What is the actual function of the vapor line used for loading liquefied hydrocarbons?
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What is the actual function of the vapor line used for loading liquefied hydrocarbons? Let’s discuss this
Balance the pressure between the storage tank and the tank truck, preventing excessive pressure in the tank truck during loading and thus avoiding difficulties in loading as well as loss due to venting
To be precise: it should be called the vapor-phase equilibrium line; as the name implies, this is what is meant by the statement on the second floor.
The gas phase tube serves to balance pressure
Its main function is to prevent an increase in pressure inside the tank during loading, which occurs due to the volatilization of light components present in the gas phase space of the tank, thereby making it difficult for the loading pump to perform its task. The presence of a return line allows for pressure balance between the tank and the container, reducing the pressure difference and thus increasing the loading speed/
The gas pipe of the tank truck serves two purposes: the first is to act as a gas-phase balance pipe, and the second is to function as an air inlet when unloading the compressor!
Gas phase equilibrium is ultimately aimed at ensuring safety during filling, as well as reducing losses.
In fact, different media have various functions. Their main functions are two: first, to provide balance; second, to serve as an interface for compressors. However, in LNG processing, gas-phase pipes are used to pressurize the tank trucks for unloading purposes!
Liquid hydrocarbons have high saturated vapor pressures, which leads to easy leakage. Gas-phase pipelines used for loading and unloading can regulate the air pressure inside trains and vehicles, facilitating these operations; at the same time, light hydrocarbons can be recovered, thereby reducing leakage.
When the tank is relatively hot, the liquefied hydrocarbons enter it and vaporize, causing pressure to rise; as a result, the liquid cannot enter. Therefore, it is necessary to return the gas phase to the storage tank to maintain balance. The LNG truck has three ports: a liquid phase port, a gas phase port, and a booster interface (for unloading)
During unloading, it is connected to the compressor outlet to pressurize the tank truck! During loading, balance the pressure, but it is usually not necessary to use it! Because when low-temperature ethylene or other substances enter the tank car, the gas phase is compressed, resulting in an increase in pressure; the boiling point of ethylene also rises, causing the gas phase to liquefy. Eventually, an equilibrium is reached, and the pressure inside the tank car does not keep increasing! But if the tanker contains non-condensable gases such as nitrogen, it needs to be emptied!