Cement Carrier

Overview

A cement carrier is a bulk ship specialised for powdered cargo: ordinary cement, fly ash and ground granulated blast-furnace slag. These powders behave nothing like grain or ore. They are fine enough to fluidise and flow like a liquid when aerated, they make choking dust if handled in the open, and they set hard if they meet water. A modern cement carrier is therefore a sealed, self-discharging plant that loads and unloads its cargo with air rather than grabs or shore cranes, and keeps the whole process dust-tight.

The cargo lives in the sealed Cement Hold tanks inside the Hull Structure. The Aeration & Fluidising System turns the settled powder back into a flowing bed; the Conveying System gathers it and lifts it; the Air Blower Plant supplies the dry air that drives it through pipes; the Dust Control System catches the displaced dust; the Propulsion Train train moves the ship; and the crew work from the Deckhouse.

The cargo holds

Each Cement Hold is a sealed steel tank, not an open bulk hold, because cement must be kept away from moisture and from the air. The floor slopes to discharge points and is paved with porous Aeration Pad panels fed from sealed Air Plenum chambers below. The hold walls carry smooth Hold Liner Panel panels so the powder does not cling and bridge over the outlets. Cement is blown in through gasketed Loading Hatch connections during loading, and a Level Probe tracks the depth. At the low point of each floor a Discharge Cone draws the fluidised cement into the conveying system.

Fluidising the cargo

The trick that makes self-discharge possible is fluidisation. Settled cement is dense and immobile, but blow a gentle stream of air up through it and the powder lifts on a cushion of air bubbles and flows like water. The Aeration & Fluidising System does this with low-pressure Fluidising Blower units feeding the floor plenums and a network of inclined Air Slide channels, whose porous decks keep the cement moving downhill toward the intakes. A Aeration Valve for each floor section lets the crew fluidise and empty the hold zone by zone rather than all at once, which keeps the air demand manageable and the trim under control.

Conveying and discharge

Once the cement is flowing, the Conveying System moves it off the ship. Enclosed Screw Conveyor units gather the fluidised powder from the hold intakes and feed a Pneumatic Lift Pump pump, a pressure vessel that injects high-pressure air to push slugs of cement up the Transport Pipe to deck level and out through a Discharge Arm to the receiving shore silo. The transport pipe is built from abrasion-resistant material and uses long-radius Wear Bend sections, because the dense air-and-cement stream wears ordinary pipe through quickly. A Diverter Valve routes the flow to whichever connection or hold is in use.

The air supply and why it must be dry

The conveying air comes from the Air Blower Plant, where high-pressure Conveying Compressor units feed an Air Receiver. The defining requirement is that the air must be dry. Cement is a hydraulic material: any moisture starts the chemical reaction that sets it, so damp conveying air would cake the powder and block the pipes solid. The plant therefore puts every compressor through an After-Cooler and an Air Dryer before the air reaches the cargo, and the holds themselves are sealed against humid sea air.

Dust control

Pushing powder around displaces air, and that air carries fine cement dust that must not escape into the harbour or the engine room. The Dust Control System vents the holds and the conveying equipment through reverse-jet Bag Filter collectors, whose Filter Bag sleeves trap the fines and are pulse-cleaned by jets of air. A Vent Fan keeps the system under slight suction, and a Rotary Airlock Valve airlock returns the collected fines to the cargo so nothing is wasted and nothing is dumped overboard.

Propulsion, power and variants

The ship is driven by the Propulsion Train train, a medium-speed Main Engine turning a Propeller. Self-discharge draws heavy electrical power for the blowers and compressors, so several Auxiliary Genset sets run during cargo operations and the whole plant is run from a Cargo Control Room room. Cement carriers range from small coastal ships of a few thousand tonnes serving local terminals up to ocean carriers of fifteen to twenty thousand tonnes. Some carry their own deck silos and bagging plant to deliver cement to ports with no shore reception facilities at all, which makes the ship a complete mobile cement terminal.