Transient flow or water hammer is the sudden change of flow conditions due to an unexpected event such as a sudden closing of a valve installed on the pipeline system. Water hammer can pose serious threats to the pipeline system, as the flow's pressure can be changed above or below the limits which the pipeline network can withstand. Controlling the water hammer pressure becomes indispensable, so a transient surge analysis is required to investigate the points of severe changes along with the pipeline system. Compressed air vessel (CAV) is a pressure control device that is used to control the positive and negative pressure changes. There are two parameters affect the sizing of the air vessel; the initial trapped air volume and throttling the aperture of the air vessel. A computer model based on the unsteady one-dimensional flow equations of momentum and continuity is established to examine the proper size of the two parameters. The equations are solved by the method of characteristics while the vessel is modeled as a quasi-one-dimensional flow system. An experimental test rig provided with a rapid closing magnetic valve and pressure sensors are used to validate the model results. Both the experimental and model results show the high capability of the air vessel to dampen water hammer pressure. In addition, the results show that the introduced throttling action has a wide effect on the size of the air vessel and that the diameter of the throttling shall not be less than 0.3 the diameter of the main pipe. Besides, the initial volume of the trapped air should be in the range of 53% to 78% of the total volume of the air vessel, so that the air vessel works effectively with optimum value of 60%.