In this two-part blog post, we will be discussing the factors that need careful consideration before undertaking any form of level measurement in the highly specialized area of LNG Full Containment Tanks.
In this first part, we will briefly discuss the evolution of the technology for level measurement, the requirements of Full Containment storage tanks, challenges we expect to face, the outmoded technologies and solutions, and finally the system make up of a robust Rosemount Full Containment Tank Gauging Solution.
Later, in the second post, we will outline the types of tanks applied to LNG tanks, Emerson’s latest in-house powerful Inventory Management Software for Full Containment Tanks Storing LNG and Liquefied Gases and finally the safety features deliberations LNG Full Containment.
How the Technology Evolved?
Over the years, technology has evolved from the large float, suspended by a perforated steel tape to the radar gauge uses microwaves for the measurement of the liquid level. Radar gauges are electronic devices without moving parts. Radar continues to be the fastest growing tank gauging technology according to independent market surveys. Non-contacting radar gauges are used to provide accurate and reliable level measurements in a broad range of challenging applications. Temperature variations, dust and pressure changes do not affect the accuracy of this technology, and because these devices are non-contacting, they are suitable for use in vessels with agitators as well as tanks containing viscous, sticky, or abrasive fluids.
The Requirements of Full Containment Storage Tanks – Long Term Storage
Full containment tanks for storing LNG are large and complex structures with a typical capacity of up to 200,000 m3. They include an inner liquid containment steel tank, an outer concrete (or steel) tank for secondary containment control, and thermal insulation between the two to minimize liquid boil-off. The planning, design, and construction of such technically advanced tanks requires significant investment, and payback times can be lengthy. For this reason, tank farm operators need to implement accurate and reliable tank gauging systems that will help to minimize operating and maintenance costs, thereby enabling a quicker return on investment.
Measurement Challenges in LNG Full Containment Tanks
Determining the level and temperature of LNG in full containment tanks presents all kinds of challenges for measurement technology.
- The large nature of the tanks and the long measurement range often over 40 meters (131 feet) makes measuring accuracy difficult to achieve.
- These closed tanks are not opened during operation, and instruments are normally not accessed for maintenance purposes during their full lifecycle. Therefore, it is crucial for measurement technology to be reliable and redundant level measurements are often vital within these applications.
- The inner structure of these tanks presents a further measurement challenge. A standard storage tank has only one vapor space; but in full containment tanks, there are two different vapor spaces – one outside the tank’s fixed suspended deck and one inside it. These large vapor spaces have different temperatures, and for inventory purposes, this needs to be considered when calculating the liquid equivalent within the spaces.
Outmoded Methods vs. Advanced Measurement
A weighing system in the servo gauge senses the tension in the wire. Signals from the weighing mechanism control an electric motor in the servo unit and make the displacer follow the liquid level movements. A transmitter sends the level information to the control room using digital communications. Although widely used, these methods have major shortcomings. For example, the accuracy of a float gauge is often poor, with multiple error sources including buoyancy differences, dead-band, backlash, and hysteresis in the mechanisms. The float gauge is a relatively simple device but has many moving parts that will require regular maintenance over its lifespan. Servo gauges generally perform better than float gauges, but also have many moving parts. Because the displacer and the wire are in contact with the liquid, servo gauges may require more attention for calibration and maintenance.
The modern approach involves using top-down non-contacting radar level gauges to provide accurate and reliable level measurement. This has become the world’s fastest growing tank gauging technology in cryogenic applications. The measurement of liquid level by non-contacting radar gauges is based on microwave signals emitted towards the surface and reflected to the transmitter. To reliably measure the level in a full containment tank, a radar gauge needs a sufficiently strong reflected signal, known as an echo, from the LNG surface. Radar gauges based on frequency modulated continuous wave (FMCW) technology transmit a radar signal with increasing frequency over time to create a signal sweep. The signal echo reflected from the surface is picked up by the antenna. Because the frequency of the transmitted signal constantly varies, the echo frequency always differs slightly to the transmitted signal at any given moment. The difference between these frequencies is directly proportional to the echo delay – i.e., the distance from the transmitter to the surface – which enables accurate measurement of the level.
The Approach: The Rosemount Tank Gauging System for Full Containment Tanks
To reliably measure the level in an LNG tank, a radar gauge needs a sufficiently strong echo from the LNG surface. , Using the Rosemount Tank Gauging system, a 4-inch still-pipe guides the FMCW radar signal, resulting in a strong, undisturbed echo from the surface of the liquid. Typically, the radar sweep is controlled by a crystal oscillator to achieve 0.5 millimeters (0.020 inches) instrument accuracy. Using a precise radar level gauge can deliver a 180% reduction in volume uncertainty over traditional methods. The long measuring distance required in full containment LNG tanks can be challenging for non-contacting radar level gauges. However, the latest devices can provide highly accurate measurements at distances of over 55 meters (180 feet), and furthermore, the measurements can be verified while the tank is in operation. This is achieved through comparing measured level values to the known distance of a reference pin mounted in the still-pipe along with a deflection plate at the end of the pipe.
A specific antenna option can be employed to enable a device to function in such an extreme environment. These devices have minimal maintenance requirements, as they have no moving parts and do not touch the liquid, and they can boast impressive reliability, with mean time between failures for critical parts measured in decades. The long measuring range required in these tall tanks can be challenging for non-contacting radar level gauges. However, the latest devices can provide highly accurate measurements at distances of over 55 meters (180 feet), and furthermore, the measurements can be verified while the tank is in operation. This is achieved through comparing measured level values to the known distance of a reference pin mounted in the still-pipe along with a deflection plate at the end of the pipe.
Online Temperature Monitoring
Non-contacting radar level gauges are integrated into high-performance LNG tank gauging systems that also include devices to monitor tank temperature and density profiles to detect stratification. This occurs when two separate layers of LNG are formed within a tank and can potentially lead to a dangerous condition called ‘rollover’, which can result in an instantaneous release of boil-off vapor. Additional temperature measurements monitor the cool-down process and support leak detection. Both functions are achieved by using multiple temperature sensor elements. These are distributed along the inner tank wall and at the tank bottom (for cool-down supervision) and within the insulation space between the inner and outer tank walls (for leak detection).
Summary and What’s Next
In conclusion, we discussed the evolution of the technology for level measurement, the requirements of Full Containment storage tanks, challenges we expect to face, the outmoded technologies and solutions, and finally the system make up of a robust Rosemount Full Containment Tank Gauging Solution.
In the second and final post, we will outline the types of tanks one encounters, Emerson’s innovation in software helps customers with specific requirements and calculations and finally the safety features on offer with the LNG Full Containment Tanks.