Coriolis Flow Meters

What’s a Coriolis Flow Meter?

Understanding the Coriolis Effect

Working Mechanism

1. Vibration of the Tube:

   An electromagnetic drive unit vibrates the flow tube at its natural frequency. This vibration occurs perpendicular to the direction of the fluid flow.

2. Fluid Flow Through the Vibrating Tube:

   As the fluid enters the vibrating tube, it begins to interact with the tube’s vibration.

3. Phase Shift Detection:

   Sensors located at the inlet and outlet of the tube detect the time difference (phase shift) between the vibrations at these two points.

4. Calculation of Mass Flow Rate:

   The phase shift is directly proportional to the mass flow rate of the fluid. The greater the mass flow, the larger the phase shift. The flow meter’s electronics compute the mass flow rate based on this phase shift.

5. Density and Temperature Measurement:

   Many Coriolis flow meters also measure the fluid’s density by analyzing changes in the vibration frequency, as well as temperature using built-in temperature sensors. This allows for additional calculations, such as volumetric flow rate and fluid composition analysis.

Tube Designs of Coriolis Flow Meter

1. U-Shaped Tubes

   • Description:Feature one or more U-shaped tubes.
   • Advantages:
     • High sensitivity due to the larger displacement caused by the U-shape.
     • Effective for measuring low flow rates.
   • Limitations:
     • Bulkier design.
     • Higher pressure drop compared to straight tubes.

2. S-Shaped (Omega) Tubes

   • Description: Resemble the Greek letter Omega (Ω) or an S-shape.
   • Advantages:
     • Compact design compared to U-shaped tubes.
     • Balanced structure reduces stress and extends meter life.
   • Limitations: Slightly less sensitive than U-shaped tubes.

3. Straight Tubes

   • Description: Consist of one or two straight tubes.
   • Advantages:
     • Minimal pressure drop.
     • Easier to clean, ideal for sanitary applications.
     • Lower risk of clogging with viscous or particulate-laden fluids.
   • Limitations:
     • Less sensitive at low flow rates.
     • May require more robust support to prevent external vibrations.

4. Dual Curved Tubes

   • Description: Incorporate two parallel curved tubes.
   • Advantages:
     • Improved sensitivity and accuracy.
     • Reduced effects of external vibrations.
   • Limitations:More complex design can increase cost.

Accuracy of Coriolis Flow Meter

• Liquids: ±0.1% to ±0.2% of the reading.
• Gases: ±0.5% to ±1.0% of the reading.

Factors Influencing Accuracy

1. Fluid Properties:

   Viscosity, density, and temperature can affect measurements, but Coriolis meters compensate for these variations effectively.

2. Flow Conditions:

   Swirling flows or asymmetrical velocity profiles can introduce errors. Proper installation helps mitigate these issues.

3. External Vibrations:

   Vibrations from nearby equipment can interfere with the meter’s operation. Isolation or damping measures may be necessary.

4. Two-Phase Flow:

   The presence of gas bubbles in liquids or liquid droplets in gases can affect accuracy.

Enhancing Accuracy

• Calibration: Regular calibration ensures the meter maintains its accuracy over time.
• Installation: Following manufacturer guidelines for installation minimizes potential errors.
• Maintenance: Routine inspections and maintenance prevent performance degradation.

Rangeability of Coriolis Flow Meter

• Turndown Ratios: Up to 100:1 for liquids, meaning the maximum flow rate is 100 times the minimum flow rate the meter can accurately measure.

Benefits of High Rangeability

• Versatility: A single meter can cover a wide range of flow rates, reducing the need for multiple meters.
• Cost-Effective: Lower installation and maintenance costs due to fewer required meters.
• Process Flexibility: Ideal for processes with variable flow rates.

Sizing and Pressure Drop of Coriolis Flow Meter

Sizing Considerations

1. Flow Rate: The meter should be sized to handle the maximum expected flow rate without exceeding its capacity.
2. Fluid Characteristics: Viscosity and density affect the meter’s operation and must be considered during sizing.
3. Process Conditions: Temperature and pressure conditions can influence material selection and meter design.

Pressure Drop

• Definition: The reduction in pressure as the fluid passes through the flow meter.
• Factors Influencing Pressure Drop:
  • Tube Design: Curved tubes generally result in higher pressure drops than straight tubes.
  • Flow Rate: Higher flow rates increase pressure drop.
  • Fluid Viscosity: More viscous fluids lead to higher pressure drops.
• Mitigation:
  • Proper Sizing: Oversizing the meter can reduce pressure drop but may affect accuracy at low flow rates.
  • Tube Selection: Choosing a straight-tube design can minimize pressure drop.