30, 60, 100 cubic meter LNG storage tanks, LNG gasification plant process flow
As shown in the figure, LNG is transported to the LNG satellite station via low-temperature truck tankers. It is pressurized in the tanker storage tanks using horizontal dedicated unloading boosters set up at the unloading platform. The LNG is then sent to the satellite station's low-temperature LNG storage tanks by utilizing the pressure difference. Under operating conditions, the storage tank booster increases the pressure of the LNG inside the tank to 0.6 MPa. The pressurized low-temperature LNG enters an air-cooled gasifier, where it exchanges heat with the air, converting to gaseous natural gas and increasing in temperature. The outlet temperature is 10°C lower than the ambient temperature, with a pressure of 0.45-0.60 MPa. If the temperature of the natural gas at the outlet of the air-cooled gasifier does not reach above 5°C, it is heated by a water bath heater. Finally, after pressure regulation (with the pressure regulator's outlet set at 0.35 MPa), metering, and odorization, it enters the city's transmission and distribution network, being supplied to various users.
1. Unloading Process
LNG is transported to the city's LNG regasification station from the liquefaction plant via road tankers or tank container trucks. The tanker's vacuum-temperature booster is used to increase the pressure in the tanker's storage tank (or the tanker container truck is pressurized using an onboard unloading booster at the station), creating a pressure difference between the tanker and the LNG storage tank. This pressure difference is then utilized to discharge the LNG from the tanker into the regasification station's storage tank. Upon completion of unloading, the gaseous natural gas is recovered from the tanker through the gas phase pipeline at the unloading platform.
During unloading, to prevent an increase in pressure within the LNG storage tank from affecting unloading speed, an upward liquid injection method is used when the LNG temperature in the tanker is lower than that in the storage tank. The low-temperature LNG in the tanker is sprayed into the tank through the upward liquid injection nozzle of the storage tank in a spray state, cooling part of the gas to a liquid and reducing the tank pressure, allowing for smooth unloading. If the LNG temperature in the tanker is higher than that in the storage tank, a downward liquid injection method is employed, with the high-temperature LNG entering the tank from the bottom injection port, mixing with the low-temperature LNG inside to cool down, preventing the high-temperature LNG from evaporating through the upward injection port and increasing the tank pressure, which would make unloading difficult. In actual operations, due to the relatively distant location of the LNG gas source from the consuming cities, the LNG temperature inside the tanker is typically higher than that in the storage tanks at the gasification stations upon arrival at the consuming cities, necessitating the use of the downward liquid injection method. Therefore, except for the first time of LNG filling, the downward liquid injection method is generally used during normal tanker unloading.
To prevent significant temperature difference stress from damaging the pipeline or affecting unloading speed due to rapid cooling during unloading, the unloading pipeline should be pre-cooled with LNG from the storage tank prior to each unloading. Additionally, it should be avoided to rapidly open or close valves to prevent sudden changes in LNG flow rate, which could cause liquid impact and damage the pipeline.
2. Tank Pressure Boosting Technology
Driven by pressure, LNG flows from the storage tank to the adiabatic gasifier, where it is converted to gaseous natural gas for customer use. As LNG exits the tank, the internal pressure continuously drops, slowing down the outflow rate until it stops. Therefore, during normal gas supply operations, it is necessary to continuously replenish gas into the tank to maintain the pressure within a certain range, ensuring the LNG gasification process can continue. The tank's pressurization is achieved using an automatic pressure boost valve and an auto-pressurizing adiabatic gasifier. When the tank pressure falls below the set opening value of the automatic pressure boost valve, the valve opens, allowing the LNG to flow into the auto-pressurizing adiabatic gasifier (the installation height of the auto-pressurizing adiabatic gasifier should be below the tank's lowest liquid level) due to the liquid level difference. In the auto-pressurizing adiabatic gasifier, the LNG is vaporized into gaseous natural gas through heat exchange with air, which then enters the tank, raising the internal pressure to the required operating pressure.
3. LNG Gasification Technology
LNG undergoes heat exchange with the atmosphere in a vacuum gasifier, transitioning from liquid to gas. By the time it reaches the outlet, it's 10℃ cooler than the ambient temperature. If it drops below 5℃, it's heated by a water bath vaporizer. The hot water for the water bath vaporizer comes from the water circulation of a hot water boiler.
4. Gas Safety Discharge Process
LNG is a liquid mixture primarily composed of methane, with a boiling point of -161.5°C at atmospheric pressure and a storage temperature of -162.3°C, with a density of approximately 430 kg/m³. When LNG vaporizes into gaseous natural gas, its critical buoyancy temperature is -107°C. Above -107°C, the gaseous natural gas is lighter than air and will rise and drift away from the leak source. Below -107°C, the gaseous natural gas is heavier than air, and the cold gaseous natural gas will accumulate downward, forming a flammable explosive mixture with air. To prevent the formation of an explosive mixture due to the downward accumulation of low-temperature gaseous natural gas from safety valve venting, a 1-unit air-temperature safety vent gas heater is set up. The vent gas is first heated by this heater to reduce its density below that of air before being released into the atmosphere.
For LNG liquefaction stations in the south without EAG heating equipment, to prevent the cold burn injury to operators caused by the release of low-temperature LNG gas-liquid mixture after the safety valve trips, the single safety valve vent pipe and the tank vent pipe should be connected to the central vent main pipe for venting.
Important considerations for the use of lng liquefied natural gas filling station storage tanks:
1. Tanks with dual root valves, and valves near the tank, should remain open.
2. During unloading, it is recommended to fill the tank to no more than 90% of its effective volume (manufacturers specify between 85%-95% of the geometric volume), and the remaining capacity should not be less than 10%.
3. High pressure in LNG tanks does not necessarily indicate an issue with the tank. Consider the overall system's heat loss and liquid retention time. Generally, when pressure exceeds 1.1 MPa, it is necessary to vent and depressurize. Long-term high pressure in the tank can affect the lifespan of the tank's vacuum insulation.
4. If there is an excessive and prolonged "sweating" or frosting on the storage tank, along with a rapid and continuous increase in pressure, consider the vacuum failure. Immediate action is required, including manual pressure relief and transferring the LNG liquid inside the tank.
5. Suggest conducting vacuum tests on the storage tank annually. For hot tanks, a vacuum level less than 1Pa is considered合格. For cold tanks, a vacuum level less than 0.5Pa (0.00375 TOF) is合格.
6. Tanks cannot be transported with liquid. When not in use for an extended period, the LNG liquid inside the tank should be drained.
7. Storage tanks are pressure vessels. According to the "Safety Supervision Regulations for Pressure Vessel Technology" implemented by the People's Republic of China, installation, maintenance, and modification of pressure vessels require units with corresponding qualifications to notify the local quality supervision department. A permit must be obtained before installation, maintenance, or modification can be carried out.
LNG Storage Tanks
LNG Tanks, Manufacturer of LNG Tanks, Factory Producing LNG Tanks
