IMO AB Three-Screw
Pump Spare Parts

A driven power rotor meshes with two idler rotors that turn in closely fitted bores. The meshing threads close off a chain of sealed cavities, and those cavities travel from the suction end to the discharge end without changing volume. Nothing is accelerated radially, so flow leaves the pump smooth and almost pulsation-free, torque stays even, and the delivered volume barely moves when discharge pressure changes. That behaviour is why these pumps sit on boiler and diesel fuel supply and booster duty, on main and auxiliary engine lube oil service, on stern tube and gearbox circulation, on hydraulic packs for deck machinery, hatch covers and ramps, and on replenishing and servo duty alongside the piston pumps of a steering gear.

Two construction details govern everything downstream in a spare parts request. The first is hydraulic balance: discharge pressure is led to the back face of each idler so that axial thrust is carried on an oil film rather than by a thrust bearing. The driven power rotor is balanced the same way, by a balance piston or drum at the discharge end, with only the small residual thrust taken by the ball bearing that also locates the shaft. That is why a worn balance piston or balance bush shows up as bearing distress rather than as a flow complaint. Running clearance in the idler bores is therefore not slack to be tolerated — it is a working dimension inside the pressure balance. Once wear opens it, the pump can still shift oil at low backpressure while quietly losing delivery at rated pressure.

The second is the matched set. Power rotor and idlers are ground and matched as one set and carry a common set identity. They are not interchangeable items picked individually from a shelf, which is why an engineer reporting one scored idler is, in supply terms, reporting a rotor set. On the current ACE and ACG Std Line pumps the maker's own component list has no separate liner: the complete power rotor, carrying its balancing piston, and the two idler rotors run directly in the pump body between front and rear covers, with a balancing bush, the shaft seal and one ball bearing beside them. For a centrifugal pump the natural order line is an impeller and a wear ring; for these IMO pumps it is the rotor set with its balancing bush, and a requisition that says insert or cartridge is checked against the sectional drawing of the pump actually fitted.

• Abrasive scoring. Two sources, two stories. Catalytic fines carried over from a purifier that is overloaded, mis-set or bypassed are fine enough to pass any suction strainer and lap the screw flanks and idler bores. Coarse rust and scale from a tank or a line, past a holed or bypassed suction strainer, cut more locally and leave visible tracking. In both cases flow at zero backpressure looks normal while delivery at rated pressure falls away. Rotor set plus bearing surfaces, usually.
• Viscosity collapse. A change to low-sulphur distillate or an over-heated fuel line thins the film separating rotor from liner. Polished bearing surfaces, bright bore marks, gradual pressure decay — often the real cause behind a pump that was "worn out" twice in a year.
• Dry running and air ingress. Seconds of running dry can pick up an idler in its bore and wreck the seal faces. A leaking suction joint produces the same damage more slowly, with noise and a swinging pressure gauge.
• Inlet starvation and cavitation. Cold heavy fuel, a choked filter or an undersized suction line stops the pump filling — at high viscosity the pump cannot draw against the NPSH it needs, and dissolved air and gas come out of solution and collapse. On distillate the same conditions produce true vapour cavitation. Either way the damage sits at the suction end: rattling noise, pitted screw crests and erosion at the liner inlet.
• Continuous relief valve recirculation. Using the integral valve to throttle output sends hot oil round in a loop, thinning it and accelerating every wear mode above. Chattered springs and hammered seats are the visible half of the problem.
• Seal and bearing distress. Fuel dilution, thermal shock and coupling misalignment show up as cracked carbon faces, leakage at the drive end, and ball bearings that spall and howl before they seize. Spalling and cage wear point at misalignment or diluted oil; true brinelling points at a hammered-on coupling or a spell of vibration in lay-up, and is worth reporting separately because it changes what else needs inspecting.

• Full designation. IMO's own sample, ACE 025N3 NVBP, reads in order: series; size, which is the power-rotor diameter in millimetres; rotor lead letter (on the ACE key L or K low, N normal, D high); design generation; body material (N, nodular cast iron); shaft seal (V Lube Line, T Fuel Line); mounting (B flange, F foot); relief valve (P fitted, E none). A non-standard pump carries an extra A-number after that block. Copy the whole string, including trailing letters and any A-number; capacity follows size and lead together, not the size figure alone.
• Serial number. The build identity. Two pumps of the same size figure can carry different rotor sets and seal executions across build years.
• Direction of rotation. Read facing the shaft end. IMO builds these pumps for clockwise rotation as standard and supplies anticlockwise ones only on special request, so an anticlockwise pump is a special build and its full designation and serial matter all the more. Arrows on the pump show the direction. Thread handing fixes which port is suction, and a rotor set of the wrong hand does not fail quietly — it reverses the pump. The suction line becomes the pressure side, so tank line, strainer and filter housing see a pressure they are not rated for, with the built-in relief valve on the wrong side to protect them, the discharge line is drawn down instead of fed, and the shaft seal, which normally runs at suction pressure, can be blown out on the first start.
• Rated viscosity. The viscosity range the rotor and bearing clearances were selected for, in cSt at working temperature. It decides bearing material and clearance grade.
• Rated differential pressure. The bar figure the balance arrangement was built for. It separates otherwise identical frames and governs the relief valve spring.
• Duty and mounting. Fuel oil, lubricating oil or hydraulic; horizontal or vertical; port orientation and coupling type. Photographs of the plate, the drive end and the removed part cover the gaps when the plate is corroded.

Tools for this equipment: the mechanical seal material code for the seal faces and the flange identifier for the suction and discharge flanges. For a pump on lubricating oil, the oil viscosity grade converter gives the viscosity range behind an ISO VG or SAE grade, to set against the viscosity the pump was rated for. Where the rated viscosity on an older data sheet is in SUS, the viscosity units converter gives it in cSt, or in cP with the density of the liquid. For the spring of the integral relief valve, the compression spring calculator works out the rate and the load at the fitted length, so a replacement can be compared with the spring taken out.

With those fields present, a request can be assessed at the right level — matched rotor set, insert, cartridge or individual wear item — instead of returning as a question. Whether the idlers run in separate bushings or directly in the housing bores varies by series and build, so the designation and serial decide that before a line is quoted. Where the plate is corroded past reading, the RFQ workspace accepts a photograph of the pump body, the drive-end bracket arrow and the removed idler, which between them recover rotation, size figure and wear state; how we work covers what is done with that evidence.