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LGTCO/LGTF exit exam questions and answers — Set 19 — Liquefied Gas Tanker Cargo Operations

Set 19 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.

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Set 19 — 30 LGTCO/LGTF exit exam questions with answers

1. Before entry into an enclosed space, at which oxygen readings can the atmosphere prove fatal or unsafe?

  • A. Only when it is below 19.5% by volume
  • B. When it is below 19.5% (asphyxiating) or above 23.5% (oxygen-enriched)
  • C. Only between 20.8 and 21.0% by volume
  • D. Only when it is above 23.5% by volume
Answer: B — When it is below 19.5% (asphyxiating) or above 23.5% (oxygen-enriched)
Below 19.5% the atmosphere is oxygen-deficient and will asphyxiate; above 23.5% it is oxygen-enriched and even materials that normally resist ignition can flash. Normal air is about 20.9%. Treating only the low reading as dangerous (first option) misses the enrichment hazard.

2. In a fixed dry chemical powder installation, what is the role of the nitrogen or carbon dioxide bottles?

  • A. They act as a propellant, pressurising the container to expel and fluidise the powder
  • B. They cool the burning steel before the powder lands
  • C. They chemically break the fire's chain reaction
  • D. They wet the powder so it clings to the flame
Answer: A — They act as a propellant, pressurising the container to expel and fluidise the powder
The nitrogen or CO₂ is a propellant only — reduced from around 200 bar to about 20 bar, it fluidises the powder and drives it out through the monitors and hoses. The powder itself, not the gas, breaks the combustion chain reaction, and the system gives essentially no cooling, so hot steel must be cooled with water separately.

3. Propane and ammonia can both be carried as liquid at ambient temperature under pressure, yet LNG cannot. Why?

  • A. Methane's boiling point is too high to allow refrigeration
  • B. Methane is chemically unstable above 0 °C
  • C. Methane's critical temperature is about −82.5 °C, and above the critical temperature no pressure will liquefy it
  • D. Methane's vapour is lighter than air and cannot be compressed
Answer: C — Methane's critical temperature is about −82.5 °C, and above the critical temperature no pressure will liquefy it
Above its critical temperature no amount of pressure will liquefy a gas; methane's is about −82.5 °C, far below any ambient temperature, so LNG can only be kept liquid by keeping it cold (around −162 °C). Propane (critical ~+97 °C) and ammonia (critical ~+132 °C) have critical temperatures above ambient, so pressure alone works for them.

4. A section of cargo liquid line is isolated with cold liquid trapped between two closed valves, and the sun warms the deck. What must be fitted to make this safe?

  • A. A sample point to bleed off pressure by hand
  • B. A relief valve discharging back to a tank or the vapour line
  • C. A blank flange at each end of the section
  • D. A drain cock left cracked open
Answer: B — A relief valve discharging back to a tank or the vapour line
Trapped liquefied gas has nowhere to expand; as it warms it boils and pressure runs away until the line bursts, so every section that can shut liquid in must carry a relief valve piped back to a tank or the vapour side. A blank flange only seals the line — it provides no pressure relief and would make the hazard worse.

5. A cargo tank is inerted to 2% oxygen and full of hydrocarbon vapour. A catalytic (pellistor) explosimeter is used to test it. What happens?

  • A. It reads accurately in % LFL, as it would in air
  • B. It reads well over 100% LFL and pegs full scale
  • C. It automatically switches to reading % by volume
  • D. It reads a false low, because without oxygen the sample cannot burn on the filament
Answer: D — It reads a false low, because without oxygen the sample cannot burn on the filament
A catalytic sensor works by burning the sample on a heated bead, which needs oxygen; in an inert atmosphere nothing burns, so the meter reads a dangerously reassuring false low. A tankscope reading % by volume must be used instead — the instrument does not switch ranges by itself.

6. At atmospheric pressure, at what temperature does ammonia boil?

  • A. About −42 °C
  • B. About −33 °C
  • C. About −104 °C
  • D. About −162 °C
Answer: B — About −33 °C
Ammonia boils at about −33 °C at atmospheric pressure. Its critical temperature is much higher at about +132 °C, so it can also be carried fully pressurised at ambient. The figure −42 °C is propane's boiling point and −162 °C is LNG's — both are common traps.

7. Why must a cargo tank be cooled down before bulk loading a fully refrigerated cargo?

  • A. To prevent violent flash evaporation and severe thermal stress on the tank structure
  • B. To raise the tank pressure ready for loading
  • C. To remove the last of the inert gas from the tank
  • D. To increase the usable volume of the tank
Answer: A — To prevent violent flash evaporation and severe thermal stress on the tank structure
Introducing cold liquid into a warm tank would flash off large volumes of vapour (over-pressure and heavy venting) and thermally shock the steel; cooling down slowly through the spray rails avoids both. Removing inert gas is the separate gassing-up step that comes before cool-down.

