POWER June 2012 - 75

CONDENSERS
As is typical of most condensers, a
rotameter located on the vacuum pump
discharge is used for manual air in-leakage
recording. Although this measurement
technique may be appropriate for spotchecking,
it does not capture real-time
data beyond the manual reading an operator
may pencil onto a datasheet during
rounds. On this unit, air in-leakage is also
monitored in real time via the RheoVac
system. Specifically, the RheoVac continuously
monitors the total mass flow rate,
temperature, pressure, and total saturation
of the sample stream. From this information,
the air in-leakage can be determined.
The sensor is installed between the condenser
takeoff and the inlet to the vacuum
pump.
All data captured by the RheoVac system
is automatically sent to the plant historian,
which is an OSIsoft PI system. Once the
data is entered into the data historian, the
air in-leakage data can be trended against
all other measured condenser performance
parameters.
Prior to implementing the current condenser
performance protocol, the staff at
the plant did not trust the data produced
by the RheoVac system. There was good
reason for their skepticism, as the condenser
problems experienced at that time
all seemed to be the result of tube scaling
and fouling. After all, the scaling and fouling
were easy to see when the condenser
was periodically opened for inspection.
Hence, the RheoVac data that showed
large amounts of air in-leakage was not
taken seriously.
Late in May 2011, the unit scheduled a
three-day planned outage during which the
LP and HP condensers were opened and the
tubes were cleaned by mechanical scraping.
Although the condenser performance
improved, the condenser did not return to
its expected design conditions. Figure 1
illustrates the post-condenser cleaning air
in-leakage data in relation to the expected
results as of the end of May 2011. Data
from the PI historian also showed that the
condenser cleanliness factor was slightly
improved after mechanical cleaning (Figure
2). Figure 3 shows condenser performance
in terms of the large difference
between the actual and design condenser
backpressure, also measured post-outage.
The loss in condenser vacuum translates
directly into lowered plant output and reduced
unit efficiency.
As control of the cooling water chemistry
improved and the air in-leakage
readings were double-checked and confirmed,
it soon became clear that the poor
condenser performance must be the result
June 2012 | POWER
of water chemistry problems on the water
and steam sides. For this case study,
our focus will be on the HP section of the
condenser.
The data convinced the plant staff to
turn their attention to air in-leakage as the
possible cause of poor condenser performance.
The plant hired an outside consultant
to conduct a condenser air in-leakage
inspection. A total of 13 vacuum leaks
were identified, with six of the leaks considered
a major source of air in-leakage
into the condenser. The list below provides
examples of the type of leaks found, as
well as their severity, based on the surveying
company's standards:
■ LP turbine gland seal housing, small
■ Boiler feed pump turbine gland seal,
medium
■ Boiler feed pump turbine shaft seal,
medium
■ LP turbine bonnet flange below grating,
large
3. Good vacuum restored. Condenser performance, in terms of the vacuum conditions
within the condenser, remained much poorer than design conditions, even after the condenser
was mechanically cleaned. Source: Nalco Co.
Actual
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
Expected
4. Trust the data. The plant heat rate improved little after a short outage was taken to clean
the condenser in late May 2011. By fixing a long list of condenser system vacuum leaks, air inleakage
was stopped and the condenser vacuum returned to near-design conditions. The impact
on the plant heat rate was significant. Source: Nalco Co.
500
400
300
200
100
-100
www.powermag.com
75
Heat rate penalty (Btu/kWh)
Condenser vacuum (in HgA)
12/23/11
11/3/11
9/14/11
7/26/11
6/6/11
4/17/11
2/26/11
1/7/11
11/18/10
11/3/11
9/14/11
7/26/11
6/6/11
4/17/11
2/26/11
1/7/11
11/18/10
12/23/11

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POWER June 2012

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