30, 60, 100 cubic meter LNG storage tanks, LNG gasification station process flow
As shown in the figure, LNG is transported to the LNG satellite station via low-temperature tanker trucks. It is pressurized in the tanker truck tanks using horizontal dedicated tanker boosters set 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 vaporizer, where it exchanges heat with the air and converts into gaseous natural gas while 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 vaporizer does not reach above 5°C, it is heated using a water bath heater. Finally, after pressure regulation (regulator outlet pressure of 0.35 MPa), measurement, and odorization, it enters the city's transmission and distribution network, serving various customers.
1. Unloading Process
LNG is transported to the city's LNG gasification station from the LNG liquefaction plant via highway tanker trucks or tank container vehicles. The tanker trucks utilize an onboard air-temperature type pressurization gasifier to increase the pressure in the storage tanks (or through a booster gasifier set up within the station for tank container vehicles), creating a pressure difference between the tanker and the LNG storage tank. This pressure difference is then used to unload the LNG from the tanker into the storage tank at the gasification station. Upon completion of unloading, the gaseous natural gas from the tanker is recovered through the vapor phase pipeline at the unloading platform.
During unloading, to prevent increased pressure in the LNG storage tank from affecting the unloading speed, an upward feeding method is used when the LNG temperature in the tank car is lower than that in the storage tank. The low-temperature LNG in the tank car is sprayed into the tank through the upward feeding pipe nozzle in a spray state, cooling part of the gas to a liquid and reducing the tank pressure for smooth unloading. If the LNG temperature in the tank car is higher than that in the storage tank, a downward feeding method is employed, with the high-temperature LNG entering the tank from the downward feeding opening, mixing with the low-temperature LNG inside to cool down and avoid the high-temperature LNG evaporating through the upward feeding opening, which would increase the tank pressure and complicate unloading. In actual operations, due to the relatively long distance between the LNG gas source and the consuming city, the LNG temperature inside the tank car is usually higher than that in the storage tank at the gasification station upon arrival at the consuming city, necessitating the use of the downward feeding method. Therefore, except for the initial LNG loading, the downward feeding method is predominantly used during normal tank car 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, as this can cause a sudden change in LNG flow rate, leading to liquid impact damage to the pipeline.
2. Storage Tank Pressure Boosting Technology
Driven by pressure, LNG flows from the storage tank to the regasifier, where it is converted into gaseous natural gas for customer use. As LNG exits the tank, the internal pressure decreases, slowing the outflow rate until it stops. Therefore, during normal gas supply operations, it is necessary to continuously replenish the tank with gas to maintain the pressure within a certain range, ensuring the continuous gasification process. The tank pressurization is achieved using an automatic pressure booster valve and an auto-pressurizing regasifier. When the tank pressure falls below the set opening value of the automatic pressure booster valve, the valve opens, and the LNG in the tank flows into the auto-pressurizing regasifier due to the liquid level difference (the installation height of the auto-pressurizing regasifier should be below the tank's lowest liquid level). In the auto-pressurizing regasifier, the LNG is vaporized into gaseous natural gas by heat exchange with air, which then flows back into the tank, raising the pressure to the required operating level.
3. LNG Gasification Technology
LNG undergoes heat exchange with the atmosphere in a vacuum vaporizer, transforming from liquid to gas. By the time it exits, it is 10℃ cooler than the ambient temperature. If the temperature falls below 5℃, it is 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 Technology
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. At temperatures 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低温 gaseous natural gas will accumulate downward, forming a flammable explosive mixture with air. To prevent the accumulation of low-temperature gaseous natural gas from safety valves, which can form an explosive mixture, one air-cooled safety vent gas heater is installed. 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 regasification stations in southern regions without EAG heating equipment, to prevent operators from being injured by the cold LNG gas-liquid mixture released after the safety valve trips, the individual safety valve vent pipes and the storage tank vent pipes should be connected to the centralized vent main pipe for venting.
Key Equipment for LNG Gasification Station
LNG Storage Tank
Tanks are the main equipment of LNG liquefaction stations, accounting for a significant portion of the construction cost. The design of tanks should be given high priority.
1.1 Selection of Materials for LNG Tanks
During normal operation, the working temperature of the LNG tank is -162.3°C, and it must be cooled to -196°C before the first use.
Liquid nitrogen pre-cools the tank, which is designed for -196°C. The inner tank must withstand both the working pressure of the medium and the low temperature of LNG, requiring the material to have excellent comprehensive mechanical properties at low temperatures, particularly good low-temperature toughness. Therefore, the inner tank material is 0Crl8Ni9, equivalent to ASME standard 304.
As a normal temperature external pressure vessel, the outer shell material is selected as low-alloy steel Q345R, low-carbon steel Q245R, and Q235B.
