Why We Need To Purge The Cargo Tanks Before Making Them Gas Free

Summary

This educational video from 'My Time Genie' delves into the critical process of purging hydrocarbons from oil tanker cargo tanks to ensure safety. It explains the dangers of hydrogen sulfide and hydrocarbons, and the importance of using inert gas to purge these gases before introducing fresh air. The video outlines three purging methods: displacement, dilution, and positive displacement, aiming to reduce hydrocarbon levels below 2% to avoid flammability. It emphasizes the use of HC detectors to measure the safety of the tank's atmosphere and inert gas, ensuring a safe environment for gas freeing operations.

Takeaways

• 🚢 The atmosphere inside a cargo tank on an oil tanker can be lethal due to gases like hydrogen sulfide and hydrocarbons.
• 🌬️ 'Perching purging' is a procedure used to expel dangerous gases from cargo tanks by introducing inert gas.
• ⛔ Hydrogen sulfide is purged to reduce its level to a point that is safe for human life.
• 🔥 Hydrocarbons are purged to prevent the risk of explosion when mixed with oxygen.
• 🔍 Punching a cargo tank is necessary before purging to avoid creating a flammable mixture with the introduction of fresh air.
• 🔄 The three stages of purging a cargo tank are: too rich (can't catch fire), flammable range (risk of fire or explosion), and too lean (safe from fire or explosion).
• 🛠️ There are three methods for purging hydrocarbons: displacement, dilution, and positive displacement.
• 💡 Inert gas is used because it lacks both hydrocarbons and oxygen, preventing the atmosphere from becoming flammable.
• 🔍 A hydrocarbon (HC) detector is crucial to ensure the atmosphere in a cargo tank is below 2% HC before introducing fresh air.
• 🌟 Once the HC level is confirmed to be below 2%, it's safe to introduce fresh air and proceed with gas freeing the cargo tank.

Q & A

What is the primary danger of the atmosphere inside a cargo tank on an oil tanker?

The atmosphere inside a cargo tank can contain hydrogen sulfide, which is dangerous for human life, and hydrocarbons, which are not only harmful if inhaled but can also lead to an explosion.

What is the purpose of purging in the context of oil tankers?

Purging is done to expel dangerous gases like hydrogen sulfide and hydrocarbons from a cargo tank, making it safe for human entry and preventing potential explosions.

Why is it crucial to reduce the hydrogen sulfide level in a cargo tank?

Reducing the hydrogen sulfide level ensures that it is no longer dangerous to human life in case it escapes onto the main deck of the tanker.

What is the significance of reducing the hydrocarbon percentage in a cargo tank before making it gas-free?

Reducing the hydrocarbon percentage prevents the atmosphere inside the tank from becoming flammable when mixed with oxygen, thus avoiding the risk of fire or explosion.

Why is it incorrect to assume that introducing fresh air into a cargo tank will automatically expel all dangerous atmospheres?

While fresh air will expel the atmosphere, it also raises the oxygen level, which, when combined with existing hydrocarbons, can create a flammable mixture if not properly managed.

What are the three stages an atmosphere in a cargo tank goes through during purging?

The three stages are: 1) Too rich, where hydrocarbons are too much and cannot catch fire; 2) Flammable range, where the hydrocarbon percentage is just right to cause a fire or explosion with a heat source; 3) Too lean, where hydrocarbons are so low that a flammable atmosphere does not form.

What is the 'lean level' referred to in the context of purging a cargo tank?

The 'lean level' is when the hydrocarbon percentage in a cargo tank is so low that introducing oxygen will not create a flammable atmosphere, thus preventing fire or explosion.

What are the three methods used to purge hydrocarbons from a cargo tank?

The three methods are: 1) Displacement method, 2) Dilution method, and 3) Positive displacement method. Each method aims to expel the existing atmosphere in the cargo tank.

Why is inert gas considered safe for introducing into a cargo tank?

Inert gas is considered safe because it lacks both the hydrocarbon fuel and oxygen, preventing the atmosphere inside the cargo tank from becoming flammable.

What is the target hydrocarbon level in a cargo tank before introducing fresh air, according to the script?

The target hydrocarbon level should be below two percent by volume to ensure that introducing fresh air will not make the atmosphere inside the cargo tank flammable.

What is the final step in making a cargo tank safe for gas freeing, as described in the script?

The final step is to confirm with gas detectors that the hydrocarbon level is below two percent by volume and that the inert gas has less than five percent oxygen before introducing fresh air into the cargo tank.