Pumps (motor) — Most Asked Questions
33 questions asked about pumps (motor) across the General and Fn 4b and Fn 3 papers, heaviest first. 1 of them are must-master.
Learn the system first
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1 Gear pump overhaul
Also asked: “Fuel pump overhaul”
Answer
A gear-pump overhaul is a clearance job: open up, measure casing-to-gear clearance and backlash, renew worn parts and prove the relief valve.
- Check clearance between casing and gear teeth with a feeler — as it grows, liquid slips back and discharge falls.
- Check backlash for heat expansion with lead wire or feelers; no backlash means damaged teeth and an overloaded motor.
- Meshing gear teeth wear with time and fluid abrasiveness, losing efficiency and pumping ability — renew them on condition, not on hope.
- Remove and pop-test the relief valve at every overhaul; inspect bushes, shafts and seals while open.
Surveyor follow-up: discharge pressure is normal but flow has fallen — clearance or relief valve, and how do you decide in five minutes?
Full theory: Overhaul, Inspection and Repair -
2 Centrifugal pump low discharge reasons
Also asked: “Centrifugal pump discharge low. reasons?”
Answer
Low discharge is starved suction, worn internals or a changed system — vacuum gauge and pressure gauge together name the culprit.
- Suction faults: choked strainer, air leak, high lift, vapour lock or too-small piping — high vacuum with low pressure.
- Internal wear: worn impeller, enlarged clearances, damaged wear rings and rough passages bleed flow back inside.
- System changes: extra resistance, partly shut or wrongly set valves, burst discharge line or higher back-pressure than designed.
- Running faults: wrong rotation after maintenance, low speed, clogged impeller eye or operation far off the best-efficiency point.
Surveyor follow-up: suction vacuum high and discharge low — blockage or air leak, and where exactly?
Full theory: Troubleshooting Index -
3 NPSH?
Answer
NPSH is the suction pressure margin above vapour: available must beat required or the pump cavitates.
- NPSHA equals atmospheric head plus static head minus suction lift, friction losses and vapour head.
- NPSHR is the pump’s own demand — run with NPSHA comfortably above it.
- Low NPSHA comes from high lift, hot liquid, choked suction, vapour lock or heavy friction.
- When stripping with a falling head, throttle the discharge to cut friction losses and hold the margin.
Surveyor follow-up: pump rattles with suction vacuum normal — cavitation or air ingress, and what single reading decides?
Full theory: Suction, NPSH, Cavitation and Priming -
4 Centrifugal pump not developing sufficient pressure, what could be the problems
Also asked: “Centrifugal pump not developing enough pressure reasons ( outside aspects are all fine”, “centrifugal pump not making pressure”, “Centrifugal pump not developing pressure reasons?”
Answer
No pressure means no head built: starved eye, slipping internals, wrong speed or a system that will not let it build.
- Suction side: air leak, choked strainer, high lift, vapour lock or hot liquid flashing at the eye.
- Pump side: worn impeller, eroded clearances, damaged wear rings, wrong rotation or low speed after maintenance.
- Impeller behaviour itself: wrong diameter, damaged blades, wrong eye size or rough finish bleeding efficiency.
- System side: burst discharge line, open bypass, or running far right of best efficiency where head collapses.
Surveyor follow-up: pressure low with ammeter low too — starved pump or open system, and what single valve move proves it?
Full theory: Troubleshooting Index -
5 Why centrifugal pump has wear ring?
Also asked: “Purpose of wear ring in centrifugal pump and how does wearing takes place”
Answer
Wear rings are the cheap sacrificial clearance: they let the impeller spin close to the casing without metal-to-metal wear.
- A tight clearance between impeller and casing stops discharge leaking back to suction.
- Some erosion is certain, so replaceable rings on impeller and casing take the wear instead of the parts themselves.
- As clearances open, leakage grows and efficiency falls — renew the rings, not the pump.
- Sketch it as casing, volute, impeller, shaft, gland and the two rings holding the running gap.
Surveyor follow-up: discharge falling with clearances double the book figure — rings, impeller or speed, and what do you measure first?
