What to check on a Screw Pump.
A screw pump is a positive displacement pump that moves liquid axially along meshing helical rotors (typically one power rotor driving one or two idler rotors) instead of trapping it between gear teeth or sliding vanes. The rotor geometry gives an almost pulsation-free, continuous flow, which matters for fuel injection supply and transfer duties where pressure spikes cause noise and wear elsewhere in the system. Compared with a gear pump, a screw pump tolerates a wider viscosity range with less efficiency loss, which is why it is the standard choice…
What sets a screw pump apart
A screw pump is a positive displacement pump that moves liquid axially along meshing helical rotors (typically one power rotor driving one or two idler rotors) instead of trapping it between gear teeth or sliding vanes. The rotor geometry gives an almost pulsation-free, continuous flow, which matters for fuel injection supply and transfer duties where pressure spikes cause noise and wear elsewhere in the system. Compared with a gear pump, a screw pump tolerates a wider viscosity range with less efficiency loss, which is why it is the standard choice for heavy fuel oil (HFO) transfer and supply, where viscosity can swing several hundred centistokes between a cold start and normal running temperature. Screw pumps also run quieter and with less shaft vibration than gear or vane types at the same flow and pressure, an advantage in engine rooms with pumps mounted close to accommodation spaces.
Main components
Power rotor and idler rotor(s)
The power rotor is driven directly by the motor; idler rotors mesh with it and are turned only by the fluid film and rotor contact, not by a separate gear train in most designs, which keeps the pump simple and reduces parts subject to wear.
Housing / stator
Precision-bored housing that the rotors run in with close clearance; this clearance is what determines volumetric efficiency and how much internal slip, and therefore capacity loss, occurs at a given viscosity.
Mechanical seal or packed gland
Seals the shaft where it passes through the housing; mechanical seals dominate on modern fuel and lube oil screw pumps for tighter leak control.
Relief valve
Fitted internally or on the discharge line, since a positive displacement pump will build pressure indefinitely against a closed valve and must have a path to relieve it.
Selection and sizing
- Viscosity range - the single most important selection parameter for fuel duty; the pump must maintain rated flow across the full range from cold-start to operating temperature.
- Differential pressure - fuel supply pumps typically run at several bar; boosting or circulating pumps in HFO systems can run higher against injection system back-pressure.
- NPSH available - suction conditions from the service tank, especially on transfer duty pulling from a nearly empty tank.
- Rotor material and clearance - tighter clearances raise volumetric efficiency but reduce tolerance to solids or wear debris in the fluid.
Regulations and class
Fuel oil supply and transfer pumps fall under the classification society's machinery rules for essential auxiliary systems, which typically require duplication of fuel supply pumps to the main engine so a single pump failure does not stop propulsion. Class also reviews the relief valve setting and pressure rating of the pump and its piping as part of the fuel system approval. There is no IMO instrument specific to screw pumps; requirements apply to the fuel and lube oil systems they serve, under MARPOL Annex I for oil-handling arrangements and under class rules for redundancy of essential services.
Typical faults
| Fault | Consequence |
|---|---|
| Rotor and housing wear (increased clearance) | falling volumetric efficiency, especially noticeable on light, low-viscosity fuel |
| Mechanical seal failure | fuel or oil leakage at the shaft, a fire and slip hazard in the engine room |
| Relief valve stuck or incorrectly set | either nuisance recirculation losing capacity, or overpressure of piping if stuck shut |
| Cavitation on suction | noise, rotor pitting and falling output, usually from a restricted suction line or a nearly empty tank |
What to look for in a supplier
- Published performance curves across the actual viscosity range the fuel or oil will see in service, not just at a single reference viscosity.
- Rotor and housing material suited to the fluid, including compatibility with low-sulphur and biofuel blends now common in fuel systems.
- Spare rotor sets and seal kits genuinely available for the model, since screw pump rotors are precision parts not produced by third parties.
A screw pump that loses capacity is almost always wear, not a blockage - rising slip through worn rotor clearance shows up first as reduced flow at normal running temperature, well before any alarm trips.