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GASCO exit exam questions and answers — Set 18 — Advanced Training for Liquefied Gas Tanker Cargo Operations

Set 18 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 18 — 30 GASCO exit exam questions with answers

1. Under the IGC Code (unharmonised survey scheme), the intermediate survey should be conducted at intervals not exceeding how many months?

  • A. 24 months
  • B. 36 months
  • C. 48 months
  • D. 30 months
Answer: D — 30 months
Under the original IGC survey requirements, an intermediate survey for the Certificate of Fitness was required at intervals not exceeding 30 months. Under the harmonised system it falls within three months of the second or third anniversary date.

2. When gassing up with which cargo would you not use inert gas from a combustion-type generator?

  • A. Propane
  • B. Methane
  • C. Ethylene
  • D. Ammonia
Answer: D — Ammonia
Combustion inert gas contains carbon dioxide, which reacts with ammonia to form solid carbamates that can block lines, valves and filters. Tanks for ammonia are therefore inerted with nitrogen, or gassed up directly from air.

3. Which is the worst condition that can lead to cavitation in a centrifugal pump?

  • A. Steady and uniform flow conditions
  • B. A high suction pressure
  • C. Vapour pockets formed in the suction flow stream
  • D. A low discharge pressure
Answer: C — Vapour pockets formed in the suction flow stream
Cavitation occurs when suction pressure falls below the liquid's vapour pressure. Vapour pockets form in the suction stream and collapse violently at the impeller, causing noise, vibration, loss of head and erosion. This is critical with boiling liquefied gas cargoes.

4. ____ means replacing an inert atmosphere in a tank with vapour from the next cargo to a suitable level to allow cooling down and loading.

  • A. Purging
  • B. Venting
  • C. Gassing up
  • D. Blanketing
Answer: C — Gassing up
Gassing up replaces the inert gas in a cargo tank with vapour of the cargo to be loaded. This removes non-condensable gases that would hinder reliquefaction and prepares the tank for cool-down.

5. What is the typical composition of inert gas produced by an inert gas generator?

  • A. Mainly nitrogen (about 85%) and carbon dioxide (about 14%), with oxygen below 0.5%
  • B. Mainly carbon dioxide (about 85%) with about 15% nitrogen
  • C. Pure nitrogen with about 5% oxygen
  • D. Mainly argon and hydrogen
Answer: A — Mainly nitrogen (about 85%) and carbon dioxide (about 14%), with oxygen below 0.5%
An oil-fired inert gas generator burning fuel with near-stoichiometric air produces roughly 85% nitrogen and 14% carbon dioxide, with small amounts of CO, SOx and water vapour. On gas carriers oxygen is kept below about 0.5%.

6. An uncontrolled liquid or vapour release can occur through?

  • A. Lightning strikes or electrical malfunctions only
  • B. Overfilling of tanks, unforeseen opening of relief valves, hose bursting or pipe fracture, and tank or pipe leakage
  • C. Excessive cleanliness of tanks
  • D. Thermal expansion of the accommodation
Answer: B — Overfilling of tanks, unforeseen opening of relief valves, hose bursting or pipe fracture, and tank or pipe leakage
The main causes of cargo release on gas carriers are tank overfilling, relief valves lifting unexpectedly, hose or pipe failure, and leaks from tanks or piping. ESD systems, high-level alarms and careful hose handling are designed to prevent these.

7. Some liquefied gas cargoes that are unstable or reactive may be stabilised before sailing by adding?

  • A. An inhibitor or additives
  • B. Water or moisture
  • C. Oxygen
  • D. Light or UV stabilisers
Answer: A — An inhibitor or additives
Monomers such as butadiene and vinyl chloride may polymerise or react, so an inhibitor is added before shipment. The IGC Code requires an inhibitor certificate giving the inhibitor's name, amount, date added and expected lifetime.

8. What is the critical pressure (bar absolute) of n-butane?

  • A. 25.0 to 25.5
  • B. 45.0 to 45.5
  • C. 30.0 to 30.5
  • D. 38.1 to 38.2
Answer: D — 38.1 to 38.2
n-Butane has a critical temperature of about 152°C and a critical pressure of about 38 bar absolute. Above 152°C it cannot be liquefied by pressure alone.

9. Where are spring-loaded relief valves commonly used on gas carriers?

  • A. In cargo pipelines, plant components and smaller cargo tanks
  • B. In the crew quarters
  • C. In the fresh water supply system
  • D. In the engine exhaust system
Answer: A — In cargo pipelines, plant components and smaller cargo tanks
Spring-loaded relief valves are used on cargo pipelines (for example to relieve liquid trapped between closed valves), on plant components such as heat exchangers and compressors, and on smaller pressure tanks. Large cargo tanks usually have pilot-operated valves.

10. The airlock should be fitted with?

  • A. No safety features
  • B. A fixed gas detection system
  • C. A high-pressure release valve
  • D. An open flame source
Answer: B — A fixed gas detection system
Airlocks between gas-dangerous and gas-safe spaces must be mechanically ventilated and monitored for cargo vapour, with gas detection and alarms. This ensures dangerous gas cannot pass into accommodation or machinery spaces.