Full theory: Centrifugal Pump Construction -
6 Centrifual pump are not self primmed why
Also asked: “Why is centrifugal pump not self priming?”
Answer
Centrifugal pumps cannot suck air: the impeller flings water, not air, so a dry casing never builds suction.
- No water in the eye means no centrifugal head — air slips without lifting the suction column.
- Leaking foot valves, high lift, small suction piping and air leaks keep her dry and rattling.
- Prime by flooding the casing through a priming funnel with vents open until solid water shows.
- Foot valves, flooded suction and self-priming tricks excepted, she starts wet or not at all.
Surveyor follow-up: casing full yet still no discharge — primed or air-bound, and which cock proves it?
Full theory: Suction, NPSH, Cavitation and Priming -
7 Liquid ring pump?
Answer
Liquid-ring pumps prime centrifugals: eccentric star rotor throws a water ring that drags air out of casings and suctions.
- Star rotor offset in its housing spins a sealing water ring round the inner wall.
- Air from the pump casing and suction pipe bleeds into the ring cells and leaves by the discharge.
- Fed with a little sealing water, the primer evacuates tanks and pipes until the main pump floods solid.
- Automatic start-stop on vacuum switches with isolating cocks and non-returns per pump served.
Surveyor follow-up: main pump air-bound with the primer running dry — sealing water, suction leak or primer, and what first?
Full theory: Suction, NPSH, Cavitation and Priming -
8 Why Centrifugal pump efficiency drops?
Also asked: “What is the reason for decrease in eff. of centrifugal pump”, “Reasons for decreasing of pump efficiency”
Answer
Efficiency dies from worn gaps, rough faces, wrong speed and off-design flow: impeller, clearances and curve position decide.
- Impeller velocity, diameter, blade count, eye size, thickness and pitch set the head the wheel can build.
- Worn wear rings and rough internal finish bleed discharge back to suction.
- Running near shutoff or far right of best efficiency burns power as heat, vibration and shaft bend.
- Overrated pumps throttled to duty waste the margin — trim impeller or speed to the system curve.
Surveyor follow-up: efficiency down with clearances correct — impeller, speed or system curve, and what reading proves it?
Full theory: Suction, NPSH, Cavitation and Priming -
9 Checks on centrifugal pump impeller and shaft
Also asked: “Checks to be done on centrifugal pump shaft”
Answer
Open-up checks read impeller, rings, shaft and balance: clearances, erosion, scoring and equal cut-back.
- Impeller-to-ring clearances with bearing play read every opening; renew rings before casings wear.
- Blade tips read for erosion at inlets — thin tips cut back equally all round past 2.4 mm minimum with rounded noses.
- Shafts read for scoring — polish light marks, renew or sleeve deep ones with packing or seals to match.
- Gouges round the volute circle mean misalignment — align, torque and prove before boxing up.
Surveyor follow-up: tips thin on one blade only — erosion, cavitation or rub, and do you cut one or all?
Full theory: Pump Principles and Pump Types -
10 Low sea water pump pressure
Answer
Low seawater pressure is starved suction or tired pump: strainers, leaks, lift and impeller in that order.
- Suction strainer choked, foot valve leaking, lift too high or air leaking into suction piping.
- Suction pipe too small or throttled with pressure falling at the nozzle and seals following.
- Worn impeller and rings bleeding discharge back to suction with amps sagging.
- Prove by suction vacuum against trial figures before opening — strainers first, pump second.
Surveyor follow-up: vacuum high with discharge low — strainer, leak or impeller, and where exactly?
Full theory: Troubleshooting Index -
11 Q3. Indicate preferred material for sea water system in general and for sea water pump parts and precautionary measures for corrosion control
Answer
Seawater wants bronze, CuNi, titanium or ductile iron — never grey iron — with coatings, insulation and cathodic cover.
- Side valves in steel or ductile bronze, spheroidal graphite or Meehanite iron to spec — grey iron banned under shock.
- Pipes and pumps in compatible metals with galvanizing, coatings and cathodic protection where mixed.
- Galvanic series rules joints: titanium noble at top, steel wasting below — insulate or sacrifice accordingly.
- Weld spatter dressed before coating; wastage watched at tank tops, welds and bronze-steel junctions.
