Any pipeline in which velocity fluctuations can occur can be subjected to large magnitude pressure transients. All pipelines are subjected to such velocity fluctuations when for example a valve is opened or closed or when an hydraulic turbine connected to it is started up or shut down. In Chapter 8 it was demonstrated that a rapid velocity fluctuation, that is one occurring in a time less than 2L/c, causes a pressure change that can be as large as 130 feet head per foot per second of velocity change. It is therefore clear that a pipe must be protected against such rapid velocity fluctuations if it is not to be excessively costly. In other words, rapid velocity fluctuations must be converted into slow velocity fluctuations so that the pressure transients so generated will be diminished to insignificant values.
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