POWER June 2012 - 76

CONDENSERS
■ HP turbine valve packing, large
■ Spillover valve packing, very large
■ Slop drain, very large
■ Vacuum pump outside seal, very large
With the leaks identified, the plant manager
arranged to have the leaks repaired
during an upcoming mini-outage, which
took place during the latter part of October
2011. Looking at Figures 1 through 3,
note the reduced air in-leakage post-outage
shown in November 2011 (Figure 1),
based the data historian for the RheoVac
system, the much improved cleanliness
factor (Figure 2), and, most importantly,
the vacuum conditions inside the condenser
returned to values close to the original
design (Figure 3). The excellent work performed
by the plant staff produced a plant
heat rate improvement of nearly 200 Btu/
kWh (Figure 4).
Today, the plant's staff trust the RheoVac
data and even use historian data on
condenser performance to plan the next
outage. Also, having real-time air inleakage
data available allowed the plant
5. Rising temperature. The vacuum pump seal water temperature prior to May 7, 2011,
was extremely high, threatening continued plant operation. Cleaning the seal water heat exchanger
returned the seal water to its expected temperature levels. Source: Nalco Co.
Average condenser water supply temperature
150
140
130
120
110
100
90
80
70
Vacuum pump seal temperature
to confirm that the money spent on repairs
had an immediate, positive impact on the
plant's bottom line.
Case Study 2: Vacuum Pump Seal
Water Temperature
The second case study involves a southestern
utility's 2,000-MW-class multiplepower-block
combined cycle plant. The
second unit at this plant, commissioned
about a decade ago, is a 2 x 1 power block
configuration producing around 500 MW.
Steam, produced by two triple-pressure
heat recovery steam generators from the
exhaust heat of the combustion turbines, is
sent to a steam turbine. The exhaust steam
from the steam turbine enters a two-pass,
single-pressure condenser, which uses
once-through cooling water from a large
pond. The pond water is very high in alkalinity,
phosphate, and hardness, which
obviously increases the potential for mineral
scaling throughout the plant's heat exchangers.
The summer pond temperatures
can rise to 92F and higher, which increases
the potential for scale formation.
Air removal from the condenser is by
6. Loss of vacuum. The high seal water temperature reduced the effectiveness of the vacuum
pump, which allowed the pressure inside the condenser to rise to the point where a steam
turbine trip was imminent. After May 7, 2011, a return to the design seal water temperature allowed
the vacuum pump to produce the design vacuum in the condenser. Source: Nalco Co.
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
means of a vacuum pump system, as is
true for virtually all combined cycle plants.
These particular units use a plate-and-frame
heat exchanger to cool the seal water prior
to its entering the actual vacuum pump
unit. Vacuum pumps rely on seal water to
maintain the seal between the body and the
rotor that actually draws the vacuum on
the condenser. The seal water temperature
is extremely important: If its temperature
ventures above the saturation temperature
at the condenser pressure, then the seal water
will flash, the seal is lost, and the pump
will no longer be able to draw a vacuum.
(See " Condenser Backpressure High?
Check Vacuum System Sizing " in the February
2012 issue or the POWER archives at
www.powermag.com.)
The seal water is actually condensate
that is formed during the air removal process.
As the steam/water stream from the
condenser is brought from a vacuum to
atmospheric pressure, a wet air mixture is
sent to a knockout tank, where the air exits
via the exhaust and the condensate is captured
at the bottom of the knockout tank.
This condensate is reused as seal water for
the vacuum pump. Any extra condensate
formation is dumped to drain. As the condensate
is circulated through the vacuum
pump, it is sent to a plate-and-frame heat
exchanger, where cooling water is used
to keep the seal water temperature under
control.
If the cooling water chemistry is not
properly controlled, fouling and scal76
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POWER
| June 2012
Condenser vacuum (in HgA)
Temperature (F)
5/19/11
5/17/11
5/15/11
5/13/11
5/11/11
5/9/11
5/7/11
5/5/11
5/3/11
5/19/11
5/1/11
5/17/11
5/15/11
5/13/11
5/11/11
5/9/11
5/7/11
5/5/11
5/3/11
5/1/11

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POWER June 2012

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