Surveyor follow-up: grey-iron sea valve offered as a cheap spare — accept, reject or test, and under what rule?
Full theory: Shipbuilding Materials, Welding and Weld Defects -
12 Sketch and describe the working principle of a double acting reciprocating pump
Answer
Double-acting means both strokes work: piston sucks one side while discharging the other through paired valves.
- Piston sealed by rings with two suction and two discharge valves, one pair each side.
- Stroke one way opens suction behind and discharge ahead; return stroke mirrors the pair.
- Delivery flows twice per revolution — steadier than single-acting with smaller pulsation.
- Sketch crank, piston, four valves with suction and delivery pipes showing flow arrows both strokes.
Surveyor follow-up: one side delivers weak with valves proved — rings, suction or discharge, and what proves it?
Full theory: Pump Principles and Pump Types -
13 In a centrifugal pump , if we use gland packing , how we prevent shaft from erosion
Answer
Glands seal on sleeves, not shafts: fit a renewable sleeve with a lantern ring fed clean sealing water.
- Packing rings sit in the stuffing box against a shaft sleeve that takes the wear instead of the shaft.
- A lantern ring between packing sets feeds sealing water that cools, lubricates and bars air.
- Compress the gland evenly so it slides home without cocking — weeping proves sealing, dryness proves overheating.
- Renew sleeves scored beyond polish with packing; a grooved shaft without a sleeve condemns the sealing plan.
Surveyor follow-up: packing dry and hot with sealing water proved flowing — overtightened, misplaced lantern ring or wrong packing first?
Full theory: Centrifugal Pump Construction -
14 Pump bearing greasing
Also asked: “Centrifugal pump bearing greesing”
Answer
Grease bearings sparingly and separately: lithium for pumps, polyurea for motors, never mixed, never packed.
- Halfway up the lowest ball or roller is full for oil baths — watch the sight glass, not the clock.
- Grease little and often to schedule; excess cooks bearings as surely as starvation.
- Keep lithium pump grease out of polyurea motor grease — mixing kills both films early.
- Teach watchkeepers the indicators: hot housings and weeping seals condemn lubrication before vibration does.
Surveyor follow-up: housing hot with grease purging past the seals — underfed, overfed or wrong grease, and what proves it?
Full theory: Centrifugal Pump Construction -
15 centrifugal pump casing crack repair
Also asked: “Centrifugal pump casing is cracked, what to do?”
Answer
Cracked casings get honesty first: prove the crack, dress and seal small pits, weld or stitch cracks by procedure, renew when condemned.
- Open with marks punched, drained and isolated; renew gaskets and packing on every opening.
- Prove corrosion, pitting and cavitation mapped; Devcon or metal putty only for pits inside limits.
- Weld or stitch cracks with class-approved procedure and survey agreement — pressure-test before return.
- Renew when cracks reach sealing faces, volutes thin past limits, or repairs keep failing the hold test.
Surveyor follow-up: crack ground out and filled with putty on a pressure casing — temporary seal or survey failure, and what next?
Full theory: Centrifugal Pump Construction -
16 Cargo pump safety as per new rule
Answer
Cargo pump rooms sail gas-tight and trip-happy: gas detection, temperature and pressure trips, remote stops and fixed smothering.
- Continuous gas detection with alarms and ventilation interlocks — dangerous versus safe spaces marked on the plan.
- Bearing and casing temperature trips with suction and discharge pressure guards stop the pump before seals fail.
- Emergency shutdown from deck and control room independent of the local starter, proved on schedule.
- Fixed fire-extinguishing cover with lighting, drainage and cofferdam discipline proved before loading.
Surveyor follow-up: gas alarm muted with one temperature trip bypassed during discharge — continue, slow or stop the pump?
Full theory: Centrifugal Cargo Pumps & Stripping Systems — Double-Entry Impellers, Steam Turbines and Vac-Strip -
17 Engage tuming gear and stop L.O pump
Answer
Never stop lube oil with the shaft moving: turning gear turns only on full pre-lubrication with the LO pump running.
- Start the LO pump and prove pressures, flows and sight glasses before the turning gear moves a degree.
