How a Solids Control System Works: A Stage‑by‑Stage Technical Explanation

In oil and gas drilling, the drilling fluid (mud) circulates continuously from the surface down the drill string, out through the bit, and back up the annulus to the surface. On its return, the mud carries drilled cuttings, formation gases, and other contaminants. If this contaminated mud were pumped directly back into the well, it would quickly become too thick, lose its ability to carry cuttings, damage pumps, and create well‑control hazards. The Solids Control System solves this problem by cleaning the mud in a progressive, multi‑stage process. This article explains how a solids control system works, step by step, from the flowline to the suction pit, and highlights why AIPU Solid Control equipment makes the process efficient and reliable.

1. The Overall Goal of a Solids Control System

The primary goal is to remove drilled solids and gas from the drilling fluid so that the cleaned mud can be recirculated. By doing this repeatedly, the system:

• Maintains mud density and viscosity within specifications.

• Reduces wear on mud pumps and downhole tools.

• Lowers chemical and barite consumption.

• Minimizes waste volume and disposal costs.

• Prevents gas‑related safety incidents.

The system works by passing the mud through a series of separation devices, each designed to remove progressively smaller particles and, if needed, entrained gas.

2. The Basic Working Principle: Progressive Separation

The solids control system operates on the principle of progressive separation: larger particles are removed first, then medium particles, then fine particles, and finally ultra‑fine particles or barite is recovered. Gas, if present, is removed after the shakers but before the fine‑separation stages.

The typical flow path is:

Flowline → Shale shaker→ Degasser (optional) → Desander → Desilter → Centrifuge → Suction pit → Mud pumps

Each stage uses a different physical mechanism: vibrating screens (shakers), vacuum (degasser), centrifugal force in hydrocyclones (desander/desilter), and high‑G centrifugation (centrifuge). Supporting equipment—agitators, shearing pumps, jet mixers, and centrifugal pumps—keeps the mud moving and conditioned.

3. Stage 1 – Shale Shaker: Removing Large Cuttings

The returning mud first lands on the shale shaker. This is a vibrating screen that separates large drilled cuttings from the liquid.

How it works: A motor drives an eccentric weight or multiple weights to create a linear, elliptical, or circular vibration. Mud flows onto the screen deck. The vibration moves the solids toward the discharge end while liquid and fine solids pass through the screen mesh. The dried cuttings fall into a cuttings box or are conveyed away.

What is removed: Particles larger than the screen opening (typically 75 µm and above). Different screen meshes (API 20 to API 325) can be used depending on the required cut point.

Why it is first: Removing large solids immediately prevents them from breaking down into smaller, harder‑to‑remove particles.

AIPU equipment example: Hunter‑MG series shale shakers (single, dual, or triple motion) with capacities from 50 to 420 m³/h. The Hunter‑MGD dual‑motion shaker can switch between linear and balanced elliptical motion for optimal performance.

4. Stage 2 – Vacuum Degasser: Removing Entrained Gas (If Present)

If the formation contains gas (methane, H₂S, CO₂), the mud will carry small gas bubbles. These bubbles reduce mud density and cause pump cavitation. The vacuum degasser removes them.

How it works: A vacuum pump creates a partial vacuum (‑0.02 to ‑0.04 MPa) inside a sealed vessel. The mud is spread into a thin film by a high‑speed rotor or baffles. Under low pressure, dissolved gas comes out of solution and existing bubbles expand dramatically. The expanded gas rises to the top and is sucked away by the vacuum pump through a vent line. The degassed mud falls to the bottom and returns to the tank.

What is removed: Entrained and dissolved gases, with an efficiency of ≥95%.

Placement: After the shaker, before the desander. Gas would interfere with hydrocyclone separation if placed later.

AIPU equipment example: APLCQ300 vertical degasser (300 m³/h, compact design) or APZCQ series tank‑mounted degassers (240–360 m³/h). H₂S‑resistant steel and ATEX certifications are available.

5. Stage 3 – Desander: Removing Medium Solids (Sand)

After degassing, the mud flows to the desander. The desander uses large hydrocyclones (10‑inch or 12‑inch cones) to remove sand‑sized particles.

How it works: A centrifugal pump feeds mud tangentially into the hydrocyclone at 0.25–0.45 MPa. The spinning motion creates centrifugal force that throws heavier sand particles outward against the cone wall. These particles spiral down and exit through the underflow (bottom). Cleaner mud moves up through the vortex finder and exits through the overflow (top), heading to the next stage. An optional bottom shaker can further dewater the separated sand.

What is removed: Particles in the range of 44–74 µm (sand and coarse silt).

Why it is placed here: Removing sand before the desilter protects the smaller desilter cones from abrasive wear.

AIPU equipment example: Hunter S series desanders (120–360 m³/h) or APCS series with polyurethane cones and optional bottom shaker.

6. Stage 4 – Desilter: Removing Fine Solids

The desilter uses smaller hydrocyclones (4‑inch or 5‑inch cones) to remove fine silt and barite‑sized particles down to about 15 µm.

How it works: The operating principle is the same as the desander, but the smaller cones generate higher centrifugal force, capturing finer particles. The underflow (solid‑rich) often goes to a small “mud cleaner” shaker to dry the solids before disposal. The overflow (cleaned mud) proceeds to the centrifuge stage.

What is removed: Particles in the range of 15–44 µm.

Placement: After the desander, before the centrifuge.

AIPU equipment example: Hunter N series desilters (120–360 m³/h) or APCN series with 100% polyurethane cones. Each 4‑inch cone handles about 12–15 m³/h, so a 240 m³/h system requires 12–16 cones.

