POWER May 2016 - 56

FUNDAMENTALS
5. Boogey man. These data show the effect a reduction in circulating water flow rate has
on the back pressure penalty (sometimes referred to as boogey). Data are for the month of
August 2014. Courtesy: Kevin Boudreaux, Nalco Co.
■ Current cooling water flow (gpm) ■ BP penalty w/design cleanliness factor (CF)% and design flow (inHg)
170,000
165,000
160,000
155,000
150,000
145,000
140,000
135,000
130,000
0.9
0.8
0.7
0.6
0.5
0.4
0.3
6. One thing leads to another. These data from the month of August 2014 show the
effect that a reduction in flow rate (see Figure 5, showing the same time frame) has on the heat
rate and megawatt penalty. Source: Kevin Boudreaux, Nalco Co.
■ Heat rate penalty % if design conditions ■ MW/hr losses/gains design CF% and design flow
1.80%
1.70%
1.60%
1.50%
1.40%
1.30%
1.20%
1.10%
1.00%
0.90%
0.80%
BP. However, a more detailed review of the
condenser performance data uncovered the
true cause: a BOP issue manifesting as a condenser
performance issue.
Instrumenting a cooling tower for proper
online monitoring is very difficult, and there
are very few plants that have this capability.
However, periodic inspection of the recirculating
pump inlet screens for cleanliness and
cooling tower fill for proper water distribution
is simple. If, for instance, during these inspections
staff find that fill flow is not distributed
properly, at least the plant has a heads up for
preparing and scheduling maintenance.
Vacuum Pump Operation Effects
on Condenser Operation
Vacuum pumps are perhaps the most underrated
piece of equipment at a power plant.
Typically, unless a catastrophic failure oc56
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
curs,
vacuum pumps are rarely monitored,
inspected, cleaned, or otherwise maintained.
This is demonstrated by the fact that it would
be very difficult to find a vacuum pump with
the necessary instrumentation for proper online
monitoring.
It is easy to understand why monitoring,
inspecting, and maintaining the main condenser
is critical and intuitive. Condensers
are massive pieces of equipment that get a
lot of press, they directly affect the steam
turbine operation, and relatively speaking,
the basic theory of how they work is well
known. Most plant personnel understand
that waterside fouling-scale, biological,
and sedimentation-of the condenser tubes
leads to performance degradation and overall
losses in plant efficiency. This is why many
condenser inspection and maintenance programs
focus heavily on tubeside and water
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box cleanliness.
What many do not understand is that
subtle changes in vacuum pump operation
greatly affect condenser efficiency. This lack
of knowledge leads to neglecting the vacuum
pumps in condenser inspection and maintenance
programs. So unless an actual event is
occurring, such as water hammer or bearing
failure, the vacuum pump remains hidden
within the boiler and turbine deck labyrinth,
unattended. And, as previously noted, these
units are usually not monitored online, meaning
that any performance degradation goes
unnoticed. Therefore, the main focus of these
systems should be online monitoring to help
guide planning and scheduling of inspections
and maintenance.
For instance, seal water temperature
greatly influences the air removal capacity
of a vacuum pump: The cooler the seal water,
the higher the capacity and, conversely,
capacity is reduced with warmer seal water
temperatures. Seal water is cooled using a
heat exchanger, which can be a shell-andtube
or plate-and-frame device, with either
once-through, closed-loop, or recirculating
water as the cooling medium. Upon entering
the exchanger, heat contained in the seal
water is transferred to the cooling medium,
in turn reducing the seal water temperature.
The cooler seal water is then pumped at a design
flow rate (also rarely monitored) to the
vacuum pump, where it provides the necessary
impeller seal, allowing the pump to draw
vacuum on the condenser.
If the heat exchanger becomes fouled in
any way-with scaling, biological, or silting
agents-heat transfer does not occur, seal water
temperature remains hotter than designed,
and (as previously discussed) air removal
capacity diminishes. When this happens, air
blanketing within the condenser occurs, the
operating BP rises, and overall turbine efficiency
declines. Just as with cooling tower basin
water temperature, seal water temperature
directly affects the overall plant heat rate.
The following example highlights how
debilitating an unmaintained vacuum pump
can be to condenser efficiency. Fortunately
for this particular unit, vacuum pump seal
water temperature is actually measured online
and sent back to the plant historian. Unfortunately,
it is no-action data, meaning that
it is not actively monitored, and there are no
setpoints/alarms around the value.
Vacuum Pump Case 1. The subject
plant, located in central Florida, uses a very
difficult (chemically speaking) makeup water
source. It is very warm, with elevated levels
of calcium, alkalinity, and phosphate, making
it extremely scaling in nature. There are
no cooling towers on site, and all condensers
and heat exchangers, including the vacuum
POWER | May 2016
Heat rate improvement
gpm
MW
inHg

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POWER May 2016

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