The LNG tank gauging system typically consists of both primary and secondary level gauges, which work together to provide accurate and reliable measurements of the liquid level in LNG (Liquefied Natural Gas) storage tanks. These systems are critical for ensuring safety, inventory management, and compliance with industry standards, particularly in facilities where precise monitoring of cryogenic liquid levels is necessary.
Here’s how the primary and secondary level gauges function in an LNG tank gauging system:
Primary Level Gauge (PLG)
The primary level gauge is the main device used to measure the level of LNG within the storage tank. It typically uses technologies such as guided wave radar or differential pressure transducers for accurate level detection.
Guided Wave Radar (GWR):
- Principle: Sends radar waves along a probe that is immersed in the LNG. The wave is reflected back from the surface of the LNG, and the time taken for this reflection is used to calculate the liquid level.
- Advantages: Provides continuous level measurement with high accuracy. It’s particularly well-suited for cryogenic temperatures in LNG tanks.
- Common Applications: Used in high-precision systems where level measurements need to be accurate for inventory management, custody transfer, and safety purposes.
Differential Pressure (DP) Transmitters:
- Principle: Measures the difference in pressure between the top and bottom of the tank. The level of the LNG can be inferred from the pressure difference.
- Advantages: Reliable and well-established method for measuring liquid levels in large tanks, including cryogenic storage tanks.
- Common Applications: Often used for level measurement in bulk liquid storage tanks, including LNG, due to its proven accuracy and long-term reliability.
Secondary Level Gauge (SLG)
The secondary level gauge is an auxiliary system used to verify the readings provided by the primary gauge. It serves as a backup to ensure the accuracy and reliability of the primary measurement.
Common Technologies Used in Secondary Level Gauging:
Float-and-Tape Systems:
- Principle: A float is placed on the surface of the LNG, and the movement of the float is measured by a tape or other mechanical device.
- Advantages: Provides a redundant level measurement to cross-check the readings of the primary gauge. While it is less common in modern systems, it is still used in some LNG applications.
Capacitance Probes:
- Principle: Measures changes in capacitance between electrodes to detect the liquid level.
- Advantages: Suitable for verifying the accuracy of the primary gauge in some systems, especially in more straightforward storage tank setups.
Ultrasonic or Radar-Based Secondary Measurement:
- Principle: Similar to the primary level gauge, but used for additional verification.
- Advantages: These sensors can be installed at different points or angles in the tank, allowing for further redundancy and reliability in the gauging system.
Conclusion:
The combination of primary and secondary level gauges in an LNG tank gauging system ensures high levels of accuracy, safety, and reliability. Both systems work together to monitor the LNG levels in the tank, detect any discrepancies, and ensure regulatory compliance. The primary gauge provides continuous, real-time measurements, while the secondary gauge acts as a backup to verify the readings and safeguard against failures.