Handling LPG And ammonia cargo - Outline of Cargo Loading Operation
preparation for loading
(1) LPG is loaded through the manifold by using different liquid lines for propane and butane into each allotted tanks by way of filling valves in the bottom.
(2) As the tank pressure rises in the process of loading, cargo is loaded with gas inside being drawn out by using compressors through the vapor suction on the top of each tank, and then returned to cargo tanks in the form of liquid after reliquefying by the reliquefaction plant on board.
(1) Air purge and leak test of loading arms in the manifold section. Air is purged from the loading arms, and at the same time leaks from the loading arm connections are checked, in either of the following two methods after their connection, and it is confirmed that the oxygen concentration is less than 1%.
i) Nitrogen gas is injected at the roots of loading arms on the shore side and air is purged from manifold drain valves on the vessel and the concentration of oxygen is measured. By the injection of nitrogen gas pressure is applied on the manifold section and leaks from the connections are also checked.
ii) A drain hose of the shore side is connected to the manifold drain valve on the ship’s side and air is purged by making use of remnant pressure in the loading rm through the following route : loading arm => manifold => drain valve => drain hose => shore. The concentration of oxygen is measured on the shore by checking gas discharged from the drain hose. As the remnant pressure is used, the pressure is not so high to conduct a leak test.
iii) Air is pushed out from manifold drain valves on the ship’s side into the atmosphere by the remnant pressure in the loading arms. In this case flammable gas is discharged on to the upper deck, and due caution is necessary to remove ignition sources.
iv) By supplying inert gas into the manifold through an inert gas line and releasing gas into the atmosphere from drain valves, the vessel can purge air by herself.
(2) Cooldown of liquid lines and arms and leak checks of lines:
When the cargo loading operation begins, liquid is transferred at a very slow rate thereby pre-cooling liquid lines and enabling leak checks. After confirming that the temperature of liquid lines are in the order of below -35C in the case of propane and 0 degree C in the case of butane, and no abnormal condition is observed, the cargo loading rate should be increased in sequence.
(3) Trim during loading operations:
Water ballast is discharged as the loading operation is in progress, and such operation should be carried out so that no excessive stresses may be imposed on the hull and that no excessive trim is created.
(4) Tank pressure during loading operations:
While loading, care is required to keep the tank pressure constant at a specified value.
(5) Disposal of remaining liquid and disconnection of loading arms after completion of loading:
After completion of cargo work, remaining liquid in the loading arms is disposed of in one of the following manners; and the arm disconnecting operation is carried out after checking to see that the concentration of flammable gas is 1 Vol % or less.
i) Method by which liquid and gas are pushed into the lines of the vessel by introducing nitrogen gas from the shore side. It is a method by which nitrogen gas is introduced at the roots of loading arms on the shore side to push remaining liquid and gas into cross over lines, and the following checks are made through drain valves : whether liquid still remains or not; and the concentration of remnant gas.
ii) Method by which remaining liquid is discharged into the sea With the manifold gate valve closed, a plastic hose is connected to the manifold drain valve, and is then led into the sea water overboard; through the hose remaining liquid is discharged into the sea. At this instant the end of the plastic hose should be slightly put into the sea; it should be adjusted so that it may not be placed too deep or above the sea surface.
iii) Method by which nitrogen gas or inert gas is let in from the m