The second function of an air valve is to admit air into the system when the internal pressure of the pipeline drops below atmospheric pressures. By admitting air into the pipeline as the internal vacuum condition develops, the magnitude of the vacuum pressure can be reduced and as a result help prevent the pipeline from experiencing excessive deflection and/or collapse as well as help prevent the formation of a full vacuum condition in which vapor cavities may form from the fluid vaporizing. The air valve used in this situation is an air & vacuum valve. The air & vacuum valve is also used to discharge large volumes of air from the pipeline system when the pipeline is initially filled and after water column separation.