Barite sag doesn’t only happen downhole. A mud tank with a dead spot drops weighting material and drilled solids to the bottom, and the mud you pump is lighter and dirtier than the mud you built. Agitator sizing is what keeps every barrel in the pit uniform — and it requires pumping capacity, rheology, geometry, placement, operating level and verified suspension; turnover is one check.
Turnover is one sizing check
A mud agitator is sized by turnover rate (TOR) — the time, in seconds, for its impeller to move a volume equal to the whole compartment. A short TOR means fast, aggressive turnover; a long TOR means gentle. Solids-removal sections want a brisk turnover to keep drilled solids in suspension so the removal equipment can catch them; storage and reserve pits can run gentler. The impeller diameter and horsepower are then chosen from the manufacturer's displacement table to hit the target TOR at the maximum mud weight expected.
Every compartment, sized to its duty
Select number and placement from actual compartment geometry, rheology, liquid levels, solids loading and OEM pumping data. Spacing around 10 ft is only a starting guide. Settling compartments have a different duty; choose impeller type and clearance for verified circulation and suspension rather than tank depth alone.
Agitators, mud guns and the shear question
Mechanical agitators do the steady suspension work; mud guns add jetting energy and blending but draw fluid from the clean suction section, so their added volume has to be accounted for in the removal-section balance or they short-circuit solids back into it. The objective of the whole agitation system is uniform suspension of all solids, homogeneous properties throughout the system, and appropriate — not excessive — shear, all for economical power. Over-agitation wastes energy and can shear solids finer; under-agitation lets them settle. Turnover is how you tune between the two.
The equation
Turnover rate for a compartment:
TOR (s) = compartment volume ÷ impeller displacement rate
Volume in gallons (with the liquid level ~1 ft below the tank top); impeller displacement (gal/s) from the manufacturer’s table for the chosen impeller diameter and mud weight. Rule of thumb: one agitator per ~10 ft of tank; size to the maximum expected mud weight.
Treat a dead compartment as a sag risk, not just a still pit. If mud weight out of a tank reads lighter than mud in, or the tank bottom is building solids, the agitation is under-sized or a compartment is unstirred. Size to turnover at the maximum mud weight, and put an agitator in every active compartment.
Common questions
How are mud agitators sized?
Define suspension and blending duty, then verify OEM pumping capacity, rheology, solids loading, active liquid volume, geometry, placement and power/shaft limits. Nominal V/Q turnover time is one screening calculation, not proof of complete suspension.
How many agitators does a mud tank need?
Select number and placement for actual compartment geometry, liquid levels, rheology and required coverage. Spacing around 10 ft is only a starting guide; settling compartments have a different duty.
Why does mud weight matter for agitator sizing?
Density affects power demand at fixed geometry and speed. Rheology and flow regime also affect pumping and suspension. Define the expected operating envelope and verify OEM limits rather than assuming 20 ppg or prescribing a larger impeller from density alone.
