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GASCO exit exam questions and answers — Set 17 — Advanced Training for Liquefied Gas Tanker Cargo Operations

Set 17 of the Advanced Training for Liquefied Gas Tanker Cargo Operations (GASCO) exit exam bank — 30 of 710 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 17 — 30 GASCO exit exam questions with answers

1. Type 3G ships are permitted to carry only ____ and ____ cargoes.

  • A. Hazardous and explosive
  • B. Nitrogen and refrigerant gas
  • C. Radioactive and flammable
  • D. Pressurised and corrosive
Answer: B — Nitrogen and refrigerant gas
In chapter 19 of the IGC Code, products such as nitrogen and refrigerant gases are assigned ship type 3G, the least stringent type, because they are non-flammable and of low toxicity.

2. A liquid with an absolute vapour pressure exceeding 2.8 bar (kg/cm2) at 37.8 degrees Celsius, and certain other substances specified in the Code, is known as?

  • A. Liquefied gas
  • B. Volatile water
  • C. High-pressure oil
  • D. Non-flammable solvent
Answer: A — Liquefied gas
The IGC Code defines liquefied gases as liquids having a vapour pressure exceeding 2.8 bar absolute at 37.8°C, together with certain other substances listed in chapter 19 of the Code.

3. Vapour produced above a cargo liquid surface due to evaporation caused by heat ingress is known as?

  • A. Condensation
  • B. Precipitation
  • C. Boil-off
  • D. Sublimation
Answer: C — Boil-off
Boil-off is vapour generated when heat leaks through the tank insulation into a refrigerated cargo, causing it to evaporate. It must be handled by reliquefaction, use as fuel or a gas combustion unit to control tank pressure.

4. A centrifugal pump sets the liquid into motion by means of ____ and ____ force.

  • A. Gravitational and magnetic
  • B. Kinetic and centrifugal
  • C. Thermal and frictional
  • D. Electromagnetic and pneumatic
Answer: B — Kinetic and centrifugal
The rotating impeller of a centrifugal pump gives the liquid kinetic energy and throws it outward by centrifugal force; the volute or diffuser then converts this velocity into pressure head.

5. The cargo compressor room is maintained at ____ air pressure.

  • A. Negative
  • B. Positive
  • C. Neutral
  • D. Vacuum
Answer: A — Negative
The compressor room is a gas-dangerous space, so its mechanical ventilation extracts air and keeps it at negative pressure relative to adjacent spaces. Any leakage flows inward, while the electric motor room is kept at positive pressure.

6. If the cargo is fully ____ at -163 degrees Celsius, the cargo tanks are made from aluminium or nickel steel.

  • A. Heated
  • B. Pressurised
  • C. Insulated
  • D. Refrigerated
Answer: D — Refrigerated
LNG is carried fully refrigerated at about -163°C at near atmospheric pressure. Materials in contact with it must keep their toughness at that temperature, so the IGC Code requires aluminium alloys, 9% nickel steel, austenitic stainless steel or invar.

7. The two main types of compressors used on gas carriers are the ____ and ____ types.

  • A. Rotary and reciprocating
  • B. Centrifugal and pneumatic
  • C. Linear and turbo
  • D. Hydraulic and diaphragm
Answer: A — Rotary and reciprocating
Cargo compressors on LPG carriers are mainly reciprocating (oil-free piston) machines or rotary screw compressors. Both are positive-displacement types used for reliquefaction, vapour return and gassing-up duties.

8. The purpose of gas freeing is to replace residues of inert gas and cargo vapour with:

  • A. Liquid nitrogen
  • B. Oxygen
  • C. Dry air
  • D. Carbon monoxide
Answer: C — Dry air
Gas freeing (aeration) replaces inert gas and cargo vapour in the tank with dry air until the atmosphere holds 21% oxygen and no harmful vapour. Dry air is used so that moisture and ice do not form.

9. What is the vapour relative density (air = 1) of methane?

  • A. 2.07
  • B. 1.55
  • C. 0.42
  • D. 0.55
Answer: D — 0.55
Methane has a molecular mass of 16 against about 29 for air, giving a vapour relative density of about 0.55, so methane vapour is lighter than air. The value 0.42 is the relative density of liquid methane.

10. Which types of valves are commonly used on gas tankers?

  • A. Piston valves, diaphragm valves, needle valves, check valves
  • B. Ball valves, globe valves, gate valves, butterfly valves
  • C. Plug valves, spool valves, pinch valves, pressure relief valves
  • D. Slide valves, control valves, solenoid valves, safety valves
Answer: B — Ball valves, globe valves, gate valves, butterfly valves
Cargo systems on gas carriers mainly use ball, globe, gate and butterfly valves, made of materials suited to low temperatures and often with extended bonnets to keep the gland seals away from the cold cargo.

