POWER May 2016 - 57

FUNDAMENTALS
7. Drop your BP. These data from October through December 2015 show how the vacuum
pump seal water temperature affects the actual operating back pressure of a surface condenser.
Source: Kevin Boudreaux, Nalco Co.
■ #1 vacuum pump seal water temp (F) ■ Back pressure (inHg)
130
125
120
115
110
105
100
95
90
85
80
pump plate-and-frame exchangers, are oncethrough
systems. Because the plant does not
treat the cooling water with antiscalant, significant
scaling throughout all systems has
been experienced.
Due to these scaling events, taking the
vacuum pump offline to clean the heat exchanger
has become routine. This evolution
4.75
4.50
4.25
4.00
3.75
3.50
3.25
3.00
2.75
2.50
is not prompted by the seal water temperature
(even though it is in fact monitored online)
but by a condenser BP high enough to
nearly trip the plant. Now, undoubtedly, this
is a very extreme case, but being extreme
makes it one of the best illustrations of how
seal water temperature directly affects actual
condenser operation.
Illustrating the effect of scaling on a vacuum
pump heat exchanger, Figure 7 shows the
seal water temperature in this case rises to approximately
128F. With the reduction in vacuum
pump capacity, the BP rises to over 4.4
inHg, initiating an exchanger cleaning. Post
cleaning, the seal water temperature drops to
98F, which is much more acceptable, given
that the inlet cooling water temperature was
87F. A reduction in seal water temperature by
30 degrees significantly improves the air removal
capacity of the vacuum pump, thus reducing
air blanketing of the condenser tubes.
More important is the decrease in BP associated
with the exchanger cleaning. Figure
7 shows the BP drops from 4.4 inHg to 3.5
inHg, correlating with a reduction in heat rate
of 1.0%, or 75 Btu/kWh.
Poor control of vacuum pump cooling water
chemistry is a prominent concern throughout
the power industry, and it is incumbent
upon generators to pay special attention to
this aspect of vacuum pump operation.
Vacuum Pump Case 2. It is important
to recognize that fouling of a vacuum pump
heat exchanger is not limited to the waterside.
In one particular case, the root cause of
condenser performance degradation was ultimately
identified as fouling on the shellside
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57
Temp (F)
inHg

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POWER May 2016

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