Auxiliary machinery: pumps, compressors, separators, heat exchangers, steering gear and deck machinery
This domain tests the machinery that keeps propulsion alive: pumps for cooling, bilge and ballast, compressors for starting and control air, separators for fuel and oil, heat exchangers for temperature control, and the steering and deck machinery that make the ship operational. Questions are usually symptom-based, so learn each machine by its failure modes.
Pumps
- Centrifugal pumps follow Q ∝ N, H ∝ N², P ∝ N³. Throttle the discharge, never the suction; cavitation from low NPSH shows as noise, vibration and impeller erosion.
- Positive displacement pumps need a relief valve on the discharge and must never run against a closed valve; they are self-priming but sensitive to dry running.
- Bilge and ballast systems rely on non-return valves, clean strums and correct valve line-up.
Compressors and separators
- Multi-stage reciprocating compressors use intercoolers and moisture separators; each stage carries its own safety valve, and unloaders allow starting without load.
- Oil and water carry-over damages starting air valves and creates oil-mist risk in air lines, so drains are blown regularly.
- A purifier removes water using a gravity disc; a clarifier removes solids only and needs no gravity disc. Excessive throughput reduces separation efficiency, and wrong back pressure prevents the bowl sealing.
Heat exchangers, steering gear and deck machinery
- Heat transfer depends on LMTD and fouling factor; plate units are compact and highly effective; sacrificial anodes protect the seawater side.
- SOLAS steering gear: two independent power units; with both running, 15° one side to 30° the other in 28 s, and 60 s with one unit. Rudder limit is 35°.
- Deck machinery: windlass, mooring winches with auto-tension, tested brakes, and warping drums used only with the brake applied.
Worked example
A seawater pump at 50 Hz delivers 80 m³/h at 45 m head absorbing 12 kW. At 45 Hz, speed ratio is 0.9: flow = 80 × 0.9 = 72 m³/h, head = 45 × 0.81 = 36.5 m, power = 12 × 0.729 = 8.7 kW. The reasoning is that flow follows speed, head its square and power its cube, but the prediction is only accurate where the system has little static head.
Traps
Throttling a centrifugal pump suction; omitting the relief valve on a displacement pump; fitting the wrong gravity disc; applying the 28 s steering figure with one power unit running; assuming affinity laws hold exactly against a high static head.