POWER November 2011 - 36

CONDENSER LIFE CYCLE
the cooling water mass flow by 20% over the
original design (the far right column).
Cooling Water Inlet Temperature. In this
case study, we used the design inlet water temperature
of 85F for all calculations. When the
plant has a marginal condenser that limits power
production during peak summer conditions, this
calculation should be done using the maximum
inlet water temperature for your analysis, as the
material choice could have a significant impact
on the project economic payback.
Condenser Backpressure. After the cooling
water, steam flow, and tube alternative parameters
have been determined, the saturation
temperature is calculated and the backpressure
is found by using the steam tables. A lower
backpressure (or higher vacuum) increases
steam turbine efficiency. HEI formulas predict
backpressure improvement of 0.12 inch Hg,
0.04 inch Hg, and 0.10 inch Hg for 22 BWG
titanium, N08367, and S44660, respectively.
In this analysis, the greatest heat rate gain
without the pump upgrade was found using
24 BWG S44660, an improvement of 0.16
inch Hg. HEI calculations show 0.36-inch
Hg reduction for the upgraded pump option.
These last two performance upgrades are
very significant.
Tube Span Calculation. Over the years,
many different vibration methodologies have
been developed to calculate a " safe span "
that will produce no tube damage. The HEI
Standard uses two methods to calculate a safe
tube span length.
The first option, referred to as the tube
spacing (ligament) method, assumes that the
steam will achieve sonic velocity (either by
very cold water, shutting down a waterbox,
or by several other means), causing tube vibration.
The tube span is adjusted to keep the
vibration amplitude equal to or less than onethird
of the tube spacing. The design allows
for an additional clearance of one-third of the
ligament, preventing tube-to-tube collisions.
The second method, extracted from the
MacDuff and Fegler method (known as the
M-F method), compares the natural frequency
of the tube versus vortex shedding and
fluid elastic whirling. To be conservative, the
designer should use the shorter length predicted
by the two calculation methods.
For this case study, the two HEI methods
were very conservative, predicting lengths
that are significantly shorter than with the
methods used for the original condenser
design. For this condenser, the M-F method
developed the shortest span of all the tube
choices. As the original condenser span was
46 inches, both HEI methods suggest that
staking will be required regardless of alloy,
including the 90-10 copper nickel replacement
option. Tube vibration was not a problem
on this condenser in the past, possibly
because of the larger steam dome that was
so common in older condensers. Your tube
vibration consultant will help you make the
final tube length decision and determine
whether tube staking is required.
Uplift Force. Copper-nickel has the highest
metal density of the common tube materials, so
all of the new materials considered will result
in a condenser with significantly less weight.
The difference in pressure across the large turbine
exhaust area can create a significant uplift
force. When this condenser is at 1.5 inches Hg
of backpressure, the uplift due to the vacuum
is approximately 700,000 lb. If another tube is
selected, the drop in tube weight could overload
the supports and possibly damage the turbine.
Switching to titanium tubing results in a weight
reduction of 260,000 lb. A specialist should be
consulted to determine if additional condenser
foundation anchoring is required.
Estimated Fuel Savings. The change in
steam turbine backpressure will have an impact
on heat rate and, ultimately, the amount
of fuel that will be consumed by the plant.
This case study assumes the coal purchased
over the 20-year analysis period costs an average
of $2.50 per million Btu.
For this plant, we determined that for each
0.1 inch of Hg change in backpressure, the
plant heat rate will change 15 Btu for each
kWh generated (see the HEI Standard for
more details). A fuel savings of $100,000 per
year is estimated if 22 BWG titanium is chosen.
When switching to the super-austenitic
N08367, $33,000 per year in fuel savings results.
The calculations find that S44660 saves
$83,000 to $133,000 per year, depending on
the tube wall thickness chosen. Adding the
pump upgrade to the 22 BWG SEA-CURE
Table 2. Twenty-year cost analysis of retubing a condenser with 90-10 copper-nickel tubes compared with
alternative alloy options. Capital costs and savings estimates were estimated during the summer of 2011. A negative number represents
a cost savings. Source: Plymouth Tube Co.
Alloy option
Wall thickness (inches)
20% pump upgrade
Estimated tube cost
Installation cost
Scrap recovery cost
Estimated staking cost
Anchoring improvement cost
Tubesheet coatings
Pump upgrade cost
Total capital cost
Fuel cost differencesa
Derate losses to fix tube leaksb
Chemical treatment $100,000/yr
Turbine/boiler cleaning every 4 years
Cost over 20 years
Lost power caused by copper plating
on high-pressure turbinec
90-10 copper nickel
0.049
No
$2,430,000
$600,000
-1,000,000
$2,030,000
Base
$7,875,000
$2,000,000
$1,000,000
$12,905,000
$26,061,000
Ti Gr 2
0.028
No
$2,200,000
$600,000
-1,000,000
$250,000
$50,000
$165,000
$2,265,000
-2,000,000
$265,000
$0
N08367
0.028
No
$2,850,000
$600,000
-1,000,000
$50,000
$165,000
$2,665,000
-665,000
$2,000,000
$0
S44660
0.028
No
$2,000,000
$600,000
-1,000,000
$165,000
$1,765,000
-1,650,000
$115,000
$0
S44660
0.022
No
$1,800,000
$600,000
-1,000,000
$75,000
$165,000
$1,640,000
-2,673,220
-1,033,220
$0
S44660
24 BWG
Yes
$2,000,000
$600,000
-1,000,000
$165,000
$400,000
$2,165,000
-6,000,000
-3,835,000
$0
Notes: a. Fuel costs from Table 1 are summed over 20 years. b. Assume one derate per year for the first five years and two per year thereafter. The cost of each derate is $225,000.
c. Assume an average 5 MW/year loss at $35/MWh, 85% load factor.
36
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POWER November 2011

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