Hydrates are crystallized, compact, porous and rather light mass, similar to compressed snow. They are made of water, hydrocarbons, H2S and CO2. Unlike ice, hydrates have an unusual characteristic: they form at a temperature that is above water freezing point . For instance, they can form at 20 °C at particular pressures. When they are exposed to air, they dissolve chugging and fizzing because of the gas that is more or less slowly freed according to the surrounding temperature. When they are lit in the air, they can slowly and completely burn, until they leave a small residue of water. This does not represent the quantity of water they actually contain, as part of the water content is lost through evaporation. Hydrates form quite easily by simple contact of gas and water and the formation is related to the conditions of temperature and pressure, according to the law of equilibrium. They can quite frequently form in lines that gather gas from the various wells and transport it to the central treatment station where it undergoes dehydration. This formation of hydrates can partially or totally obstruct the lines, limiting or hindering the transportation of gas.
If the refrigerated cargo or tank system is not adequately dried, residual moisture can cause problems with icing and hydrate formation when the tanks are cooled down.
To illustrate the potential problem, air at 30°at atmospheric pressure and 50% humidity, can contain 15 g/ m3 of water.
This would equate to approximately 15 litres of water per 1,000 m3 of air. For a large LPG carrier, of 80,000 m3 capacity, it would be equal to approximately 1.200 litres of water in the cargo containment system.
The objective of drying is to lower the dew point temperature, in the tank to a temperature below that at which the cargo vapour will be introduced during the gassing-up operation.
Malfunction of cargo pumps, valves and spray nozzles due to ice or hydrate formation is often the result of an inadequately dried cargo system. Parts of Hydrates with sufficient momentum in the pipe can seriously damage pipes or other important equipment. In this subject for prevent hydrates, While the addition of antifreeze may be possible at pump suctions and other parts of the cargo system, it is not generally considered a substitute for thorough drying. Antifreeze is not permitted to be used with all cargoes and, where it is, it may not be effective at the carriage temperature for the cargo being carried.