Mass flow measurement is the lifeblood of various industries, playing a pivotal role in recipe formulations, material balance calculations, billing, and custody transfer operations. Given the critical nature of these applications, the reliability and precision of mass flow detection are of utmost importance.
The Evolution of Mass Flow Measurement
In the past, mass flow was often derived from the readings of a volumetric flow meter and a densitometer. These measurements were based on the relationship between process pressure, temperature, and density, which were not always precisely known, leading to inaccuracies.
Early mass flow meters employed angular momentum, utilising a motor-driven impeller to impart angular momentum to the fluid. This angular velocity was directly proportional to density, with a spring-held stationary turbine measuring the resulting torque as an indication of mass flow. However, these meters were complex and costly to maintain.
The Coriolis mass flow meter, developed in the 1970s, marked a significant advancement. It is widely regarded as the most accurate type of mass flow meter, finding extensive use in industrial applications. These meters operate based on the Coriolis effect. In this fascinating phenomenon, a mass moving within a rotating system experiences a force perpendicular to its motion and the axis of rotation.
Understanding the Coriolis Principle
The Coriolis effect was first observed by G.G. Coriolis, a French engineer, who noted that objects moving on Earth’s surface tend to veer to the right in the Northern Hemisphere and the left in the Southern Hemisphere due to the planet’s eastward rotation. This phenomenon plays a crucial role in ocean tides and weather patterns.
In the context of fluid flow, when a fluid passes through a pipe subjected to Coriolis acceleration, a deflecting force is generated based on the mass flow rate of the liquid. This force acts perpendicular to the flow direction and tube rotation. The Coriolis force is harnessed for mass flow measurement.
To illustrate, imagine a particle travelling inside a rotating tube. The particle moves under the influence of centripetal and Coriolis accelerations. The Coriolis force generated by this motion can be used to determine mass flow rate.
How a Coriolis Flow Meter Operates
Coriolis mass flow meters operate on the principle of inertia. A fluid, be it a liquid or gas, flows through a tube that is vibrated by a small actuator. This vibration introduces a Coriolis acceleration into the flowing stream, causing a measurable twisting force on the tube, resulting in a phase shift. This twisting force is directly proportional to the mass of the fluid, allowing the meter to measure mass flow accurately. Coriolis flow meters can measure flow in both forward and reverse directions.
Typically, the tube is anchored at two points and vibrated between these anchors, similar to a spring and mass assembly. Once set in motion, this assembly vibrates at its resonant frequency, which is determined by the mass of the assembly. This resonant frequency minimises the driving force required to maintain tube vibration.
Coriolis meters come in various tube designs, including U-shaped and straight forms, with some allowing self-draining when mounted vertically. Two parallel tubes can split the flow, while a single continuous tube design does not. In either case, drivers connected to the tubes induce vibration, and sensors detect the resulting motion, which correlates to the flow rate.
Coriolis Meter Evolution and Improvements
The first-generation Coriolis meters utilised single curved, thin-walled tubes. However, these faced challenges such as vibration interference and meter failure in corrosive and erosive services. Two-tube designs were subsequently introduced to reduce external vibrations and improve performance. Recent advancements include using various tube shapes, thicker walls, full-bore diameters, and heavy manifolds for improved reliability and ruggedness.
Interferences and Accuracy
Coriolis flow meters provide exceptional accuracy, typically within 0.1-2% of the rate for mass flow measurement. However, they are sensitive to air or gas pockets in the fluid, which can affect measurement accuracy. Density changes due to temperature fluctuations are continuously compensated using temperature sensors.
Sizing and Pressure Drop
The wide rangeability of Coriolis flow meters, from 30:1 to as high as 200:1, allows for flexible sizing. Downsizing is acceptable when the pipe is oversized and the fluid is clean with low viscosity. These meters generally have higher pressure drops than traditional volumetric meters due to their reduced tubing diameter and circuitous flow path.
Applications and Limitations
Coriolis mass flow meters are highly versatile, finding applications in various industries for measuring gases and liquids, including corrosive substances. They excel in mass flow, density, temperature, and viscosity measurements. However, they may not be competitive in simple flow measurement applications where volumetric sensors suffice.
In summary, Coriolis flow meters offer unmatched precision in mass flow measurement and find extensive use across industries due to their versatility and accuracy. Their ability to provide multiple measures in one instrument and compatibility with a wide range of fluids make them invaluable tools in modern industrial processes.