What to check on a Cargo Pump (Screw).
A screw pump is a positive-displacement machine: two intermeshing screws rotate inside a close-fitting casing and physically carry cargo from suction to discharge in a series of sealed pockets, rather than relying on centrifugal force to impart velocity. That difference matters most on cargoes a centrifugal pump handles poorly — heavy fuel oil, bitumen, vegetable oils, molasses and other viscous or shear-sensitive products. A centrifugal pump's output collapses as viscosity rises; a screw pump's output stays close to constant across a wide viscosity range and a wide range of discharge…
What makes a screw pump the right choice for difficult cargoes
A screw pump is a positive-displacement machine: two intermeshing screws rotate inside a close-fitting casing and physically carry cargo from suction to discharge in a series of sealed pockets, rather than relying on centrifugal force to impart velocity. That difference matters most on cargoes a centrifugal pump handles poorly — heavy fuel oil, bitumen, vegetable oils, molasses and other viscous or shear-sensitive products. A centrifugal pump's output collapses as viscosity rises; a screw pump's output stays close to constant across a wide viscosity range and a wide range of discharge pressure, and because the cargo is carried rather than thrown through an impeller, shear-sensitive products suffer far less mechanical damage passing through it.
Main components
Twin intermeshing screws
Precision-machined rotors that trap cargo in sealed cavities as they turn, moving it steadily along the pump axis with minimal internal recirculation.
Timing gears
Keep the two screws synchronized without the rotors actually touching each other, so wear is taken up by the gears and bearings rather than by contact between the screws themselves.
Mechanical seal
Seals the rotating shaft where it exits the casing; seal material selection matters more here than on many other pump types because screw pumps are so often chosen for aggressive or abrasive cargoes.
Relief valve
A mandatory fitting on any positive-displacement pump: because the pump will keep building pressure against a closed downstream valve rather than simply losing flow the way a centrifugal pump does, a relief valve routed back to suction or a tank is required to prevent the discharge line from being over-pressurized.
Selection and sizing
| Parameter | Why it matters |
|---|---|
| Viscosity range | The main reason to choose a screw pump over a centrifugal in the first place |
| Flow at rated speed | Positive-displacement output is largely independent of discharge pressure, unlike centrifugal output |
| Self-priming capability | Screw pumps generally prime themselves and tolerate some entrained air, useful on stripping duty |
| Relief valve setting | Must be matched to the pipeline's actual pressure rating, not left at a generic factory default |
| Shear sensitivity of the cargo | Determines how tight the internal clearances and rotor speed need to be kept |
Regulations and class
Class rules require a relief arrangement on every positive-displacement cargo pump for the reason above, and where screw pumps are used for chemical cargoes the IBC Code's material compatibility and segregation requirements apply just as they do to any other pump type on that trade. There is no separate SOLAS chapter specific to screw pumps; the applicable rules are the general cargo system and pump room requirements that cover the vessel type.
Typical faults
- Timing gear or bearing wear from years of abrasive cargo, gradually increasing internal clearance and reducing volumetric efficiency even though the pump still turns and produces some flow
- Seal failure on aggressive or hot cargoes, a common cause of pump room entry on ships trading heavy or chemical products
- Relief valve seizure from a viscous or waxy cargo solidifying inside it, defeating its purpose exactly when a blocked line needs it most
- Rotor damage from tramp metal or debris entering the suction line, since the close clearances that make a screw pump efficient also make it intolerant of foreign objects
What to look for in a supplier
- Documented performance across the vessel's actual viscosity and temperature range, including the coldest cargo the ship is likely to load
- Seal materials proven against the specific cargo list, not a generic elastomer selection
- Relief valve sized and certified for the installed pipeline pressure rating
- Spare rotor and bearing sets held on board or readily available, since a worn rotor set is the main driver of falling output over time
Never run a screw pump against a closed discharge valve and assume the relief valve will save the line — check that valve is free and correctly set at every survey, because a seized relief valve turns a routine blockage into an overpressure event.