Marine Auxiliaries Oral Exam Questions — Pumps, Air, Purifiers, Boilers & Steering

Why no relief valve on a centrifugal, but mandatory on PD pumps?

Centrifugals slip and churn at shut head with nowhere for pressure to build. PD pumps displace regardless — trapped liquid bursts casings, so discharge relief bypasses to suction.

Centrifugal characteristics? Why not making pressure after overhaul?

Flow rises with speed while head rises with speed squared — throttling discharge raises pressure, starving suction destroys it. No pressure means running backwards, air-bound casing, worn wear ring, choked suction or relief passing: prove rotation, prime, gauges, then open the suction path.

Cavitation — how do bubbles form, where does damage land?

Starved NPSH drops local pressure below vapour point, bubbles form and implode on the impeller eye — noise, vibration, pitting eaten exactly where they collapse. Cure is suction head and flow, never discharge throttling alone.

Wear ring purpose, where fitted? Checks after overhauling a centrifugal?

Sacrificial clearance seal between impeller and casing against recirculation — renewed when clearance exceeds book. After overhaul: rotation, priming, pressures, seal dryness, current, vibration, then performance against baseline. A PD pump after overhaul adds relief-lift proof and timed delivery check.

Shell-and-tube leak test? Backflush valve purpose?

Covers off the seawater side, circulate the shell-side fluid and watch dry tube ends weep — halo test for traces, pressure test to confirm. Backflush reverses inlet flow to expel debris without opening the cooler.

Expansion in shell-and-tube vs plate units? Heat exchanger types?

Shell-and-tube grows along floating heads, bellows or U-bends. Plate packs absorb growth in sliding plates and bellows clearances, tightened to book dimension — one uneven pull warps plates for good. Families: shell-and-tube, plate, tubular and air-cooled.

Expansion tank in jacket cooling — why? JCW expansion bellow?

The tank takes thermal expansion, holds system head for pump NPSH, vents air and receives makeup — level falling means leak, rising means gas or oil ingress. Bellows in the piping flex with that same growth so flanges never carry it.

Why multistage compression with inter and aftercoolers?

Single-stage heat would cook oil and kill density — stages with cooling between keep each ratio manageable and delivery cool and dense. Intercooler between stages, aftercooler before the bottle.

Intermediate-stage pressure — how calculated?

Geometric mean of suction and delivery for two equal stages — each stage shares the ratio so work and heat split evenly. Quote book pressures, never derive them in the viva.

Bumping clearance — check and adjust?

Lead wire crushed at TDC, mic'd against book clearance (order of half to one percent of bore), shimmed true — too tight strikes, too loose bleeds efficiency. Failed bumping on one stage shows as that stage running hot with low delivery.

Interstage relief lifting means what? Leaky suction vs discharge valves?

Next stage refusing air (valve passing, cooler choked) or this stage over-delivering. Leaky suction blows back to the inlet filter; leaky discharge overheats the head and drops delivery — feel the valve pockets, read the stage gauges.

Bursting disc — purpose, position, your action on rupture?

Sacrificial copper disc in cooler water spaces against tube-burst pressure. Ruptured: stop, renew identical spec, find the tube failure — one capped set gets you through manoeuvring only.

Compressor safeties? Air-side safeties? Fusible plug material?

Stage reliefs, bursting discs, 120 °C-class fusible plugs in white metal, LO, water, motor trips with HP, LP and air-temperature alarms. Air side adds bottle reliefs, drains and melting-plug venting — air released goes to atmosphere, never bottled into the engine room.

Unloader uses? Start with a dead unloader?

Vents HP delivery on start so the motor spins up unloaded, drains moisture at stops, blows condensate on timer. Dead unloader: start with the discharge cracked to atmosphere and load gently once running — then repair it.

Safeties on the bottle? Why water in the bottle?

Relief valve, pressure gauge, drain, fusible plug, stop valves with slow-opening procedure. Water is condensed charge moisture plus cooler carryover — drained on watch, because water in starting air hydraulic-locks start valves.

Compressor valve assembly — parts?

