The behavior of air valves can be best viewed using a profile view.
A value less than the vapor pressure indicates that no flow will occur. WaterGEMS/CAD provides a warning message if the pressure drops below zero and a more severe warning when it drops below the vapor pressure of the fluid (32 ft, 9.8 m for water). This is generally not recommended as flexible pipes may collapse and intrusion of questionable fluids can occur in water distribution systems. If there is no air valve at a high point and the pressure drops below zero, the pipeline will behave as a siphon. For transient analysis (available in Bentley HAMMER), there are numerous other properties that can come into play.
The "Treat as Junction" property is the only property in steady and EPS runs that is different between a junction and an air valve. For valves being analyzed, this property should be set to False. The valves that are almost certain to remain closed in the analysis can have a property "Treat as Junction?" set to True and the air valve will behave as a junction node in the model run. In systems with multiple valves, it is best to focus the analysis on those valves which are likely to open and close. The more air valves, the more likely this is to occur. In sewer force mains, irrigation systems and raw water transmission systems, pressure can drop when pumps turn off or head loss becomes excessive and they can often be in the open position and pipes can be partly full, immediately downstream of the high point.īecause air valves can change status during a model run, they can introduce instability in the run. In water distribution systems which must maintain a positive pressure, the valves are almost always closed. While air valves are often placed on pumps to remove air, their primary use is at high points in pressure piping systems, which are the first locations which can experience negative pressure in water systems and are the most likely places where air can accumulate. They are also used in filling and draining of pipelines which is not addressed in WaterGEMS/CAD. While they are useful in normal operation, they are especially important in transient analysis (available in Bentley HAMMER). The underlying behavior of the valves is to be closed when then hydraulic grade is above the valve elevation and to open to maintain atmospheric pressure when the hydraulic grade would otherwise have dropped below the valve. An Air Valve is the model element used to represent air relief valves, air release valves, vacuum breaker valves and combination air valves.
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