- Engage the gear with the indicator cocks open and the shaft free — grinding a dry bearing scores it in one turn.
- Keep oil circulating the whole time the gear is engaged; stopping the pump strands bearings, gears and reduction teeth dry.
- Disengage and lock the gear with oil still flowing after blow-through, logging times both ends.
Surveyor follow-up: LO pump stopped with the gear still engaged and the shaft mid-turn — restart oil, finish the turn or stop everything first?
Full theory: Two-Stroke Main Engine Standby & Start Preparation: Preheating, Turning Gear, Blow-Through & Bridge Control Changeover -
18 With reference to centrifugal pumps: (a)0) Sketch typical discharge characteristics showing variation of throughput as the
Answer
Sketch H/Q falling with flow, power rising with flow, efficiency humped between: shutoff head highest, runout thinnest, best point marked.
- Head-discharge curve falls as throughput rises — steep for high shutoff head, flat where steady pressure matters.
- Power curve starts lowest at zero flow with the discharge shut, climbing as flow grows.
- Efficiency curve humps convex with the duty point at its peak — running far off it wastes fuel and invites cavitation.
- Drawn at constant speed by throttling discharge against measured head; system curves for long pipes and high tanks cross it at the working point.
Surveyor follow-up: duty point sitting far right of the hump with amps high — throttle, trim the impeller or accept it?
Full theory: Pump Curves and Speed Rules Made Simple -
19 centrifugal pump not taking suction reasons
Also asked: “Screw pump not taking suction reasons”
Answer
No suction means no prime or no headroom: air in, strainer choked, lift too high, or NPSH below the pump’s demand.
- Air leaks in suction joints, leaking foot valves and empty strainers break the prime before flow starts.
- Excess suction lift, undersized or elbow-choked suction piping starves inlet pressure into cavitation.
- Wrong selection shows as NPSH available below NPSH required — the pump was never going to suck on that duty.
- Prove by venting, strainer inspection, suction-gauge reading and valve line-up before opening the pump.
Surveyor follow-up: suction gauge fluttering with the strainer clean and valves lined up — air leak, lift or selection first?
Full theory: Centrifugal Pump Construction -
20 Air lock system n where is it used
Answer
An air lock is a trapped air pocket in a liquid line — it sits at the high points of steam-heated heaters and heating coils.
- On open hotwell systems, feed pump control problems pull air in, and it mixes with the feed water into the boiler.
- More air also enters through packing gland air leaks on the pump suction, drawing it down the line.
- The trapped air collects at the top of a riser or a coil and the steam inside stops heating as the circuit is no longer full.
- Bleed it from the high-point vent, then find the leak — an air lock removed twice is a leak still in place.
Surveyor follow-up: coil cold with vents open and water rising at the high point — air lock or steam trap first?
Full theory: Centrifugal Pump Construction -
21 Anti healing pump tank capacity
Answer
Anti-healing tank capacity is set by the volume the pump must not uncover: enough liquid to keep the seal wetted at the lowest level.
- Healing means the seal recovers itself — the tank must hold enough liquid that the pump never draws air at the worst level.
- Capacity is calculated from seal demand, evaporation and the minimum level at which the pump is still primed.
- Too small and the seal runs dry, scores and weeps; too large and the tank stagnates and grows sludge.
- Set level marks, sight glass and a vent so the tank breathes without pressurising the water inside.
Surveyor follow-up: seal weeping with the tank full — capacity, level setting or the vent line first?
Full theory: Centrifugal Pump Construction -
22 Construction of a centrifugal pump
Answer
A centrifugal pump is a rotating impeller in a stationary casing, with a small designed clearance that the wear rings make renewable.
- Rotating impellers inside stationary casings turn kinetic energy into pressure in the volute discharge.
- A small clearance between impeller and casing is kept deliberately — too tight seizes, too loose leaks back.
- Wearing rings fitted to impeller or casing give that clearance as a replaceable item, so erosion is renewed not the pump.
- The shaft is supported in bearings and the stuffing box with gland packing and lantern ring keeps the pumped liquid in.
Surveyor follow-up: pump rebuilt with new rings at maker clearance and the current unchanged — correct, or back to standard clearance?
