It is important for engineers to be fully aware of the need of protection of water and wastewater pipeline from negative pressure occurring in case of pipe drainage or burst, in presence of undersized or poorly located air valves, and in case of transients associated to flow rates changes, like valves operation or pump trip.
Failing in doing that may lead to the entrance of contaminated water into the system, pipe collapse, cavitation and column separation because the negative pressure can be just as damaging to the network as high pressure.
Alleviation of sub-atmospheric conditions commonly requires the introduction of properly located air valves at the high points, changes in slopes, long segments and whenever the risk of column separation may appear.
The air flow capacity through air valves depends on the effective section of the seat, following physical laws and mathematical formulas. Engineers should therefore always base their analysis on the air flow charts provided by the manufacturer and not only on the connection which may lead to inaccuracies and great danger.
The following pictures shows, purely for illustrative purposes, the use of two sewage air valves both with flanged DN 150 mm. On the image on the right CSA full bore model with an intake capacity of approx. 5000 m3/h with a 0,5 Dp, suitable for the application of pipelines up to 700/900 mm , while on the left a model with reduced passage on top, features a capacity less than 300 m3/h, strongly advised for pipeline not larger than 150/200 mm. If used for larger pipes the latter will inevitably lead to negative pressure.
The video below shows the power of vacuum as a reminder of the great danger coming from a poor judgment and air valves assessment.
CSA AVS sizing program has been designed to help users in the choice and location of air valves, in case of pipe drainage and burst, then seamless integrated with the CSA surge water hammer analysis program, able to carry our dynamic simulations and transient analysis.