In differential pressure-based flow measurement systems using orifice plates, the choice of pressure tapping method significantly affects accuracy, installation complexity, and maintenance. The three most common tapping configurations are Corner Tapping, Flange Tapping, and D–D/2 Tapping. Below is a detailed explanation of each method and their respective advantages and disadvantages.
1. Corner Pressure Tapping
Configuration:
The pressure taps are located at the upstream and downstream faces of the orifice plate, aligned with the pipe wall. In single-tap designs, each pressure tap is placed at a distance equal to half the tap diameter from the respective orifice face. When using annular chambers, the spacing equals half the annulus width.
Advantages:
High Measurement Accuracy: Positioned in regions of steep pressure gradients, especially suitable for annular tapping, ensuring better pressure averaging and higher differential accuracy.
Stable Discharge Coefficient: When Reynolds number exceeds the critical threshold, the discharge coefficient mainly correlates with the diameter ratio (β).
Minimal Effect from Line Pressure Losses: Minor fluctuations in pressure drop along the pipe have limited impact on measurement.
Disadvantages:
High Installation Requirements: Precise positioning is critical. Misalignment can significantly compromise accuracy.
Prone to Clogging: Particularly with contaminated fluids, pressure taps may become blocked and are difficult to clean.
2. Flange Pressure Tapping
Configuration:
Pressure taps are positioned 25.4 mm (1 inch) upstream and downstream from the orifice plate faces, directly on the orifice flanges. This method is often referred to as the “1-inch tap” configuration.
Advantages:
Simple Construction: Uses standardized orifice flanges, making fabrication and installation straightforward.
Easier Maintenance: Taps are accessible, making it easier to clear blockages when measuring contaminated media.
Wide Medium Compatibility: Suitable for a broad range of fluids, including gas, liquid, and steam.
Disadvantages:
Higher Cost: Requires specially machined flanges, typically rated at 300LB or higher.
Discharge Coefficient Influenced by Pipe Size: Aside from β and Reynolds number, the coefficient also depends on pipe diameter (D).
3. D–D/2 Pressure Tapping
Configuration:
The upstream pressure tap is located one pipe diameter (D) upstream from the orifice plate face, while the downstream tap is placed at 0.5D downstream.
Advantages:
Good Clogging Resistance: Tap locations are less likely to accumulate contaminants, and cleaning is relatively simple.
Versatile Application: Works with most fluid types and installation environments.
Disadvantages:
Lower Differential Pressure Signal: Because of the extended tap spacing, the pressure difference is smaller. This may affect measurement sensitivity in low-dP applications.
Conclusion
Selecting the appropriate pressure tapping method is essential for ensuring flow measurement accuracy and long-term system reliability. The choice should consider:
Desired measurement precision
Medium cleanliness
Installation constraints
Maintenance accessibility
The diagram below provides a visual comparison of the three configurations: