What to check on a Submerged Cargo Pump.
A submerged cargo pump, the type most associated with the Framo system, puts the whole drive unit down in the tank with the pump itself: a hydraulic motor, fed by oil pumped down from a power pack on deck, sits directly above or alongside the centrifugal stages at the tank bottom, with no long line shaft running the height of the tank. That is the opposite arrangement to a deepwell pump, which keeps an electric motor on deck and drives the stages through a shaft supported by product-lubricated bearings all…
What sets a submerged pump apart
A submerged cargo pump, the type most associated with the Framo system, puts the whole drive unit down in the tank with the pump itself: a hydraulic motor, fed by oil pumped down from a power pack on deck, sits directly above or alongside the centrifugal stages at the tank bottom, with no long line shaft running the height of the tank. That is the opposite arrangement to a deepwell pump, which keeps an electric motor on deck and drives the stages through a shaft supported by product-lubricated bearings all the way down. Removing the line shaft removes its bearings and alignment issues, and because the drive is hydraulic rather than electric, pump speed and therefore flow rate can be varied smoothly by throttling hydraulic oil flow rather than needing a separate frequency converter. The trade-off is that hydraulic oil is now circulating through lines that run down into the cargo tank, so preventing that oil from ever mixing with the cargo becomes a design requirement in its own right.
Main components
Hydraulic power pack
An electric motor on deck drives a hydraulic pump, building the flow and pressure that is distributed to one or more submerged pump units through a manifold. A single power pack commonly supplies several tanks, giving the arrangement some redundancy if one power pack fails.
Supply and return lines
Hydraulic oil travels down to the submerged unit and back through dedicated piping run inside the tank, typically double-walled or otherwise arranged so a line failure cannot let hydraulic oil escape directly into the cargo or vice versa.
Submerged pump unit
The hydraulic motor and centrifugal pump stages form a single compact unit at the tank bottom, connected to deck only by the hydraulic lines, a discharge riser pipe and control or monitoring cables, with no rotating shaft passing through open cargo space.
Selection and sizing
| Parameter | What it decides |
|---|---|
| Hydraulic pressure and flow available from the power pack | Sets maximum achievable pump speed and therefore capacity |
| Number of pump units per power pack | Trades redundancy against deck space and piping complexity |
| NPSH at minimum tank level | Limits stripping depth before cavitation, similar to a deepwell pump |
| Variable speed range | Allows one pump size to cover both high-rate discharge and slow, controlled stripping |
Regulations and class
Wetted materials and cargo compatibility follow the same IBC Code cargo list logic as any other cargo pump on chemical tankers, and class rules for hydraulic piping running through cargo tanks require arrangements, generally double containment or an equivalent barrier, that prevent hydraulic oil and cargo from mixing in the event of a single line failure, following IMO guidance issued after incidents where hydraulic oil contaminated cargo. Class surveys examine the hydraulic system's pressure testing and leakage monitoring records alongside the pump's own performance record.
Typical faults
- Shaft seal wear on the submerged hydraulic motor letting hydraulic oil leak into the cargo, a contamination event that can condemn an entire tank's cargo
- Hydraulic line fatigue cracking from vibration where lines are not adequately clamped over their full run down the tank
- Power pack overheating under sustained high-flow discharge if the oil cooler is undersized or fouled
- Cavitation from throttling the pump too aggressively at low tank level instead of reducing hydraulic flow
What to look for in a supplier
- Documented double containment or equivalent barrier arrangement for hydraulic lines through the cargo tank
- Leakage monitoring or oil-in-cargo detection fitted to the system, not left as an assumed feature
- Interchangeability of submerged pump units across the fleet's tank sizes to simplify spares holding
- Service history and hydraulic oil specification matched to the temperature range of the cargoes actually carried
Treat any unexplained hydraulic oil level drop on the power pack as a cargo contamination risk first and a pump fault second; by the time oil shows up in a cargo sample, it has usually been leaking for a while.