Full theory: Centrifugal Pump Construction -
23 Emergrcy pump regulation
Answer
Emergency pumps are regulated on capacity and independence: enough output, its own power, and a place no single fire can reach.
- Emergency fire pump delivers at least 40% of the main pump and never less than 25 m³/h, feeding two independent jets.
- Installed outside the engine room, usually forward of the collision bulkhead, at the steering gear flat or the tunnel.
- Diesel-driven with its own fuel for 3 hours plus a reserve for 15 more, or electrically driven from the emergency generator.
- A compartment fire must not be able to put every fire pump out of action — the separation is the point of the rule.
Surveyor follow-up: emergency pump proven running but unable to deliver two independent jets — start proved, duty proved?
Full theory: Ship Fire Main & Emergency Fire Pump System -
24 How distillate pump take suction when condenser in vacuum?
Answer
The distillate pump keeps the condenser under vacuum by pulling from it, and an eductor or vent handles the air it takes with the liquid.
- The pump draws distillate from the condenser shell and discharges it to the sight glass and onward to the evaporator.
- Air in the stream would break the vacuum, so an eductor or vent is arranged on the suction side to carry it off.
- The pump must be primed before the condenser is closed down, or air enters and the vacuum collapses.
- Sustained loss of vacuum points to air inleakage, a leaking gland or a failing pump seal — not to a dirty condenser.
Surveyor follow-up: condenser vacuum falling with the eductor running and the pump seal proved — where else is air entering?
Full theory: Fresh Water Generator — Vacuum Boiling, Operation & Faults -
25 How to take the carrier bearing wearout in rotary vane
Answer
Carrier bearing wear-down is read the same way as any bearing: the clearance is measured, and the carrier is cut or renewed to suit.
- Wear-down is taken with a bridge gauge and feelers against maker limits, not judged by eye.
- Where the carrier is the adjustable element, it is shimmed or re-faced rather than the casing being machined.
- Excess clearance makes the rotor float, the vane timing wander and the pump lose volume and heat.
- Renew the bearing shells and the vanes together, then prove the pump holds pressure with the relief valve at its setting.
Surveyor follow-up: carrier clearance high with vanes renewed and the pump losing pressure — carrier, casing or the relief first?
Full theory: Rotary Vane Steering Gear — Principle, Pressure Limit and Ram Comparison -
26 Hydrophore pump is getting cutin/cutout very frequently, what will you check and actions
Answer
Short-cycling hydrophores are pressure-control faults: bladder precharge, pressure switch settings or a leaking vessel.
- Check the bladder precharge against the maker figure — a soft bladder reads low and cuts the pump in early.
- Check the pressure switch cut-in and cut-out settings and the differential between them; too tight a band short-cycles.
- A vessel that will not hold pressure, or a stuck or failed foot valve, makes the pump run constantly with no demand.
- Correct the setting, recharge the bladder, then log the run hours — frequent starting is itself a life-shortening fault.
Surveyor follow-up: hydrophore cycling hourly with the vessel holding pressure and the bladder correct — what is left to check?
Full theory: Hydrophore & Water Supply — Pressure on Demand -
27 Hypernist testing and type of pump
Answer
Hypermist uses a high-pressure plunger pump — 200 litres per minute at about 70 bar — with zoned solenoids and misting nozzles.
- A plunger-type reciprocating pump supplies the high pressure; each zone is opened by its own solenoid valve.
- Nozzles deliver roughly 12–20 litres per minute at about 50 bar, breaking the water into a fine cooling fog.
- Two detectors must agree before activation, and the alarm panel then zones the fire and calls the control panel.
- Test the sensors with the approved test agent and prove each zone in turn — a system proved only by the panel is half proved.
Surveyor follow-up: one zone silent on test with the pump proved — solenoid, nozzle or the zone pipe first?
Full theory: Ship Sprinkler & Hypermist (Water Mist) Systems -
28 Meat room temperature down same reason centrifugal pump overhaul and clearance. Bunkar plan and sample collection safe steering system
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29 Pump gland leaking what ll u do
Answer
A leaking pump gland is cured by adjustment, not force: bed the packing in, then bring it down a fraction of a turn at a time.
