An LNG (Liquefied Natural Gas) tank gauging system is crucial for accurately measuring the level, temperature, and pressure of LNG in storage and fuel tanks. These systems help ensure safe and efficient handling of LNG, a cryogenic liquid stored at extremely low temperatures. When selecting or designing an LNG tank gauging system for a fuel tank, several factors must be considered to ensure optimal performance and safety.
Key Components of an LNG Tank Gauging System:
- Level Measurement
- Radar Level Gauges: Commonly used due to their non-contact nature, radar gauges work by sending microwave signals to the surface of the LNG and measuring the time taken for the signal to return. This provides a highly accurate reading of the LNG level.
- Servo Tank Gauges: These use a mechanical displacer that is lowered into the tank to measure the LNG level. The force needed to maintain the displacer at the liquid surface is used to determine the level.
- Temperature Measurement
- Temperature Probes: Multiple sensors are placed at different levels in the tank to monitor temperature gradients. Since LNG is cryogenic, monitoring temperature ensures the liquid remains within safe storage conditions and prevents boiling or vaporization.
- Pressure Measurement
- Pressure Transmitters: These measure the pressure in the tank vapor space. Monitoring pressure is critical, as changes in pressure can indicate vaporization, tank integrity issues, or the need for venting.
- Density Measurement
- Differential Pressure Gauges: Used to measure the density of LNG in the tank, which helps in assessing the amount of gas stored. Density measurement is important to monitor LNG stratification and the quality of the stored product.
- Tank Volume Measurement
- Strapping Tables: A table or chart is often used in conjunction with the level measurement system to calculate the volume of LNG in the tank based on the tank’s geometry.
- Vapor Phase Measurement
- Vapor Monitoring Sensors: These sensors detect the concentration of gas in the vapor phase above the LNG. This is crucial for both safety and optimizing the LNG’s boil-off management.
Technologies Used in LNG Tank Gauging:
- Guided Wave Radar (GWR)
- Function: A probe is inserted into the tank, and microwave pulses are transmitted down the probe. These pulses are reflected back by the LNG surface and provide an accurate level reading.
- Advantages: High accuracy, no moving parts, and the ability to work under extreme cryogenic conditions.
- Capacitance Level Measurement
- Function: A probe measures the capacitance between itself and the tank wall, which varies with the level of LNG.
- Advantages: Effective for measuring low dielectric liquids such as LNG, with a quick response time.
- Differential Pressure Transmitters
- Function: Measures the difference in pressure at two points (usually at the top and bottom of the tank) to determine the liquid level.
- Advantages: Widely used for liquids, though they require calibration for varying LNG densities.
- Float Gauges
- Function: Uses a float on the surface of the LNG that moves with the liquid level.
- Advantages: Simple and cost-effective, but not suitable for tanks with complex shapes or cryogenic conditions.
Applications for Fuel Tanks:
- Fuel Supply for LNG-Powered Vehicles: LNG is increasingly used as fuel for heavy-duty vehicles and ships due to its environmental benefits. Accurate gauging helps ensure efficient fuel management and prevents fuel shortages.
- LNG Bunkering Stations: In LNG bunkering, gauging systems ensure safe and efficient transfer of LNG fuel to ships.
- Storage Tanks at LNG Terminals: For large-scale storage, reliable gauging is essential for inventory management and maintaining optimal operating conditions.
Recommendations:
- Rosemount Tank Gauging System: Known for precision in cryogenic applications, offering both radar and servo tank gauges that are LNG-compatible.
- Proservo NMS5: A high-performance servo gauge for LNG tanks, providing accurate level, interface, and density measurement.
- Emerson’s Micro Motion LNG Density and Viscosity Meters: For continuous and precise LNG density measurement, ensuring accurate fuel readings.