Equipment Guide · Deepwell / Hydraulic Cargo Pumps (Framo type)
On a product or chemical tanker there is no pumproom at all. Instead, every cargo tank carries its own submerged centrifugal pump at the tank bottom, spun by a hydraulic motor fed with high-pressure oil from a power pack on deck — and between the cargo and the oil sits a purgeable void, the cofferdam, that turns every seal leak into something you can see. Tap a step or press "Play" to follow the power down the pipe stack and the cargo up — then take the simulator and strip a tank yourself without dry-running the seal.
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Hydraulic power is flow times pressure — in the units a deck officer actually sees: P (kW) ≈ Q (l/min) × Δp (bar) ÷ 600. Worked backwards: a pump discharging 300 m³/h of SG 0.80 cargo against 100 m of head is lifting about 65 kW of cargo. Allow roughly a quarter for motor slip and pipe friction (η ≈ 75%) and the oil must deliver ≈ 87 kW — at 250 bar, that is ≈ 210 l/min of hydraulic oil going down that one pipe stack. Run six pumps like that and the power pack is finding well over half a megawatt; this is why the units cut in and out with demand, and why every pump slows when you ask the main for more than it has.
You have one deepwell pump on one tank. The STC speed sets how fast the impeller turns; the backpressure is what the shore line asks of it. Head falls with speed² — too slow and the pump cannot reach the manifold at all; too fast on a low level and the well cannot feed the impeller: it cavitates and runs the cargo seal dry. Wear is invisible while you pump — only the purge check shows what the seal has been doing. Strip the tank slow, then purge and read the outlet.
What hurts the cargo seal, in order — low level at speed >60% · cavitation (well starved) · dry run (empty tank, pump turning). Each one you allow adds hidden wear; past the limit the seal passes cargo into the cofferdam.
The impeller is flooded and primed at any level — that part is free. What is not free is the end of the discharge: as the level falls the little well can only feed so much, and an impeller asking for more cavitates; an impeller turning in an empty tank runs its cargo seal dry. Strip at low speed (and drop the backpressure so a slow pump can still open the line — head ∝ speed²), then run the purge check: trace = healthy, cargo at the outlet = the seal is telling you it is done.
Strip the tank slow, then purge and read the outlet.
Working principle — explained in our own plain words with our own schematic from general marine knowledge (not copied from any OEM manual). Conceptual; varies by ship/model. Reviewed and verified by a Master Mariner.