Valve plate, suction and delivery reeds or plates with springs, seats, lift limiters and gaskets — lapped seats, free lift, correct springs or the stage breathes backwards.

Purifier vs clarifier? Separation principle?

Purifier splits two liquids across an interface with a gravity disc; clarifier strips solids seal-less with no disc and no interface. Spin sorts by density across the disc stack — heaviest to the wall, clean oil to the centre.

Gravity disc without the chart? Oil density vs disc diameter?

Start largest, shrink till the seal holds — interface outward means too big, inward means too small. Disc diameter runs inverse to oil density: denser oil needs a smaller hole, read off density with temperature and throughput.

Purifier overflowing — checks? Back-pressure valve where and why?

Disc wrong, throughput too high, temperature low, sludge space full, seal lost — cut feed, check interface side, desludge, correct disc. Back-pressure valve sits on the clean-oil outlet to keep the paring disc dipped so it can pump onward.

Friction clutch purpose? Purifier safeties?

Slip-drive run-up protecting motor and gears — worn or oily pads show as slow speed on the revolution counter. Safeties: overspeed trip, vibration switch, high-temperature and low-pressure alarms, hood interlock, operating-water proving.

FWG vacuum, ejector, salinometer in one breath? Vacuum not coming?

Ejectors hog air out to high vacuum so jacket-heat water boils near 50 °C; brine and air ejectors sustain it; the salinometer diverts off-spec distillate at its sample valve. No vacuum: air leaks, ejector nozzle worn or motive steam weak, condenser scaled, overboard choked — prove steam, then hunt air.

Low FWG output? Distillate pump NPSH?

Low vacuum, scaled evaporator, weak heat, high salinity dump, air ingress — read salinity first, vacuum second. The distillate pump sucks from a vacuum chamber, so its NPSH comes from the condenser's static head and a sealed suction — any air leak kills it.

TEV role? Back-pressure valve in the reefer plant — where, why?

TEV meters feed to hold 3–6 °C superheat at the evaporator outlet. The back-pressure (evaporator-pressure) valve holds warmer rooms at their higher setpoint while colder rooms pull down — one compressor, different room temperatures.

Under vs overcharge? Moisture and air signs?

Under: low pressures, glass bubbles, low amps, warm rooms, endless running. Over: high condenser level with HP trips. Air: high discharge with jumping needle. Moisture: TEV ice starvation with HP climbing — evacuate, renew drier, recharge by weight.

Short cycling — causes? Fridge running continuously?

LP-chatter rapid start-stop from low charge, wrong cut-in/out, blocked strainer, leaking valves or drier. Continuous running is the mirror: short of gas, iced coil, weak compressor, door or insulation failure, condenser starved of cooling.

Fridge LO reading vs air compressor? Gauge temperature scales?

Fridge reads running differential (pump minus crankcase), air reads absolute delivery — different datums. Dual-scale gauges map saturation pressure to temperature for instant superheat reading.

Belt drive on the reefer compressor — why? Defrost drains — gooseneck speciality?

Belts cushion motor drive cheaply with slip as built-in overload warning. Defrost water drains through a gooseneck loop that traps cold-room air inside while letting water out — no straight pipe breathing warm moist air back in.

LO sampling done right? Tests on LO?

Sample hot running oil mid-stream into sterile bottles, labelled and lab-bound — never the drain dregs. Lab reads viscosity, TBN/TAN, water, insolubles, metals and flash point; onboard: crackle, blotter and density.

Why treat boiler water? Chloride limit? Blowdown purpose and destination?

Treatment stops scale, corrosion and carryover — scale insulates, chlorides pit, oil films overheat tubes. Chlorides capped in low ppm per book. Bottom blows sludge, surface (scum) blows oil and foam — to the blowdown tank, never bilges.

Boiler water tests? Reduce chloride content? Oil in the boiler?

Chlorides, pH, alkalinity, phosphate/sulphite reserve, conductivity — cooled samples at the salinometer valve. Cut chlorides by blowdown plus clean feed; cut carryover by lowering level and load. Oil means stop firing, find the condenser or heater leak, blow down and boil out — oil blanketing overheats tubes to failure.

Foaming vs priming?

