Cargo Pump (Tanker)

What defines a tanker cargo pump

The cargo pump is the machine that actually discharges a tanker's cargo, distinct from the ballast pumps that move seawater and from the stripping pump or eductor that clears the last residue a main cargo pump cannot reach. Product tankers commonly use deep well pumps, one per tank, driven from deck through a long vertical shaft, giving good segregation between grades since each tank has its own dedicated pump. Crude carriers more often use large steam turbine-driven centrifugal pumps in a pump room, feeding a common manifold, prioritising discharge rate over segregation because crude is usually carried in fewer, larger parcels.

Main components

Pump unit

Deep well pumps are multistage centrifugal units hung inside the tank on a long shaft from a deck-mounted driver. Submersible pumps integrate an electric motor at the bottom of the same column, in a pressurised, gas-tight housing, avoiding the long shaft altogether. Large crude carriers typically use horizontal centrifugal pumps in a dedicated pump room.

Drive

Hydraulic drive is common on deep well pumps for the flexibility of variable speed without electrical equipment inside the tank. Steam turbine drive remains standard on many crude carriers because it avoids any electrical machinery in a hazardous area. Direct electric submersible motors are increasingly used where their explosion-protected design is accepted for the cargo range carried.

Shaft and bearings (deep well type)

Line bearings support the long shaft at intervals down the pump column; these are lubricated by the cargo itself in most designs, which means bearing life depends heavily on the cargo's lubricating properties.

Stripping arrangement

A separate stripping pump, eductor, or a stripping stage built into the main pump clears the residual cargo the main pump cannot draw down to, which matters directly for how clean a tank is left for the next cargo.

Selection and sizing

Discharge rate, the figure charterers care about most, is set by pump capacity together with how many pumps can run simultaneously without starving each other on a shared manifold or shore connection. Cargo viscosity is the deciding factor for which pump type suits a given trade: centrifugal pumps lose capacity sharply as viscosity rises, while positive displacement or purpose-built high-viscosity centrifugal designs handle heavy, viscous products a standard pump cannot move at a useful rate. Net positive suction head available inside the tank, which falls as the tank empties, limits how far down a pump can draw cargo before it loses suction, directly affecting how much stripping is left to do.

Regulations and class

Cargo pump rooms and hydraulic or electrical equipment inside cargo tanks fall under strict hazardous area classification, with class and IACS UR requirements governing gas-tight integrity of any electrical machinery submerged in cargo. Pump room ventilation, gas detection and emergency shutdown are surveyed items, and MARPOL Annex I sets requirements for stripping performance tied to the ship's approved discharge and clean ballast arrangements.

Typical faults

What to look for in a supplier

• Performance curves covering the actual viscosity range of the cargoes the ship is intended to carry, not just clean product data
• Bearing and seal materials proven on comparable cargo grades and documented service life
• Explosion-protection certification appropriate to submersible or electrically driven designs
• Realistic stripping performance figures, since this is what actually determines how clean a tank is left for the next cargo

Track discharge rate against tank level, not just against elapsed time; a rate that falls sharply well before the tank is empty usually means cavitation, not a valve left throttled.