Lube Oil and Fuel Oil Systems

What the rule requires

Loss of oil pressure is a trip-level casualty

Bearings fail within seconds of losing oil pressure, so any fall in pressure is investigated at once — oil level, suction strainer, relief valve, pump, and external leaks or internal dilution — and the engine is never run on low oil pressure NAVEDTRA 14075 §3-3.

On auxiliary machinery the indication is split between two places: a pressure gauge in the feed line and a thermometer in the return line show how the oil system is functioning, and thermometers are often fitted in the bearings themselves as a warning against overheating NAVEDTRA 14104 Ch. 10 — Lubricating Systems. The required responses to those two indications are not the same. A decided drop in oil pressure means shut the equipment down immediately. A moderate rise in oil temperature means investigate. Sump quantity is read from an oil-level float gauge.

The lube oil temperature on an operating auxiliary has come up moderately but pressure is steady. What are you required to do?

Investigate the rise. A moderate rise in oil temperature calls for investigation; immediate shutdown is the required response to a decided drop in oil pressure. Candidates who treat every abnormal reading as a trip lose this one.

What the oil is doing, and the path it takes

The lubricating oil reduces friction and wear, cools the moving parts, carries away metal and combustion debris, cushions bearing shock loads, and helps seal the piston rings . In a wet-sump engine a gear pump draws from the sump through a suction strainer and delivers under pressure through a full-flow filter and a lube-oil cooler to the main bearings, connecting-rod bearings, camshaft and valve gear, and by drilled passages or spray nozzles to the underside of the piston crowns for cooling. A relief valve holds system pressure; a bypass allows oil to keep flowing if the filter or cooler becomes restricted. Large engines use a dry sump with a separate lube-oil tank and both pressure and scavenge pumps.

On auxiliary units the system is the same set of parts — sump or reservoir, pump, strainer, cooler, temperature and pressure gauges, and the piping to and from the bearings — arranged to lubricate the bearings, cool them, and flush wear products out of them . The pump is generally gear type, and the pressure relief valve returns excess oil to the pump suction. Dual strainers are commonly fitted so the system can run on one while the other is cleaned. The cooler is generally tube-in-shell with seawater through the tubes and oil around them, and oil temperature is controlled by adjusting the valve regulating seawater flow through the tubes — not by throttling the oil.

Keeping the charge fit for service

Oil is held at the correct level, grade, and operating temperature. Too cold and it is thick and does not reach the clearances; too hot and the film weakens . It degrades in service from fuel dilution, water contamination, oxidation, and the acidic products of combustion, so it is sampled and analysed for viscosity, water, fuel, total base number — its reserve of acid-neutralising alkalinity — and wear metals, and renewed on condition rather than on a calendar. Every part of the lubricating oil system must be kept properly maintained at all times if all engine parts are to receive adequate lubrication NAVEDTRA 14075 Ch. 3 — Lubricating System.

Grade is read off the symbol number, which states class and viscosity: symbol 2190 is a number 2 class oil of 190 SSU, the viscosity figure being the seconds required for 60 cc of oil at 130 °F to pass a standard opening in a Saybolt viscosimeter NAVEDTRA 14104 Ch. 10 — Lubricating Oils and Greases. 2190TEP is used for all propulsion turbines and reduction gears, the TEP additives increasing the oil's ability to displace water from steel and inhibiting oxidation. Internal combustion engines, gasoline and diesel, use symbol 9110, 9170, 9250, or 9500. Mineral oils are the best lubricant for machinery at high speed and high temperature; animal and vegetable oils lubricate well but cannot withstand high temperatures. Where retention of oil would be difficult — throttle links, pump bearings, small boat steering links — grease is used instead. When no lubrication chart is posted, the manufacturer's technical manual for that unit is the basic reference for the correct oil.

The fuel oil train, in flow order

Fuel is drawn from bunker (storage) tanks, may be transferred to settling tanks where water and heavy sediment settle out and are drained, then to service (day) tanks that feed the engine NAVEDTRA 14075 §3-2. Heavier fuels are heated to bring viscosity down to the value needed for good pumping and atomisation. From the service tank a supply pump sends fuel through duplex filters — arranged so one is cleaned while the other stays in service — to the injection equipment, with a water separator removing free water that would otherwise corrode injectors and disrupt combustion. Injection plunger and nozzle clearances are measured in microns, and a trace of grit ruins them; that is the whole reason for the settling, filtering, and separating.