11. How are the pump suction capacity and the resulting discharge rate regulated?

  • A. By increasing the pump rate, opening the discharge valve fully and using a bypass
  • B. By disconnecting the pump, closing the discharge valve and increasing discharge pressure
  • C. By stopping the pump, opening the suction valve fully and reducing discharge pressure
  • D. By reducing the pump speed, throttling the discharge valve or using a bypass (return valve)
Answer: D — By reducing the pump speed, throttling the discharge valve or using a bypass (return valve)
The discharge rate of a cargo pump is controlled by changing pump speed where possible, throttling the discharge valve, or recirculating part of the flow back to the tank through a bypass or return valve. The suction is never throttled, as this causes cavitation.

12. Vapour removed from contact with its liquid and heated beyond its boiling temperature is called?

  • A. Subcooled vapour
  • B. Condensed liquid
  • C. Superheated vapour
  • D. Saturated vapour
Answer: C — Superheated vapour
Saturated vapour is in equilibrium with its liquid at the boiling temperature. Once vapour is separated from its liquid and heated above that temperature, it becomes superheated vapour.

13. Air-driven lamps of an approved type may be used in dangerous or hazardous areas. To avoid accumulation of static electricity at the appliance, which precautions should be observed?

  • A. The air supply should be connected to a high-voltage power source
  • B. The supply hose should be made of insulating material
  • C. The air supply should have a water trap, the supply hose should be of low electrical resistance, and permanently installed units should be earthed
  • D. No precautions are needed, as air-driven lamps are inherently safe
Answer: C — The air supply should have a water trap, the supply hose should be of low electrical resistance, and permanently installed units should be earthed
ISGOTT advises that air-driven lamps have a water trap in the air supply and a low-resistance supply hose, and that permanent installations be earthed. This stops static charge building up on the lamp, which could cause an incendive spark.

14. Absolute zero temperature is equal to how many degrees Celsius?

  • A. 0 degrees Celsius
  • B. 273.15 degrees Celsius
  • C. -273.15 degrees Celsius
  • D. 100 degrees Celsius
Answer: C — -273.15 degrees Celsius
Absolute zero is 0 kelvin, the lowest possible temperature, at which molecular motion is at a minimum. Since 0°C equals 273.15 K, absolute zero is -273.15°C.

15. What does the initial survey include before a ship is put into service or before the Certificate of Fitness is issued for the first time?

  • A. Inspection of crew qualifications
  • B. Assessment of environmental conditions around the ship
  • C. Verification of the ship's insurance cover
  • D. A complete survey of the structure, equipment, fittings, arrangements and material of the ship
Answer: D — A complete survey of the structure, equipment, fittings, arrangements and material of the ship
Under the IGC Code the initial survey, carried out before the ship enters service or before the International Certificate of Fitness is first issued, is a complete survey of the ship's structure, equipment, fittings, arrangements and materials covered by the Code.

16. Access from a gas-dangerous zone on the open weather deck to a gas-safe space must be via an air lock consisting of ____ substantially gastight steel doors spaced at least 1.5 m but not more than 2.5 m apart.

  • A. 2
  • B. 0
  • C. 4
  • D. 1
Answer: A — 2
The IGC Code requires an air lock to have two self-closing, substantially gastight steel doors spaced at least 1.5 m and not more than 2.5 m apart, so that both cannot easily be open at once.

17. What is the critical temperature of n-butane?

  • A. -82.6 degrees Celsius
  • B. 153 degrees Celsius
  • C. 96.7 degrees Celsius
  • D. 9.2 degrees Celsius
Answer: B — 153 degrees Celsius
n-Butane has a critical temperature of about 152 to 153°C, well above ambient, so it can easily be liquefied by pressure alone. Methane, propane and ethylene all have lower critical temperatures than butane.

18. Why are cargo vaporisers required on gas carriers?

  • A. To produce vapour that may be needed for gassing up tanks
  • B. To increase the liquid cargo temperature before discharge
  • C. To cool down the cargo tanks during loading
  • D. To release excess pressure from the cargo tanks
Answer: A — To produce vapour that may be needed for gassing up tanks
A vaporiser turns liquid cargo into vapour. It is used to gas up tanks, especially when no shore vapour is available, and to supply vapour that keeps tank pressure up during discharge without vapour return. Warming liquid is the job of the cargo heater.

19. What are the main categories of cargo containment systems?

  • A. Unattached tanks, temporary tanks, external tanks
  • B. Permeable tanks, composite tanks, externally insulated tanks
  • C. Integral tanks, independent tanks, membrane tanks, semi-membrane tanks, internal insulation tanks
  • D. Conductive tanks, radiant tanks, exothermic tanks
Answer: C — Integral tanks, independent tanks, membrane tanks, semi-membrane tanks, internal insulation tanks
IGC Code chapter 4 classifies cargo containment systems as integral, membrane, semi-membrane, independent Type A, B and C and internal insulation tanks. Each type has its own design and secondary barrier requirements.

