What to check on a Cargo Pump (Deepwell).
A deepwell cargo pump is a vertical, multistage centrifugal pump built into a long column that is lowered through a trunk in the tank top so the pumping element itself sits near the tank bottom, with a long shaft running up to a motor mounted on deck. This differs from submersible cargo pumps, which put the motor itself down in the tank sealed against the cargo, and from eductor or stripping systems, which move cargo using a jet of driving liquid rather than a rotating impeller. Deepwell pumps are the…
What Sets a Deepwell Pump Apart
A deepwell cargo pump is a vertical, multistage centrifugal pump built into a long column that is lowered through a trunk in the tank top so the pumping element itself sits near the tank bottom, with a long shaft running up to a motor mounted on deck. This differs from submersible cargo pumps, which put the motor itself down in the tank sealed against the cargo, and from eductor or stripping systems, which move cargo using a jet of driving liquid rather than a rotating impeller. Deepwell pumps are the standard fit on product and chemical tankers where each tank needs its own independent, segregated pump so different grades can be loaded and discharged simultaneously without cross-contamination. Because the motor stays above deck, deepwell pumps avoid the sealed-motor cargo compatibility questions that come with submersibles, at the cost of a long shaft that must run true through multiple line-shaft bearings.
Main Components
Pump bowl assembly
Multiple impeller stages stacked in a column at the base of the unit build the discharge head progressively; stage count is matched to the required total dynamic head for the tank's discharge line run and shore connection pressure.
Line shaft and bearings
The shaft runs the full depth of the tank inside a shaft tube, supported by intermediate bearings that are typically lubricated by the cargo itself, which is why compatibility between the bearing material and the cargo grade matters.
Top unit and deck seal
The above-deck top unit houses the motor coupling, a mechanical seal or stuffing box sealing the shaft where it passes through the deck, and often a bearing lubrication or cooling arrangement for cargoes that cannot lubricate the shaft themselves.
Stripping stage / eductor
Many deepwell pumps include a low-flow stripping stage or a separate eductor connection to remove the final residual cargo from the tank bottom once bulk flow can no longer be maintained by the main stages.
Selection and Sizing
Sizing is set by cargo compatibility as much as by hydraulic duty, since the wrong material choice fails long before the pump reaches its rated flow:
- Rated capacity (m³/h) and total dynamic head at the design discharge rate to shore or to a cargo manifold
- Cargo compatibility of wetted materials, seals and shaft bearing bushes with the specific cargo grades the tank carries
- Number of stages and NPSH available at minimum tank level, since deepwell pumps must maintain suction as the tank empties
- Stripping capability and the residual quantity it can achieve, which matters for grade-critical chemical cargoes
Regulations and Class
Cargo pump installations on tankers fall under SOLAS Chapter II-2 fire safety requirements for cargo pump rooms and, for chemical tankers, the IBC Code requirements on cargo containment and segregation that drive the independent-pump-per-tank arrangement in the first place. Class rules require periodic tightness testing of the shaft seal at the deck penetration and inspection of the pump room ventilation and gas detection systems that protect against vapour accumulating around the top unit. Cargo piping and pump certification is also checked against the vessel's certificate of fitness for the cargoes it is approved to carry.
Typical Faults
| Fault | Cause | Consequence |
|---|---|---|
| Line shaft bearing wear | Cargo with poor lubricating properties, running dry during stripping | Shaft vibration, eventual seizure requiring full pump withdrawal from the tank |
| Deck seal leakage | Worn mechanical seal faces, misalignment after reinstallation | Cargo vapour escape at deck level, gas hazard around the top unit |
| Loss of suction / cavitation | Low tank level below NPSH margin, blocked suction strainer | Reduced discharge rate, pitting damage to impellers over repeated occurrence |
| Cross-contamination between grades | Incomplete stripping or line clearing between cargoes | Off-spec cargo, rejected discharge, commercial claim |
What to Look for in a Supplier
- Material compatibility charts for the specific cargo list the vessel trades, not a generic chemical resistance table
- Documented stripping performance and residual quantity figures for comparable tank geometry
- Availability of line shaft bearings and mechanical seal kits as stocked spares, since these are the parts that wear fastest
- Experience servicing pumps of this depth and stage count, since deepwell units longer than the standard range need column handling equipment at overhaul
Never run a deepwell pump into stripping mode without confirming actual tank level against the gauge — a false-empty reading and a dry line shaft bearing is one of the fastest ways to write off a pump mid-voyage.