Air trapped in the fuel system after a filter change or after running a tank dry is bled off, because vapour in the injection line prevents the pump from delivering solid fuel.

Injection timing

Timing sets when fuel is delivered relative to piston position, and it decides whether the engine makes full power without knocking or overheating . Injection too early — advanced — burns the fuel before the piston reaches top centre, so the engine knocks and the bearings are overloaded. Too late — retarded — leaves combustion incomplete, exhaust temperatures rise, and power and economy fall. Timing is checked against the maker's marks, and set electronically on modern engines. Injectors are tested on a pop-tester for correct opening pressure and spray pattern, and cleaned or renewed when they dribble, spray unevenly, or open at the wrong pressure.

Learn it as one line: early knocks, late runs hot. A question that hands you rising exhaust temperature with falling power is a retarded-timing question.

An engine knocks under load and the exhaust temperatures are normal. Advanced or retarded injection?

Advanced. Early injection burns the charge before the piston reaches top centre — the engine knocks and the bearings are overloaded. Retarded injection produces the opposite signature: no knock, but rising exhaust temperature with lost power and economy.

What 46 CFR forbids outright

These are the flat prohibitions and the numbers attached to them, and they are the ones written into questions 46 CFR §56.50-60:

Oil pumps must have no discharge connections to fire mains, boiler feed systems, or condensers. Cargo and fuel oil piping is kept separate from other piping as far as practicable, with positive means to prevent interconnection in service; the two may be combined only where the cargo system carries Grade E oils only.
Test cocks must not be fitted to fuel oil or cargo oil tanks.
Valves for drawing fuel or draining water from fuel are not permitted, except that one such valve is allowed on diesel driven machinery if it is located within the machinery space away from any potential source of ignition and fitted with a cap or plug to prevent leakage.
Fuel oil tanks overhanging boilers are prohibited.
No part of the fuel oil system carrying heated oil at a pressure above 180 kPa (26 psi) may be concealed where defects and leakage cannot be readily observed, and each machinery space containing part of that system must be adequately illuminated.
Heating coil drains and oil heater drains must run to an inspection tank or other suitable oil detector, so that a leaking coil is detected rather than dumping oil into the water side.
Oil piping must not run through feed or potable water tanks, and feed or potable water piping must not pass through oil tanks. Oil piping must not be led through accommodation spaces, except fill piping not normally used at sea that is steel, all welded, and not concealed.
Piping subject to internal head pressure from oil in the tank gets a positive shutoff valve at the tank, operable manually at the valve and remotely from an accessible, safe location outside that compartment. Valves outside the tank are steel, ductile cast iron to ASTM F1155, or a ductile nonferrous alloy melting above 1,700 °F.
Piping is run away from hot surfaces; where that is unavoidable, joints are welded or shielded, and piping carrying fuel or lube oil near an open flame or parts operating above 500 °F must be seamless steel.
Drains, strainers, and other equipment subject to normal oil leakage get drip pans or equivalent to keep oil out of the bilge.

Telling it apart

The three tanks in the fuel train

The criterion is what the tank does to the fuel, not where it sits NAVEDTRA 14075 §3-2.

Bunker (storage) tank — holds the ship's fuel as received. Nothing is being conditioned here; fuel is drawn from it for transfer.
Settling tank — water and heavy sediment separate out by gravity and are drained off. This is where water is removed from fuel by draining, which is why no test cock is needed or permitted to find it.
Service (day) tank — the tank that feeds the engine. The supply pump takes suction here, not from the settling tank. Most often misfiled: candidates answer that the settling tank supplies the engine.

Relief valve against filter bypass

Both are in the lube oil system and both open on pressure, but they answer different faults NAVEDTRA 14075 §3-3.

Relief valve — holds system pressure; on auxiliary systems the excess it passes recirculates to the pump suction NAVEDTRA 14104 Ch. 10 — Lubricating Systems.
Bypass — opens when the filter or cooler becomes restricted, so oil keeps flowing to the bearings even though it is flowing unfiltered or uncooled.

Working a question

You have the oiler's watch. Lube oil pressure on an operating auxiliary diesel drops sharply while temperature and load are unchanged. What is required of you?

