AWWA M51:2001 pdf download.Manual of Water Supply Practices M51
A power failure or line break may result in a sudden change in the flow velocity because of column separation and gravity flow. The gravity flow may result in excessive vacuum conditions occurring at the adjacent high points. Most small and medium‐size pipes commonly used in the water industry can withstand a complete vacuum; however, because low stiffness, large‐diameter pipelines may collapse from negative internal pressures. Therefore, sizing air valves for gravity flow conditions is important to maintaining the integrity of the pipeline. Air valves at high points should be sized to allow the inflow of air to minimize negative pressures in the pipeline and prevent possible damage to pump seals, equipment, or the pipeline itself.
When sizing an air valve orifice for gravity flow, the pipe slope will determine the volume of air required to prevent excessive vacuum. An appropriate air valve should be provided at the nearest high point with the orifice size to allow the require inflow of air to replace the water in the pipeline. Figure 4‐3 illustrates the required inflow of air required for various pipe sizes and slopes.
The orifice sizing of an air valve for inflow is typically based on the lower of 5 psi (34 kPa) or the allowable negative pressure below atmospheric pressure for the pipeline with a suitable safety factor. Sonic flow will occur when the outlet‐to‐inlet pressure ratio (ASME, 1971) fall below 0.53. knowing that the inlet pressure is atmospheric pressure (14.7 psia [101 kPa]), then any negative pipeline pressure below 7.8 psia (54 kPa [absolute]) or ‐7 psig (48 kPa) (vacuum) will produce sonic flow. Because the flow will be sonic and restricted, flow volume will not increase beyond ‐7 psi (48 kPa) diferential.
If gravity flow occurs in a pipeline with a change in slope where the pipeline lower section has a steeper slope than the upper section, then an air/vacuum valve should be considered at the location where the pipeline change slope. The gravity flow will be greater in the pipeline section with the steeper slope. The air/vacuum valve orifice should be sized so that the inflow of air at this point equals the difference in the two flow rates at the allowable negative pressure.