Modeling One and Twophase Water Hammer Flows - Advances in Control and Automation of Water Systems - page 44

3.2 mAteriAls And metHods
3.2.1 two Phase flow
For Two phase flow systems with free gas and the potential for water-column separa-
tion, the numerical simulation of hydraulic transients is more complex and the com-
puted results are more uncertain. Small pressure spikes caused by the type of tiny
steam bubbles or vapor pockets that are difficult to simulate accurately seldom result
in a significant change to the transient envelopes. Larger vapor-pocket collapse events
resulting in significant upsurge pressures are simulated with enough accuracy to sup-
port definitive conclusions. The cause of a hydraulic transient is any sudden change in
the fluid itself or any sudden change at the pressurized system's boundaries, including:
• Changes in fluid properties
• Changes at system boundaries
Hydraulic transients can result in the following physical phenomena: High or low
transient pressures—These can be applied to piping and joints in a fraction of a second
and they often alternate from high to low and vice versa . High pressures resulting from
the collapse of vapor pockets are analogous to cavitations in a pump: they primarily
accelerate wear and tear, but they can burst a pipe by overcoming its surge-tolerance
limit. Sub atmospheric or even full-vacuum pressures can combine with overburden
and groundwater pressures to collapse pipes by buckling failure. Groundwater can
also be sucked into the piping.
Hightransientlows —these can result in significant degradation of water quality
as deposits and rust are loosened and entrained at high velocities. This is aggravated
whenever flows reverse direction during a transient event. High-velocity flows also
exert rapidly moving pressure pulses result in temporary, but very significant, transient
forces at bends and other fittings, which can cause joints to move. Even for buried
pipe, repeated deflections combined with pressure cycling can wear out joints and
result in leakage or outright failure. Thrust blocks are typically sized for steady-state
forces plus a safety factor—not transient forces—and typically resist thrust in only one
direction. In pump stations, low pressures on the downstream side of a slow-closing
check valve may result in a very fast closure known as valve slam. A 10 psi (69 kPa)
pressure differential across the face of a 16 in. (400 mm) valve can result in impact
forces in excess of 2,000 lb. (8,900 N).
Column separation —Water columns typically separate at abrupt changes in pro-
file or local high points due to sub atmospheric pressure. The space between the water
columns is filled either by the formation of vapor (e.g., steam at ambient temperature)
or air, if it is admitted to the pipeline through a valve. With vaporous forces at pipe
bends Transient forces—cavitations, a vapor rocket forms and then collapses when
the pipeline pressure increases as more flow enters the region than leaves it. Collapse
of the vapor pocket can cause a dramatic high-pressure transient if the water column
rejoins very rapidly, which can, in turn, cause the pipeline to rupture. Vaporous cavita-
tions can also result in pipe flexure that damages pipe linings. High pressures can also
result when air is expelled rapidly from a pipeline, which tends to repeat more times
than when a vapor pocket collapses.