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LGTCO/LGTF exit exam questions and answers — Set 15 — Liquefied Gas Tanker Cargo Operations

Set 15 of the Liquefied Gas Tanker Cargo Operations (LGTCO/LGTF) exit exam bank — 30 of 599 questions, with the correct option marked and a one-line explanation for every one. Read it end to end, jump to a single set, or search it — then sit the same bank as a timed paper. The real DG Shipping exit exam is 30 marks with 15 to pass.

These are the bank's original groupings. Practising deals the whole bank out again every time you reshuffle, so a practice “Set 3” is a different 30 questions each time — the set numbers below are the source file's, and are only used here so the key stays navigable. Options are listed in their original order with fixed letters; when you practise, those move too.

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Set 15 — 30 LGTCO/LGTF exit exam questions with answers

1. An owner wants a Certificate of Fitness to carry chlorine — the most acutely toxic of the common gas cargoes. Which IGC ship type must the vessel be?

  • A. Type 3G
  • B. Type 2G
  • C. Type 2PG
  • D. Type 1G
Answer: D — Type 1G
The IGC Code types a ship by the hazard of its permitted cargoes, and the most dangerous products demand Type 1G — maximum preventive measures and survival of damage anywhere in the ship's length. Chlorine sits there; LNG, by contrast, is only a 2G cargo, which is why 2G is not enough for chlorine.

2. Of the four containment types, which alone is exempt from any secondary barrier, and on what grounds?

  • A. Type A — its prismatic shape spreads load so it cannot crack
  • B. Type C — as a pressure vessel with a large design margin, a leak is not credible
  • C. Membrane — the inner hull already forms a complete second skin
  • D. Type B — the drip tray beneath it already catches any leakage
Answer: B — Type C — as a pressure vessel with a large design margin, a leak is not credible
Type C is a true pressure vessel (about 5–7 barg semi-pressurised, up to ~18 barg fully pressurised) designed with a large safety margin, so failure and leakage are not considered credible and no secondary barrier is required. Type A and membrane need a full barrier and Type B a partial one, because they are low-pressure tanks where a leak is credible.

3. A Moss-sphere LNG carrier carries only a partial (drip-tray) secondary barrier rather than a full one. Which design philosophy justifies this?

  • A. Fully pressurised pressure-vessel design
  • B. Integral tank construction forming part of the hull
  • C. Double-hull redundancy around the cargo block
  • D. Leak-before-failure crack-propagation analysis
Answer: D — Leak-before-failure crack-propagation analysis
Independent Type B tanks are validated by detailed fatigue and crack-propagation analysis so any flaw will leak slowly and be detected long before it can fail — the 'leak-before-failure' basis that permits only a partial barrier. A full second skin is reserved for Type A and membrane tanks, which are not proven this way.

4. A part-full, sealed propane tank is left in the sun. No cargo is added, yet the pressure gauge steadily climbs. The best explanation is that:

  • A. Warming the liquid raises its saturated vapour pressure
  • B. The ullage space shrinks as liquid expands into it
  • C. The relief valve setting has quietly drifted upward
  • D. More liquid in the tank always means more pressure
Answer: A — Warming the liquid raises its saturated vapour pressure
Saturated vapour pressure depends only on the liquid's temperature: warm the propane and more molecules escape the surface, so the pressure rises even though nothing was added. Option 3 is a common error — SVP is independent of how full the tank is, so a 5% and a 95% full tank at the same temperature read the same pressure.

5. LNG is carried at about −162 °C. Which material family is used for its cargo tanks, chosen to stay tough at that cryogenic temperature?

  • A. Ordinary mild shipbuilding steel
  • B. 3.5% nickel steel
  • C. 9% nickel steel or stainless steel
  • D. High-tensile carbon-manganese steel
Answer: C — 9% nickel steel or stainless steel
Methane's −162 °C demands 9% nickel steel or stainless steel, which retain ductility at cryogenic temperatures. Ordinary mild steel would become brittle and could shatter; 3.5% nickel steel is only good to about −48 °C, suiting fully refrigerated LPG rather than LNG.

