In a great number of engineering problems one is interested in the perturbation of a given flow. The motion of the perturbations relative to the fluid is called the wave motion, and the velocity of propagation the wave velocity. In the preceding sections the equations required to treat this phenomenon have already been introduced. A description of such a system requires the use of the continuity equation,
Thus it is possible to recognize a rather strict analogy between the propagation of sound waves in gases and elastic waves in liquids. On the basis of this analogy it becomes possible to calculate wave propagation and waterhammer in both crude oil and natural gas pipelines.