Air apparent

New design guidance on reducing air problems in water and wastewater pipelines from HR Wallingford

Pipelines conveying water and wastewater can be severely affected by the presence of air in the flow. The list of potential problems is long and includes reduction in flow capacity (under performance); disruption to the flow; reduction in pump and turbine efficiency with associated increased operational costs; and difficulties or reduction in efficiency in filter or screen operation.

Vibration and structural damage due to the phenomenon of blow-back, an accumulation of air at hydraulic jumps which is unable to travel downstream, is another reported problem.

Other problems, such as generation of waterhammer pressures, potential for false readings on metering devices, buoyancy effects in underwater pipes, increased potential for corrosion of ferrous pipelines and increased biological activity due to the presence of air have also been reported. Some of these problems are not confined to pumping mains but can also occur in pipelines operating under gravity.

In an attempt to achieve satisfactory operation of pipelines, considerable engineering effort and cost goes into providing the minimum gradients to enable the transport of air to points of release, and in releasing the air at these points by means of air valves and chambers.

The guidance on which these engineering decisions are based is extensive but fractioned and gives a wide range of results. For cost and feasibility reasons, most studies have involved laboratory work where air movement is investigated in the form of bubbles (as opposed to the more critical case of air pockets), in smaller diameter pipes under a restricted range of pipe slopes. Field data, which would eliminate uncertainties in terms of scale effects, is also difficult to conduct. And the reliability of the data is affected by large uncertainties in the volumes of air present resulting from lack of visual access.

As a result of a two-year collaborative study part-funded by the Department of Trade and Industry, Thames Water and United Utilities, HR Wallingford has produced a guidance manual on how to prevent problems arising from the presence of air in water and wastewater pipelines. The manual includes contributions from the University of Liverpool and Black & Veatch, as well as from a panel of contractors, designers and specifiers. The scope of the study involved a comprehensive literature review of existing published information and practical knowledge, as well as targeted experimental and numerical studies to fill identified gaps in knowledge.

New design manual

The manual provides practical information to designers, contractors and operators dealing with gravity pipes and pumping mains which convey raw or potable water, cooling water, wastewater or stormwater. It gives an introduction to the complex, and still not fully understood, subject of multiphase flow, and presents existing and new information in a way which will enable engineering decisions to be made with increased confidence.

In order to prevent problems associated with the presence of air, it is important to understand the various ways in which air can enter a pipeline. The manual describes four main sources: dissolved air; air entrained from the atmosphere at drop chambers, inlets, intakes and outfalls, at hydraulic jumps, at sections with negative pressure;

accumulated air resulting from filling and emptying operations; and, although less common, air that is pumped into a pipeline to reduce cavitation pressures.

Depending on the relative amounts of air and water in the flow, different air or water patterns will arise, and these are also influenced by the pipe slope. For most water-engineering applications, where maximising the the capacity for transport of water is the prime objective, the flow patterns most commonly encountered are bubble and plug (air pocket) flow regimes. Since bubbles are easier to transport and will coalesce into air pockets, the guidance manual focuses on the transport of air pockets and provides newly collected information on the influence of pipe slope on air pocket shape. In horizontal or near horizontal pipes, pockets tend to be very elongated (and therefore may be more easily trapped at joints or bends) whereas they take a wedge-like shape as the slope increases.

Detailed experimental work conducted on 150mm diameter pipes set at various slopes provided new evidence that a minimum flow velocity (critical velocity) is required to move air along downward sloping pipes and even for horizontal pipes. This velocity is dependent on the pipe slope as well as on the volume of the air pocket. The manual presents new formulae and other design guidance on air transport and air entrainment at hydraulic jumps that will help engineers achieve the minimum velocities or gradients required to minimise air-induced problems and make more informed decisions on the need and location of air valves in pipelines. Guidance is given on how to design for exclusion of air at pump inlets, pipes and chambers and to control it by means of air valves, air vents and hydraulic removal.

The effects of entrapped or entrained air on pressure surges experienced by a pipeline when interruptions to the flow occur are also covered in the manual. Recent numerical modelling has shown that the presence of single air pockets of a critical size at a particular location along a pipeline – but especially at the upstream end – can result in significant exacerbation of peak transient pressures.

The manual alerts engineers to the need to seriously consider this as a potential factor for existing pipeline failures and to take measures to prevent detrimental effects on new designs.

The manual Air Problems in Pipelines: a Design Manual is available from HR Wallingford.

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