Fluid Hammer
When the flow of fluid through a system is suddenly halted at one point, through valve closure or a pump trip, the fluid in the remainder of the system cannot be stopped instantaneously as well. As fluid continues to flow into the area of stoppage (upstream of the valve or pump), the fluid compresses, causing a high-pressure situation at that point. Likewise, on the other side of the restriction, the fluid moves away from the stoppage point, creating a low pressure (vacuum) situation at that location. Fluid at the next elbow or closure along the pipeline is still at the original operating pressure, resulting in an unbalanced pressure force acting on the valve seat or the elbow.
The fluid continues to flow, compressing (or decompressing) fluid
further away from the point of flow stoppage, thus causing the leading edge of
the pressure pulse to move through the line. As the pulse moves past the first
elbow, the pressure is now equalized at each end of the pipe run, leading to a
balanced (i.e., zero) pressure load on the first pipe leg. However, the
unbalanced pressure, by passing the elbow, has now shifted to the second leg.
The unbalanced pressure load will continue to rise and fall in
sequential legs as the pressure pulse travels back to the source (or forward to
the sink). The ramp up time of the profile roughly coincides with the elapsed
time from full flow to low flow, such as the closing time of the valve or trip
time of the pump. Since the leading edge of the pressure pulse is not expected
to change as the pulse travels through the system, the ramp down time is the
same. The duration of the load from initiation through the beginning of the
down ramp is equal to the time required for the pressure pulse to travel the
length of the pipe leg.