Chemical Engineering May 2022 - 36

Then, note the distance
between the orange and
red lines. This low-level
water height is the vertical
head used to calculate
NPSHA. As such,
floor pumps without
a flash tank can pressurize
and have a low
NPSHA.
There is so little
fill
FIGURE 15. A flash vessel reduces condensate temperature and mitigates
pressure buildup in the pump receiver
FIGURE 16. Non-electric pumps do not cavitate, but still
require flash vessels for proper operation
head available (approximately
0.1 psi) to
provide ample NPSHA
to the pumps. Is it any
wonder why these pumps experience
issues whenever the condensate
temperature is near-tosteam?
This pump needs a vapor
pressure of 10.68 psia from a
non-flashing condensate temperature
of 196ºF to avoid cavitation,
but unless the load is extremely
low (for cooling), it is unlikely, due
to the undersized vent line.
Reliable system design requires
key components, proper elevation,
minimal leaked live steam
infiltration, good flashing and effective
venting, as seen in Figure
15. To mitigate cavitation, condensate
pumps should first have
the flash steam effectively handled
in a properly sized flash vessel,
then receive condensate from
an appropriate height to provide
ample NPSHA.
FIGURE 17. Elevating the receiver of non-electric pumps
can increase capacity by filling the pump body quickly
the red, orange and blue lines displayed
on the receiver in Figure 14.
The blue line represents a high-level
trigger point that alerts the pumps
to operate, and the orange line represents
a low-level trigger to stop
pumping and thereby prevent losing
the water seal over the pumps.
36
Non-electric options
Non-electric secondary pressure
drainers can be used in lieu of
electric pumps. Figure 16 shows
an example of a Type 1 secondary
pressure drainer (SPD-1) that
can provide effective condensate
recovery without the use of electrical
components [9,10,11]. They
are mechanical pumps that use
pressurized steam or air to discharge
a certain displacement
volume of condensate into the
return header.
With no impellers to rotate and
cause flashing, it is not possible
for SPD-1 devices to cavitate during
pump operation. Even so, for
proper system performance, they do
require a separate receiver as part
of their installation to enable flashing
of high-temperature condensate
prior to pump entry. When properly
designed, SPDs can provide a significant
reliability improvement over
electric pumping systems.
Although cavitation is not a concern
with an SPD-1 system, there
can be a benefit to installing its flash
receiver at a higher elevation. This
accelerates condensate flow into
the pump and increases its capacity
compared to the same unit if its
tank were situated at a lower level,
as seen in Figure 17.
The sizing and overall design of
the flash tank/receiver, flash vent
and balance line are also key to support
reliable SPD-1 operation, so it
is recommended to consult with the
respective manufacturer of electric
or
non-electric pumps for design
recommendations relative to their
systems and in accordance with applicable
recognized standards.
Turn flash into cash
There is a great deal of readily available
information relating to the design
of pressurized flash-recovery
systems. These provide a great benefit,
because often as much as 10%
of flash steam can be recovered and
used for low-pressure purposes.
One key dependency for flash recovery
to be valuable is a definite need
for use of this supplemental steam
elsewhere so that it reduces boiler
load. If the boiler demand is not reduced,
then the alleged savings do
not materialize.
Another key dependency for
implementation is to understand
whether the elevated backpressure
limits the process equipment performance
or increases maintenance
costs. If equipment performance or
reliability are hindered, then those issues
may preclude its use.
These questions can be reviewed
by knowledgeable engineers who
can perform appropriate design and
steam-balance analyses to confirm
if the anticipated benefits can
be achieved.
However, even if it is not possible
to install a pressurized flash-recovery
system to use flash steam elsewhere,
there is still the possibility to
recover valuable portions of the flash
that would otherwise be wasted
to atmosphere.
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