20. As per the IGC Code, cargo tank temperature indication requires at least ____ devices.

  • A. 0
  • B. 2
  • C. 1
  • D. 5
Answer: B — 2
The IGC Code requires each cargo tank to have at least two devices for indicating cargo temperature, one at the bottom of the tank and one near the top below the highest allowable liquid level, so that temperature can be monitored during cool-down and voyage.

21. Which is a typical hydrate inhibitor?

  • A. Nitrogen
  • B. Carbon dioxide
  • C. Water
  • D. Methanol
Answer: D — Methanol
Methanol (like glycols) acts as an antifreeze. It lowers the temperature at which hydrates form and is injected into pump suctions and lines on LPG carriers to stop hydrates blocking pumps, valves and filters.

22. Compared to globe and angle valves, gate valves will:

  • A. Give a higher pressure drop when fully open
  • B. Operate with little or no pressure drop when fully open
  • C. Have a similar pressure drop to globe and angle valves
  • D. Always function better in high-temperature conditions
Answer: B — Operate with little or no pressure drop when fully open
A fully open gate valve leaves a straight, unobstructed flow path, so its pressure drop is very small. Globe and angle valves force the flow to change direction through the seat, which creates much higher resistance.

23. In all gas trades it is essential that the ship and terminal operators are:

  • A. Unaware of each other's emergency procedures
  • B. Lacking knowledge of the cargo handling equipment
  • C. Unfamiliar with emergency shutdown procedures
  • D. Familiar with the basic characteristics of each other's facilities, aware of the division of responsibilities and able to communicate effectively
Answer: D — Familiar with the basic characteristics of each other's facilities, aware of the division of responsibilities and able to communicate effectively
SIGTTO and ISGOTT guidance stress that safe cargo transfer depends on ship and terminal understanding each other's equipment, agreeing who is responsible for what in the ship/shore safety checklist, and keeping effective communication throughout the port call.

24. How many non-return valves, or equivalent, are fitted in the inert gas main to prevent backflow of cargo vapour?

  • A. 5
  • B. 20
  • C. 2
  • D. None
Answer: C — 2
The IGC Code requires two non-return valves or equivalent devices in the inert gas main where it connects to the cargo system. This prevents cargo vapour or liquid flowing back into the inert gas plant in gas-safe spaces.

25. The main hazards of a liquefied gas vapour cloud are?

  • A. Transparency
  • B. Invisibility
  • C. Flammability, toxicity and low temperature
  • D. Buoyancy
Answer: C — Flammability, toxicity and low temperature
A vapour cloud from spilled liquefied gas may be flammable, toxic or asphyxiating, and very cold, causing frostbite and brittle fracture of steel. These hazards decide the emergency response and the area that must be kept clear.

26. Hydrates are snow-like compounds of?

  • A. Carbon dioxide and methane
  • B. Hydrocarbon and water
  • C. Oxygen and nitrogen
  • D. Sulphur and hydrogen
Answer: B — Hydrocarbon and water
Hydrates are ice-like crystalline compounds that form when free water combines with hydrocarbons such as propane or butane under certain conditions of pressure and temperature. They can block pumps, valves and instrument lines.

27. What is the critical pressure (bar absolute) of propane?

  • A. 58.4
  • B. 36.2
  • C. 42.6
  • D. 25.8
Answer: C — 42.6
Propane has a critical temperature of about 96.7°C and a critical pressure of about 42.5 bar absolute. Fully pressurised LPG tanks are designed for a vapour pressure of around 18 bar, well below this.

28. One source of pollution from gas cargoes is the release of ____ vapours from the LPG condenser of the reliquefaction plant.

  • A. Sulphur-containing
  • B. Water-soluble
  • C. Odourless
  • D. Incondensable
Answer: D — Incondensable
Non-condensable gases such as nitrogen, inert gas and some cargo vapour collect in the condenser and must be vented. This release carries cargo vapour to atmosphere, so careful gassing up and purging are needed to keep incondensables to a minimum.

29. Which factors influence a pump's suction condition?

  • A. The colour of the liquid
  • B. The ambient temperature only
  • C. The pump's NPSH, the cargo tank pressure, the liquid level and the cargo vapour pressure
  • D. The ship's draft
Answer: C — The pump's NPSH, the cargo tank pressure, the liquid level and the cargo vapour pressure
The available suction head depends on tank pressure, liquid level above the pump and the cargo's vapour pressure. This must exceed the pump's required NPSH or the cargo boils at the impeller and cavitates, which is critical with boiling liquefied gases.

30. The ratio of the density of a liquid at a given temperature to the density of fresh water at a standard temperature is called?

  • A. Dynamic viscosity
  • B. Specific gravity
  • C. Vapour pressure
  • D. Surface tension
Answer: B — Specific gravity
Specific gravity, or relative density, is the density of a liquid divided by the density of fresh water at a standard temperature. It has no units, and cargo calculations use it to convert volume into mass.

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