LNG gasification station

After the LNG tanker enters the gasification station, the unloading hose is used to connect the tanker with the gas-liquid two-phase pipeline on the unloading platform. Depending on the unloading booster in the station, a certain pressure difference is formed between the tanker and the LNG storage tank, and the LNG is discharged into the storage tank through the liquid inlet pipe. After the tank car is unloaded, the gas-liquid valve is switched to recycle the residual gas in the tank car through the gas-phase pipeline.

The LNG in the tank is sent to the LNG air temperature gasifier for gasification by pressurizing the tank supercharger, and then enters the natural gas main pipe through pressure regulation, metering and odorization for downstream users.

1 unloading process

LNG is transported from the LNG liquefaction plant to the LNG gasification station of the city using gas through the highway tank car or tank container car. The air temperature pressurized vaporizer on the tank car is used to boost the tank car storage tank (or the unloading pressurized vaporizer set in the station is used to boost the tank container car), so as to form a certain pressure difference between the tank car and the LNG storage tank, The pressure difference is used to unload the LNG in the tank car into the storage tank of the gasification station. At the end of unloading, the gas-phase natural gas in the tank car is recovered through the gas-phase pipeline of unloading platform.

When unloading, in order to prevent the pressure rise in the LNG tank from affecting the unloading speed, when the temperature of LNG in the tank car is lower than the temperature of LNG in the tank, the upper liquid feeding mode is adopted. The low temperature LNG in the tank car enters the tank in spray state through the liquid inlet nozzle on the tank, which cools part of the gas into liquid and reduces the pressure in the tank, so that the unloading can be carried out smoothly. If the temperature of LNG in the tank car is higher than that of LNG in the tank, the lower liquid inlet mode is adopted. The high temperature LNG enters the tank from the lower liquid inlet and mixes with the low temperature LNG in the tank to reduce the temperature, so as to prevent the high temperature LNG from entering the tank from the upper liquid inlet to evaporate and increase the pressure in the tank, resulting in the difficulty of unloading. In practice, Doer to the fact that the LNG source is far away from the city where the gas is used, the temperature of LNG in the tank car is usually higher than that in the storage tank of the gasification station when the long-distance transportation arrives at the city where the gas is used, so the downward liquid feeding mode can only be adopted. Therefore, in addition to the way of upper liquid feeding when LNG is filled for the first time, the way of lower liquid feeding is basically adopted when unloading the tanker normally.

In order to prevent the pipeline from being damaged or the unloading speed from being affected by large temperature difference stress caused by quenching during unloading, the LNG in the storage tank should be used to precooling the unloading pipeline before each unloading. At the same time, it is necessary to prevent liquid hammer from damaging the pipeline Doer  to sudden change of LNG flow rate caused by quick opening or closing of the valve.

2 storage process

Driven by the self weight of liquid and the internal pressure of the tank, LNG flows from the tank to the air heated gasifier, which gasifies into gaseous natural gas and supplies it to users. With the outflow of LNG from the tank, the pressure in the tank decreases, and the speed of LNG out of the tank gradually slows down until it stops. Therefore, in the normal gas supply operation, it is necessary to continuously supply gas to the tank and maintain the pressure in the tank within a certain range in order to make the LNG gasification process continue. The pressurization of storage tank is realized by using automatic pressurization regulating valve and self pressurization air temperature gasifier. When the pressure in the storage tank is lower than the set opening value of the automatic pressurization valve, the automatic pressurization valve will open, and LNG in the storage tank will flow into the self pressurization air temperature gasifier according to the liquid level difference (the installation height of the self pressurization air temperature gasifier should be lower than the minimum liquid level of the storage tank). In the self pressurization air temperature gasifier, LNG will be gasified into gaseous natural gas through heat exchange with air, and then the gaseous natural gas will flow into the storage tank, Raise the pressure in the tank to the required working pressure. The LNG in the storage tank is sent to the main air temperature gasifier for gasification by using this pressure, and then the natural gas after gasification is regulated (usually adjusted to 0.2-0.4mpa), metered and odorized, and then sent to the urban medium pressure transmission and distribution network to supply gas for users.

