Water hammer phenomenon is usually happened in water distribution networks due to pumps power failure or sudden closure, opening valves which lead to generate massive positive and negative pressures that may be catastrophic. In this research Bentley HAMMER Vi 8 is used to study the effect of roughness coefficient on water hammer wave in pipes. There are different methods to control and reduce these transient effects such as surge tanks, air vessels and pressure reducing valves. Choosing one of

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... hese methods depends on the design criteria, location and topography that strongly affect the decision maker. Air vessel was chosen in this study as a protection method for its ability to deal with negative pressure more effectively and more economical than other forms of water hammer protection devices. This study was applied on Bosat Kareem El Dein water pipe network. The research concluded that water velocity, length of pipe, modulus of elasticity, inside diameter of pipe, pipe wall thickness and roughness coefficient are the main factors affecting the severity of water hammer. Also increasing the size of vessels decrease the damping time of the surge pressure wave, but this should be regarded from the economical point of view.