8. Which of these cargoes does NOT require an added stabiliser (inhibitor) to be carried safely?

  • A. Butadiene
  • B. Vinyl chloride monomer
  • C. Propane
  • D. Isoprene
Answer: C — Propane
Inhibitors are needed for polymerisable cargoes — butadiene, vinyl chloride and isoprene can react with themselves or with oxygen to form unstable products, so they carry an added stabiliser. Propane is a saturated hydrocarbon that cannot polymerise, so it needs no inhibitor.

9. What arrangement is provided at the cargo vent masts to deal with a vent fire?

  • A. A bursting disc that blows the flame clear
  • B. A pressure gauge and a drain valve
  • C. A seawater deluge ring around the mast head
  • D. A nitrogen fire-smothering connection together with a flame screen
Answer: D — A nitrogen fire-smothering connection together with a flame screen
Vent masts are protected by a nitrogen (fire-smothering) connection and a flame screen/arrester: if a vent ignites you smother it with nitrogen rather than trying to blow it out. A bursting disc relieves pressure but does nothing to extinguish or arrest a flame.

10. Which method is most widely used to measure a toxic gas at the parts-per-million level in a tank atmosphere?

  • A. Chemical indicator (detector) tubes drawn with a bellows pump
  • B. A catalytic explosimeter reading % LFL
  • C. A tankscope reading % by volume
  • D. A paramagnetic oxygen cell
Answer: A — Chemical indicator (detector) tubes drawn with a bellows pump
Chemical detector tubes (for example Dräger tubes) change colour to give a direct ppm reading, which is exactly the low range toxic limits sit in. An explosimeter (% LFL) and a tankscope (% volume) both work far above that range and cannot resolve toxic concentrations, and an oxygen cell measures only oxygen.

11. In a cascade reliquefaction plant or a refrigerated air drier, what is Freon?

  • A. A flammable hydrocarbon refrigerant that must be kept clear of the cargo
  • B. A non-flammable halocarbon refrigerant that circulates in a separate loop and never contacts the cargo
  • C. An inert gas used to purge the cargo tanks
  • D. A foam-forming agent used to fight refrigerant fires
Answer: B — A non-flammable halocarbon refrigerant that circulates in a separate loop and never contacts the cargo
Freon is a non-flammable halocarbon refrigerant used as the working fluid in the secondary (cascade) loop and in air driers; it stays in its own circuit and never touches the cargo. It is not a hydrocarbon and is not flammable, so the first option is wrong.

12. What is the liquid relative density (water = 1) of ethylene oxide at its atmospheric boiling point?

  • A. About 0.57
  • B. About 1.20
  • C. About 0.68
  • D. About 0.90
Answer: D — About 0.90
Ethylene oxide has a liquid relative density of about 0.90. It is one of the most hazardous common gas cargoes — a Type 1G product carried in Type C tanks with a nitrogen-padded vapour space because it is flammable over an exceptionally wide range and can even decompose without air.

13. A term describes the condition in which the blood can no longer carry enough oxygen to the brain. What is it?

  • A. Narcosis
  • B. Frostbite
  • C. Asphyxia
  • D. Chemical burn
Answer: C — Asphyxia
Asphyxia is the failure to get enough oxygen to the brain; a simple asphyxiant cargo causes it by displacing air, and it gives no warning — the victim simply loses consciousness. That absence of warning is why an enclosed space is never entered without a gas test and a permit.

14. Which is the fast, normal method of discharging a gas carrier, rather than a slow fall-back?

  • A. Discharge by the ship's cargo pumps
  • B. Discharge by pressurising the tank with its own vapour
  • C. Discharge using vapour supplied from shore
  • D. Discharge by boil-off alone
Answer: A — Discharge by the ship's cargo pumps
Pumped discharge, using the submerged cargo pumps, is the quick and normal method. Pressure-displacement methods — pushing the liquid out with vapour, whether the ship's own or shore-supplied — are slow and are reserved for small pressurised ships or when pumps are unavailable.

15. Roughly what is the mass of 1 standard cubic metre (SCM) of natural gas?

  • A. About 0.075 kg
  • B. About 0.75 kg
  • C. About 7.5 kg
  • D. About 75 kg
Answer: B — About 0.75 kg
One standard cubic metre of natural gas weighs about 0.75 kg (methane is roughly 0.68 kg/m³ at 15 °C, with natural gas a little higher depending on composition). This is an order-of-magnitude figure; 0.075 kg is ten times too light and 7.5 kg ten times too heavy.

