Describe cargo cooling routines on board of gas carriers.
07 Jun '17, 03:52
On fully refrigerated gas carriers a low pressure and temperature of the cargo must be maintained. Cargo cooling plants procedure at sea depend on the plant itself, type of cargo, cargo tank insulation, the ambient temperature and the length of the sea voyage. The fewer cargo plants and hours the plant is running, the lower fuel consumption resulted, and maintenance cost will be reduced.
As long as the cargo cooling plants are running, there must be no vacuum in the cargo tanks. A positive cargo tank pressure e.g. 0,01 bar or higher to be maintained to avoid air leaks into the cargo tank. Some cargoes like ethylene and butadiene, will be contaminated if the content of oxygen is too high. To avoid a vacuum in the cargo tank there may be a pressure switch on the suction side of the cargo compressor or on the cargo tank connected to the cargo compressor, that stops the compressor when there is too low suction pressure.
On semi-refrigerated gas carriers there are a few more options how to handle cargo at sea, than with a fully refrigerated gas carrier. It is possible either maintain the cargo temperature or cool down the cargo, depending on the charter party. The number of cargo cooling plants to be used depends on the cargo, the ambient temperature, capacity of the cooling plant, duration of the sea voyage and the insulation of the cargo tanks. Cargo should be cooled down to the discharge temperature as soon as possible and then maintained at this temperature the rest of the voyage. It gives a possibility to discharge the cargo earlier if discharge port changed . As long as the cargo is onboard it may be sold to another customer, so it must be prepared to discharge the cargo earlier than planned. Must always be kept in mind that cooling down the cargo demands a lot of energy. Read the charter party/ loading order carefully and run the cargo cooling plants as economically as possible.
Always check the weather forecast and air temperatures for your voyage. The seawater temperature has a major influence on your cooling capacity. Higher seawater temperature results in reduced cooling capacity. Vessel pitching and rolling result in additional rise of cargo tank pressure. Try to cool down the cargo as mush as possible before you enter into bad weather. When the cargo cooling plant is running the cargo-cooling log must be filled out. In the cargo-cooling log recorded various pressures, temperatures and ampere for each of the cargo cooling plants. Also remember to record when condensate was changed and to which tank pumped. This is to avoid overfilling of cargo tanks and interrupt the cargo cooling process.
On fully pressurized gas carriers only cargo tank pressure can be controlled if there is cargo compressor onboard. If not vapor could be vented the to the atmosphere. The cargo tank steel of fully pressurized gas carriers normally designed for minimum temperature of -10C. That means we normally control the cargo temperature and pressure and do not lower it.
07 Jun '17, 05:09