Demystifying Air Valves and How to Use and Size Them
Proper sizing of air valves is essential for maintaining the efficiency and safety of pipeline systems. Air valves are available in three basic types: air release, air vacuum, and combination. Numerous factors, such as pipeline materials, flow conditions, water or wastewater, and pipeline profile, contribute to the correct sizing of air valves. In this article, we will explore the technical aspects of sizing air valves, with particular emphasis on the importance of vacuum protection and the role of air vacuum valves in preventing dangerous vacuum conditions.
Types of Air Valves
Before delving into the sizing process, it is important to understand the three main types of air valves:
Air Release Valves (ARV’s): These valves are designed to release small volumes of air that accumulate in pipelines during normal operation, reducing the risk of air entrapment and improving system efficiency.
Air Vacuum Valves (AVV’s): These valves serve to admit or release large volumes of air during pipeline filling or draining, preventing the formation of dangerous vacuum conditions that could cause pipeline collapse or damage.
Combination Air Valves (CAV for clean water, SCAV for sewage): These valves combine the functions of air release and air vacuum valves in a single unit, offering a versatile solution for various pipeline applications.
Of course, there are others but this article will focus on these three.
Factors to Consider When Sizing Air Valves
The following factors should be taken into account when sizing air valves for a pipeline system:
Pipeline Materials: Different materials, such as ductile iron, PVC, or steel, may require different air valve sizing approaches due to their varying resistance to vacuum conditions and pressure surges.
Flow Conditions: The flow rate, velocity, and pressure in the pipeline system play a critical role in determining the appropriate size and type of air valve required to maintain efficient and safe operation. Also important are the speed at which the pipeline is filled or drained.
Water or Wastewater: The type of fluid being transported in the pipeline, whether it is water or wastewater, can influence the sizing and selection of air valves, as the presence of solids or chemicals may require specialized valves or additional considerations.
Pipeline Profile: The elevation changes and layout of the pipeline system can significantly impact the risk of vacuum formation and the required placement and sizing of air valves. In general air valves are installed at localized high points in a piping system. That is where air will accumulate.
Vacuum Protection and Air Vacuum Valves
The most critical consideration when sizing air valves is ensuring adequate vacuum protection. Dangerous vacuum conditions can develop in a pipeline during draining or a pipeline rupture, and significant pipeline elevation changes can further increase this risk. Air vacuum valves are specifically designed to prevent the formation of such vacuum conditions by admitting or releasing large volumes of air as needed. To properly size air vacuum valves for optimal vacuum protection, the following steps should be followed:
Analyze the pipeline profile to identify areas with significant elevation changes or potential for vacuum formation.
Determine the maximum allowable vacuum pressure for the specific pipeline material and the desired safety factor. An important part of this is to determine what percentage of pipeline rupture will be planned for. Greater air intake and larger or more air vacuum valves are required if the planning assumption is 100% line break rather than 50%.
Calculate the air inflow and outflow rates required to ensure the maximum allowable vacuum pressure will not be exceeded during filling, draining or pipeline rupture events.
Select the appropriate air vacuum valve size based on the calculated air inflow and outflow rates, ensuring that the valve can adequately admit and release the required air volumes to prevent dangerous vacuum conditions.
Air Release Valves
The importance of Air Release Valves is related to system efficiency. Air pockets build up at high points of a pumping system. If they cannot be removed they essentially create a blockage. This means the pumps now have to work harder and the system capacity is reduced. Energy efficiency is a critical design factor these days and energy consumption of pumps is an important factor.
Air release valve sizing can be difficult at times with orifices based on flow conditions, confusing charts and shades of slide rules lurking. These days things are easier, particularly if using a modern valve like the DeZURIK ASU which automatically sizes itself.
To properly size air release valves:
Determine the media. Wastewater is about 7% air by volume whereas clean water is much less. Air coming out of solution is one of our targets. More importantly, wastewater ARVs require a minimum of 2” inlets so the media does not regularly clog the valve internals.
Analyze the pumping system and profile. Things such as pressure, flow, pipeline size, and profiles are important. Air Release valves are installed at system high points.
Calculate the required air release rate which will dictate orifice size.
Bringing it all together - The Combination Air Valve
Combination Air Valves perform both functions previously discussed. We have already discussed how to size these so the article is done…well not so fast. Modern designs such as the DeZURIK ASU automatically size the Air Release function based on real-time pipeline conditions. This makes the discussion primarily about vacuum protection. A single-body combination air valve can be used where the air inflow requirements dedicated by sizing the vacuum protection needed result in an outlet of 6” or less. If more volume is needed the solution is to provide a large air vacuum valve and pipe an air release valve to the side of it. These are called, not surprisingly, dual-body combination air valves.
Sizing air valves correctly is crucial for ensuring the efficient and safe operation of pipeline systems. By taking into account factors such as pipeline materials, flow conditions, water or wastewater, and pipeline profile, engineers and system designers can select the appropriate size and type of air valve to maintain optimal system performance and prevent dangerous vacuum conditions. In particular, using air vacuum valves is essential for providing adequate vacuum protection and safeguarding pipelines from potential collapse or damage. Most often combination air valves are used which simplifies the process. Your local DeZURIK representatives are experts at sizing air valves and can answer your questions.