16. On a membrane LNG carrier, what is the purpose of the secondary barrier?

  • A. To provide the tank's main insulation against boil-off
  • B. To add cargo capacity to the primary space
  • C. To strengthen the primary barrier during loading
  • D. To temporarily contain any leakage of cold liquid and keep it off the hull steel
Answer: D — To temporarily contain any leakage of cold liquid and keep it off the hull steel
The secondary barrier is there to contain any envisaged leakage through the primary membrane for a defined period, stopping the −162 °C liquid from reaching and embrittling the hull steel. It is a containment safeguard, not insulation, which is a separate function of the insulation layers.

17. The leak at a burning deck flange has been isolated and the steel is being cooled. Which agent is used to actually put the flame out, and how does it work?

  • A. Straight seawater jets, by cooling the flame below its ignition temperature
  • B. High-expansion foam, by blanketing the gas flame
  • C. Dry chemical powder, by breaking the combustion chain reaction
  • D. CO₂ from portable extinguishers, by cooling the gas as it expands
Answer: C — Dry chemical powder, by breaking the combustion chain reaction
Dry chemical powder is the agent for a gas fire: it absorbs free radicals and breaks the combustion chain reaction. Water cools steel and screens against heat but must never be played onto a burning gas jet or pool, and foam is for a confined liquid pool, not a gas flame.

18. How is hydrate formation in the cargo system best prevented during loading?

  • A. By thoroughly drying the system, using an approved antifreeze only with the shipper's written permission
  • B. By adding water to the cargo to dilute it
  • C. By raising the cargo temperature above 0 °C
  • D. By increasing the loading rate to flush the lines
Answer: A — By thoroughly drying the system, using an approved antifreeze only with the shipper's written permission
Hydrates are ice-like solids that block lines and jam valves, and the primary defence is to dry the system so there is no water for them to form from. Antifreeze such as methanol may only be added with the shipper's written agreement because it contaminates the cargo — adding water would do the opposite of what is required.

19. The harm a toxic gas does to a person in air depends on which of the following?

  • A. The concentration in air only
  • B. Both the concentration and the duration of exposure
  • C. The duration of exposure only
  • D. The ambient temperature only
Answer: B — Both the concentration and the duration of exposure
Toxic effect depends on both how much and for how long, which is exactly why exposure limits come in pairs — a time-weighted average over the working day and a short-term limit over 15 minutes. Concentration alone ignores that a low level breathed for a long time can be just as harmful as a high level briefly.

20. A crew member's hand is splashed by liquid LNG and shows a cryogenic (cold) burn. What is the correct first-aid action?

  • A. Rub the area briskly to restore circulation
  • B. Apply direct heat from a heater or water above 45 °C
  • C. Gently flood the area with copious lukewarm water and get medical help, without rubbing
  • D. Wrap the frozen tissue tightly and leave it for several hours
Answer: C — Gently flood the area with copious lukewarm water and get medical help, without rubbing
Cold burns are rewarmed gently with plenty of lukewarm water and prompt medical help; the tissue is frozen, so rubbing it or applying direct heat causes further damage. Clothing frozen to the skin is left in place rather than pulled away.

21. Where are each seafarer's individual responsibilities in an emergency defined?

  • A. In the charter party
  • B. In the cargo manifest
  • C. In the classification society's rules
  • D. In the Safety Management System, with individual duties set out on the Muster List
Answer: D — In the Safety Management System, with individual duties set out on the Muster List
Emergency responsibilities live in the ship's Safety Management System under the ISM Code, and each person's specific duty and station appear on the Muster List. The point is to know your station before the alarm sounds — commercial documents like the charter party or manifest have no such function.

22. A jet fire burns at a deck flange with hot pipework around it; the leak is isolated and you have both a water and a dry-powder monitor. What is the correct sequence?

  • A. Cool the hot steel with water first, then extinguish the flame with dry powder, then keep cooling
  • B. Apply powder and sprayed water together for double the effect
  • C. Kill the flame with powder first, then cool the steel afterwards
  • D. Use water only, since powder is only for engine-room fires
Answer: A — Cool the hot steel with water first, then extinguish the flame with dry powder, then keep cooling
Dry powder has almost no cooling effect, so hot metal must be cooled with water first or it re-ignites the gas the instant the powder clears; the order is cool, extinguish, cool. Powder and sprayed water must never be used together because the water washes the powder out of the flame.

23. During discharge, how is the suction capacity — and therefore the discharge rate — of a cargo pump correctly regulated?

