A transient is a temporary flow and pressure condition that occurs in a hydraulic system between an initial steady-state condition and a final steady-state condition. When velocity changes rapidly in response to the operation of a flow-control device(for instance, a valve closure or pump start), the compressibility of the liquid and the elasticity of the pipeline cause a transient pressure wave to propagate throughout the system. If the magnitude of this transient pressure wave and the resulting transient flow variation is great enough and adequate transient-control measures are not in place, a transient can cause system hydraulic components to fail (for instance, a pipe burst). In general, transients resulting from relatively slow changes in flow rate are referred to as surges, and those resulting from more rapid changes in flow rate are referred to as water hammer events. Surges in pressurized systems are different than tidal or storm surges, flood waves, or dam breaks, which can occur in open-water bodies. A water hammer wave travels much faster in a pressurized system and it can burst even the strongest pipes. In general engineering practice, the terms surge, transient, hammer, and water hammer are synonymous.