Chemical Engineering May 2022 - 34

FIGURE 7. The NPSHR for this pump is 9.5 ft (point D)
FIGURE 9. This particular design configuration is not recommended because its lack of a flash receiver
can have a negative impact
TABLE 2. THE IMPACT OF DECREASING CONDENSATE
TEMPERATURE
Condensate temperature reduced to 200ºF
New vapor pressure is 11.54 psia
Calculation
+1.3 psi
+14.7 psi
-11.54 psi
-0.2 psi
Fill head
Receiver pressure
Vapor pressure
Frictional loss
= 4.26 psi
Corresponds to 9.83 ft NPSHA
FIGURE 8. Increasing receiver height increases NPSHA
ceiver by 6.5 ft - from 3 ft to 9.5 ft
- increases NPSHA to 10.1 ft, which
is more than sufficient for the NPSHR
of 9.5 ft. Alternatively, keeping the
receiver elevation at 3 ft, but reducing
the condensate temperature by
10ºF (from 210 to 200ºF) increases
NPSHA to an acceptable value of
9.83 ft, as outlined in Table 2.
Vent problems away
An example of a common application
design is shown in Figure 9,
with a heat exchanger discharging
condensate that flashes when exit34
ing
the steam trap into
a floor-mounted, electric
condensate pump.
The drawing is provided
with the notice
that this installation is
typically not recommended.
Since the receiver
is clearly vented,
why is this particular
example not recommended?
What is the
potential cause for this
pump to cavitate? This
installation is similar to
the pump shown in Figure
2 that experienced
significant maintenance
issues causing spillage.
Figure 10 provides clarification
about some
of the issues with this
layout, namely the role
of the 1-in. pipe at the
top of the installation.
Other considerations
for Figure 10 include
the following:
* The small receiver collects condensate
at the pump inlet
* The small vent equalizes internal
pressure as water level rises
and falls
* Condensate pressure
and temperature
are increased if flash is
not vented before
* An increased
temperature results in
lower NPSHA, potential
cavitation, motor overheating
and damage
Pump
manufacturer
1
2
3
to
pump seals
There can be a misconception
about the vent found on condensate
pump tanks like this floor-mounted
example. That vent is sometimes
considered by designers to be a
flash steam vent, but in actuality, its
purpose is to allow balancing of the
tank to the atmosphere so that condensate
can freely enter and replace
the vapor space within the receiver.
Commonly, the floor receiver's vent
is too small to handle flash steam
velocity appropriately - ideally to
less than or equal to 50 ft/s, or at
a maximum, 70 ft/s. Note that this
selection depends on actual site
procedures and recommendations
from a knowledgeable engineer
[7, 8]. A correctly sized vessel
should be added to the system to
vent the flash steam prior to entry
into the condensate pump receiver
(Figure 11).
As can be seen in Figure 9, there is
no separate flash vessel, and the hot
condensate is discharged directly
into the floor-mounted receiver.
There are several issues with this
approach - the first being elevated
condensate temperatures and the
second relates to insufficient filling
TABLE 3. VENT SIZE RESULTS FROM SELECTED ELECTRIC
PUMP MANUFACTURERS
Vent
size, in.
1.25
1.5
2
Velocity,
ft/s
715
526
319
Pressure
drop over
12-ft pipe
length, psi
4.8
2.1
0.6
Steam
temperature,
ºF
227
219
214
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