A weak underflow looks almost normal but removes little. Across a bank it means drilled solids aren't being discarded — LGS climbs and dilution rises while the cones appear to be working.
Likely causes
Apex worn open / over-sized — too little classification at the apex.
Apex set too small for genuine discharge (choked toward roping).
Feed head marginal — cones forming a weak air core.
Bank fed thin mud (already low solids reaching the cones).
Low solids in the feed — nothing for the cones to discharge (can be normal).
High viscosity hindering classification.
Upstream removal already stripping the load before the cones see it (rare — usually the opposite).
How to diagnose it
Calculate feed head and compare against ~75 ft.
Inspect apex wear and size against the duty.
Sample the underflow — solids content vs a watery discharge.
Trend LGS against cone performance.
Confirm the feed isn't already stripped or short-circuited upstream.
The fix — step by step
Restore feed head to form a proper air core.
Set the apex to the load — size for a solids-laden umbrella, not a thin spray or a rope.
Replace worn-open apexes that under-classify.
Confirm the cones are seeing the solids (check distribution / bypass upstream).
If viscosity is hindering separation, address rheology upstream.
Confirm it's fixed
✓ Verify: A solids-laden umbrella discharge restored, with feed head in band and the apex sized to the load.
Field note. A thin underflow is easy to call ‘fine' because the cone is spraying — but a spray of mostly water removes almost nothing. Judge cones by what's in the discharge, not by the fact that something is coming out the apex.
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.