Secure the machinery. A decided drop in oil pressure requires immediate shutdown NAVEDTRA 14104 Ch. 10 — Lubricating Systems. Any answer that has you continue running while you diagnose is wrong before the diagnosis begins, because bearings fail within seconds NAVEDTRA 14075 §3-3.
Check the oil level. Lowest-effort item and the most common cause. A sump below suction gives exactly this indication.
Check the suction strainer. A choked strainer starves the pump; pressure downstream falls.
Check the relief valve. Stuck open, it dumps delivery back to suction and holds no pressure.
Check the pump. With level, strainer, and relief valve accounted for, the pump is the remaining pressure source.
Look for external leakage and for internal dilution. Oil thinned by fuel dilution or water contamination will not hold pressure, and it will show up in the sample as changed viscosity, fuel, or water content.
Do not restore load on low pressure. The engine is not run on low oil pressure at all.

Steps 2 through 6 are the source's own investigation order, and they walk the system from the sump outward. Working them in that order is what keeps you from pulling a pump that was never the fault.

Where candidates lose the point

Securing the machinery for a temperature rise. The candidate has "shut it down" in mind from the pressure rule and applies it to heat. A moderate rise in oil temperature is investigated; immediate shutdown belongs to a decided drop in oil pressure.
Reading 26 psi as a system pressure limit. It is not a maximum working pressure. Above 180 kPa (26 psi), heated fuel oil piping may not be run concealed where leakage cannot be readily observed.
Answering that no drain valve is ever allowed in a fuel oil system. One valve is permitted on diesel driven machinery, sited away from any ignition source and capped or plugged. The item that is flatly prohibited on fuel and cargo oil tanks is the test cock.
Fitting a test cock or sounding a tank to find water in the fuel. Water is settled out and drained at the settling tank, and heating coil drains lead to an inspection tank or oil detector so a leaking coil is caught.
Assigning high exhaust temperature to advanced timing. Advanced injection knocks and overloads bearings. Retarded injection is what drives exhaust temperature up while power and economy fall.
Cranking an engine to clear air after a filter change instead of venting the system. Vapour in the injection line stops the pump delivering solid fuel; the air is bled off.
Reaching for a heavier oil to protect a worn bearing. Oil that is too thick does not reach the clearances. Grade comes from the lubrication chart or the manufacturer's technical manual for that unit, and internal combustion engines take symbol 9110, 9170, 9250, or 9500 — 2190TEP is propulsion turbine and reduction gear oil.
Controlling cooler outlet oil temperature with the oil valve. On a tube-in-shell cooler the seawater flow through the tubes is what is regulated.

Check yourself

You are lining up to transfer fuel and find a proposed cross-connection from the fuel oil transfer pump discharge to the fire main. Permitted?

No. Oil pumps must have no discharge connections to fire mains, boiler feed systems, or condensers, and oil piping is kept separate from other piping as far as practicable with positive means against interconnection in service 46 CFR §56.50-60.

Heavy fuel is being burned and the engine will not atomise properly. Which conditioning step is at fault, and what is it supposed to achieve?

Heating. Heavier fuels are heated to bring viscosity down to the value needed for good pumping and atomisation NAVEDTRA 14075 §3-2. Cold, over-viscous fuel gives poor atomisation regardless of injector condition.

An oil analysis report comes back with reduced total base number. What does that tell you, and what governs whether the charge is changed?

Total base number is the oil's reserve of alkalinity for neutralising the acidic products of combustion, so a fall means that reserve is being consumed NAVEDTRA 14075 §3-3. The charge is renewed on condition — viscosity, water, fuel, TBN, and wear metals — rather than on a fixed calendar interval.

Fuel oil fill piping must pass through an accommodation space on a small vessel. Under what conditions, if any, is that acceptable?

Only fill piping not normally used at sea may pass through accommodation spaces, and then only if it is of steel construction, all welded, and not concealed . Oil piping generally must not be led through accommodation spaces. A shutoff valve is fitted at each filling end.

A lube oil filter is heavily loaded and the differential across it is high, yet bearing oil pressure is holding. What is happening, and why is it not reassuring?

The bypass has opened to keep oil flowing past the restricted filter . Flow is maintained, but the oil reaching the bearings is unfiltered, so the debris the oil carries away is going back through the running gear. The filter is changed, and air bled where the system requires it.

Which is the correct suction for the engine supply pump, and what has been removed from the fuel by the time it gets there?

The service (day) tank. By that point water and heavy sediment have settled out and been drained in the settling tank; downstream of the supply pump the fuel still passes duplex filters and a water separator before reaching the injection equipment .

An inspector asks why the heating coil drain on a fuel oil tank leads to an inspection tank. What is the answer?

So that oil leaking from the coil is detected. Each drain from a heating coil, and each drain from an oil heater, must run to an inspection tank or other suitable oil detector .