Gas tanker basics part 2 reliq plant, heater n dcp

Summary

This video script provides an in-depth look at the operations of a liquefied gas carrier, focusing on cargo loading, boil-off gas re-liquefaction, and pressure and temperature control. It describes the process of handling cargo such as LPG and LNG, the functioning of compressors, condensers, and expansion valves to maintain liquid state, and the importance of controlling boil-off gas to prevent cargo loss. The video also covers auxiliary systems like booster pumps, cargo heaters, and fire-fighting equipment to ensure safe and efficient operations in a challenging environment with extreme temperatures.

Takeaways

• 😀 The script explains the process of handling cargo and boil-off gas on refrigerator ships.
• 😀 A prismatic cargo tank is used for storing liquefied gases, such as LPG and LNG, on these ships.
• 😀 Boil-off gas occurs when a liquid is heated above its boiling point, turning into vapor. This is a key consideration when transporting gases like propane, which has a boiling point of -42°C.
• 😀 To manage boil-off gas and avoid losses, the gas is re-liquefied using a simple three-step process: compression, cooling in a condenser, and expansion.
• 😀 Compression of gas makes it easier to convert it into a liquid, while condensation cools the gas, making it suitable for storage again.
• 😀 The expansion of compressed liquid causes a decrease in temperature, following principles like P1V1 = T1, similar to how air conditioners work.
• 😀 For gases that are difficult to compress in a single cycle, multiple-stage compressors (two or three stages) are used to reach the required temperatures.
• 😀 Cargo loading involves controlling the boil-off gas and maintaining pressure and temperature using the equation plant to ensure the cargo remains at optimal conditions.
• 😀 Additional equipment, such as methanol pumps, are used to prevent hydrate formation, which could damage pumps by crystallizing water in the gas.
• 😀 The cargo system includes booster pumps to maintain sufficient pressure for long pipelines and cargo heaters to adjust temperatures to meet delivery requirements.
• 😀 Safety measures on these ships include a fixed firefighting system using dry chemical powders, fire-fighting monitors, and portable hoses to handle potential emergencies.

Q & A

What is boil-off gas and how is it related to the cargo tank?

Boil-off gas is the vapor generated when the liquid cargo inside the tank heats up above its boiling point. For example, propane has a boiling point of -42°C, and when the liquid in the tank exceeds this temperature, it turns into vapor. This gas needs to be controlled to prevent losses and manage tank pressure and temperature.

Why is it important to re-liquefy boil-off gas?

Re-liquefying boil-off gas is crucial to prevent the loss of cargo in vapor form and to maintain the pressure and temperature inside the tank. Without re-liquefaction, the pressure would increase, and the temperature would become unstable, affecting the cargo.

How does the re-liquefaction process work in a gas carrier?

The re-liquefaction process involves three main steps: compressing the gas to increase its pressure, cooling it in a condenser to reduce its temperature, and then expanding it through an expansion valve, which reduces the pressure and further cools the liquid, turning it back into liquid state.

Why is compression needed to make gas into liquid?

Compression is needed to increase the gas's pressure and reduce its volume. This makes it easier for the gas to transition into a liquid state. The higher the pressure, the more the gas molecules are forced closer together, aiding in the liquefaction process.

What are the different stages of compression used in re-liquefaction?

Depending on the cargo and required temperature, compression can be done in single, double, or triple-stage compressors. These stages allow for a more gradual reduction in volume, enabling the gas to reach the desired liquid state and temperature, especially for gases that require very low temperatures like LNG.

How do temperature and pressure relate to each other in the re-liquefaction process?

In the re-liquefaction process, temperature and pressure are inversely related. As the gas is compressed, its temperature increases. Once cooled in a condenser, the gas is expanded through a valve, where pressure decreases, and temperature also drops, allowing the gas to return to its liquid state.

What role do methanol pumps play on LPG vessels?

Methanol pumps are used as anti-freeze agents to prevent hydrate formation in LPG vessels. Hydrates can form when water reacts with the gas at low temperatures, potentially damaging the pumps. Methanol is injected to prevent this issue.

What are cargo heaters and why are they necessary on gas carriers?

Cargo heaters are used to increase the temperature of the cargo when needed, such as when the cargo is stored at very low temperatures like -45°C and needs to be heated to -42°C for discharge. These heaters help maintain the required temperature for the cargo at the discharge point.

How does the fixed firefighting system on gas carriers work?

The fixed firefighting system on gas carriers typically uses a dry chemical powder system for extinguishing fires, particularly on the deck. It is activated using pilot bottles and valves, and the system can be triggered remotely to manage fires effectively.

What is the purpose of the booster pump in the cargo line?

The booster pump is used to enhance the pressure of the liquid cargo, especially when there is back pressure or when the cargo needs to be pushed through long pipelines. It helps to maintain adequate pressure for smooth cargo transfer.