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To begin to understand surge wave travel, let’s consider an example of a single pump, a simple level pipeline, and a large reservoir. The normal condition consists of a running pump producing flow in the pipeline in the direction of the reservoir. We will assume a large reservoir where the pressure does not change as fluid is pumped into it. The areas of normal pressure are shaded blue, the areas of reduced pressure, or downsurge, are shaded gray and he areas of increased pressure, or upsurge are shaded red. There is minor contraction and expansion of pipe during downsurge and upsurge. When the pump is shut down, the flow into the pipeline comes to a stop and the fluid velocity changes to zero as the fluid comes to rest. This velocity change initiates a downsurge that begins at the pump and travels in a manner similar to wave travel toward the reservoir. The pipe contracts as the downsurge travels through it. The surge wave travels at the speed of sound in water, modified by the material and size of pipe. When the downsurge reaches the reservoir, the entire pipeline is at a reduced pressure. The entire pipeline is contracted from is normal diameter, and the surge velocity is momentarily at zero. Since the reservoir pressure is now greater than the pressure in the pipe. As the pressure tends to equalize in the pipe, there is a surge in the opposite direction at normal pressure. It should be noted that surge waves travel in any direction, regardless of fluid flow direction in the pipe. As the pressure wave reaches the closed check valve the wave velocity is instantly stopped. The surge wave has now made one round trip of the system. The time it takes for the surge wave to make a round trip is an important concept known as a surge period, or critical period. The sudden velocity change that occurs when the reverse flow is halted at the check valve causes an upsurge that begins at the check valve, expands the pipe and begins to travel back down the pipe toward the reservoir. When the upsurge wave reaches the reservoir, the highest pressure is exerted on the pipeline. For an instant, the upsurge reflects back toward the pump, followed by normal pressure from the reservoir. The pressure in the pipeline behind the upsurge is at the normal reservoir pressure. Two surge periods have been completed. The pressure conditions are similar to right before the pump shutdown, except this time the pump is not running and the check valve is closed. The next cycle begins.