Nuclear gauges and alternative level technologies in critical refinery applications (Ri 2023)

In order for important equipment in an oil refinery to function in a smooth and productive manner, accurate level measurement is an essential component.

David Williams

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Article Summary

Making sure that the readings are accurate not only improves the efficiency of the refining process but also keeps the workers, the equipment, and the environment safe. The desalter, the delayed coker, and the fluidised catalytic cracker are three units that greatly rely on this precision for a variety of reasons. The accurate and reliable measurement of levels in critical applications is paramount to ensuring not only the efficiency of industrial processes but also the safety of both personnel and the environment. The choice of appropriate technology for these measurements can directly influence the robustness of a system and, in many scenarios, the margin between successful operation and catastrophic failure. Within this context, nucleonic gauges have emerged as a frontrunner, boasting a degree of reliability unmatched by many other level measurement technologies.

Refineries employ a combination of various level measuring methods selected according to the particular needs of the application. Variables like the make-up of the process fluid, the environment, the required level of precision, and the cost of installing and maintaining the instruments frequently have an impact on the decision.

The technique of measuring level using differential pressure (DP) is known for its versatility. The versatility of this technology extends from storage tanks to reactor vessels, supported by its extensive historical background and well-known principles. Nevertheless, it is important to acknowledge that this approach has certain constraints. For example, DP measurements might be susceptible to variations in fluid density or external air pressures. In the event that there are changes in the specific gravity of the fluid, it becomes necessary to perform recalibration in order to maintain the accuracy of measurements.¹

The utilisation of displacer level measurement provides a reliable and consistent method of measurement, even in scenarios where there are fluctuations in liquid densities. This characteristic makes it a very suitable option for accurately measuring interfaces or in circumstances where there are changes in liquid-specific gravity. One potential drawback is that, due to its mechanical composition, the system has movable components that may experience wear and tear over time, requiring periodic maintenance. The presence of very viscous liquids or slurries may impede the displacer’s motion, potentially impacting the accuracy of the measurement.¹

Capacitance level measurement
Capacitance level measurement is preferred in situations where prioritising minimal maintenance is crucial, especially owing to the absence of mechanical components. Moreover, it demonstrates proficiency in managing a wide range of circumstances, including specific substances with corrosive properties. Nonetheless, the reliance on a uniform dielectric constant of the medium in order to get precise measurements might pose a constraint. In addition, the accumulation of probe residue may occasionally impact measurements, necessitating periodic maintenance.¹

Radar level monitoring
Radar level monitoring is widely recognised for its non-contact characteristic, rendering it highly suitable for substances that are volatile or prone to corrosion. The robustness of this technology, demonstrated by its capacity to maintain dependability when encountering various obstacles such as vapours, high temperatures, or fluctuations in liquid characteristics, reinforces its prominent role in several refinery operations. However, occasionally, using technology may come at a slightly higher cost compared to using alternative strategies. Moreover, intricate settings may require a process of calibration or tuning in order to guarantee precise measurements.¹

Benefits of Nucleonic gauges
Nucleonic gauges stand out in the domain of level measurement due to their non-intrusive nature, making them ideal for high-temperature, high-pressure, or corrosive situations. Their non-invasive technique not only protects them from potential wear and corrosion but also assures an amazing level of stability unaffected by changes in the qualities of a substance. This non-contact technique of measuring also makes them resistant to changes in fluid parameters such as density and viscosity.

A really non-contact device is characterised by its lack of interference and non-intrusiveness to the process. To illustrate the distinctions among non-intrusive, non-invasive, and non-contact, it should be noted that a non-invasive device does not disrupt the flow of a process fluid. While non-intrusive implies that the devices do not make direct contact with the process fluid, it can be either invasive or non-invasive.² Occasionally, the phrase ‘non-contact’ is employed to indicate that a device does not physically touch the process fluid. However, it should be noted that such instruments, like through air RADAR, nonetheless intrude into the vessel.

This unique ability assures constant and trustworthy readings even in changing process circumstances, removing the possible dangers associated with erroneous data. Precision is praised for these gauges, which promote predictable and consistent industrial processes. The lack of moving components, along with a non-contact mode of operation, results in low maintenance needs. Furthermore, their resistance to external disturbances such as foam or vessel internals emphasises their toughness.

Fundamentally, each level measuring technique comes with a unique set of advantages, but it also has built-in limitations. The ultimate choice is contingent upon the particular demands of the given application, prevailing environmental factors, the properties of the fluid in question, and financial factors.

Case studies
One of the most common malfunctions identified in a study by Kister³ was that tower failure was related to faulty level measurement or control, which caused excess base level and premature tower flooding. For instance, in one case, it describes a propane deasphalting unit where the level controller failed, causing the tower to flood. The remedy was to install a nuclear level gauge and a redundant level control valve. Similarly, another case describes a crude distillation unit where the level transmitter failed, causing the tower to flood, exceeding the reboiler return. The remedy was to install a nuclear level gauge and a redundant level transmitter.

This study highlights the importance of proper level measurement and control to prevent tower malfunctions. By learning from past malfunctions and implementing preventive measures, engineers and operators can avoid falling into the same traps and ensure safe and efficient tower operation. The study recommends installing better level measurement, primarily nuclear levels, and ensuring adequate level indication to prevent excess base level and premature tower flooding. By following these recommendations, operators can minimise the risk of tower malfunctions and ensure safe and efficient operation.³

The inaccuracies in the level measurement of the coke drums can exert a substantial influence on both the quality of the product and the output of the delayed coker. The coking process facilitates the conversion of heavy residuum, leading to the generation of lighter and more economically valuable gasoils. The optimisation of feed rate is dependent upon an accurate determination of the level, as it enables the maximisation of yields while simultaneously limiting the potential incidence of a drum foam-over. The foaming over of a drum in the delayed coking unit can have a substantial impact on the refinery’s overall profitability through numerous means, including unexpected shutdowns, equipment damage, reduced throughput, and increased maintenance costs. All of these elements have the potential to lead to an increase in operational expenses.⁴

The time frame of a unit shutdown after a foam-over can vary depending on the severity of the foam-over. In extreme cases, it may be shut down for several weeks to physically clean the solidified foam in the overhead line, fractionator tower strainers, and other relevant components. The previously mentioned expenses not only result in an increase in maintenance costs but also contribute to a decrease in the operational availability of the delayed coking unit. In the event of a delayed coker shutdown, the refinery may need to decrease the total throughput to the crude distillation unit. Additionally, other units, such as the fluidised catalytic cracker and mild hydrocrackers, may also see reductions in their feed rates. This reduction in operational units can greatly reduce the operational margins of a refinery.⁵

Nucleonic gauges, suitable for severe conditions, excel at monitoring levels in crucial applications. By incorporating these measuring devices, companies may improve operational effectiveness and safety criteria, eventually safeguarding valuable assets and ensuring the safety of individuals. Nucleonic gauges emerge as a trusted ally in volatile environments with flammable or explosive materials, considerably lowering ignition hazards. Because of their unrivalled dependability, they are a favoured choice for safety shutdown systems.

This short article originally appeared in the 2023Refining India Newspaper, which you can VIEW HERE

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