LPG Carriers brief overview
LPG carriers breif overview: One of the stars of the Constellation Marine Service team is a Gas Tanker Master mariner who has sailed on board LPG carriers with reputed companies like A.P. Moller Maersk and MOL ships. He has also sailed onboard oil tankers and different ship types with Maersk. Based on his LPG Carriers experience of 9 years including 2 years master’s experience, a brief overview of LPG carrier and its operations are highlighted by him below to some extent.
Let’s start off by saying that contrary to many people’s belief, the gas is not carried in vapor form but instead in liquid form which is maintained in that state either due to temperature or pressure of the cargo. Thus there are 3 different types of LPG carriers, based on how the cargo is carried: Fully Refrigerated, Semi Refrigerated/ Semi Pressurized and Fully Pressurized Gas carriers. When one says LPG cargo it refers to Liquefied Petroleum Gas Cargoes which normally incorporates basic cargoes such as Propane (commonly referred to as C3) and Butane (Commonly termed as C4) and its derivatives such as Propylene, Butene, Butadiene and so on. Propane is lighter than Butane and they are used in many common domestic products such as cooking gas and for making rubber products. The type of LPG cargoes which can be carried on board are mentioned in the Certificate of Fitness.
LPG Carriers brief overview-cargo tanks
The cargo tanks are made of low temperature stainless steel material capable of carrying cargoes at low temperatures based on the cargo tank design criteria. Each cargo tank dome is fitted with Fusible plugs which will melt if the temperature of the surrounding area reaches between + 98 to 104 deg C causing an emergency shut down of the cargo system. The cargo pumps usually found onboard are Deep well centrifugal pumps or fully submerged centrifugal pumps. No cargo lines or valves are present inside or passing through the cargo tanks and it is a completely closed system as some of the LPG cargoes are carcinogenic. All the pipes are penetrating the cargo tanks on the tank dome and each tank has a centerline bulkhead differentiating the port and starboard section of the tank. There are 2 MARVS (Maximum Allowable Relief Valve Setting) per tank to release the excessive pressure in case of an emergency and not to cause rupture of the tank. The MARVS are connected to the Vent line proceeding to the vent mast to release the gas at a safe height on the main deck in the gas safe zone. Each tank has various vapor inlet points at various levels in the tanks to determine the level of Hydrocarbon content during grade change.
The entire deck area is fitted with Water spray system which is used as a medium to prevent brittle facture on deck in case of cargo spill on deck due to its low temperature of cargo. Vessel also have emergency deck shower and eye wash station on deck for use by crew in case of accidental contact with the cargo. The compressor room encompasses the cargo compressors, condensers and cargo vaporizers which are used to reduce the tank temperature
The major difference between oil and gas tanker cargo calculation is that the weight of vapor is also calculated and accounted for as that forms part of the cargo. The Shrinkage factor of the cargo tank and fixed measuring Ullaging tape should also be accounted for and is also another aspect which is mentioned in the tank calibration tables. At all times the crew should monitor the pressure and temperature of the cargo tanks. The relation between temperature and pressure is critical and has to be well understood, which when simply put – they are inversely proportional to each other. If the tank pressure is high then the cargo temperature would be low and vice-versa. Boiling point terminology of products to be well understood so that we can ensure safe cargo loading, care of cargo during transit and during unloading of the cargo.
One of the basic important consideration before joining LPG carrier is a clear understanding of the basic chemistry aspects of liquefied petroleum products (LPG). A must read guides are the tanker safety guide Liquefied gases issued by ICS and Society of International Gas Tanker and terminal operators (SIGTTO) publication.
LPG Carriers works on the basic principle of NAIL which is very essential for Gas change operation signifying the density of products, which stands for –
N for Nitrogen/ Ammonia (NH3),
A for Air
I for Inert Gas
L for LPG cargo.
As a general rule, heavier cargo has to be put at the bottom of the cargo tank and lighter cargo should be displayed from the top part of the tank. More explanation of the grade change process will be explained in another separate blog.