Chemical Engineering March 2021 - 31

areas where those contaminants come
through if a vacuum is present inside the
pipe. Engineers working with aseptic
processes especially need to be vigilant
for these conditions.
Transient event mitigation
Hopefully, the prior sections of this article
have made it evident that waterhammer
is a serious and complex issue. From a
single transient event, a wide range of
damaging conditions can arise. So the
question now becomes how to mitigate
the effects of waterhammer. While the
instigating events may be unavoidable,
engineers can design countermeasures
to account for the effects of waterhammer
and maintain compliance with the
ASME code.
A common way to mitigate waterhammer
is to manage the initiating event
itself, if possible. The simplest solution is
to close the valve over a longer period.
The previously mentioned Joukowsky
equation predicts the pressure rise from
an instantaneous change in flow. If that
change in flow is spread over a longer
time, then the resulting pressure wave
is less extreme. The longer the time interval
for fluid-velocity change, the lower
the magnitude of acceleration, and the
lower the resulting force and pressure.
Like most of life, the reality is more
complicated. Each type of valve has
unique characteristics, and the exact
timing of how to close the valve depends
on those characteristics. The
suggestion
for
valve
closure
mainly
relies on how the Cv (flow coefficient)
changes with valve position, and then
how that valve position changes over
time. If Cv (and thus, flow) changes dramatically
near the end of valve closure,
it is very important to close it slowly during
that period.
There is a common valve closure
practice known as the " 80/20 rule. "
That is, take 20% of the total valve-closure
time to close the first 80% of the
valve, and then take the last 80% of the
time to close the final 20% of the valve.
Closing the valve more slowly toward
the end mitigates the surge because
this is usually the moment of largest velocity
change.
However, even this rule of thumb has
its limitations. If the valve has a large
pressure drop relative to the pressure
drop of the entire system, then this
valve may already have tight control of
the flow. Changing the valve position
affects the system's flow significantly.
In this case, the valve should be closed
very slowly throughout the entire closure,
because every change in position
is significant. Understanding your
valves and their interaction with the
system is important, but difficult. Software
for waterhammer analysis can be
helpful in this regard as well.
Surge suppression equipment
Slowing valve closures as a waterhammer-mitigation
method require a planned
closure. But of course, an unplanned
need to close a valve can also occur, leading
to unexpected transient flow events.
For instance, a power outage may cause
a pump trip, which then causes a check
valve slam (rapid closure). In cases like
these, in-line equipment is required to
mitigate the waterhammer.
Relief valves or rupture disks are common
in-line components, because they
are a logical resolution to high-pressure
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Chemical Engineering March 2021

Table of Contents for the Digital Edition of Chemical Engineering March 2021