6. A fully refrigerated LPG and ammonia carrier uses Type A prismatic tanks of 3.5% nickel steel. The lowest cargo temperature this grade permits is about:

  • A. −162 °C
  • B. −90 °C
  • C. −104 °C
  • D. −20 °C
Answer: B — −90 °C
Under the IGC Code material tables, 3.5 % nickel steel is acceptable down to a design temperature of about −90 °C. That covers fully refrigerated LPG and ammonia (about −48 °C and −33 °C) but not ethylene (−104 °C) or LNG (−162 °C).

7. A length of cryogenic cargo liquid line can be isolated with a closed valve at each end. What fitting is mandatory on that trapped section, and why?

  • A. A sample cock, so cargo quality can be checked
  • B. A blank flange, to seal the line positively
  • C. A thermal relief valve, because trapped liquid warms, expands and could burst the pipe
  • D. A drain plug, so the section can be emptied by gravity
Answer: C — A thermal relief valve, because trapped liquid warms, expands and could burst the pipe
Cold liquid shut in between two valves warms from ambient heat, expands and — being nearly incompressible — will burst the line by hydraulic lock unless relieved. A thermal (safety) relief valve venting back to a tank or the vapour line is therefore required on every isolatable section; a blank or drain does nothing to relieve the expansion.

8. On a cargo loading line a valve is fitted that lets liquid pass one way and shuts automatically the instant flow tries to reverse. This is a:

  • A. Non-return (check) valve
  • B. Globe valve
  • C. Gate valve
  • D. Needle valve
Answer: A — Non-return (check) valve
A non-return (check) valve permits flow in one direction only and closes on reverse flow, guarding against back-flow to a pump or stripping line. Globe, gate and needle valves are all operated deliberately by hand or actuator; none closes by itself against reversal.

9. Which valve is best suited to throttling and regulating a cargo flow, rather than simply isolating a line fully open or fully shut?

  • A. Gate valve
  • B. Ball valve
  • C. Butterfly valve
  • D. Globe valve
Answer: D — Globe valve
A globe valve throttles well because its disc seats against the flow, giving controllable, stable regulation across the stroke. Gate and ball valves are isolation valves meant for fully open or fully closed service and erode if throttled; a needle valve does the same fine job as a globe but only on very small-bore lines.

10. On an LNG carrier on a normal loaded passage, which compressor delivers the steady boil-off gas to the machinery space to be burned as fuel?

  • A. The high-duty compressor
  • B. The low-duty compressor
  • C. The cascade booster compressor
  • D. The stripping eductor
Answer: B — The low-duty compressor
The low-duty (LD) compressor handles the modest, continuous boil-off during the loaded and ballast voyage and sends it to the boilers or dual-fuel engines as fuel. The high-duty compressor is reserved for the far larger vapour flows of cool-down and loading, so it is not the sea-passage machine.

11. During cargo-tank cool-down and bulk loading, when very large volumes of vapour must be moved quickly, the duty falls to the:

  • A. High-duty compressor
  • B. Low-duty compressor
  • C. Reliquefaction expansion valve
  • D. Inert-gas deck blower
Answer: A — High-duty compressor
The high-duty (HD) compressor is sized for the big vapour flows generated during cool-down and loading. The low-duty compressor deals only with the small, steady sea-passage boil-off, so it cannot cope with the surge produced when warm tanks are chilled or filled.

12. Ethylene must be held at about −104 °C. A reliquefaction plant able to reach that deep cold uses which arrangement?

  • A. A single direct cycle condensed by seawater
  • B. Simply burning the boil-off as fuel
  • C. A cascade cycle in which one refrigerant cools another in stages
  • D. Pressurising the tank until the vapour condenses
Answer: C — A cascade cycle in which one refrigerant cools another in stages
A cascade plant chains refrigerants so that each stage cools the next, reaching temperatures a single direct cycle cannot. A direct cycle condensed by seawater is adequate for ordinary LPG but cannot reach −104 °C, and pressurisation is useless because ethylene is near its critical temperature.