In the process of self pressurization, with the continuous inflow of gaseous natural gas, the pressure of the storage tank increases continuously. When the pressure rises to the closing pressure of the automatic pressurization control valve (about 10% higher than the set opening pressure), the automatic pressurization valve closes and the pressurization process ends. With the continuous gasification process, when the pressure in the tank is lower than the opening pressure set by the pressurization valve, the automatic pressurization valve will open to start a new round of pressurization.

3 gasification process

LNG at - 162 ℃ is heat exchanged with air through the device to gasify into gas-phase ng, which is then sent to the pipe network through pressure regulation and metering.

Air heated gasifier is the main gasification facility for LNG gasification station to supply gas to city. The gasification capacity of the gasifier is determined by the peak hour gas consumption with a certain margin, usually 1.3-1.5 times of the peak hour gas consumption. The gasification capacity of a single gasifier is calculated as 3000m3 / h. two sets are in a group, and two groups are configured in the design to switch each other.

According to the local climate characteristics of Guangxi, the minimum temperature in a year is 5.5-15.2 ℃, and the air temperature gasifier can be used all the year round without adding other heat exchange equipment.

4bog recovery process

During unloading operation, when all liquid LNG is unloaded into the tank, close the liquid inlet valve of the tank, and the tank car is full of gas phase natural gas; At the end of unloading, part of the remaining LNG in the liquid phase pipeline can not fully enter the storage tank, and part of the LNG must be left in the unloading supercharger; When the liquid enters the tank, the pressure of the tank will increase with the liquid phase entering, and the gas phase will be recovered through the bypass valve; There is also a certain amount of natural gasification rate of LNG in the storage tank during use; The temperature of these parts of gaseous natural gas is very low when they are recovered, and they can be recycled only after further heat exchange with the external environment through bog gasifier. After pressure regulation, metering and odorization, they are sent to the medium pressure pipeline outside the station.

5eag gas emission process

LNG is a liquid mixture mainly composed of methane, with boiling point temperature of - 161.5 ℃ and storage temperature of - 162.3 ℃ under normal pressure, and density of about 430 kg / m3. When LNG is gasified to gaseous natural gas, the critical buoyancy temperature is - 107 ℃. When the temperature of gaseous natural gas is higher than - 107 ℃, the gaseous natural gas is lighter than air and will float away from the leakage. When the temperature of gaseous natural gas is lower than - 107 ℃, gaseous natural gas is heavier than air. Low temperature gaseous natural gas will accumulate downward and form combustible explosives with air. In order to prevent the low-temperature gaseous natural gas vented by the safety valve from accumulating downward to form explosive mixture, an air temperature type safe release gas heater (EAG) is set up. The released gas is heated by the heater to make its density less than air, and then it is introduced into the upper air.

6 pressure regulating and metering process

The skid is mainly composed of ball valve, filter, pressure regulator, flowmeter and other devices. According to the two-way design and considering the backup, the pressure regulator is the self operated pressure regulator produced by Fisher company of the United States, and the flowmeter is the turbine flowmeter.

After gasification, the natural gas enters the pressure regulating metering skid, and the outlet pressure is reduced to 0.38mpa after pressure regulating. After pressure regulating, metering and odorization, the natural gas is sent to the downstream pipeline for downstream users.

The pressure regulating and metering skid fully considers the overall gas demand of the project, and the pressure regulator and flowmeter are set in two ways, one for use and one for standby; Large range ratio flowmeter is selected for the flowmeter, which can measure normally when the gas consumption is small, without setting small flow bypass. At the same time, in order to ensure the safety of gas use, the automatic odorization device is equipped, and Tetrahydrothiophene is used as odorant.