11. Which type of ship is most suitable for carrying smaller cargoes of LPG and chemical gases on short-sea and near-sea routes?

  • A. Fully pressurised ships
  • B. Fully refrigerated ships
  • C. Membrane LNG carriers
  • D. Large ethylene carriers
Answer: A — Fully pressurised ships
Fully pressurised ships have small Type C pressure vessels and carry LPG and chemical gases at ambient temperature without reliquefaction. They are simple and cheap to run, so they suit small parcels on short coastal routes.

12. An enclosed space in the cargo area, external to a cargo containment system, other than a hold space, ballast space, fuel oil tank, cargo pump or compressor room, or any space in normal use by personnel, is called?

  • A. Habitable zone
  • B. Restricted area
  • C. Confined space
  • D. Void space
Answer: D — Void space
This is the IGC Code definition of a void space: an enclosed space in the cargo area outside the cargo containment system that is not a hold, ballast, fuel, cargo pump or compressor room or a space normally used by personnel.

13. On a fully refrigerated gas tanker carrying liquid butane, how is the tank pressure controlled?

  • A. By the reliquefaction plant
  • B. By adjusting the ship's draft
  • C. By manual manipulation of the tank valves
  • D. By the cargo pump discharge pressure
Answer: A — By the reliquefaction plant
On fully refrigerated LPG ships the boil-off vapour is drawn off by the cargo compressors, condensed in the reliquefaction plant and returned to the tank, keeping cargo temperature and tank pressure within the design limits required by IGC chapter 7.

14. The temperature at which the heat from combustion of burning vapours is capable of producing sufficient vapour for combustion to continue is called?

  • A. Fire point
  • B. Ignition temperature
  • C. Flash point
  • D. Auto-ignition temperature
Answer: A — Fire point
At the flash point vapour ignites only momentarily. At the fire point, slightly higher, the liquid gives off vapour fast enough to keep the flame burning once it has been ignited.

15. The pressure at which a substance exists in the liquid state at its critical temperature (the saturation pressure at the critical temperature) is called?

  • A. Saturation pressure
  • B. Critical pressure
  • C. Vapour pressure
  • D. Triple point pressure
Answer: B — Critical pressure
Critical pressure is the saturation vapour pressure at the critical temperature. Above the critical temperature a gas cannot be liquefied by pressure alone, however high that pressure is.

16. The gas codes call for sampling intervals from any one space generally not exceeding how many minutes?

  • A. 40 minutes
  • B. 35 minutes
  • C. 38 minutes
  • D. 30 minutes
Answer: D — 30 minutes
For sequential sampling fixed gas detection systems, the IGC Code requires that each sampling point be sampled at intervals not exceeding 30 minutes, so a leak in any monitored space is detected promptly.

17. Inhibiting the flame is the most ____ way to extinguish a free-burning liquefied gas fire.

  • A. Inefficient
  • B. Unreliable
  • C. Effective
  • D. Expensive
Answer: C — Effective
Dry chemical powder interrupts the combustion chain reaction, inhibiting the flame. For free-burning liquefied gas fires this is the most effective method of extinguishment, which is why the IGC Code requires a fixed dry powder system on deck.

18. Which law states that equal volumes of different gases at the same pressure and temperature contain the same number of molecules?

  • A. Boyle's law
  • B. Avogadro's law
  • C. Charles's law
  • D. Gay-Lussac's law
Answer: B — Avogadro's law
Avogadro's law states that equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. It is why one mole of any ideal gas takes up about 22.4 litres at STP.

19. A ship whose gas cargo is listed in chapter 19 of the IGC Code and has very severe safety and environmental hazards, requiring maximum preventive measures to preclude escape of the cargo, is called which type of ship?

  • A. Type 2G
  • B. Type 3G
  • C. Type 1G
  • D. Type 4G
Answer: C — Type 1G
The IGC Code defines a Type 1G ship as a gas carrier intended for products needing maximum preventive measures to prevent their escape, such as chlorine. Type 2G and 3G ships are for progressively less hazardous products.

20. What is the purpose of heat exchangers in a gas carrier's cargo handling system?

  • A. As air and inert gas driers, lubricating oil coolers, liquid cargo heaters, vaporisers, and condensers and intercoolers in the reliquefaction plant
  • B. Compression of inert gas
  • C. Generation of electricity
  • D. Cooling of accommodation spaces
Answer: A — As air and inert gas driers, lubricating oil coolers, liquid cargo heaters, vaporisers, and condensers and intercoolers in the reliquefaction plant
Heat exchangers are used across the cargo plant: driers for air and inert gas, oil coolers, cargo heaters and vaporisers for discharge and gassing up, and condensers and intercoolers in the reliquefaction cycle.

