What to check on a Deepwell Cargo Pump.
In a deepwell cargo pump, the driving motor sits on deck, well clear of the cargo, and a long line shaft runs down through the tank to a multistage centrifugal pump bowl assembly near the tank bottom. This is the reverse of a submersible cargo pump, where the motor itself is submerged in the cargo inside a pressure-tight canister. The deepwell arrangement keeps electrical components out of the cargo entirely, which matters for tankers carrying products where any risk of an ignition source inside the tank is unacceptable, and it…
What sets a deepwell pump apart from a submersible cargo pump
In a deepwell cargo pump, the driving motor sits on deck, well clear of the cargo, and a long line shaft runs down through the tank to a multistage centrifugal pump bowl assembly near the tank bottom. This is the reverse of a submersible cargo pump, where the motor itself is submerged in the cargo inside a pressure-tight canister. The deepwell arrangement keeps electrical components out of the cargo entirely, which matters for tankers carrying products where any risk of an ignition source inside the tank is unacceptable, and it allows the pump bowls and shaft to be pulled for overhaul through the pump room or a deck trunk without gas-freeing and entering the tank itself, unlike a submersible unit that has to be lifted clear through its own deck fitting.
Main components
Driving motor and top plate
Mounted on deck or in the pump room, coupled to the line shaft through a mechanical seal or a hydraulic drive arrangement that keeps any vapour from the tank away from the motor's electrical parts.
Line shaft and bearings
A long shaft, often in several sections with intermediate line-shaft bearings lubricated by the cargo itself or by a separate system, transmits torque from the deck-mounted drive down to the pump bowls. Shaft length and the number of bearings scale with tank depth.
Pump bowls
A multistage centrifugal stack near the tank bottom, sized to develop the head needed to lift cargo from the tank to the manifold against the full static height and pipeline friction loss.
Suction bell and strum box
A strainer at the pump inlet keeps debris and sediment out of the bowls, and the suction bell is set close to the tank bottom to minimise the unpumpable residual left after stripping.
Selection and sizing
- Capacity and head must match the cargo's viscosity range and the discharge rate the loading terminal or charter party requires within the allowed time.
- NPSH available at the pump bowls falls as the tank empties, so the pump's stripping performance at low tank level is as important as its rated capacity at a full tank.
- Material selection for wetted parts must suit the full range of cargoes the vessel is certified to carry, since a stainless or duplex specification for chemical cargoes differs from a standard bronze or cast steel unit for crude or clean petroleum.
- Line shaft length is fixed to the specific tank depth, so a deepwell pump is not a stock item interchangeable between tanks of different depths without re-engineering.
Regulations and class
Cargo pump installations on tankers fall under the vessel's cargo system survey regime, with pump room and tank entry procedures governed by class and by SOLAS and MARPOL requirements for gas-freeing and enclosed space entry before any bowl or shaft work inside the tank becomes necessary. Where cargoes fall under IBC or IGC Code carriage requirements, the pump's material compatibility and any vapour-tight shaft seal arrangement are checked against the specific cargo list the vessel is certified to carry, not just a generic petroleum specification.
Typical faults
| Fault | Cause | Consequence |
|---|---|---|
| Loss of suction near end of discharge | Insufficient NPSH as tank level drops below the pump's stripping limit | Cavitation, incomplete cargo discharge, high remaining on board |
| Line shaft bearing failure | Loss of cargo lubrication or misalignment after a repair | Vibration, shaft seizure, pump out of service until pulled |
| Reduced discharge rate | Worn wear rings or eroded impellers in the bowl stack | Longer discharge time, potential demurrage exposure |
| Seal leakage at deck plate | Mechanical seal wear at the top of the line shaft | Vapour or cargo leakage on deck, potential cargo loss and hazard |
What to look for in a supplier
- Bowl and shaft material certificates matched to the vessel's actual cargo list, not a generic specification.
- Documented stripping performance at low tank level, since this is often the operationally limiting figure rather than rated full-tank capacity.
- Availability of line shaft sections and bearings as spares, since these are the components most exposed to wear over a pump's service life.
- Pull-and-overhaul procedure compatible with the vessel's pump room or trunk arrangement, confirmed at the specification stage rather than discovered at the first overhaul.
Track discharge rate against tank level on every cargo operation a gradual fall-off in rate at a consistent tank level, watch after watch, is the earliest sign of bowl wear, well before the pump fails to strip.