damaging failure is invisible. The cyclone is mechanically intact, the differential reads in window, and the seal still sees abrasive flow. AFPM Question 64 documents a refinery where catalyst concentration in the FCC pumparound doubled after a slurry catalyst separator was commissioned — catalyst fines at ~1.5 SG in 0.9 SG slurry oil fall below the 2:1 threshold. Pumps & Systems put the heuristic clearly: silica in water separates cleanly; clay or calcium carbonate in thickened slurry does not.
- Reversed flow on suction-pressure spike — a transient dropping discharge pressure below chamber pressure back-feeds the dirty fraction to the seal. The 25 psi minimum is the margin that prevents it; check-valve chatter or dual-pump transfers are the usual triggers.
- Internal erosion and partial plugging — hard slurries above design wear the apex and inlet. A temperature check across the cyclone is the fast field diagnostic — if downstream runs appreciably cooler than upstream, the cyclone is partially obstructed.
When API Plan 31 Fails and the Cyclone Separator Needs Plan 32 or Plan 41
Plan 31 has two natural escalations within the API 682 piping plan taxonomy, triggered by different breach types:
- Plan 32 — fluid escalation — external clean flush replaces the recirculation. Escalate when solids loading exceeds 10 percent, particle distribution sits dominantly below d50, or SG ratio is under 2:1 (FCC catalyst fines, coke fines, density-matched solids). The process fluid itself cannot be conditioned by a cyclone in these cases.
- Plan 41 — temperature escalation — cyclone plus downstream cooler. Escalate when the four-parameter envelope fits but flush-stream temperature exceeds elastomer limits or pushes pumped fluid too close to its vapor-pressure margin.
- Decision rule — Plan 41 is the temperature escalation; Plan 32 is the fluid escalation. Try Plan 41 first if the only breach is heat; commit to Plan 32 when the slurry itself fails the SG / size / loading screen.
Where API Plan 31 Cyclone Separator Specifications Go Wrong
The recurring mistake is treating the 2:1 specific gravity rule as a sufficient screen rather than one of four parameters. A slurry meeting the SG threshold but loaded above 10 percent solids plugs within weeks; one dominated by sub-d50 fines passes through regardless of density; one running on inadequate differential reverses flow on the first suction spike. Check all four parameters against the slurry’s actual particle distribution, solids loading, and pump curve at the spec stage — Plan 32 or Plan 41 then is cheaper than replacing seals later.
