ESDU 85009 gives a graphical means of estimating the maximum mass inflow rate, and hence the pressure drop below atmospheric, through an air inlet/outlet valve following pump trip or valve closure. The maximum volume of air in the resulting air cavity is also predicted, and hence the time taken for the air to be released on restarting the line. The method assumes a constant diameter liquid-filled line carrying fluid from an upstream sump to a downstream constant pressure point. A program in Basic is also given to provide the information, and covers the case when the line is additionally protected by an air vessel. The use of the information is illustrated in a practical worked example. Guidance on the use and location of air valves in pipelines for both normal use and surge suppression is included.