Chemical Engineering August 2012 - 39

Engineering Practice
Now ask yourself, what happens to
this energy?
The answer is that it goes into making
steam. Consider that vaporizing
an entire pound of 250°F condensate
would require about 900 Btu (per the
steam table). Since only 88 Btu of excess
energy are available, about 10% of
each pound of 338°F condensate leaving
the 100-psi steam main for the 15psi
condensate system will vaporize to
saturated steam, while the remaining
90% of condensate discharge remains
in liquid form. So, by mass, there are
nine parts water to one part steam entering
the condensate return line.
But the masses of the two phases
are not what we would notice if trap
discharge were visible. We would notice
the relative volume of the two
phases. Volume wise, the specific volume
of saturated steam at 15 psig is
about 800 times that of an identical
mass of liquid water. Thus, by volume,
the ratio of steam to water looks like
800 parts steam to 9 parts water, or 89
to 1. Therefore, what you would see exiting
the trap is predominantly steam
with a fine water mist interspersed in
the steam.
What we essentially have, then,
when the HP condensate discharge enters
the CR main is a large volumetric
flow of saturated steam at 250°F entering
a pumped CR line full of 200°F
water at 15 psi. The water is subcooled
roughly 50°F with respect to the entering
steam. This is enough subcooling
to support condensation-induced waterhammer.
In other words, if the entering
steam is able to blow a sizable
bubble, which is subsequently surrounded
by subcooled condensate, the
bubble can abruptly collapse, thereby
allowing the surrounding water to
rush in to the void left behind by the
disappearing steam and smack into itself.
Depending on the size of the void,
the overpressure from this waterhammer
event can exceed 1,000 psi.
Is waterhammer guaranteed?
The answer, surprisingly, is no. And
this explains why some steam systems
are able to get away with injecting HP
condensate into LP pumped condensate
returns without severe waterhammer.
What determines whether the two
mixing flows will hammer as they mix
If Rc/s can continuously be kept >1.0 - plus a
margin for imperfect mixing - waterhammer
due to insertion of flash steam into subcooled
condensate can be avoided
is the ratio of the condensing capacity
of the condensate flow to the heating
capacity of the incoming steam flow.
Researchers at Creare, Inc. first defined
this ratio as Rc/s, although their
notation and definition are modified
slightly here for the scenario being discussed.
Rc/s is defined as follows:
terhammer would be possible. This is
explained below).
(1)
Where:
ms = the mass flow of flash steam, lb/h
mc = the mass flow of subcooled condensate,
lb/h
cp = the heat capacity of water (1 Btu/
lb-°F)
ΔTbe low saturation temperature = the degrees
of subcooling below the saturation
temperature, °F
hfg = heat of vaporization, Btu/lb
* If Rc/s is < 1.0: there is not enough
flowing condensate-heat capacity to
condense all incoming steam flow,
so steam bubbles will remain in
the mix. The resulting two-phase
mixture will not collapse in a waterhammer
because there is not enough
condensing capacity to allow it to do
so. But, the flow is susceptible to
hammering downstream if another
subcooled-condensate flow merges
with the bubbly mixture so that Rc/s
then goes over 1.0.
* If Rc/s is initially > 1.0 and there is
perfect mixing of the two streams:
All steam will be condensed as it enters
the flowing condensate return
main and no steam bubbles will remain
to collapse. Thus, if flows remain
steady, the mix will not hammer.
Call this the " stable " mixing
region with respect to Rc/s. (Note,
however, if Rc/s is just slightly above
1.0 with imperfect mixing or stratification
of the flows, some steam bubbles
may persist temporarily and wa34
CHEMICAL ENGINEERING WWW.CHE.COM AUGUST 2012
The complicating factor in any normal
steam system is this: Flows do not remain
steady. Condensate pumps cycle
on and off to maintain their receiver's
tank level; blast discharge traps fire,
then dwell, then fire again; and downstream
in the CR main other condensate
streams may tee-in heading back
to the steam plant. All these events
change the Rc/s of the overall mixture
stream. In what the author considers to
be a landmark paper written in the mid
1980s for the nuclear power industry,
each of these shifting conditions was
tested as a function of Rc/s with varying
liquid-flow velocities to see when
waterhammer occurred [2]. In these
tests, steam was injected coaxially and
cocurrently through a 2-in. injector pipe
that was mounted to discharge axially
down the middle of an 8-in. pumped
condensate line as shown in Figure
2. One hundred and fifty tests for waterhammer
were performed, of which
about one-half exhibited waterhammer.
The tests showed the following:
Case A: Sudden increase in condensate
flow. If Rc/s was initially
< 1.0 in a mixture of constant steam
and condensate flows (so that steam
bubbles persisted in the mix) and condensate
flow was suddenly increased
(as if an additional condensate pump
were started) so that Rc/s exceeded
1.0, a waterhammer always occurred.
The situation was similar if the condensate
flow was ramped up from a
lower flow to a higher flow, although
water hammer did not occur in every
instance. One waterhammer did occur
with initial Rc/s as high as 1.3 into the
stable region - presumably the result
of incomplete initial mixing, which allowed
some bubbles to persist.
Case B: Abrupt shutoff of steam
flow. Likewise, if Rc/s was initially
still < 1.0 in the mixture but instead of
condensate flow increasing, steam flow
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Chemical Engineering August 2012

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