Oil tankers are designated to carry petroleum-based substances in liquid form. Inside these vessel tanks, there is a tendency for gaseous vapour formation above the level of the liquid cargo. Even after cargo offloading, when we have empty tanks, these gases are left behind and often pose hazardous conditions.
These gaseous contents, rich in oxygen concentration, are highly inflammable and explosive in nature, often aggravating the risks of combustion, which can be extremely dangerous.
Hence, as per regulatory guidelines, all tankers should be equipped with an Inert Gas System (IGT) that reduces this risk dramatically.
What are inert gases?
Inert gases, also known as noble-type gases, are non-reactive or negligibly reactive gases that do not have combustion or oxidation characteristics.
In other words, these gases do not react to cause a fire. Ideal inert gases are helium, neon, argon, krypton, and so on. Other than these, gases with low oxygen content and those that lack combustion capabilities are also classified as inert gases as they have negligible risks of explosion or fire.
What is Inert Gas System?
An inert gas system employs the concept of forcefully blowing inert gases inside the tank holds through an extensive piping arrangement to reduce the concentration of flammable oxygen trapped inside them and make them lesser reactive to combustions. Essentially, by pumping in inert gases inside the tankage, there is a mixture of inert gases and oil vapours is created that is far less reactive.
For all practical purposes, for a gas to be qualified as inert gas onboard a vessel, as per SOLAS requirements, it should have a maximum oxygen level content of 8% by volume.
Furthermore, the resultant mixture of gases should have a worst-case maximum oxygen concentration of 10%. For all practical purposes, any concentration greater than 11-12% is considered harmful and explosive.
Also, the amount of inert gases to be supplied inside the tank holds cannot be indefinite or in excess only for the purpose of solely reducing inflammability risks. This is because an excess build-up of pressure can also have adverse effects and lead to tank bursts, structural failure, and other modes of damage.
For practical purposes, for a fully filled oil tank with some clearance above the liquid level, a maximum of 5% margin of inert gas content can be accommodated. Of course, depending on the level of cargo in tanks, this can be more.
The main sources of these inert gas supplies are:
- Flue or run-off gases from boilers and exhaust systems are high in carbon content and have less reactive properties.
- A dedicated inert gas Generator
- Turbines for supplying these inert gases.
What is a deck seal?
A deck seal is like a preventive system or stopper mechanism that prevent the backflow of the inert gas and oil vapour mixture to its sources.
Now, as we know from above, inert gas is produced and supplied from the given sources. As all these mechanisms or systems are situated close to or near the engine and machinery spaces due to their purpose and powering requirement, any backflow of combustible substances in the form of inert gas and vapour mixture is highly risky and combustible.
At this point, a question may arise: How the gases with oxygen concentration levels within 10% be of potential risk? The answer lies in the fact that an 8-10% concentration of oxygen is still quite significant in areas with a high heat index and electrical activity. Unlike ideal inert gases like helium or neon, the concentration of these waste gases is not almost null. Therefore, these mixtures are low-combustible or moderately combustible but not completely incombustible!
The machinery spaces and all types of regions in the way of the engine room(s) have a higher temperature, multiple ignition sources, powering lines, and a network of equipment and systems in constant operations.
So, in case of any kind of leakage or build-up of concentrated and flammable gases, and that too under consistent exposure to a risk-prone environment over a prolonged amount of time, the risks multiply manifold. Hence, all care should be taken to ensure that any form of gases from the cargo holds is not reverted to their sources through the same network or released into hazardous environments.
The IGT systems usually have valves (of non-return type) in their supply lines. But often, valves are not 100% reliable, and in case of any form of failure or leakage, the risk comes back into the picture.
Thus, deck seals ensure protection against the risky backflow or reverse flow of inert gas and oil vapour mixture, acting as a reliable secondary safety barrier.
There are three types of deck valves used
Wet Valve: This is the most common type of deck seal mechanism used. This is based on the physics of pressure differentials. The inlet line of gases is made to flow into a container partially filled with water and an outlet for the gases to the deck main or the tankage. Now, if the positive or upstream pressure of the gases is greater than the hydraulic pressure of the water column, there is no issue.
However, in the reverse case, when the outlet has a higher pressure or a resultant negative pressure, the gases tend to flow back, and that’s when the water column acts as a barrier, preventing the risky backflow of the gases to the inlet line.
Here, essentially, the water acts as a padding or plug against the gases by maintaining a positive pressure differential. This is also less expensive. The fundamental concept is based on a venturi meter. Disadvantages include corrosion effects, lesser flow rates of inert gas through the lines due to the water constraints, and often water droplets and particles passing through along with the gases.
Semi-dry type: The principle of this second type works based on the suction of water whenever the net pressure of the chamber becomes negative; that is, there is a chance of water backflow to the source route through the piping. However, the fundamentals of the venturi meter remain the same. In this design, essentially, the passage of inert gas from the source to the deck spaces and tanks remains dry without any obstruction.
However, there is a separate compartment attached to this piping that holds water. During positive airflow, that is, when the gas flows from the source to the cargo tanks, the water remains in its designated chamber.
However, in the event of a backflow or negative pressure differential, the water from the hold is drawn or sucked in due to suction or vacuum action. Now, the water column, like in the wet valve type seal, acts as a barrier and prevents water from backflowing. The semi-dry type of seal can be considered an improvement over the wet type.
Dry type: This is the most advanced form of deck seal mechanism. It combines the principle of the above two in an automated manner. The passage of inert gases through the lines mostly remains dry. And the water is separately stored in a separate adjoining tank known as the holding tank. In the event of negative pressure, or reverse flow, there are detecting sensors which get activated. These, in turn, activate a release mechanism in the form of an automated valve control to the holding tank, which releases water into the pipelines.
This water, once again, acts as a barrier and prevents the backflow of gases to the source. All actions of filling and drainage of water are done by automated valves. The only disadvantage of this system is that, like all other automated systems, a failure can render the system defunct and increase the risk of gas backflow. So, these kinds of seals require regular maintenance and inspections. They are commonly installed in most modern tankers.
Deck seals are usually placed at intermitted locations in the way of the inert gas pipeline network at designated intervals. They externally appear like an enclosed cylinder or metal casing that rises above the main deck level and are strictly air and watertight.
They have inlets and outlets for the piping and are often maintained at a definite pressure using pressure monitoring systems. They also have openings or small manholes for repair and maintenance work. All the boundary edges of the sealed container are either tightly welded or bolted as per requirement. They are often protected against corrosive effects and external and internal influences.
From a structural point of view, as these encasings for the deck seal compartment impose extra loads on the deck, they are preferably fitted in the way of underdeck stiffening members. Deck seals are usually fitted in later stages of construction as outfit or piping items. To avoid risks, they are inspected and maintained regularly for the oil tankers.
Common components of the deck seal system other than the mechanism mentioned above are:
- Relief valves
- Inlet and outlet valves
- Dain valves
- Inlets and outlets for water and gases
- Pressure monitoring apparatus
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Subhodeep is a Naval Architecture and Ocean Engineering graduate. Interested in the intricacies of marine structures and goal-based design aspects, he is dedicated to sharing and propagation of common technical knowledge within this sector, which, at this very moment, requires a turnabout to flourish back to its old glory.