What to check on a Deepwell Cargo Pump.
A deepwell cargo pump is a vertical, multistage centrifugal pump whose electric motor sits on deck, in the open air, while the pump bowls and impellers hang at the bottom of the cargo tank on a long line shaft. The motor never touches the cargo; only the shaft, bearings and pump stages are submerged. That is the key difference from a submerged pump of the Framo type, where the drive itself, a hydraulic motor, sits down in the tank next to the pump and only a hydraulic power fluid line…
What sets a deepwell pump apart
A deepwell cargo pump is a vertical, multistage centrifugal pump whose electric motor sits on deck, in the open air, while the pump bowls and impellers hang at the bottom of the cargo tank on a long line shaft. The motor never touches the cargo; only the shaft, bearings and pump stages are submerged. That is the key difference from a submerged pump of the Framo type, where the drive itself, a hydraulic motor, sits down in the tank next to the pump and only a hydraulic power fluid line and control cables run to deck. Deepwell design keeps electrical equipment out of the cargo, which matters for products with a low flash point, but it commits the ship to a long, slender rotating shaft running through the full tank depth, supported only by product-lubricated bearings, and that shaft is the part that fails first if the pump is ever run without cargo covering it.
Main components
Pump bowl assembly
A stack of two to six centrifugal stages, each with its own impeller and diffuser, sized to build the head needed to push cargo from the bottom of the tank to the manifold. More stages mean more head but also a longer, heavier shaft to support.
Column pipe and line shaft
The column pipe both carries the discharge flow up to deck and forms the housing for the line shaft. Intermediate bearings, usually rubber or a composite bearing material, are spaced along the shaft and rely entirely on the cargo itself for lubrication and cooling.
Top drive motor and deck seal
An explosion-protected electric motor, typically flame-proof or increased-safety rated, drives the line shaft from a pedestal on deck. Where the shaft passes through the deck head into the tank, a mechanical or lip seal keeps cargo vapour from escaping into the motor space.
Discharge head and foot valve
The discharge head at deck level connects the pump to the ship's cargo piping and carries the pressure gauge and often a vibration monitor. A foot valve or strainer at the pump's suction prevents debris entering the first stage and can provide non-return protection during stripping.
Selection and sizing
| Parameter | What it decides |
|---|---|
| Rated capacity and head | Matched to tank turnaround time and discharge pressure required at the manifold |
| Number of stages | Sets achievable head; more stages add shaft length and bearing count |
| NPSH available at tank bottom | Limits how low the tank can be stripped before the pump cavitates |
| Wetted material | Stainless or duplex steel for chemicals and IMO Type 2 products, carbon steel acceptable for some petroleum cargoes |
Regulations and class
Cargo pump electrical equipment above and around the tank falls under the hazardous area zoning in SOLAS and IACS rules, and on chemical tankers the pump's wetted materials and cargo compatibility are governed by the IBC Code cargo list for the products the ship is certified to carry. Class surveys check the deck seal, the motor's explosion-protection certificate and the pump's performance test record at each renewal survey.
Typical faults
- Line shaft bearings running dry during stripping, wearing rapidly and increasing vibration until the shaft seizes or breaks
- Deck gland seal leakage, letting cargo vapour into the motor space and risking an ignition source in a hazardous area
- Stage wear ring erosion reducing capacity and increasing internal recirculation, most common on abrasive or crystallising cargoes
- Motor overload from running against a closed valve, since a deepwell pump has no bypass and dead-heads quickly
What to look for in a supplier
- Documented NPSH curve for the specific stage count and speed, not a generic family curve
- Explosion-protection certificate for the exact motor model matching the ship's zone classification
- Line shaft bearing material rated for the cargo grades the ship actually carries, since some elastomers swell or degrade in certain solvents
- Spare parts commonality across the fleet's pump models, given how many tank sizes a single class of ship can carry
Never let the pump run once the suction bell is uncovered during stripping; a few seconds without cargo on the bearings can mean a shaft replacement that keeps the tank out of service for days.