  • A. By throttling the pump's suction valve
  • B. By throttling the discharge valve and varying the pump speed
  • C. By adjusting the cargo temperature
  • D. By changing the shore tank level
Answer: B — By throttling the discharge valve and varying the pump speed
The rate is controlled on the discharge side, by throttling the discharge valve and varying pump speed. The suction is never throttled — starving a cryogenic pump on the suction side causes cavitation that quickly destroys it.

24. A cargo compressor room reads 19% oxygen before entry. Why is a cartridge/filter respirator not acceptable, and what must be worn?

  • A. A filter respirator is fine provided the space is ventilated for five minutes first
  • B. A simple dust mask is adequate at 19% oxygen
  • C. A cartridge respirator, because it adds oxygen to the air
  • D. A self-contained breathing apparatus, because a filter cannot make oxygen and the level may keep falling
Answer: D — A self-contained breathing apparatus, because a filter cannot make oxygen and the level may keep falling
A filter or cartridge respirator only cleans the air passing through it — it makes no oxygen — so it is useless where oxygen is already deficient and possibly still falling. Only a self-contained (or air-line) set that supplies its own air is safe; below 19.5% you must carry your own air.

25. Gas is burning as a controlled jet at an isolatable leak. Why is simply extinguishing the flame while gas still flows the wrong first move?

  • A. Because the flame is too hot to approach safely
  • B. Because dry powder is in short supply on board
  • C. Because it leaves an unignited, invisible vapour cloud that can drift to an ignition source and explode
  • D. Because the relief valves will lift as soon as the flame goes out
Answer: C — Because it leaves an unignited, invisible vapour cloud that can drift to an ignition source and explode
Kill the flame while gas keeps escaping and you replace a visible, controlled fire with an invisible vapour cloud that spreads low across the deck toward any ignition source — a vapour-cloud explosion is far worse than the fire it replaced. The rule is to stop the leak (isolate the source) first, then cool and extinguish.

26. Flame is impinging on the vapour space of a pressurised cargo tank still part-full of liquid. What is the danger, even though a relief valve is fitted?

  • A. A BLEVE — the dry wall in the vapour space overheats and tears while the tank is full of superheated liquid, which flashes in a fireball
  • B. The liquid will freeze solid and crack the tank base
  • C. The relief valve will reliably prevent any failure
  • D. The cargo will simply boil off harmlessly through the mast
Answer: A — A BLEVE — the dry wall in the vapour space overheats and tears while the tank is full of superheated liquid, which flashes in a fireball
Where liquid wets the wall it carries heat away, but the dry wall in the vapour space has no such cooling: it weakens and fails while the tank is still full of liquid far above its atmospheric boiling point, which flashes instantly — a Boiling Liquid Expanding Vapour Explosion. A relief valve protects against pressure, not against a weakened wall, so it cannot prevent a BLEVE.

27. How must the fixed dry chemical powder system on a gas carrier be arranged for coverage of the cargo area?

  • A. A single nozzle at the funnel covering the whole ship
  • B. Coverage of the engine room only
  • C. So that powder reaches any exposed cargo piping, manifold and process unit, from two independent units, by monitors and hand hoses
  • D. For accommodation fires only
Answer: C — So that powder reaches any exposed cargo piping, manifold and process unit, from two independent units, by monitors and hand hoses
The IGC Code requires that powder can reach any exposed cargo piping, manifold and gas process unit — the likely leak and fire points — from two independent units, through fixed monitors and hand-hose lines along the deck. A single point of delivery could not guarantee that coverage.

28. What is the vapour relative density of ethane (air = 1)?

  • A. 0.55
  • B. 2.01
  • C. 1.05
  • D. 0.60
Answer: C — 1.05
Ethane's vapour relative density is about 1.05, so it is slightly heavier than air and a leak settles into low spaces. 0.55–0.60 would be lighter-than-air values (closer to methane); 2.01 is far too high for a two-carbon gas.

29. What is the critical temperature of ammonia?

  • A. +36 °C
  • B. +132 °C
  • C. +97 °C
  • D. −33 °C
Answer: B — +132 °C
Ammonia's critical temperature is about +132 °C. Because that is well above ambient, ammonia can be liquefied — and carried — under pressure alone. The tempting −33 °C is its atmospheric boiling point, not its critical temperature.

30. Which of these cargoes does NOT require a stabilising additive (inhibitor)?

  • A. Butadiene
  • B. Vinyl chloride monomer
  • C. Isoprene
  • D. Propane
Answer: D — Propane
Propane is a saturated hydrocarbon (an alkane) and cannot polymerise, so it needs no inhibitor. Butadiene, vinyl chloride monomer and isoprene are unsaturated and polymerisable, so each is shipped with an inhibitor and a maximum carriage temperature.

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