13. In a cascade reliquefaction plant and in refrigerated air driers, the refrigerant Freon is best described as:

  • A. A flammable hydrocarbon that blends with the cargo vapour
  • B. An inert gas used to purge the cargo tanks
  • C. A foam-forming firefighting agent
  • D. A non-flammable halocarbon that never contacts the cargo
Answer: D — A non-flammable halocarbon that never contacts the cargo
Freon is a non-flammable halocarbon that works in the secondary loop of a cascade plant and in air driers, always kept separate from the cargo. It is neither a hydrocarbon nor an inert purging gas, so the escaping-cargo and tank-purging descriptions are wrong.

14. Heat leaking into a refrigerated tank supplies the energy for the liquid to change into vapour at constant temperature. That energy is the cargo's:

  • A. Sensible heat
  • B. Latent heat of vaporisation
  • C. Specific gravity
  • D. Critical pressure
Answer: B — Latent heat of vaporisation
Latent heat of vaporisation is the energy a substance absorbs to change state without any temperature rise — precisely what boil-off consumes. Sensible heat, by contrast, changes the temperature you can read on a thermometer; it is not what drives the phase change.

15. Venting a cargo tank makes the remaining liquid colder. This self-cooling effect, driven by the latent heat drawn from the liquid as it boils, is called:

  • A. Auto-refrigeration
  • B. Cascade cooling
  • C. Rollover
  • D. Cavitation
Answer: A — Auto-refrigeration
As pressure is released the liquid boils harder, and the latent heat for that boiling is taken from the liquid itself, chilling it — auto-refrigeration, used deliberately during cool-down. Rollover is a sudden mixing of stratified layers, and cavitation is vapour forming at a pump; neither describes this cooling.

16. As an order-of-magnitude figure, the mass of one standard cubic metre (SCM) of natural gas is closest to:

  • A. 0.075 kg
  • B. 0.75 kg
  • C. 7.5 kg
  • D. 75 kg
Answer: B — 0.75 kg
One standard cubic metre of natural gas weighs roughly 0.75 kg (methane is about 0.68 kg/m³, natural gas 0.7–0.8 depending on composition). The other values are out by a factor of ten or more — 0.075 kg would be far too light for a hydrocarbon gas and 7.5 kg far too heavy.

17. One cubic metre of LNG, if allowed to warm and vaporise completely, produces roughly what volume of gas — the fact that makes liquefaction worthwhile?

  • A. About 6 m³
  • B. About 60 m³
  • C. About 260 m³
  • D. About 600 m³
Answer: D — About 600 m³
Liquefying methane shrinks it by roughly 600 to 1, so one shipload of LNG represents about 600 shiploads of gas — the whole economic point of the trade. The price of that compactness is that the cold liquid is always trying to boil back to vapour, driving the need for insulation and boil-off management.

18. Propane (vapour RD 1.55) and butane (vapour RD 2.0) escape on deck. Compared with a warm methane leak, these vapours will:

  • A. Rise faster and clear the deck
  • B. Disperse in exactly the same way
  • C. Sink and collect in low spaces such as the pump-room bilge
  • D. Neither rise nor fall but stay put
Answer: C — Sink and collect in low spaces such as the pump-room bilge
Being well over air density, propane and butane vapours flow downhill and pool in bilges, trenches and other low, poorly ventilated spaces, creating a flammable and asphyxiation hazard. Warm methane (RD ~0.55) is lighter than air and rises, so the two behave oppositely — making 'same way' wrong.

19. Ethylene oxide, one of the most hazardous gas cargoes carried, has a liquid relative density (water = 1) at its atmospheric boiling point of about:

  • A. 0.57
  • B. 0.90
  • C. 1.20
  • D. 0.45
Answer: B — 0.90
Ethylene oxide's liquid relative density is about 0.90, so it floats on water. Its extreme hazard — flammable, toxic and reactive — is why it demands a Type 1G ship, Type C tanks, an inert vapour space and both flammable and toxic detection; 0.57 and 0.45 are the far lighter ethylene and methane liquids.

