Marine Auxiliary Machinery and Systems: Pumps, Purifiers, Boilers, Steering and Refrigeration
This domain tests the machinery that keeps the main engine running: pumps, fuel and lube-oil purifiers, boilers and feed systems, air compressors, steering gear, and refrigeration and air-conditioning plant. Questions are typically about operating limits, correct adjustment, and what a given symptom tells you.
Core rules
- Centrifugal pumps deliver flow against a system head curve; throttling discharge moves the duty point, and running a centrifugal pump against a closed discharge causes overheating and seizure.
- Positive-displacement pumps must have a relief valve protecting a blocked discharge.
- Purifier separation depends on the gravity disc bore, correct back pressure in the clean-oil outlet, and fuel temperature that fixes density and viscosity.
- SOLAS requires the main steering gear to move the rudder from 35 degrees on one side to 30 degrees on the other in not more than 28 seconds at maximum ahead service speed, with two independent power units; the auxiliary gear must give 15 degrees in 60 seconds.
- Air entering a refrigeration circuit collects in the condenser, raising discharge pressure and reducing capacity; it is purged, never tolerated.
Worked example
A cooling-water pump rated 100 m3/h at 1200 rpm and 40 m head is slowed to 900 rpm. Speed ratio is 0.75, so flow becomes 75 m3/h, head becomes 40 x 0.75 squared = 22.5 m, and absorbed power becomes 0.75 cubed = 0.422 of the original. The head reduction, not the flow, is what stalls a cooling system sized on the original duty.
Traps
Assuming a centrifugal pump holds flow when discharge head rises; selecting the gravity disc by fuel grade name rather than by density at the working temperature; and diagnosing a low boiler water level from the gauge glass without first blowing it through.
Steering questions often ask which single failure still leaves compliance, so know the two-power-unit arrangement.