POWER May 2016 - 58

FUNDAMENTALS
8. Dropping dissolved oxygen. Data from November and December 2015 show the
effect load cycling had on condensate dissolved oxygen levels. The green line indicates the date
of vacuum pump heat exchanger cleaning. Source: Kevin Boudreaux, Nalco Co.
■ Gross production (MW) ■ Hotwell/condensate dissolved oxygen (ppb)
800
700
600
500
400
300
200
100
225
200
175
150
125
100
75
50
25
9. Dramatic drop. Data from November and December 2015 show the effect the vacuum
pump heat exchanger cleaning had on condensate dissolved oxygen levels and back pressure
penalty. Source: Kevin Boudreaux, Nalco Co.
■ Hotwell/condensate dissolved oxygen (ppb) ■ BP penalty clean/current flow (inHg)
225
200
175
150
125
100
75
50
25
of a shell-and-tube heat exchanger.
Data analysis revealed that condensate
dissolved oxygen (Figure 8) and cation conductivity
levels rose rapidly as the plant cycled
from full load to partial load, suggesting
that the cause of condenser degradation was
air binding. (Air removal during low-load operation
is very common and due to a change
in steam pattern and a reduction in suction
pressure to the vacuum pump, thus reducing
its capacity.) However, performance degradation
during these times is either overlooked
or accepted because most plants only operate
at less than full load when grid demand
is low. But that does not mean it is a no-harm
event. With the consensus being that poor air
removal was the root cause, focus shifted to
the vacuum pumps in an effort to determine if
and why the system was underperforming.
Instrumentation is not available on these
58
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
0.00
units, so historical, online data were not available.
Consequently, all troubleshooting would
be done with logbooks and visual inspections,
requiring isolation of the vacuum systems.
These particular vacuum pumps have a
shell-and-tube heat exchanger using oncethrough
cooling water from an on-site lake.
Because the cooling water is well treated
with biocide and antiscalant, waterside fouling
was not suspected to be the culprit. However,
further troubleshooting continued to
suggest vacuum pump underperformance.
The plant decided to clean the shellside
of the vacuum pump because of how easy
it was to accomplish (there are two vacuum
pumps in place; however, time, resources, or
both caused the plant to thoroughly clean A
and half-clean B). Even without a thorough
cleaning on both exchangers, the data clearly
show the instant improvement (Figure 9).
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The 4-hour cleaning translated into a BP improvement
of approximately 0.5 inHg, which
amounted to a 1% efficiency improvement
when considering heat rate.
Prior to the cleaning, deposit samples were
not taken; therefore, it is unknown what the
cleaning actually removed from the shell side.
I have seen shellside bio/organic type fouling
in previous cases, and though it may seem
unlikely, it is important to understand that the
shellside is seeing a steam/air mixture that is
loaded with amine (whether ammonia or organic).
As these units are brought offline, for
either lead-lag operation or total plant shutdown,
the condensate that forms in these exchangers
sits stagnant, and very warm. With
the iron and a nitrogen sources present, bio
growth does not seem implausible. However,
without deposit analyses, it is impossible to
determine the nature of the foulant.
Perhaps even more important is the fact
that after the fix, the plant is still cycling as
heavily as it was before; however, improved
vacuum pump capacity has led to lower dissolved
oxygen concentrations in the cycle
chemistry, as shown in Figure 9. Although
calculating a monetary value for decreasing
oxygen levels in cycle chemistry is difficult,
anyone working in a power plant knows the
cost of feedwater corrosion on boiler cleanings,
tube failures, and forced outage time.
Air removal issues at this plant have been
occurring for many years. However, because
there is no online data, issues go unreported
until significant plant performance deficits
occur. Because plant staff now understand
vacuum pumps' complexity and relevance
to plant efficiency, that equipment is now a
priority, and inspection updates and cleaning
protocols have been introduced.
Maintenance Well Worth the Effort
All of the case studies discussed clearly show
the importance of considering the whole operation,
not just the condenser, when it comes
to online monitoring and offline inspection
and maintenance programs. Every attempt
should be made to properly monitor components
that have a direct effect on the condenser
operation-especially cooling towers and
vacuum pumps. Not only does online monitoring
identify problems before they become
crippling, it also can be used in planning
downtime inspections and maintenance.
Given the cost of doing nothing, whether it
is a simple walk-down or a full-blown instrumentation
package, time and money spent
monitoring these systems is typically recovered
with the identification and resolution of
a single incident. ■
-Kevin Boudreaux (kjboudreaux@nalco.
com) is an industry technical consultant
for Nalco Co.'s Power Business Unit.
POWER | May 2016
Ppb
MW
Ppb
inHg
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POWER May 2016

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