20. A 2PG ship carries the same hazard class of cargo as a 2G ship, but is additionally restricted to:

  • A. Over 150 m, Type A tanks and a MARVS below 2 bar
  • B. Any length, membrane tanks and a design temperature of −55 °C
  • C. Over 200 m and Type B tanks only
  • D. Not more than 150 m, Type C tanks, MARVS at least 7 bar and design temperature not below −55 °C
Answer: D — Not more than 150 m, Type C tanks, MARVS at least 7 bar and design temperature not below −55 °C
2PG is a size-limited sub-type of 2G: 150 m or less, Type C pressure vessels, a relief-valve setting of at least 7 bar and a design temperature no colder than −55 °C. The other options invert these limits — 2PG uses Type C, not Type A or membrane tanks, and is capped at 150 m rather than being for larger ships.

21. When a gas carrier arrives to load, which document states her exact permitted product list and her IGC ship type (1G, 2G, 2PG or 3G)?

  • A. The International Certificate of Fitness
  • B. The Safety Management Certificate
  • C. The cargo manifest
  • D. The classification society hull certificate
Answer: A — The International Certificate of Fitness
The Certificate of Fitness confirms compliance with the relevant Gas Code and names both the ship type and every product she may carry — nothing off that list may be loaded. The Safety Management Certificate covers the ISM system, and the manifest merely records a particular parcel, so neither fixes the ship's permitted cargoes.

22. In calm weather, roughly what fraction of a refrigerated cargo turns to vapour each day — a figure that rises by about 15% in heavy seas?

  • A. About 1.6% per day
  • B. About 0.016% per day
  • C. About 0.16% per day
  • D. About 5% per day
Answer: C — About 0.16% per day
Typical calm-weather boil-off is around 0.16% of the cargo per day, climbing roughly 15% when a seaway stirs the liquid and adds energy. This steady vapour drives tank pressure toward the relief setting, which is why every refrigerated ship must reliquefy, burn or vent it; 1.6% and 5% overstate the loss by an order of magnitude.

23. Anhydrous ammonia must be kept away from which materials in valves and fittings, because it attacks them?

  • A. Stainless steel and nickel steels
  • B. Copper, zinc and their alloys such as brass
  • C. Carbon steel and cast iron
  • D. PTFE and polyethylene
Answer: B — Copper, zinc and their alloys such as brass
Ammonia corrodes copper, zinc and alloys such as brass and bronze, so these must be excluded from ammonia service and steel components used instead. Carbon and stainless steels are compatible with ammonia, which is precisely why they are chosen, making the other options wrong.

24. Entering an enclosed space can prove fatal or dangerous when the oxygen reading is:

  • A. Exactly 20.9%, the normal value
  • B. Between 20.8 and 21.0%
  • C. Only when above 23.5%
  • D. Below 19.5% (asphyxiation) or above 23.5% (oxygen-enriched)
Answer: D — Below 19.5% (asphyxiation) or above 23.5% (oxygen-enriched)
Below 19.5% the atmosphere is oxygen-deficient and can asphyxiate you without warning; above 23.5% it is oxygen-enriched, so even hard-to-ignite materials flash readily. Normal air is about 20.9%, so options 0 and 1 describe a safe atmosphere, and 23.5% alone ignores the deficiency hazard.

25. A gas carrier's fixed dry-chemical powder system knocks down flame by which mechanism, with the nitrogen or CO₂ serving only to:

  • A. Cool the burning steel; wet the deck
  • B. Smother the fire with foam; dilute the vapour
  • C. Interrupt the combustion chain reaction; propel the powder out
  • D. Displace oxygen in the tank; inert the space
Answer: C — Interrupt the combustion chain reaction; propel the powder out
The powder itself breaks the chemical chain reaction of combustion, rapidly knocking the flame down, while the nitrogen or CO₂ is merely the propellant that expels it. It gives no lasting cooling, so hot steel can re-ignite the gas — which is why isolating the leak and cooling with water spray must accompany it.

