Page 52 - Valve Selection Handbook

Fundamentals 39

Figure 2-11. Incipient Cavitation Characteristics of Various "In-Line" Valves.
{Courtesy of The Institution of Engineers, Australia. 13)

Waterhammer from Valve Operation

When a valve is being opened or closed to change the flow rate, the
change in kinetic energy of the flowing fluid column introduces a transient
change in the static pressure in the pipe. In the case of a liquid, this tran-
sient change in the static pressure is sometimes accompanied by a shaking
of the pipe and a hammering sound—thus the name waterhammer.
The transient pressure change does not occur instantaneously along the
entire pipeline but progressively from the point at which the change of
flow has been initiated. If, for example, a valve at the end of a pipeline is
closed instantaneously, only the liquid elements at the valve feel the
valve closure immediately. The kinetic energy stored in the liquid ele-
ments then compresses these elements and expands the adjoining pipe
walls. The other portion of the liquid column continues to flow at its
original velocity until reaching the liquid column which is at rest.