POWER May 2010 - 14

The final dimension of the laser-drilled and -shaped component
is the finished dimension, as all the coatings were applied earlier
in the process. The mechanical impact of the laser drilling and
shaping process on the TBC is negligible (Figure 2).
Today, Chromalloy uses this single-step laser drill and shape
technology to produce a significant number of turbine airfoils
for industrial gas turbine operators plus other commercial and
military aircraft operators.
-Contributed by Frank J. Wos (fwos@chromalloy.com), director
of laser technology, Chromalloy.
Competitive Maintenance Strategies,
Part III
In the previous two installments of " Competitive Maintenance
Strategies, " we examined several best practices that should be
part of the workday routine for high-performing plants. In Part
I, we discussed the characteristics of a good reliability-centered
maintenance program, the use of root-cause analysis as a maintenance
tool, and ways to improve your predictive maintenance
program (March 2010). In Part II, we focused on combined-cycle
plants, especially heat-recovery steam generators, to identify several
operating practices that are directly linked to the cost of plant
maintenance. One specific area discussed was the need for a good
valve maintenance program (April 2010). This third and final installment
addresses three more areas where an investment in good
maintenance practices pays operating availability dividends.
Give Batteries More than Specific-Gravity Tests
The time-honored method of assessing the overall condition of
lead acid, wet-cell battery banks is to manually measure cell
voltage, current, and electrolyte specific gravity, and to visually
check fluid level and terminal post connections. While this is satisfying
to some, most experts say that you should conduct more
tests to get a true measure of the health of their batteries.
One way to do this is to deep-discharge new batteries once
every five years, increasing the frequency of deep-discharge later,
as the batteries reach the end of their 20-year life. A typical
flooded-cell battery is rated for 50 deep discharges in its 20-year
life, so 10 to 15 deep discharges for testing are not detrimental
to the battery. In addition, a discharge serves to agitate the
electrolyte, which is good for the battery.
But many substation engineers are opposed to deep discharges,
partly because they feel this takes away from the life
of the battery, and also because the battery string must be
taken off-line to discharge it. Discharge tests are costly as well,
because a utility must invest in its own battery-loading bank to
absorb the energy. Consequently, utilities have always wanted
a simpler indicator of battery strength and remaining life that
is easy to apply, less labor-intensive, and less risky to both
personnel and batteries.
Hence, battery monitors, as contrasted with single-function
testing devices, should be included in any battery room. Singlefunction
testers, such as hydrometers or voltmeters, check just
one parameter, such as specific gravity or voltage. By contrast,
battery monitors continuously check a variety of parameters,
such as voltage, temperature, fluid level, float current, and cell
impedance or resistance. Two developments have promoted the
use of monitors: automation through electronic sensors coupled
with computers, and the downsizing of large, labor-intensive
maintenance organizations to reduce costs.
Hunt Down Sources of Condenser Air In-leakage
Air leaking into a steam turbine condenser robs a plant of capacity
and efficiency and contributes to corrosion because of the
higher levels of dissolved O2
and CO2
in the condensate. It also
can lead to serious tube-freezing incidents if the condenser is
air-cooled and located in a cold-weather climate. To stay on top
of the problem, some power plant engineers have installed online
instruments that determine air in-leakage rates based on gas
concentrations in the condensate. However, the instruments can
be problematic, and they only indicate the presence of a leak,
rather than pinpointing the actual leak sites.
Most plants find it more effective to routinely test the steam
Air SyStEmS
LimitEd
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POWER | May 2010
components that are under vacuum, using mass spectrometry
and helium as a tracer gas. One technician sprays helium around
potential leak sites-such as expansion joints, flanges, and
valves-while another uses the mass spectrometer to detect the
inert gas at the discharge of the air-removal system: air ejector,
vacuum pump, or hogger. Mass spectrometry is considered more
effective than other inspection methods-such as ultrasound,
infrared, decay tests, and condenser flooding-because it can:
■ Pinpoint the location of leaks as small as 3 x 10-4
per minute, thanks to helium's tiny molecular size.
cubic feet
■ Be performed online (in fact, 15% or higher turbine load is
optimum).
■ Function without removing pipe insulation, equipment barricades,
or other obstacles.
■ Be repeated quickly and inexpensively to verify that leaks have
been fixed.
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POWER May 2010

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