Process and Control Today | Radar is the right choice for liquefied gas level measurements

Nov 05, 2024

04/11/2024 Emerson Automation Solutions

Johan Sandberg from Emerson explains why the latest non-contacting radar level gauges are more suitable than servo technology for overcoming the measurement challenges of LNG and LPG applications.

In tank gauging systems for vessels storing liquefied natural gas (LNG) and liquefied petroleum gas (LPG), accurate and reliable level measurement instruments are essential, not just for inventory control and custody transfer purposes but also for overfill prevention. LNG and LPG are both considered hazardous fuels, so filling a storage tank above its capacity could cause a spill, which would present a clear safety risk to personnel and the immediate environment. It is therefore essential that organisations deploy an overfill prevention system (OPS) in compliance with the International Electrotechnical Commission’s global functional safety standard IEC 61511.

Monitoring the level of liquefied gases also enables early detection of storage tank leaks and is therefore vital in minimising product loss and averting potential safety incidents. Preventing overspills and detecting leaks early also helps organisations in ensuring compliance with environmental regulations.

Measurement Challenges

The tanks in which liquefied gases are stored present various measurement challenges. For example, LNG is stored in cryogenic full containment tanks with a capacity of as much as 200,000 cubic metres. These are huge, complex vessels with an inner liquid containment steel tank, an outer concrete or steel tank for secondary containment, and thermal insulation between the two to avoid liquid boil-off. The vast size of these tanks means that the required range for level measurement devices can be greater than 40 metres (131 feet), making a high degree of accuracy difficult to achieve.

LPG is stored in either pressurised or refrigerated tanks. During certain conditions – such as rapid depressurisation or temperature changes – it can vaporise, which can affect the apparent level in the tank. Pressurised LPG tanks also have a vapour space at the top, which makes it challenging to obtain accurate level measurements. Tank gauging systems should therefore incorporate level measurement devices that can penetrate vapours and account for the presence of a vapour space.

Variations in temperature and pressure affect the density and volume of LPG, so tank gauging systems must compensate for this to ensure accurate and reliable measurements. The fact that LPG boils during filling, creating a turbulent surface, can also cause inaccurate level measurements.

Tanks storing liquefied gases are not opened during operation, which creates complexity when there is a need to access instruments for maintenance purposes. It is therefore important to select a level measurement technology that provides extreme reliability, a long mean time between failures (MTBF) and minimal maintenance requirements.

In liquefied gas applications, changes in liquid density can significantly impact the accuracy and reliability of some level measurement technologies, as can the dielectric constant of the liquefied gas, so consideration should be given to instruments that can meet these challenges.

Servo Level Gauges

Servo technology has historically been used to perform level measurement in liquefied gas applications. A small displacer float suspended by a thin wire is connected to a servo gauge on top of the storage tank, and a weighing system in the gauge senses the tension in the wire. Signals from the weighing mechanism control an electric motor in the servo unit and make the displacer float follow the liquid level movements. An electronic transmitter then uses fieldbus communications to send the level readings to a control room.

Servo level gauges do, however, have some major shortcomings. Because they have numerous moving parts, they are susceptible to mechanical wear, which increases the likelihood of failures. The MTBF of servo level gauges can actually be as low as five years. Regular maintenance and calibration are required to keep devices in good condition, and companies need to keep a large stock of spare parts. A further issue is that the accuracy of these devices is impacted by changes in liquid density.

Servo level gauges can be isolated from a liquefied gas storage tank by a ball valve, with a chamber above the ball valve enabling technicians to gain access. However, because it is difficult to determine if the displacer is fully wound up above the ball valve, the wire can sometimes break when closing the valve, with the displacer and wire then falling to the bottom of the tank. This not only results in a new displacer and wire having to be fitted and calibrated after every breakage, but creates the potential for broken parts to cause serious problems through being sucked into pumps and valves.

Non-contacting Radar Level Gauges

The modern approach to achieving accurate and reliable level measurements in liquefied gas storage tanks involves deploying non-contacting radar level gauges. These instruments emit microwave signals towards the liquid surface, with a precise measurement achieved by analysing the signals reflected back to the transmitter. A strong reflected signal, or echo, is essential for radar devices to provide accurate measurements. Low dielectric constants can weaken echoes, but the latest radar level gauges have been designed to perform reliably across a wide range of dielectric constants, including the low values of both LNG and LPG.

Non-contacting radar level gauges are much more reliable than servo devices, with MTBF for critical parts measured in decades. Also, because they are non-contacting and lack moving parts, their maintenance requirements are minimal. Non-contacting radar level gauges used in liquefied gas applications are therefore likely to have a very long service life, usually operating for more than 20 years without requiring maintenance or adjustment, and often reaching close to 100% availability during their long lifespan. This reduces their cost of ownership and maximises system and tank availability. In the unlikely event of a failure occurring, diagnostic software will typically identify it and take the device to a safe state.