- Open the flush to the packing and start the pump, letting it leak generously while the packing swells and seats.
- Adjust the gland nuts by hand first, then with a wrench a sixth of a turn at a time, every 15 minutes or so.
- Target about one drop per second per inch of shaft diameter, and watch the packing temperature while doing it.
- Too many adjustments crystallise and burn the packing — if it will not settle, strip and renew it with fresh rings.
Surveyor follow-up: gland tightened hard, packing hot and still dripping — adjust back, renew, or fit a mechanical seal?
Full theory: Centrifugal Pump Construction -
30 Sea water pump gland is leaking action to be taken
Answer
Salt water at a gland means erosion is already happening: fix the seal, then check what the water is eating.
- Stop the pump, slacken the gland to restore a drop-by-drop leak, and check the lantern ring is delivering water.
- If the packing is glazed, crystallised or grooved out, strip the gland and renew the rings with a shaft sleeve.
- A grooved shaft is the real fault — renew or sleeve it, or new packing will cut the same groove again.
- After repair, run and confirm the gland drips steadily rather than weeping into the sea as a curtain.
Surveyor follow-up: new packing on an old grooved shaft, still leaking after an hour — packing, sleeve or shaft renewal?
Full theory: Centrifugal Pump Construction -
31 What is air lock n its location
Answer
An air lock is a pocket of air trapped in a liquid line, and it collects at the high points where steam and water separate.
- Feed pump control trouble and gland air leaks both draw air into the system, and it rises to the highest point it can reach.
- In steam-heated heaters and coils the trapped air stops heat transfer, so the coil runs cold while the pump looks healthy.
- High-point vents, stop cocks and drain cocks exist to bleed exactly this, and must be proved free.
- Blow it down and then find the ingress — air that returns in a day is a leak still open somewhere.
Surveyor follow-up: coil cold every morning and warm by noon — air lock, pump cavitation or thermostat first?
Full theory: Centrifugal Pump Construction -
32 What would happen to the amperage of a Centrifugal Pump if it were run with the discharge Valve shut? (4) (b) What is the indication in the Exhaust Gas of Good Combustion? (4) (c) How often is the Emergency Steering Test
Also asked: “What would happen to the amperage of a Centrifugal Pump if it were run with the discharge Valve shut? (4) (b) What is the indication in the Exhaust Gas of Good Combustion? (4) (c) How often is the Emergency Steering Tested? (4) (d) Reasons for carrying out Boiler Feed Water Tests? (4”
Answer
A centrifugal pump against a shut discharge runs on shut-off head: power falls, the pump heats and vibrates itself apart.
- With the discharge shut the pump delivers no flow, so the power absorbed is least — starting current is limited, not raised.
- But running there puts the machine on its shut-off head, creating an intensive radial load and deflecting the shaft.
- The energy becomes heat and vibration, so casings can overheat, heat switches trip, and seals, bearings and sleeves suffer.
- A well-tuned exhaust reads clear or faintly sooty with even colour across the units; steering gear emergency tests are weekly.
Surveyor follow-up: pump restarted with the discharge shut for ten minutes — check the seal and bearings first, or run it again?
Full theory: Centrifugal Pump Construction -
33 in st/gr, when only one pump is in use, how is the standby pump prevented from rotating in opposite direction?
Also asked: “• in st/gr, when only one pump is in use, how is the standby pump prevented from rotating in opposite direction?”
Answer
Non-reverse locking gear in the coupling stops the idle pump being back-driven: pawls on the motor rim drop into place and hold.
- With two pumping units fitted, both may run together for faster response, though normally only one is used.
- The idle pump would otherwise be driven backwards by oil pressure from the running one.
- Non-reverse locking gear sits in the flexible coupling, with steel pawls thrown out to the rim by the rotation itself.
- In reverse they drop into the notches and lock the pump, so pressure cannot push it backwards against the running pump.
Surveyor follow-up: standby pump turning backwards on trial with the pawls fitted — is the pawl damaged or the coupling free?
Full theory: Ram Type Steering Gear — Two and Four Rams, Valve Chests and Change-Over
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