Cement does not flow out of the silo — bridging, rat-holes and lumps
Symptom
The butterfly valve is open but little or no cement reaches the screw, flow starts and stops, or lumps block the outlet.
Likely causes
- A bridge (arch) of cement has formed over the cone outlet.
- A rat-hole, where cement flows only through a narrow central channel.
- Aeration pads blocked, air line blocked, or check valves failed.
- Water in the silo or in the aeration air, forming lumps.
- Cement compacted after a long stop.
Most likely and cheapest to check first.
Checks
- Tap the cone with a rubber mallet and switch the aeration on.
- Check air pressure and flow at each aeration pad.
- If the level drops only in the centre, it is a rat-hole.
- Look for the water source - hatch seals, roof seams, air dryer.
Fix
- Aerate the cone before discharging, especially after a long stop.
- Run the vibrator only while the valve is open, in short bursts.
- Clear blocked lines; replace blocked pads and failed check valves.
- Stop the water at its source, then clean out the lumps.
- Run the silo lower more often to break up rat-holes.
Stop the equipment if
Anyone proposes to enter the silo to clear a blockage. Never enter a cement silo.
Parts you may need
Never enter the silo
Cement can bridge above a person and then collapse, burying them. Every clearing method in this manual works from outside the silo: aeration, vibration, tapping, and running the silo lower.
DANGERNever enter a cement silo to clear a blockage. If outside methods fail, call a specialist silo-cleaning company.
CAUTIONAeration pads must get dry, clean air only. Wet air turns the cement at the pad into a crust and is a main cause of lumps.
FIELD TIPRecord the time each screw needs to deliver a standard cement weight once a week. A sudden jump points to a bridge, a valve or a blocked vent; a slow rise over months is flight wear.
⚠ Every check inside a machine starts with full lockout. Safety disclaimer