Solids Control Equipment

Separation by particle size

Returns from the flow line reach a header box that splits them across the shale shakers. On high-rate rigs a scalping shaker with coarse screens takes the load off the primary shakers first. Screens set the cut point on a shaker, and everything finer goes through with the mud.

Hydrocyclones come next. A centrifugal feed pump drives mud tangentially into each cone. The spin throws heavier solids to the wall and out of the apex as underflow, while cleaned mud leaves through the overflow at the top. Desanders use larger cones, commonly 10 or 12 in (254 or 305 mm). Desilters use smaller cones, commonly 4 or 5 in (102 or 127 mm), and cut finer. A mud cleaner puts desilter cones over a fine-screen shaker so the screen returns barite and liquid from the underflow to the system.

A decanting centrifuge spins a bowl, and a scroll conveyor turning at a small speed difference pushes settled solids to the discharge end. Run at lower speed, it recovers barite from weighted mud. Run at higher speed, it removes fine low-gravity solids.

Configurations

• Shaker motion. Linear and balanced elliptical motion dominate primary shakers. Circular motion survives mainly on older units and scalpers.
• Decks and screens. Single, dual and triple decks, with pre-tensioned panel screens or hook-strip screens. The panel fits only its own basket design.
• Centrifuge speed control. Fixed-speed units use belt changes. Variable-speed units run bowl and conveyor on separate drives.

Key specifications

Standards and certification

API RP 13C defines the API screen number, found by comparing a screen's cut point with standard test sieves, and conductance, a measure of how easily fluid passes. Use both to compare replacement screens from different makers. The same document sets procedures for evaluating processing equipment performance.

An enclosed shaker room is usually classed as Zone 1 under the IMO MODU Code and IEC 61892-7, because the mud there has not yet passed the final degassing point. Vibrator motors, centrifuge motors and their junction boxes need IEC 60079 or ATEX certificates for that zone. Most solids control equipment holds little pressure, so class review on a classed unit centres on the electrical equipment and its foundations.

Failure modes and inspection

• Shaker baskets. Fatigue cracks grow at welds, cross members and screen supports. Use dye penetrant or magnetic particle testing on suspect welds.
• Vibrators and mounts. Check vibrator motor bearings, eccentric weight settings, isolation springs and rubber mounts. Measure G-force with an accelerometer against the OEM figure.
• Hydrocyclone cones. The apex and lower cone wear first. A worn apex changes the cut point and the underflow shape.
• Centrifuge conveyor. Hard-facing or tungsten carbide tiles on the flights wear and drop out. Measure flight-to-bowl clearance against OEM limits.
• Centrifuge gearbox and bearings. Check gearbox backlash, main bearing condition and vibration at operating speed. Ask for a dynamic balance report from the last overhaul.

Documentation

Ask for the model and serial number, motor nameplates and Ex certificates, screen part numbers, and for centrifuges the overhaul, balance and vibration reports with the operating hours since.

Integration and compatibility

Check the flow line and header box connections, the deck footprint and the screen supply for the model. Vibrator motors built for 50 Hz run faster on 60 Hz and raise G-force, so confirm the rating for your supply. A centrifuge needs a feed pump sized to its processing rate and a discharge route for its solids.

Refurbishment and recertification

A shaker rebuild replaces deck rubbers, springs, vibrator bearings or motors and seals, and repairs basket cracks to the OEM procedure. A centrifuge overhaul retiles the conveyor, rebuilds the gearbox, replaces bearings and rebalances the rotating assembly. A used shaker with a sound basket and current motor certificates is a low-risk buy. Buy a used centrifuge only with a recent overhaul report. Common offshore OEMs are NOV Brandt, SLB's M-I SWACO and Derrick. Cuttings from the shakers may go on to a dryer in mud treatment systems, and total screen capacity has to cover peak output from the mud pumps.