In this paper boundary conditions for the most critical function of air valves (the outlet air phase) are reviewed. The isothermal behavior, proposed by the standard books, of the trapped air into the pipe is compared with the adiabatic process, a more realistic approach for fast transients. It is realized that the unfavorable hypothesis (the adiabatic one) is not used in practice. This is because the pressure of the water hammer due to the chock of the water- column against the dead end of the pipe (higher in the adiabatic analysis) exceeds the maximum peak of pressure reached by the air trapped (bigger under the isothermal hypothesis).
With the size of the air valve, the water hammer increases whereas the maximum pressure of the air decreases. The final selection will look for the most favorable combination, a compromise between both effects. Then, in order to select the appropriate air valve, a correct analysis of the transient with the appropriate hypothesis should be performed.