Air-Pocket Clearing in Downward Sloping Pressurised Pipelines: A Comparative Study of Criteria for Air-Valve Assessment

Sammanfattning

Air pockets in pressurized pipelines may reduce hydraulic capacity, increase head losses, and affect system operation. In downward sloping pipe sections, a key question is whether the water flow can transport a trapped air pocket downstream or whether the air is likely to remain stationary, which would require an air valve to evacuate the air. This thesis compares selected criteria and guidance sources for air-pocket transport and clearing in downward sloping pressurized pipes. The Kalinske and Bliss criterion, the Kent-type criterion, the HR Wallingford method, and a German diagram-based guidance source are examined under a common set of pipe diameters and slope angles. The comparison is based on critical velocity as the main output variable. The results show that the selected criteria can give different clearing velocities because they are based on different physical interpretations, including hydraulic-jump-related transport, stationary gas-pocket clearing, and practical self-venting guidance. The study also shows that pipe diameter, downward slope angle, and assumed air-pocket parameters affect the predicted threshold. The findings support a cautious screening approach for air-valve assessment. Critical-velocity criteria can help identify possible air-accumulation risks, but they should be combined with valve-function checks, installation geometry, manufacturer data, and field inspection.