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 design of tanks should be given high priority.
1.1 Material Selection for LNG Tanks
Normal operation temperature for the LNG storage tank is -162.3°C, and it needs to be pre-cooled to -196°C before the first use.
Liquid nitrogen pre-cools the tank, requiring the tank to be designed for -196°C temperatures. The inner tank must withstand both the working pressure of the medium and the low temperatures of LNG, necessitating that the inner tank material possesses excellent comprehensive mechanical properties at low temperatures, particularly good low-temperature toughness. Therefore, the inner tank material is selected as 0Crl8Ni9, which is equivalent to ASME (American Society of Mechanical Engineers) standard 304.
As a constant temperature external pressure vessel, the outer tank material is selected as low-alloyed container steel Q345R, low-carbon steel Q245R, and Q235B.
2.2 Takeover Design
Tapped ports on the inner tank include: inlet liquid port, outlet liquid port, liquid discharge port, vapor port, level measurement port, upper level gauge port, lower level gauge port, and 8 process manholes. All tapping materials on the inner tank are made of 0Cr18Ni9.
For easy regular measurement of vacuum levels and evacuation, a vacuum evacuation port is provided on the bottom seal of the outer canister (the port is sealed after evacuation). To prevent vacuum failure and the leakage of the inner canister medium into the outer canister, a burst-proof device is set on the outer canister seal.
2. Unloading Pressure Vapourizer
Due to the absence of pressurization equipment on the LNG tank trucks, an onboard gasification regasifier with a capacity of m3/h is set up 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 Pressure VapORIZER
After filling a 100m3 LNG storage tank with 90m3 LNG, calculate the pressure increase from the 0.4MPa unloading state to the 0.6MPa working state 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℃ and the gaseous natural gas exiting the booster vaporizer at -145℃.
Designs commonly feature one LNG tank paired with one booster gasifier. Multiple tanks can also share one or a set of gasifiers for boosting, which can streamline the process, reduce equipment, and lower costs through valve switching.
4. Air-temperature type gasifier
The air-temperature 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 set at 1.3 to 1.5 times the peak-hour gas consumption. They are typically grouped in sets of 2 to 4, with 2 to 3 sets designed for alternating use.
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 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
The design capacity of the BOG air-temperature heater is based on the maximum gas phase natural gas released after the unloading of recycling tank cars, with the unloading time calculated at 30 minutes. For instance, with a 40m3 tank car pressure dropping from 0.6MPa to 0.3MPa, the required capacity of the BOG air-temperature vaporizer is 240m3/h. Typically, the number of tank cars a gasification station can simultaneously handle determines the selection of the BOG air-temperature heater. The heating capacity of BOG heaters usually ranges from 500 to 1000m3/h. In winter, the BOG is used as fuel for hot water boilers in water-bath natural gas heaters, while during other seasons, it is sent into the city's transmission and distribution network.
7. BOG Buffer Tank
For peak-shaving type LNG regasification stations, to recover excess gas from unloading tankers during off-peak periods or BOG (Boil Off Gas) from 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 capacity of the tank is set according to the recovered excess gas from the tankers.
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 Regulating, Flow Measuring, and Odorization Equipment
Select pressure-reducing devices based on the scale of the LNG gasification station. Typically, two pressure-reducing systems are installed, with self-contained regulators equipped with controllers and overpressure cut-off features being chosen for the pressure regulators.
The measurement is done using a turbine flow meter. The odorant is tetrahydrothiophene, and the dosing is powered by a diaphragm metering pump, injecting the odorant into the gas pipeline based on flow signals.
Filter
Filter structure characteristics
The site 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 differential pressure gauge, indicating the pressure difference between the inlet and outlet gases of the filter. This indicates 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.
Section 9: LNG Trailer-mounted Refueling Station Process Flow
Section 10: L-CNG Vehicle Filling Station Process
Eleven. Multi-functional CNG Station Process Flow
Chapter 12: CNG Filling Station Process Flow
Thirteen. Operations
What are LNG cars and CNG cars?
What is the function of an LNG liquefaction plant?
3. What are the classifications of gas stations for vehicles?
4. Briefly describe the four process flows of an LNG gasification station.
5. When should the upward liquid filling method be used during unloading, and when should the downward liquid filling method be used? Why?
What method is used for self-pressurization of the storage tank?
Why use EAG heaters during gas venting?
Why is a dike set up around LNG tanks and their auxiliary equipment?
9. Briefly describe the main equipment of an LNG liquefaction station and its functions.
10. Briefly describe the main equipment of an LNG vehicle refueling station.
11. Briefly describe the main process flow of a multifunctional vehicle refueling station.
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