gas tanker basics part 1

Summary

This video script provides a detailed overview of the operation of gas tankers, including the processes of inerting, purging, and loading gas cargos like LNG. It emphasizes the importance of controlling moisture levels, pressure, and temperature during cargo loading, especially for fully refrigerated ships. The script also covers the critical role of cargo control systems, safety features like relief valves, and monitoring equipment. With a focus on tank design, cargo transfer, and cooling systems, it highlights the technical precision required for safely transporting liquefied gases under extreme conditions.

Takeaways

• 😀 Gas carriers, like oil and chemical tankers, require inert gas to avoid hazardous conditions while loading cargo.
• 😀 Inert gas generators used on gas carriers produce gas with oxygen content below 1%, ensuring safe conditions inside the tanks.
• 😀 Cargo gas must be kept dry to prevent ice crystal formation, which could damage pumps or disrupt discharge operations.
• 😀 The dew point of the inert gas must be lower than the cargo temperature, ensuring no moisture is present in the cargo tanks.
• 😀 Inerting the cargo tanks involves replacing air with inert gas, and the oxygen level must be reduced to below 5%.
• 😀 Purging is done by releasing inert gas vapor into the atmosphere or to a terminal to remove residual inert gas from the system.
• 😀 To prevent damage from thermal stress when loading refrigerated gas at very low temperatures, the cargo tank must be gradually cooled down.
• 😀 The cool-down process involves careful monitoring of the pressure and temperature, starting with spraying liquid cargo at a controlled rate.
• 😀 The cargo control room (CCR) on gas carriers monitors various parameters like pressure, temperature, and oxygen content in the tanks.
• 😀 The cargo system includes various safety features like relief valves and sensors, ensuring the tank’s integrity under fluctuating pressures and temperatures.

Q & A

What is the purpose of inert gas in the cargo tanks of gas carriers?

Inert gas is used to avoid creating an explosive atmosphere in the cargo tanks. It is a precautionary measure to ensure the oxygen concentration is below 1%, preventing the risk of fire or explosion during the gas loading process.

How does the inert gas generator work in gas carriers?

The inert gas generator produces a pure inert gas that is used to displace air in the cargo tanks. It consists of a burner, scrubber, and dryer to ensure the gas is free of moisture, which could cause problems at low temperatures.

Why is moisture control crucial when loading gas on tankers?

Moisture must be controlled because if water mixes with the gas during the cooling process, ice crystals may form, causing blockages in the pump or issues during discharge. The gas needs to be absolutely dry to prevent these issues.

What is the role of the purging process in gas tanker operations?

Purging is the process of removing inert gas from the tanks and replacing it with cargo vapor. This is necessary because inert gas cannot be compressed in the refrigeration system, and excess inert gas needs to be expelled before cargo can be loaded.

What is the purpose of the cool-down process in gas tanker operations?

The cool-down process slowly lowers the temperature of the cargo tanks to match the temperature of the liquid gas (around -42°C for propane). This helps avoid excessive pressure buildup and tank damage, ensuring the cargo is safely loaded.

How is the temperature controlled during the cool-down process?

Temperature control is achieved by spraying liquid cargo into the tanks, starting from the bottom, then the middle, and finally the top. This gradual process ensures even cooling and prevents thermal stresses on the tank.

What monitoring systems are used in the Cargo Control Room (CCR) of gas carriers?

The CCR is equipped with gas detection systems, pressure gauges, temperature monitors, and emergency shut-down alarms. It allows operators to control and monitor critical parameters such as cargo levels, pressures, and the condition of the pumps and compressors.

What types of cargo tanks are used in gas carriers?

Gas carriers typically use prismatic-shaped tanks. These can be fully refrigerated, fully pressurized, or semi-pressurized, depending on the type of gas being transported. The tanks are designed to handle the specific pressure and temperature requirements of different gases.

What safety mechanisms are in place to protect the cargo tanks from pressure buildup?

Cargo tanks are equipped with safety relief valves (Mars valves) that automatically open when pressure exceeds a certain limit, allowing gas to be vented to the atmosphere or directed to the shore manifold. These valves are essential for maintaining tank integrity under high pressure.

How do gas carriers ensure the integrity of the cargo tanks during transportation?

Gas carriers use independent tanks that are not welded directly to the ship's structure. These tanks are supported by chocks and are surrounded by primary and secondary barriers to prevent leaks. Insulation, such as perlite, is used to maintain the temperature and protect the tanks from thermal stress.