The latest non-contacting radar level gauges leverage frequency modulated continuous wave (FMCW) technology, with a sensitivity more than 30 times greater than devices using older pulse modulation techniques. This level of sensitivity increases the signal strength of FMCW devices and enables them to deliver excellent measurement accuracy and reliability.

In liquefied gas applications, non-contacting radar level gauges use robust antennas specially designed to optimise their measurement accuracy and meet the specific challenges presented by LNG and LPG. These specialised antennas optimise signal transmission and reception, even when the dielectric constant is low, and can operate effectively even in cryogenic conditions. They can also handle the dynamics of vapour spaces, including changes in vapour pressure and density, ensuring consistent performance and measurement accuracy in conditions where conventional antennas might struggle.

Tank filling and emptying operations can disturb the surface of liquefied gases and impact level measurement accuracy. To mitigate the effects of this, the signal from non-contacting radar level gauges can be guided within a still-pipe. This produces a more robust echo from the surface, further optimising measurement accuracy.

A good example of the measurement accuracy of non-contacting radar technology is provided by the Rosemount™ 5900S Radar Level Gauge from Emerson. This device provides level measurement accuracy to within ±0.5 millimetres (0.020 inches) and can measure at distances of more than 60 metres (196 feet). Its measurements can even be verified in real-time while a tank is in operation, by comparing the measured values to the known distance of a verification pin mounted in the still-pipe, along with a reflective device at the end of the pipe.Radar Fears Unfounded

Despite the disadvantages of servo gauges in liquefied gas applications, some companies continue to use them because they have concerns about high gas density in the vapour space potentially impacting the performance of radar level gauges. However, radar level gauges can compensate for the influence of vapour and high gas density on signal strength, thereby ensuring that measurement accuracy is not compromised. In fact, non-contacting radar level gauges excel in measuring liquefied gas levels and are today used on more than 10,000 LNG and LPG tanks around the world, including marine gas tankers. During four decades of service on liquefied gas tanks, no Emerson radar installation has experienced any safety- or reliability-related problems because of vapour or high gas density.

Safety Functions

In terminals storing large volumes of liquefied gases, the safety functions provided by level measurement technology are vitally important. In these applications, common practice involves installing three level gauges, with primary and secondary gauges supporting the tank gauging system and a third providing information for the OPS. The safety instrumented system (SIS) may integrate input from all three gauges in a set-up where SIS alarms are triggered on a two-out-of-three voting scheme.

There is a common misconception within the tank storage industry that the level measurement instrumentation used to support the tank gauging system and the OPS must be based on different technologies – for instance, a combination of radar level gauges and servo level gauges. This is typically referred to as diverse separation. However, this is not a requirement of IEC 61511, which instead confirms that the same technology can be used for both the tank gauging system and the OPS. This is known as identical separation and is widely accepted and practiced.

Although diverse and identical separation are both legitimate options, implementing servo gauges can impact safety, as these devices offer lower levels of reliability than radar level gauges. Diverse separation also increases the likelihood of human error, as it introduces extra complexity by requiring personnel to be trained in installing, configuring and proof-testing two different technologies rather than just one. Standardising on a single level measurement technology not only reduces the likelihood of human error, thereby helping to prevent safety incidents, but also simplifies maintenance and reduces spare part requirements.

Remote Proof-testing Capability

Level measurement devices used in a SIS are required to be periodically proof-tested to ensure that they will function correctly when a demand occurs. These tests have traditionally been carried out by technicians in the field and verified by a worker in the control room. This time-consuming method can involve workers having to climb tanks to access devices, putting their safety at risk. However, the latest non-contacting radar level gauges have functionality that enables operators to perform proof-tests remotely from the control room, using powerful and easy-to-use inventory management software.

The device remains installed and overfill conditions are simulated to activate the detector and generate an alarm signal. Built-in functionality guides an operator through inputting a straightforward sequence of settings and commands from their interface, enabling a device to be proof-tested in less than five minutes. This eliminates the need for liquid to be moved in and out of the tank to perform the test, avoids the risk of spills, saves a significant amount of time, and eliminates the need for personnel to climb tanks and be exposed to the tank contents, thereby increasing worker safety and efficiency.

Servo level gauges can also be proof-tested remotely, but the procedure is more straightforward and efficient with radar level gauges, due to their non-contacting measurement method, fewer mechanical parts and advanced digital diagnostics. Servo level gauges, with their mechanical components, present additional challenges that can make remote proof-testing more complex.

Conclusion

In liquefied gas applications, modern non-contacting radar level gauges provide significant benefits in comparison with older and less sophisticated servo technology. A strong radar signal ensures accurate and reliable measurements, while a lack of moving parts makes for an almost maintenance-free device. With technology diversification not being a requirement of the relevant global safety standard, radar gauges can legitimately be used for both tank gauging and OPS purposes. Visit Tank Gauging System | Emerson US for more information about Emerson’s level measurement solutions for LNG and LPG storage tanks.

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For more information, please contact:Emerson Automation SolutionsMeridian EastMeridian Business Park 7LeicesterLE19 1UXTel: +44 (0)870 240 1978Email: [email protected] Web: https://www.Emerson.com

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