26. A cargo vapour cloud drifting across the deck presents which hazard?

  • A. Flammability alone
  • B. Toxicity alone
  • C. Oxygen displacement alone
  • D. It may be flammable, may be toxic, and can displace oxygen
Answer: D — It may be flammable, may be toxic, and can displace oxygen
A drifting cloud can reach an ignition source, may be toxic, and can displace the oxygen needed to breathe — all three at once for many cargoes. The flammable cloud also extends beyond the visible white condensation fog, so treating the mist as the boundary is dangerous; that is why no single-hazard answer is complete.

27. A stainless drip tray under the cargo manifold serves two protective functions during connection and transfer. They are to:

  • A. Catch cargo leakage and keep cryogenic liquid off the mild-steel deck
  • B. Collect rainwater for the fire main and store spare gaskets
  • C. Act as a cofferdam between tanks and trim the vessel
  • D. Cool the manifold and earth static charge to the hull
Answer: A — Catch cargo leakage and keep cryogenic liquid off the mild-steel deck
The tray contains any liquid leakage at the connection and, being stainless, keeps cryogenic cargo from touching the ordinary mild-steel deck, which would otherwise embrittle and could crack. It also stops the spill spreading across the deck or overboard — a pollution-prevention role — so the rainwater and cofferdam answers miss its purpose entirely.

28. A loaded LNG cargo tank reads well above the upper flammable limit. A watchkeeper insists this is the most dangerous possible state. Why is it in fact the safe carriage condition — and when would it turn dangerous?

  • A. Any spark would ignite it at once, so it is safe only by luck
  • B. It is below the lower flammable limit, so there is too little fuel to burn
  • C. It is over-rich — above the UFL, too little oxygen to burn — and turns dangerous only if air enters and dilutes it back down through the flammable range
  • D. Once above the upper flammable limit it can never burn again under any condition
Answer: C — It is over-rich — above the UFL, too little oxygen to burn — and turns dangerous only if air enters and dilutes it back down through the flammable range
An atmosphere above the UFL is over-rich: there is not enough oxygen to support combustion, so it will not ignite as it stands. The danger appears when air is admitted — during gas-freeing or through a leaking valve — and the mixture falls back through the flammable range. The 'below the LFL' option has it backwards: the tank is over-rich, not too lean.

29. Three sample points in a methane/air atmosphere read 3%, 9% and 20% methane by volume. A spark occurs at each. Given methane's flammable range, which one ignites?

  • A. 9% by volume — the only reading inside methane's roughly 5–15% flammable range
  • B. 3% by volume, which is within the range
  • C. 20% by volume, which is within the range
  • D. All three, because methane is always flammable
Answer: A — 9% by volume — the only reading inside methane's roughly 5–15% flammable range
Methane burns only between about 5% and 15% by volume in air. 3% is too lean and 20% is too rich, so only the 9% sample lies inside the flammable band. A mixture must sit within that band to ignite — being flammable in principle is not enough.

30. An engineer is warned that an unlagged exhaust can ignite a butane leak with no spark or flame present. Which property makes that possible, and which cargo has the lowest such figure?

  • A. The flash point; n-butane's, about −60 °C, is the lowest
  • B. The auto-ignition temperature; n-butane's, about 365 °C, is the lowest of the common cargoes
  • C. The boiling point; butane ignites as soon as it boils
  • D. The critical temperature; butane's is the lowest
Answer: B — The auto-ignition temperature; n-butane's, about 365 °C, is the lowest of the common cargoes
Above a cargo's auto-ignition temperature its vapour ignites on a hot surface alone — no spark needed — and n-butane's, about 365 °C, is the lowest of the common gases, so an unlagged exhaust can reach it. The flash point is the temperature at which a liquid gives off ignitable vapour, not the hot-surface ignition point.

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