Foaming is surface froth from oil, salts and high alkalinity; priming is water carried bodily with steam from high level or violent load swings — both wet the steam line, so treat water, hold level, change load gently.

Safety valve setting, anti-tamper, accumulation test?

Set within about 3% over working, locked and seal-wired with easing gear for manual lift. The accumulation test proves full-fire pressure stays within 10% — the valve's capacity certificate in action. EGB valves set above the auxiliary drum's.

Gauge glass blow-through and breakage safety? Why two glasses, ball on water side only?

Steam then water cocks through the drain, refill water-first, steam back last. The water-side ball checks slam shut on fracture — no ball on steam side because steam flow alone must never slam it. Two local glasses give one always-readable level; guards and remote handles keep crew clear.

EGB circulating pump — why before the engine? Soot-fire care? Uptake fire?

Water must already move when exhaust heat arrives or tubes dry-fire — pump starts before the engine, stops after. Soot-blow running, water-wash in port, bypass at low load against sticky fouling. On uptake fire: stop soot-blowing, keep water circulating, starve air, boundary-cool — never open hot casing to air.

EGB leaking — how known, how stopped?

Makeup rising, header pressure falling, steam in the casing drains, white plume at the funnel. Prove by isolating and pressure-testing; stop by bypassing the gas side or plugging tubes per manual, then repair at the stop.

Pre-purge before firing? Flame failure then? Blow-back causes?

Purge clears unburnt fuel-air bombs from the furnace before the spark. Flameout trips fuel instantly with post-purge — relight only after finding why it died (fuel, air, atomiser, flame eye). Blow-back is delayed ignition hurling flame outward: purge short, atomisation bad, timing wrong — purge fully and prove fuel shut before every light.

Boiler mountings? Pressure testing when and how?

Safety valves, main stop, feed checks, gauge glasses, pressure gauges, blowdown, scum, salinometer, air release — each with its own isolation. Hydraulic test after repair or at survey interval to book pressure with hatches and mountings proved tight.

Economiser vs boiler? Bourdon gauge?

An economiser recovers exhaust heat without its own firing or steam space; a boiler fires (or EGB-heats) its own drum to raise steam. A Bourdon gauge reads pressure through an oval tube straightening under pressure to drive the needle.

Steering safeties and safematic behaviour?

Twin units, auto-changeover, relief, bypass and isolation valves, failure and low-level alarms, local trick-wheel backup — any single failure leaves steerage, hence safematic logic. Emergency steering is the aft trick wheel with communications proved.

Rudder types? Semi-balanced advantage?

Unbalanced (stock aft), semi-balanced (part of the area forward), balanced or spade (area both sides). Semi-balanced trims torque without full spade complexity — lighter gear, still responsive helm.

Main vs auxiliary governors? Overspeed trip check?

Main governors hold propeller-law speed with droop; auxiliary governors hold fixed-frequency isochronous speed. Overspeed trips the rack above rated speed and latches — proved by manual test rig per schedule.

Two springs on auxiliary valves — why? What checked at overhaul?

Nested pair fights surge at speed and still closes the valve if one breaks. Check free length, squareness, cracks and seat pressure against book — renewed as a pair.

Why two inlet and two exhaust valves on a generator head?

Four small valves breathe more area than two big ones with lower lift, lighter springs and cooler seats — better filling, faster closing, surviving one burnt seat.

Lantern ring — what? Sacrificial anodes — where, maintenance?

Lantern ring is the lantern (lantern-ring) spacer in a packed gland admitting sealing or cooling fluid between packing sets. Anodes bolt in coolers, condensers and ballast-side chests — renewed when wasted past book percentage, never painted.

Rudder bearing clearance — why limited?

Excess clearance hammers the stock, leaks the seal and wanders the helm — measured at drydock against book limits, shimmed or bushed true.

Six temperature methods? O2 analyser calibration?

Mercury and gas expansion, bimetallic, resistance (RTD), thermocouple, pyrometer and fusible or colour-change sticks — contact against remote-reading split. Calibrate an O2 analyser on reference gas (air at 20.9%) per manual before trusting its flue reading.