Three-Screw Pump

Overview

A three-screw pump is a rotary positive-displacement pump built around a single driven Power Rotor meshing with two undriven Idler Rotors inside the close-fitting trefoil bore of a Liner Sleeve. The intermeshing screw threads divide the liquid into a series of sealed cavities that travel axially from the suction end to the discharge end at constant velocity. Because the cavities neither expand nor contract during transit, delivery is continuous and essentially free of pulsation, which is why three-screw pumps dominate lube-oil circulation, fuel-oil transfer and burner feed, hydraulic elevator drives, and crude-oil pipeline boosting where smooth flow and low noise matter.

The architecture dates to the Imo pump patented by Carl Montelius in the 1930s, and the geometry is still often called an Imo-type profile. Sizes range from small cartridge pumps delivering under 1 m³/h to pipeline units above 250 m³/h at differential pressures of 40 bar and more.

How it works

Torque enters through the Shaft Key on the Power Rotor Assembly. Only the power rotor is mechanically driven; the two idler screws in the Idler Rotor Set are rotated by the hydraulic pressure of the liquid acting on their flanks. The rotor profile is designed so the idlers carry almost no metal-to-metal contact load: they float on a hydrodynamic film of the pumped liquid, which is why the pump can only handle media with some lubricity and must never run dry.

As the screws turn, the meshing line between power-rotor flanks, idler flanks, and the Liner Cartridge bore forms a chain of closed cavities. Each shaft revolution advances every cavity by one lead, so flow is directly proportional to speed. Slip back through the 0.03–0.08 mm running clearances is the only internal loss; it grows with differential pressure and falls steeply with viscosity, giving volumetric efficiencies above 90% on typical lube oils.

Axial hydraulic thrust is the dominant rotor load. On the power rotor it is cancelled by the Balance Piston, a stepped extension that exposes discharge pressure to a balancing area; the small residue is carried by the Thrust Washer and the angular-contact Ball Bearing in the Bearing Housing Assembly. Each idler is floated by its Idler Balance Cup, which feeds discharge pressure under the rotor end; a bronze Idler Thrust Shoe supports the face only during start-up before the hydrostatic film builds.

A useful side effect of the balance piston is that the seal chamber behind it sees only suction pressure. The Mechanical Seal Assembly can therefore be a simple single seal: a carbon Seal Rotating Face spring-loaded against a silicon-carbide Seal Stationary Seat retained by the Seal Gland.

Construction

The Casing Assembly is the pressure boundary. The Casing Body is typically EN-GJL-250 cast iron rated to 40 bar, with an axial suction inlet and a top discharge port; nodular iron or cast steel is used for API 676 builds. The rotor bores are not machined into the casing directly. Instead a replaceable Liner Sleeve with three honed intersecting bores slides into the casing, located angularly by Liner Dowel Pins and sealed with an O-Ring Set. When clearances wear open, overhaul means exchanging the liner cartridge and rotor set rather than scrapping the casing.

Rotors are case-hardened or nitrided steel, ground on the flanks; the power rotor carries a double-start thread, the idlers single-start threads of opposite hand. The Bearing Housing sits outside the pumped liquid, with an Oil Seal on the atmospheric side and a locknut clamping the bearing.

Most units carry an Integral Relief Valve machined into the casing: a spring-loaded Relief Valve Poppet on a hardened Relief Valve Seat that bypasses flow back to suction if discharge is blocked. It is a safety device, not a control valve; sustained bypassing overheats the pump within minutes.

Operating limits and failure modes

The pump is self-priming once wetted, drawing suction lifts to about 5 m. Dry running destroys the hydrodynamic films and gallls the idler flanks within seconds, so low-level and low-pressure interlocks are standard. Abrasive solids wear the liner bore and rotor flanks, opening the slip path; three-screw pumps are unsuitable for slurries. At very high viscosity, speed must be reduced to keep cavity filling complete and avoid cavitation noise; manufacturers publish speed-viscosity derating curves for this purpose. Typical service life between liner-and-rotor overhauls on clean lube oil exceeds 40,000 hours.