Introduction :

Liquid level measurement is a fundamental aspect of process control in various industries, including chemical, petrochemical, and oil and gas. Traditionally, liquid levels in storage tanks, reactors, and process vessels have been the primary process variable for monitoring and control. However, this white paper presents a compelling case for shifting the focus from level measurement to volume or mass measurement, unlocking new possibilities for enhanced process control and optimization.

The Limitations of Level Measurement :

While level measurement has been the standard approach for decades, it comes with inherent limitations. In vertical cylindrical tanks, the relationship between level and volume is linear, making level measurement a reasonable proxy for volume. However, this linearity does not hold true for horizontal cylindrical tanks (such as LPG Bullets) or spherical tanks (like Horton Spheres). In these cases, relying solely on level measurement can lead to inaccuracies and suboptimal control strategies.

The Power of Volume and Mass Measurement :

By leveraging the capabilities of modern distributed control systems (DCS), it is now possible to calculate the volume or mass stored in a tank based on the measured level. This approach offers several advantages over traditional level measurement:

1. Enhanced Process Insight: Measuring volume or mass provides a more meaningful representation of the actual quantity of material in the tank, enabling better process understanding and decision-making.

2. Improved Control Performance: Tuning level control loops becomes more straightforward when using volume or mass as the process variable. The controller gain remains consistent across all levels, simplifying the tuning process and ensuring optimal control performance.

3. Increased Efficiency: By accurately tracking the volume or mass of material, process engineers can optimize inventory management, reduce waste, and improve overall process efficiency.

Implementing Volume and Mass Measurement :

To convert level measurements to volume or mass, the DCS can utilize mathematical formulae based on the tank's shape. For example, in a horizontal cylindrical tank, the volume can be calculated using the cylindrical volume formula, taking into account the tank's diameter and the measured level.

In cases where a level vs. volume calibration chart is available, such as in custody transfer applications, the DCS can directly use the calibration data to convert level to volume. This approach ensures high accuracy and eliminates the need for complex calculations.

Benefits for Process Engineers and Plant Managers :

Adopting volume or mass measurement as the primary process variable offers significant benefits for process engineers and plant managers:

1. Enhanced Process Control: By using volume or mass measurements, process engineers can design more precise and responsive control strategies, leading to improved process stability and product quality.

2. Simplified Tuning: With consistent controller gain across all levels, tuning level control loops becomes more straightforward, reducing the time and effort required for loop optimization.

3. Increased Plant Efficiency: Accurate volume or mass measurements enable better inventory management, optimized production scheduling, and reduced waste, ultimately leading to increased plant efficiency and profitability.

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