7. Stage 5 – Decanter Centrifuge: Ultra‑Fine Control or Barite Recovery

The centrifuge is the final and most precise solid‑liquid separation device. Its role depends on whether the mud is weighted (contains barite) or unweighted.

How it works: A horizontal bowl spins at high speed (1500–3200 rpm). Mud enters through a feed tube and is accelerated to bowl speed. The high centrifugal force (up to 2000 G) settles solids against the bowl wall. A screw conveyor (scroll) rotates at a slightly different speed, pushing the settled solids toward the discharge ports while the clarified liquid overflows at the other end.

Two modes of operation:

• Unweighted mud: The centrifuge removes ultra‑fine solids (2–7 µm) that hydrocyclones cannot catch. This prevents viscosity buildup and improves filter cake quality.

• Weighted mud (barite): The centrifuge separates barite (SG ≈4.2) from low‑gravity drilled solids (SG ≈2.6). The barite‑rich stream returns to the system; the fine solids are discarded.

What is removed: Particles as small as 2–7 µm, depending on bowl speed and feed rate.

Placement: Last stage, before the suction pit.

AIPU equipment example: APLW355X1257‑N (35 m³/h, 2–5 µm) for unweighted mud; APLW600X1019‑N (60 m³/h, 5–7 µm) for barite recovery. Multiple models cover 35–65 m³/h.

8. Supporting Equipment That Makes the System Work

The five main stages above cannot function without supporting components:

Mud Agitators

• Purpose: Keep solids suspended in each tank compartment.

• How they work: Low‑speed, high‑torque impellers create a flow that lifts solids from the bottom.

• AIPU APMA series: 5.5–15 kW, single or dual impeller, one per 3 m of tank length.

Centrifugal Pumps

• Purpose: Move mud between stages and feed desanders/desilters/jet mixers.

• How they work: Impeller rotation converts mechanical energy into fluid pressure and flow.

• AIPU APSB series: API 7th edition, 40–255 m³/h, vertical or horizontal.

Shearing Pump

• Purpose: Hydrate polymers and disperse clay to prevent “fish eyes” and save chemicals.

• How it works: High‑speed rotor and shear plate create intense shear (1900–2200 rpm).

• AIPU APJQB series: 100–155 m³/h, reduces hydration time from 4 hours to 15 minutes.

Jet Mud Mixer (Jet Hopper)

• Purpose: Rapidly add barite, bentonite, or dry chemicals.

• How it works: Venturi effect creates a vacuum that sucks powder into a high‑velocity fluid stream.

• AIPU APSLH series: Up to 315 kg/min barite, 180 kg/min clay.

Mud Tanks

• Purpose: Compartmented vessels that hold the mud and direct its flow from one stage to the next.

• How they work: Weir plates force mud to overflow from one compartment to the next, ensuring each piece of equipment receives feed in the correct order.

• AIPU custom tanks: Sloped or V‑shaped bottom, three‑layer marine coating, H₂S‑resistant steel option.

9. The Complete Working Sequence – A Summary

Let us follow one barrel of mud from the well back to the well:

1. Mud returns from the annulus through the flowline.

2. It flows onto the shale shaker; large cuttings (>75 µm) are removed.

3. The mud enters the degasser (if gas is present); gas bubbles are expanded and sucked away (≥95% removal).

4. A centrifugal pump sends the mud to the desander; sand‑sized particles (44–74 µm) are removed.

5. The overflow goes to the desilter; fine solids (15–44 µm) are removed.

6. The mud then enters the centrifuge; ultra‑fine solids (2–7 µm) are removed, or barite is recovered and returned.

7. Clean, conditioned mud arrives at the suction pit.

8. Mud pumps send it back downhole.

9. Throughout the process, agitators keep solids from settling, centrifugal pumps provide motive force, and shearing pumps / jet mixers are used when adding new materials.

10. Why AIPU Solids Control Systems Work Better

AIPU Solid Control has over 20 years of experience in the industry, with products delivered to more than 30 countries and cumulative shipments exceeding 300 system‑equivalent units. AIPU holds API, HSE, and multiple product certifications, plus independent invention patents.

Key reasons why AIPU systems perform reliably:

✅ Full‑range equipment – Shakers, degassers, desanders, desilters, centrifuges, agitators, shearing pumps, jet mixers, centrifugal pumps, and mud tanks – all from one source, all matched for capacity.

✅ Engineered for harsh conditions – Three‑layer marine anti‑corrosion coating, H₂S‑resistant steel (316L/2205), ATEX/IECEX certification for hazardous areas.

✅ Customizable – Any voltage/frequency, any tank size, any number of compartments.

✅ Global support – AIPU provides engineering drawings, installation guidance, and after‑sales service worldwide.

11. Conclusion

A solids control system works by passing drilling fluid through a progressive series of separation devices: shale shaker (large solids), vacuum degasser (gas), desander (sand), desilter (fine solids), and centrifuge (ultra‑fine solids or barite recovery). Each stage uses a different physical principle – vibration, vacuum, centrifugal force, and high‑G centrifugation – to achieve progressively finer cleaning. Supporting equipment (agitators, pumps, shearing pumps, jet mixers, and tanks) ensures the mud moves correctly and remains conditioned.

When all components are properly sized, sequenced, and integrated, the system delivers clean mud that protects pumps, stabilizes well control, reduces costs, and increases drilling efficiency.

AIPU Solid Control designs and builds complete solids control systems that follow this proven working principle. From individual components to fully integrated packages, AIPU ensures that every stage works in harmony.

Choose AIPU – and see how a properly working solids control system transforms your drilling performance.