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Describe vapor return usage during cargo operations.

21 Jun '17, 21:01

June 21, 2017, 9:01 p.m.
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Answer:

Loading without vapor return

When loading without vapor return, only the liquid line is connected to the terminal. On some terminals also a vapor line has to be connected but it is only for emergency use and it goes directly to flare. The cargo liquid is pumped or pressured to vessel's cargo tanks through the liquid lines.
To avoid high pressure in the vessel's cargo tanks the cargo cooling plant used. During the entire loading check the tank pressure, and do your utmost to avoid uncontrolled venting. Uncontrolled venting happens when the cargo tanks pressures rise to the set point of the safety relief valves and they open. The cargo tank vapor will then be led to the vessels vent mast. To avoid uncontrolled venting reduce the loading rate or stop loading if the cooling capacity can not be increased.

Loading without vapor return but with the use of cargo cooling plant.

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When commencing loading, always start with a slow rate to check that there is not any leakage on the terminal lines/ arms/ hoses or the vessel lines/ valves. When sure that there is no leakage's and the cargo tank shell is close to the same temperature as the cargo, increase the loading rate slowly to the agreed maximum rate. While loading rate increasing watch the cargo tank pressure carefully.
In order to avoid having too much pressure in the cargo tanks when loading, either reduce the loading rate or stop loading, if the cargo cooling plant is at its maximum capacity. If the cooling plant capacity can be increased, do it before reducing the loading rate. It is important for the vessel to load at the rate that is stated in the charter party or is agreed to by the loading master. If loading rate reduced due to foul gas, you then have to clarify it with the loading master and it must be noted in the deck logbook. When we reduce the loading rate the cargo temperature from shore will increase. On all types of gas carriers, it is important to check the cargo tank pressure all the time while loading. Do your outmost to avoid uncontrolled venting.

Loading with vapor return.

The safest and fastest way to load is with vapor return, and that can be done on all types of gas carriers. When loading with vapor return the liquid hose/ arm is connected to the vessel's liquid manifold and the vapor hose/ arm connected to the vessel's vapor manifold. The cargo liquid is pumped or pressured onboard through the vessel's liquid lines and to the cargo tanks that are to be loaded. The cargo tanks excess pressure is evacuated through the vessel's vapor lines to shore.
Before commencing of evacuating any vapor from the vessel be sure that the vapor is returned to the shore tank and not to flare. If the vapor is evacuated to flare, the vessel will be charged for the amount of vapor that is burned. Be aware that if evacuated more vapor than is agreed to, the terminal can develop problems with the shore tank pressure.

Loading with cargo cooling plant and vapor return.

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In addition to the vapor return, the vessel's cargo cooling plant used. If an indirect cargo cooling plant installed, cool down the cargo tank shell or the vapor phase in the cargo tank. If there is a direct cargo cooling plant - condense the vapor in the cargo condenser and the condensate is pressured to the cargo tank.
While loading try to keep the cargo tank pressure as low as possible, in doing so the vapor return will help alot. When vapor return exists, it is possible to increase or reduce the amount of vapor to shore by throttling the vapor manifold valve. The amount of vapor which could be sent to shore must be agreed upon with the loading master before the commencement of loading. It is the terminal's capacity to receive vapor that determines the rate of vapor the vessel can send to shore while loading.
Before opening the vapor manifold valve to send vapor to shore be sure that the vapor goes back to the shore tank and not to flare. If the vapor is sent to a flare, the vessel will be charged for the amount that is burned in the flare.
On fully pressurized gas carriers do not send to shore too much vapor that the cargo is chilled down to less than -10oC. As an example, if propane loaded and the cargo tank pressure is taken down to near 0 bars, the cargo temperature will be about -42C.
After the loading is completed the terminal loading hose/ arm has to be free of liquid. To evacuate the liquid from the loading hose/ arm either use the vessel cargo compressors and blow hot vapor, or the terminal uses nitrogen and blows onboard. If the terminal is using nitrogen, minimize the amount of nitrogen to the cargo tanks. Try to blow the line into one cargo tank only. If we get too much nitrogen in the cargo tanks, it will develop too high condenser pressure and ship's cargo compressors may stop, and in the worst case it could be an uncontrolled venting.
When loading fully refrigerated or semi-refrigerated gas carriers try to evacuate as much vapor to shore as possible in order to get the lowest possible cargo tank pressure. When loading with a high rate, the shore tank liquid level and the cargo temperature will be reduced and the vapor phase increased. When the liquid level and the cargo temperature is reduced, the terminal can take more vapor from the vessel. The maximum loading rate depends on the cargo temperature, temperature of the cargo tank shell before commencing loading, cargo cooling plant capacity, size of the loading lines and the ambient temperature.

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21 Jun '17, 21:56

June 21, 2017, 9:56 p.m.
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