LNG liquefaction plant main equipment
LNG Storage Tank
Tanks are the main equipment of LNG liquefaction stations, accounting for a significant portion of the construction cost. The tank design should be highly prioritized.
1.1 Material Selection for LNG Tanks
During normal operation, the working temperature of the LNG tank is -162.3°C; it must be pre-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 constant-temperature, external-pressure vessel, the outer shell material is selected to be low-alloyed container steel Q345R, low-carbon steel Q245R, and Q235B.
2.2 Takeover Design
Tappings on the inner tank of the storage tank include: feed-in liquid ports, discharge liquid ports, outlet ports, vapor phase ports, level measurement ports, upper level gauge ports, lower level gauge ports, and 8 process manholes. The material of the tappings on the inner tank is all 0Cr18Ni9.
To facilitate regular measurement of vacuum levels and evacuation, a vacuum evacuation port is provided on the bottom closure of the outer casing (which is sealed after evacuation). To prevent vacuum failure and the leakage of internal medium into the outer casing, a safety device is installed on the top closure of the outer casing.
2. Unloading Vehicle Booster Vaporizer
Due to the absence of pressure boosting equipment on the LNG tank trucks, an onboard gasification 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, while the temperature of the gaseous natural gas exiting the regasifier is -145°C.
3. Storage Tank Pressure VapORIZer
After filling a 100m3 LNG storage tank with 90m3 of LNG, the pressure in the 10m3 vapor space is increased from 0.4 MPa at the unloading state to 0.6 MPa for working conditions within 30 minutes. Calculations show that one 200m3/h air-temperature gasifier is used per tank for pressure increase, with the LNG entering the pressurized gasifier 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-cooled Vaporizer
The air-temperature gasifier is the main gasification facility for LNG gasification stations to supply gas to the city. The gasification capacity of the gasifier is determined based on the peak-hour gas demand, with a certain margin, usually 1.3 to 1.5 times the peak-hour gas demand. They are typically grouped in sets of 2 to 4, with 2 to 3 groups designed for use in rotation.
5. Water Bath Vaporizer
When the ambient temperature is low and the outlet 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 natural gas after vaporization [5, 6]. The heating capacity of the heater is determined as 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 unloading from the recycling tank cars, the design capacity of the BOG air-temperature heater is calculated accordingly, with the unloading time for the gaseous natural gas from the tank cars set at 30 minutes. Taking a 40m³ tank car with a pressure drop from 0.6MPa to 0.3MPa as an example, the required capacity of the BOG air-temperature gasifier is calculated to be 240m³/h. Generally, the number of tank cars that can be unloaded simultaneously at the gasification station determines the choice of the BOG air-temperature heater. Typically, the heating capacity of the BOG heater ranges from 500 to 1000m³/h. During winter, the BOG is used as fuel for hot water boilers in water-bath natural gas heaters, while during the rest of the year, it is fed into the city's transmission and distribution network.
7. BOG Buffer Tank
For peak-shaving LNG regasification stations, to recover surplus gas from non-peak卸槽 vehicles and BOG (Boil Off Gas) in storage tanks, or for natural gas blending stations to achieve uniform blending, it is common to add a BOG buffer tank at the outlet of the BOG heater. The tank's capacity is set according to the recovered surplus gas volume from the vehicles.
8. Safety Discharge Gas (EAG) Heater
The EAG air-temperature heating equipment capacity is calculated based on the maximum safe venting volume of a 100m³ tank. The calculated safe venting volume 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 Regulation, Measurement, and Odorization Equipment
Select pressure regulating devices based on the scale of the LNG gasification station. Typically, two pressure regulating devices are set up, with self-contained regulators equipped with controllers and over-pressure cut-off features being chosen as pressure regulators.
Metering is done using a turbine flow meter. The odorant used is tetrahydrothiophene, and it is injected into the gas pipeline using a diaphragm metering pump as the power source, based on flow signal.
Filter
Filter Structure Features
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 reused after cleaning. The filter is equipped with a pressure gauge that indicates the pressure difference between the inlet and outlet of the filter, indicating the degree of clogging of the filter core. The precision of the filter cores is generally 50um, and the volume of the water collection chamber is greater than 12% of the filter's volume.
