Scope and Applicability of the Item This Item provides simple@ approximate methods that may be used to guide the selection of air inlet/outlet valves required to protect liquid-filled lines from excessive sub-atmospheric pressures following pump trip or valve closure. Graphical data are provided that allow conservative estimates to be made of the maximum mass flow rate@ and hence volume flow rate@ of dry air through the valve and the maximum volume of the resulting air cavity. The data given in this Item are applicable to air valves that are located along a liquid-filled line such as that shown in Sketch 2.1. In this model@ it is assumed that liquid is discharged from the pump along a single line from the upstream sump to a downstream constant pressure point@ such as a large tank or reservoir. It is assumed that the pipe diameter and pipe wall friction factor are constant along the length of the line and there are no reflections of the pressure waves from the pipe bends and fittings@ including the air valve inlet. Component losses are assumed to be small compared with the pipe friction losses and are neglected. The pump is protected by a one-way valve (commonly referred to as a reflux valve or check valve in the water industry) that prevents reverse flow through the pump by closing completely when the pump discharge falls to zero. It is assumed that the one-way valve remains shut and does not allow any further forward flow. The model will allow systems with pressurised tanks to be considered@ provided suitable adjustments are made to the tank head values. The data given in this Item are also applicable to the downstream side of a closing valve in a liquid-filled line. A guide to the use and location of air valves in pipelines@ both for normal operation and for surge suppression@ is given in Section 3 of this Item. Guidance on the nature of pressure surges in pipelines protected by air valves is also given in that Section. In Section 4@ data are presented that may be used to estimate the maximum mass flow rate into a line through an air valve located at a high point in a pipeline to protect against extreme sub-atmospheric pressures. Data are also presented in this Section that allow estimates to be made of the maximum volume of air at that location on the pipeline following the surge event. A worked example is presented in Section 5@ and the equations that describe the air flow behaviour through air inlet/outlet valves are presented in Appendix A. The computer program@ ESDUpac A8727@ introduced in Item No. 87027 may be used to investigate the transient pressures after pump trip in a pipeline protected by air valves@ and the effect of an air vessel placed at the pump outlet on those pressure changes.