Chemical Engineering March 2021 - 30

800
600
400
200
0.01
0.02
Time (sec)
FIGURE 2. The graph depicts a pressure surge following a
rapid valve closure in a pipeline
800
600
400
200
Pressure
Vapor Volume
0.5 1 1.5
Time (sec)
2
FIGURE 3. After a rapid valve closure, such as the one represented
in Figure 2, the pipe can experience a vapor-collapse
scenario, as shown here in graphical form
changes as the pressure wave cycles.
When the wave reflects off the originating
valve, the fluid changes from negative
velocity to zero velocity. This " increase "
in velocity is met with a decrease in
pressure, below the steady-state value.
A low-pressure wave will always follow a
high-pressure wave. The severity of the
low-pressure wave depends on dampening
from friction and mitigation efforts.
Another case of low-pressure concern
in waterhammer is the area immediately
downstream of the closed valve. Similar
to the pump trip, there is no momentum
transfer to the downstream fluid once
that valve has closed, but the fluid still
has inertia driving it forward until the
system corrects toward equilibrium.
This creates a low-pressure area and
disturbs the line's pressure balance.
Whatever the cause of low-pressure
waves, they are frequently not considered
in a transient analysis. Thinking
intuitively, one might imagine that the
low-pressure situation cannot be nearly
as bad as the high-pressure effects. It
is common to associate high-pressure
scenarios with higher overall pipe stress
and lower-pressure occurances with
less pipe stress. While that thinking is
somewhat true, there are other factors
for which to account.
30
2
1.5
1
0.5
0.03 0.04
Vapor formation
If the negative-pressure wave drops the
line pressure below the vapor pressure
of the fluid, the fluid will flash (partially
evaporate) and form a short-lived vapor
pocket. When the pocket is large enough
to form a clear boundary between the
upstream and downstream liquid, the
situation is referred to as " liquid column
separation. " This term is also used more
generally to describe any liquid flashing.
Formation of vapor pockets inside a
pipe can create a situation that is worse
than the initial high pressure. Pipes tend
to resist internal high pressure much better
than they resist internal low pressure.
Often, vapor pockets create vacuum
conditions where the ambient pressure
is much higher than the internal pressure.
This pressure differential can actually
be more strenuous on a pipe than
if the pressures were reversed (higher
pressure inside the pipe than outside the
pipe). These pressure differences due to
these vacuum conditions may cause
pipe collapse and harm the integrity of
the pipe wall.
Even if the pipe does not collapse, the
vapor pocket will. The collapse causes
yet another high-pressure wave to burst
forth. These resulting pressures can be
even larger than the original pressure
wave from the valve closure. The vapor
pocket collapses with so much power
that it often bursts nearby pipe joints.
The concept is very similar to pump
cavitation. The collapse of the tiny
vapor bubbles happens with enough
force to cause " pitting " and damage to
the impellers. The same thing happens
with liquid column separation, but on a
larger scale. Figure 3 shows the vapor
collapse behavior downstream of the
same fast valve closure shown in Figure
2. Compare the resulting pressures here
with the initial pressure seen in Figure 3.
The large pressure from vapor pocket
collapse is not predicted by conventional
calculations.
Stress on a pipe and its components
due to vacuum conditions and vapor
collapse may not be the most critical
concern. In many situations, such as
wastewater treatment or food processing,
the process itself is the concern.
Even mild vacuum conditions can pull
outside contaminants into the pipes.
Joints are the main concern here, because
the seals and gaskets will be the
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Pressure (psia)
Pressure (psia)
Vapor Volume (ft3)
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Chemical Engineering March 2021

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