Chemical Engineering May 2022 - 32

FIGURE 3. As the impeller spins, dynamic increase causes
static pressure drop, which begins vaporizing the hightemperature
condensate
FIGURE 4. High head pressure (NPSHA) mitigates condensate
vaporization, maintaining liquid for pumping
FIGURE 5. When NPSHA decreases due to high rotation, some
liquid flashes into steam, thereby causing cavitation
column over the impeller and the
surface pressure acting over that
incoming condensate, as well as
negative head factors like the vapor
pressure of the condensate (how
easily the condensate can vaporize)
and inlet piping friction loss. Sufficient
NPSHA is important to keep
the condensate from vaporizing as
it experiences a static pressure drop
when the dynamics increase from
impeller rotation (Figure 4). Although
condensate may be in liquid form entering
the impeller, portions can flash
into steam vapor, causing cavitation
when the NPSHA head pressure is
reduced (Figure 5).
Pump selection
Pump manufacturers normally provide
the specific NPSHR for reliable
performance, which requires that the
NPSHA always meets or exceeds
the NPSHR to prevent cavitation [5].
NPSHA calculations can be relatively
simple to perform, and one of
the main values needed is the vapor
pressure at the expected temperature
of the condensate being discharged,
as shown in Table 1.
Consider a hypothetical new installation
that is designed to pump
210ºF condensate from an atmo32
spheric
receiver elevated 3
ft above the impeller (Figure
6). The temperature corresponds
to a vapor pressure of
14.14 psia in Table 1, and
assuming a minimal pressure
drop of 0.2 psi for the
inlet piping, the NPSHA can
be estimated as 3.83 ft. With
NPSHA known, it is just necessary
to check the pump curve
to determine if the NPSHA is
suitable for the NPSHR of the
selected model.
Suppose that the proposed
pump has performance
curves as shown in Figure
7. The pump is rated for
37.5 gal/min at 30 psi total
discharge pressure (TDP).
Since the TDP curve ends at
25 psi, if the pump resistance
is less than 25 psi, it may burn
out the motor. It can be seen
at point D that this pump has
an NPSHR of 9.5 ft, which is
significantly higher than the
NPSHA of 3.83 ft. Although
the pump may meet the discharge
rate and pressure, it can be expected
to cavitate severely due to insufficient
head to prevent the entering
condensate from flashing as the impeller
rotates. Typically, high-NPSHR
pumps tend to operate at high
speed (around 3,500 rpm), and lowering
the rotation dynamics by using
low-speed models (for instance,
1,750 rpm) can reduce the drop in
static pressure. Such lower-speed
pumps are commonly referred to as
" low NPSH " models.
There can be several caveats
when selecting
low-rpm pumps
to avoid cavitation.
One is that
these models,
when selected
for a certain
TDP, tend to be
more sensitive
to changes in
backpressure
or total dynamic
head (TDH), and
another is that
their cost can
be substantially
higher.
TABLE 1. ABSOLUTE VAPOR PRESSURE OF
WATER AT VARIOUS TEMPERATURES [6]
Temperature, ºF
180
190
195
200
205
210
212
215
220
225
Pressure, psia
7.52
9.35
10.40
11.54
12.78
14.14
14.71
15.61
17.20
18.93
Given those potential concerns, it
can be useful to consider alternative
methods to prevent cavitation using
lower-cost, high-rpm models or nonelectric
secondary pressure drainers
instead, which are explained later in
this article.
Consider a hypothetical electric
pump with a curve similar to Figure
7 with insufficient NPSHA. It is clear
that the model shown in the pump
curves will not be appropriate (since
NPSHA is less than NPSHR), so how
can this be improved? When the
NPSHA is insufficient, it is necessary
to increase its value to use the highrpm
pump unit, and there are two
possible methods to achieve this objective.
The first is to increase the fill
head by elevating the receiver, and
this is often possible when the pump
is located at a much lower level than
the source of the condensate. The
second option is to reduce the condensate
temperature.
The example shown in Figure 8
illustrates that just elevating the reFIGURE
6. The NPSHA of 3.83 ft is calculated for 210ºF condensate with an atmospheric
receiver and 3-ft filling height over the center point of the impeller
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