21. Equipment incapable of releasing sufficient electrical or thermal energy to ignite a specific hazardous atmospheric mixture in its most easily ignited concentration is known as?

  • A. Flameproof
  • B. Intrinsically safe
  • C. Non-incendive
  • D. Explosion proof
Answer: B — Intrinsically safe
Intrinsically safe equipment limits electrical and thermal energy so that, in normal use or under specified faults, it cannot ignite the gas mixture. Flameproof or explosion-proof equipment instead contains an internal explosion within its enclosure.

22. A catalytic explosimeter should not be used in a/an:

  • A. Oxygen-enriched atmosphere
  • B. Hydrogen atmosphere
  • C. Inerted atmosphere
  • D. Well-ventilated area
Answer: C — Inerted atmosphere
A catalytic filament explosimeter needs oxygen to burn the sample gas on its filament. In an inerted or oxygen-deficient atmosphere it gives false low readings, so hydrocarbon content in inerted tanks must be measured with an infrared or refractometer-type instrument.

23. Which areas does the water spray system cover on gas carriers?

  • A. Tank domes, cargo deck, manifolds, deck houses and superstructure boundaries facing the cargo area
  • B. Engine room
  • C. Accommodation areas
  • D. Cargo holds
Answer: A — Tank domes, cargo deck, manifolds, deck houses and superstructure boundaries facing the cargo area
IGC Code chapter 11 requires a water spray system to cool and protect exposed cargo tank domes, cargo tanks on deck, manifolds, cargo deck houses and superstructure boundaries facing the cargo area against fire.

24. The introduction of fresh air into a tank to remove toxic, flammable and inert gases and raise the oxygen content to 21% by volume is called?

  • A. Ventilating
  • B. Purging
  • C. Aerating
  • D. Inerting
Answer: C — Aerating
In gas carrier terminology aeration is the final stage of gas freeing. Fresh dry air replaces inert gas and any remaining cargo vapour until the oxygen content reaches 21% by volume, so the tank is safe for entry.

25. When cargo is being discharged from a refrigerated ship into pressurised storage, it is necessary to ____ the cargo, usually to at least 0 degrees Celsius.

  • A. Cool or chill
  • B. Warm or heat
  • C. Freeze
  • D. Evaporate or vaporise
Answer: B — Warm or heat
Shore pressurised tanks are usually made of carbon steel that is not designed for low temperatures. Refrigerated cargo is therefore passed through the booster pump and cargo heater to raise it to about 0°C or above before it enters the shore tank.

26. The depth of liquid in a tank, measured from the bottom, is called the?

  • A. Draft
  • B. Ullage
  • C. Sounding
  • D. Freeboard
Answer: C — Sounding
A sounding is the depth of liquid measured from the tank bottom or datum up to the liquid surface. An ullage is the empty space measured from the top of the tank down to the liquid surface.

27. Which cargoes can cause a chemical burn in addition to a cold burn?

  • A. Butadiene
  • B. Propylene
  • C. Methane
  • D. Ethylene oxide, ammonia and chlorine
Answer: D — Ethylene oxide, ammonia and chlorine
Ethylene oxide, ammonia and chlorine are corrosive or reactive to body tissue. On skin contact they cause chemical burns as well as frostbite from the cold liquid, while hydrocarbons such as methane and propylene cause only cold burns.

28. When heat is added to or removed from a substance during a reversible process, the heat divided by the absolute temperature of the substance is called?

  • A. Internal energy
  • B. Enthalpy
  • C. Entropy
  • D. Gibbs free energy
Answer: C — Entropy
Entropy change is defined as dS = dQ/T for a reversible process, where dQ is the heat transferred and T is the absolute temperature. Entropy charts are used when analysing reliquefaction cycles.

29. Which cargo inhibitor is highly toxic, so that great care must be taken when handling it?

  • A. Sodium hydroxide
  • B. Diethyl ether
  • C. Phosphoric acid
  • D. Tertiary butyl catechol (TBC)
Answer: D — Tertiary butyl catechol (TBC)
Tertiary butyl catechol is the inhibitor commonly added to butadiene and other monomers to prevent polymerisation. It is toxic and corrosive to skin and eyes, so full protective equipment is needed when handling it or the inhibitor certificate samples.

30. Under what conditions can a gas be liquefied?

  • A. High temperature (heating) and low pressure (expansion)
  • B. Low temperature (cooling) and high pressure (compression)
  • C. Low temperature (cooling) and low pressure (expansion)
  • D. High temperature (heating) and high pressure (compression)
Answer: B — Low temperature (cooling) and high pressure (compression)
A gas is liquefied by cooling it below its boiling point and/or compressing it, provided it is below its critical temperature. Reliquefaction plants use both compression and cooling to condense boil-off vapour.

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