FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

MB Power has a total generating capacity of about 2500 MW of which
approximately b0% is fossil fuel, 35« hydro and 15% nuclear. The Eddy Current
inspection program has been concentrated on the larger fossil and nuclear units
in which aluminum brass, cupro-nickel and titanium tubes are used in a wide
range of different condenser sizes and designs. Station ages range from
2 years to 20 years and several condensers have been partially retubed in the
last five years as a result of the program. Station details and corresponding
condenser tube materials are shown in Table 1.
Because of the type of corrosion detected, it has been found beneficial to
inspect most Al-brass and Cu-Ni tubed condensers on an annual basis. Results
obtained from such inspections have been used in:
- establishing accurate corrosion rates; which in turn have been used tc
monitor chemistry control effectiveness and to calculate tube life
expectancy:
- defining susceptible areas within the condenser; such as, air removal zone,
regions affected by flow channelling, regions effected by flow induced
vibration, poor performance due to variable tube quality, etc., and:
- guiding preventive maintenance by identifying individual tubes for plugging
and in defining the extent of zones for retubing.
Titanium tubed condensers have received an initial inspection and then, since
corrosion damage is minimal, have only been reinspected to investigate specific
technical problems occurring during service. As described later, tube damage
due to vibration required a major inspection program. At another station
damage due to weld spatter which occurred during construction was investigated
when discovered at a later stage.
Although several techniques for mechanizing inspection have been tried, RPC
have always reverted to manual inspection methods as being the most versatile
for condenser inspection. An experienced team of an inspector and helper can
achieve an inspection rate of 100-150 tubes per shift, the slower rate of
inspection being required for tubes exhibiting numerous significant
indications. An immediate diagnosis of each tube is made at the time of
inspection and all signals are recorded for later review. Tubes with defects
deeper than 25?; of wall thickness are noted for future reference and those with
deeper than 7b% recommended for immediate plugging.
The first lesson learned in the program was how to be cost effective.
At the outset of the program it was thought that condenser tube leakage could
be brought down to extremely low frequencies by inspecting very large numbers
of tubes. This illusion was soon dispelled by the realization that a
significant number of leaks developed quickly by random events (impact damage,
erosion at mussel shells, etc.) for which inspection provided no forewarning.
It also became clear after the first few inspections that a large sample
approaching a complete inspection was uneconomical both in cost and outage
time. Experience has now shown that a sample of 300 to 600 tubes (3 to 7'= of
the condenser) if judiciously chosen, is both adequate for monitoring generic
degredation and of a short enough duration to be acceptable to plant
operations. With this sample size, the program objective became clarified as
monitoriny the overall condition of the condenser rather than preventing
individual tube leakage.