1.2 Takeover Design
Tapped openings on the inner tank of the storage tank include: liquid inlet, liquid outlet, vapor outlet, level gauge outlet, fullness gauge outlet, upper level gauge port, lower level gauge port, and 8 process manholes. All tapping materials on the inner tank are made of 0Cr18Ni9.
For ease of regular vacuum measurement and evacuation, a vacuum evacuation port is provided on the bottom sealed cap of the outer casing (the port is sealed after evacuation). To prevent vacuum failure and the leakage of the inner casing medium into the outer casing, a pressure-relieving device is set on the sealed cap of the outer casing.
2. Unloading Pressure Vapourizer
Due to the absence of pressurization equipment on the LNG tank trucks, an onboard regasifier with a capacity of m3/h is installed at the station to increase the tank truck pressure to 0.6 MPa. The temperature of the LNG entering the regasifier is -162.3°C, and the temperature of the gaseous natural gas exiting the regasifier is -145°C.
3. Storage Tank Booster Vaporizer
After filling a 100m³ LNG tank with 90m³ of LNG, calculate the pressure increase from the unloading state's 0.4MPa to the working state's 0.6MPa within 30 minutes for the 10m³ gas phase space. According to the calculation results, one 200m³/h air-temperature vaporizer is selected for each tank to boost pressure, with the LNG entering the booster vaporizer at -162.3°C and the gaseous natural gas exiting at -145°C.
Designs often feature one LNG tank paired with one booster gasifier. Multiple tanks can also share a single or a set of gasifiers for boosting, which can simplify the process, reduce equipment, and lower costs through valve switching.
4. Air-bleed Vaporizer
The atmospheric vaporizer is the main vaporization facility for LNG gasification stations to supply gas to the city. The vaporization capacity of the vaporizer is determined based on the peak-hour gas consumption, with a certain margin, usually 1.3 to 1.5 times the peak-hour gas consumption. They are typically grouped in sets of 2 to 4, with the design featuring 2 to 3 sets, alternating for use.
Water Bath Vaporizer
When the ambient temperature is low and the outlet gas temperature of the air-cooled vaporizer is below 5°C, a water bath-type natural gas heater is connected in series after the air-cooled vaporizer to heat the gaseous natural gas after vaporization [5, 6]. The heating capacity of the heater is determined at 1.3 to 1.5 times the peak hour gas consumption.
6. BOG Heater (Boil Off Gas)
Essentially an air-temperature vaporizer
Due to the largest BOG volume occurring after the unloading of reclaim trailers, the design capacity of the BOG air-temperature heater is calculated based on this, with the time for unloading the gaseous natural gas from reclaim trailers set at 30 minutes. Taking a 40m3 trailer with a pressure drop from 0.6MPa to 0.3MPa as an example, the required capacity for the BOG air-temperature gasifier is calculated to be 240m3/h. Generally, the BOG air-temperature heater is selected based on the number of trailers that can be unloaded simultaneously at the gasification station. Typically, the heating capacity of BOG heaters ranges from 500 to 1000m3/h. During winter, the BOG is used as fuel for hot water boilers in water bath-style natural gas heaters, while the rest is sent into the urban transmission and distribution network for the rest of the year.
7. BOG Buffer Tank
For peak-shaving LNG gasification stations, to recover excess gas from unloading trucks during non-peak periods and BOG (Boil Off Gas) from storage tanks, or for natural gas blending stations to achieve uniform blending, a BOG buffer tank is often added at the outlet of the BOG heater. The capacity of the tank is set according to the recovered gas volume from the trucks.
8. Safety Release Gas (EAG) Heater
The EAG air temperature heater equipment capacity is calculated based on the maximum safe venting quantity of a 100m3 storage tank. The calculated safe venting quantity for the 100m3 tank is 500m3/h, and a single air temperature heater with a gasification rate of 500m3/h was selected in the design. The incoming gas temperature to the heater is -145°C, and the outgoing gas temperature is -15°C.
9. Pressure Regulating, Metering, and Odorization Equipment
Select pressure-reducing devices based on the scale of the LNG gasification station. Typically, two pressure-reducing lines are set up, with self-contained regulators equipped with controllers and over-pressure cut-off features being chosen as the pressure regulators.
Metering is conducted using a turbine flowmeter. The odorant is tetrahydrothiophene, and it is added by a diaphragm metering pump powered by the flow signal, injecting the odorant into the gas pipeline.
10. Filter
Structural Features of Filters
The station uses a mesh core filter for filtering out particulate impurities and water from the outgoing natural gas. The filter cores are replaceable, and the removed cores (made of stainless steel mesh) can be cleaned and reused. The filter is equipped with a pressure differential gauge, indicating the pressure difference between the inlet and outlet gases. This represents the degree of clogging in the filter cores. The precision of the filter cores is typically 50um, and the water collection chamber has a volume greater than 12% of the filter's volume.
