SUMMARY The operations consisting of filling and emptying pipe systems involve the movement of big quantities of water and air. This kind of operations, together with the possible fluid pressure alterations during the normal operation of the installation are some of the possible causes that may provoke the presence of trapped air. The trapped air pockets in a pipes system reduce the transport capacity of the pipes, at the same time they can produce damage in the pipe and in the different elements and accessories of the pipes system, like water-metres, valves etc. Moreover, in the hydraulic transients with trapped air pressures bigger than the Allievi pressure can be generated. It is also true that in some occasions and in certain circumstances, the air pockets may mitigate the waterhammer produced in a hydraulic transient acting as a cushion. The elements used usually in the pipe systems to eliminate or to admit air in the pipes are the air valves. The conventional air valves open and close automatically, so its behaviour can’t be modified externally. Actually, there are user’s manuals like the one proposed by the American Water Works Association (AWWA) or manufacturers catalogues, based in the experience obtained through the years, that serve like a guide for selecting the adequate air valve diameter and the its optimum place in the pipe system. On the other hand, there is no a norm or an accepted standard related to this question, so, the pipe system user is the person who must take the final decision. In this way, the nominal air valve diameter and the discharge or admission coefficient are the determinants parameters to select the adequate air valve for a specific use in a pipe system. Quite often, the manufacturers offer discharge coefficient numbers that are not correct. Moreover, there is little information about the air valves behaviour in no steady flow conditions, and even less information about the effects and damage on the system during a hydraulic transient. In fact, this work tries to provide more information about this item by carrying out laboratory tests, in a large installation. In these tests, the influence of the air valves on the registered pressures during the transient generated under controlled conditions has been analysed. Dynamic discharge and admission tests of air valves were realized. The air valves diameter were 50 mm and 100 mm, and the pipe diameters were 200 mm and 500 mm respectively. Complementarily, tests for determining the air valves discharge or admission capacity have been done. With the results obtained in the laboratory tests comparisons, related to the impact introduced in the transient, between a pair of air valves that have the same nominal diameter and that have been tested under similar conditions can be established. With the aim of performing a deeper study of the dynamic behaviour of the air valves and its influence in the pressure and velocity final results a mathematical model has been developed. With the aid of this mathematical model, the hydraulic transient produced during the discharge of air through an air valve situated at the extreme of a pipe while this pipe is being filled of water, is studied. Moreover, the model allows performing a sensitivity analysis of all the parameters taking part in the process with the objective of determining the degree of influence of every one of them on the transient final result. Finally, this work is completed with a software compilation that some air valves manufacturers offer and that aid in the determination, selection and positioning of the air valves into the pipe system or a hydraulic transient analysis.