Roping indicates restricted or overloaded underflow and can impair classification; it does not quantify removal by itself. Check actual feed duty, loading, apex condition and solids recovery before assigning the cause or financial impact.
Likely causes
Compare calculated feed head with the specified OEM range at the measured mud density and flow. A 75-ft rating is a design point for some cones, not a universal loss-of-separation threshold; check pump condition, piping, loading and cone geometry.
Worn feed-pump impeller / volute — head quietly lost even though the pump “runs.”
Apex too small for the load — overloaded, it ropes instead of sprays.
Cone plugging — large particles or trash lodged at the apex.
Too many cones for pump capacity — head split too thin.
Suction air leak — pump losing prime/pressure.
How to diagnose it
Compare calculated feed head with the specified OEM range at the measured mud density and flow. A 75-ft rating is a design point for some cones, not a universal loss-of-separation threshold; check pump condition, piping, loading and cone geometry.
Use underflow appearance as a diagnostic prompt. A spray does not prove separation, and a rope or intermittent discharge requires checks of loading, restrictions, flow and OEM operating duty.
Check manifold pressure against design and trend it for drift.
Inspect the feed pump — impeller/volute wear is the usual hidden cause.
Count cones running vs pump capacity.
The fix — step by step
Restore the specified OEM feed duty after checking pump, piping and loading. Repair worn components only after confirming the fault; reassess apex configuration and solids removal.
Open or replace the apex to suit the load (umbrella, not rope).
Clear plugged cones; fit a trash screen/desander upstream if trash recurs.
Match cone count to pump capacity — don’t run more cones than the pump can feed.
Confirm it's fixed
✓ Verify: Every cone showing a steady umbrella spray with a visible air core, manifold pressure giving ≥75 ft feed head, and a discard that’s solids-laden — not whole mud. Re-check head after any pump or cone change.
Field note. Running is not cutting. The single most expensive solids-control fault I see is a cone bank “running” at low head — everyone sees the cones spinning, nobody calculates the head, and fines quietly recirculate for a whole section. Two minutes with h = 19.2p/ρ settles it.
The 10 Solids-Control Numbers Every Engineer Must Track — what to measure, the formula, the field target and the red flag. Grounded in API RP 13B-1, RP 13C, EPA 40 CFR 435 & OSPAR.
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