In the field of instrumentation, theoretical knowledge is like the wrench in your hand – it may seem insignificant, but if it fails at a crucial moment, the consequences can be severe! Whether you’re studying for a certification exam or working in the field, concepts like circuit diagrams, sensor principles, and troubleshooting logic are foundational and cannot be ignored. This series will continue to update with helpful insights that will solidify your theoretical foundation and practical skills. Even in fragmented time, you can gradually build a solid foundation for your work. If you want to avoid mistakes and quickly fill in any gaps in your knowledge, let’s strengthen the basics and be more confident in your work!
1. Components of a Coriolis Mass Flow Meter and Their Functions
A Coriolis mass flow meter consists of three main parts:
Sensor: The sensor is made of a measuring tube, which can have different shapes depending on the manufacturer. Some designs are optimized for performance, while others are simply designed to differentiate from competitors’ patents.
Transmitter: The transmitter’s function is to process the low-level or binary signals from the sensor, amplify them, and output a standardized 4–20 mA signal (or frequency/pulse or digital signal) proportional to the flow rate and density.
Display: The display unit receives signals from the transmitter and typically shows the instant flow, cumulative flow, density, and temperature of the measured fluid.
The sensor is based on the principle that the Coriolis force caused by the fluid flowing through the vibrating tube causes a phase shift, which is then measured by electromagnetic or optical sensors. This shift is used to calculate the mass flow rate and fluid density.
2. How Does a Straight Tube Coriolis Mass Flow Meter Work?
A straight tube Coriolis mass flow meter typically consists of two symmetrical titanium (or zirconium) tubes welded onto a connector. These tubes are excited electromagnetically to vibrate at a frequency of around 600-1100 Hz. When there is no flow, both sides of the tube vibrate symmetrically. However, when a fluid flows through, the Coriolis force causes the vibration speed to change along the tube’s length, leading to a phase difference between the two detection systems. The greater the flow rate, the greater the phase difference.
This type of flow meter is simple in design and made from titanium, offering excellent wear resistance and corrosion resistance. Titanium’s superior elasticity compared to stainless steel allows for shorter measurement tubes and higher vibration frequencies, thus improving interference resistance and ensuring stable zero points.
3. Advantages of Coriolis Mass Flow Meters
Despite being a relatively new technology (less than 30 years of development), Coriolis mass flow meters have made significant advancements. The international standard for these meters has been adopted by the International Organization for Standardization (ISO) and the International Legal Metrology Organization. The advantages of Coriolis mass flow meters include:
Direct mass flow measurement: Unlike other flow meters, Coriolis meters are unaffected by temperature, pressure, viscosity, or density changes, offering high accuracy (up to ±0.1%).
No moving parts: There are no moving mechanical parts in the sensor, ensuring that the measurements are not affected by friction.
Obstacle-free flow path: Coriolis meters can measure fluids containing solid particles and are easy to clean.
Wide range of applications: They can measure various media, including high-viscosity liquids, slurries, and gases.
Insensitive to flow profile: These meters are not affected by laminar or turbulent flow conditions, and they don’t require straight pipe runs for installation.
Density measurement: In addition to measuring mass flow, Coriolis meters can also provide accurate density readings of the fluid.
4. Key Considerations When Installing Coriolis Mass Flow Meters
Minimize external vibrations: Since Coriolis meters rely on the vibration of the sensor tube, installation should be away from significant vibration sources, such as large motors, pumps, or transformers. Ensure that the piping around the sensor is properly supported to reduce vibrations.
Magnetic interference: The sensor uses electromagnetic excitation, so it should not be installed near large magnetic fields. Keep a distance of at least 0.6–1.0 meters from large transformers, motors, or pumps.
No stress on the sensor: Ensure that the sensor is welded onto the pipe without any external stress, such as torsion. This can be achieved by welding the sensor to a properly supported pipe.
Vertical installation preferred: For straight tube meters, vertical installation is ideal. This allows the tube to be empty when the meter is not in use, preventing scale buildup. If horizontal installation is necessary, the measurement tubes should be aligned on the same horizontal plane.
Proper orientation for liquid or gas measurement: For curved tube meters, ensure the sensor is oriented correctly. When measuring liquid, the housing should be facing downward to avoid gas accumulation. When measuring gas, the housing should face upward to avoid condensation buildup.
5. Why Does the Zero Point Drift in Coriolis Mass Flow Meters?
Zero drift in Coriolis mass flow meters is a known issue, and several factors contribute to this phenomenon:
Sensor design: The sensor typically uses two tubes, and any mechanical differences in the tubes (e.g., welding, stiffness, or resonance frequency) can cause asymmetry in vibration. Even with fine-tuned resonance frequencies when the tubes are empty, fluid flow can still lead to zero drift due to viscosity changes.
Viscosity effects: Changes in the fluid’s viscosity can affect the vibration damping and resonance frequency, contributing to drift in the zero point. This is particularly true for high-viscosity fluids.
Structural imbalance: Any imbalances in the structure, such as non-uniform material properties or manufacturing tolerances, can cause measurement inaccuracies when fluid properties (such as flow rate or viscosity) change.
Zero drift is difficult to completely eliminate due to the sensor’s structural characteristics, but with precise design and manufacturing, it can be minimized. When selecting a Coriolis meter, it is important to choose one with appropriate specifications for the expected flow range and fluid properties.