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 comes down to one number: turnover.
Turnover rate is the sizing number
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
The field rule is roughly one agitator per 10 feet (about 3 m) of tank, and every active compartment except the shaker/sand-trap section should be agitated — dead compartments are where sag starts. Heavier mud needs a bigger impeller and more power to hold the same turnover, so agitators are sized to the maximum anticipated mud weight (use 20 ppg if it is unknown) for a safety margin. Deep tanks (more than about 6 ft) use canted impeller blades rather than flat turbine blades for better top-to-bottom motion.
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?
By turnover rate — the time for the impeller to move a compartment's volume. Pick a target TOR for the duty (brisk for solids-removal sections), then choose the impeller diameter and horsepower from the manufacturer's table to hit it at the maximum expected mud weight.
How many agitators does a mud tank need?
Roughly one per 10 feet (about 3 m) of tank length, with every active compartment agitated except the shaker/sand-trap section. Unstirred compartments are where barite sag and solids settling begin.
Why does mud weight matter for agitator sizing?
Heavier mud is harder to keep in motion, so it needs a larger impeller and more power to hold the same turnover. Agitators are sized to the maximum anticipated mud weight — use 20 ppg if it is unknown — so they don't fall short as the mud is weighted up.
