Air-Pocket Clearing in Downward Sloping Pressurised Pipelines: A Comparative Study of Criteria for Air-Valve Assessment
(2026) In TVVR 5000 VVRM01 20261Division of Water Resources Engineering
- Abstract
- 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... (More) - 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. (Less) - Popular Abstract
- Water mains are often imagined as being completely full of water. In practice, air can enter during filling, draining, maintenance, or pressure changes. If this air is not removed, it can gather at high points, reduce the useful flow area, increase energy losses, and disturb operation.
This thesis asks a practical question: when water flows through a downward sloping pressurised pipe, will the flow carry a trapped air pocket away, or will the air stay in the pipe? The answer matters because a trapped air pocket may need to be removed by an air valve.
The study compared several ways to estimate the minimum water velocity needed to clear air pockets. This minimum velocity is called the critical velocity. The comparison included the... (More) - Water mains are often imagined as being completely full of water. In practice, air can enter during filling, draining, maintenance, or pressure changes. If this air is not removed, it can gather at high points, reduce the useful flow area, increase energy losses, and disturb operation.
This thesis asks a practical question: when water flows through a downward sloping pressurised pipe, will the flow carry a trapped air pocket away, or will the air stay in the pipe? The answer matters because a trapped air pocket may need to be removed by an air valve.
The study compared several ways to estimate the minimum water velocity needed to clear air pockets. This minimum velocity is called the critical velocity. The comparison included the Kalinske and Bliss criterion, a Kent-type criterion, the HR Wallingford method, and a German diagram-based guidance source.
The main result is that there is no single fixed velocity that can be used in all cases. The criteria can give different answers because they describe the air-water problem in different ways. The calculated value should therefore be read as a warning range, not as a final yes-or-no answer.
The calculations also showed that pipe diameter, downward slope, and assumed air-pocket size can change the predicted clearing velocity. A valve that looks unnecessary under one criterion may still need further checking under another.
The thesis proposes a three-step screening approach: compare the normal operating velocity with selected critical velocities, check the actual valve function and installation condition, and consider larger hydraulic events such as filling, draining, pressure-reducing valves, and pump shutdowns.
Water utilities and consultants can use the framework as a first filter. It does not replace manufacturer data, field inspection, or transient modelling. It helps decide which air-valve locations need more attention before an air valve is removed, replaced, or left unchanged. (Less)
Please use this url to cite or link to this publication:
https://lup.lub.lu.se/student-papers/record/9243587
- author
- Liu, Pan LU
- supervisor
- organization
- course
- VVRM01 20261
- year
- 2026
- type
- H2 - Master's Degree (Two Years)
- subject
- keywords
- air pockets, air valves, pressurised pipelines, critical velocity, downward sloping pipes, air-pocket clearing
- publication/series
- TVVR 5000
- report number
- TVVR26/5012
- ISSN
- 1101-9824
- language
- English
- additional info
- Examiner: Rolf Larsson
- id
- 9243587
- date added to LUP
- 2026-06-29 08:13:35
- date last changed
- 2026-06-29 08:13:35
@misc{9243587,
abstract = {{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.}},
author = {{Liu, Pan}},
issn = {{1101-9824}},
language = {{eng}},
note = {{Student Paper}},
series = {{TVVR 5000}},
title = {{Air-Pocket Clearing in Downward Sloping Pressurised Pipelines: A Comparative Study of Criteria for Air-Valve Assessment}},
year = {{2026}},
}