April - Library

AMMJ Lessons Learnt In 45 Years of Condition Monitoring 2
These tubes are then led
out of the furnace space
through gaps in the roof
tubes into the dead space,
where they connect to
primary superheater inlet
tubes. There are 80 of these
superheater sections across
the gas path, so each has 6
tubes.
Excessive metal
temperatures lead to
considerable reduction in
creep rupture life. At these
temperatures, an increase of
only 11 C° can halve the life,
so operational monitoring is
important. Manufacturers
use thermocouples installed
in tube walls, sometimes in
special sections (BEI) to try
and measure the maximum
metal temperature. Such
sophistication was not
available when these boilers
were built, so at several
sections, 5 thermocouples
were fixed across the gas path
to primary superheater outlet
tubes in the dead space,
and the limits for operation
derived by calculation.
Two boilers built almost at
the same time exhibited quite different temperature behaviour at otherwise similar operating conditions. One
was often close to alarm limits, and operation was adjusted to keep within them. The other showed no such high
temperatures. After some years of service the “good” boiler suffered a spate of superheater tube failures due
to overheating and creep rupture, and the complete superheater had to be replaced. Why should two identical
boilers be so different?
Close investigation and painstaking tracing of tube path layouts showed that the hottest tubes from the outside of
the platen array led mostly to leading tubes, but sometimes to the tube behind it in the primary superheater, as the
number of platen tube banks is less than the number in the primary superheater. The monitoring thermocouples
were installed on leading tubes. Unfortunately, in the “good” boiler, the thermocouples were installed on leading
tubes that did not come from the hottest tubes out of the platen. Presumably, the installer was given set distances
in from the furnace wall rather than specific tube numbers. The lesson here is to check such points in detail if two
“identical” plant items show quite different behaviours.
Lesson #16 In critical cases, do not believe everything you read in the control room without
verification of labels and actuality at and inside the plant.
Conclusions and final lessons
Figure 7 Boiler cross-section above furnace.
Condition monitoring can be a key contributor to higher reliability and availability when set up properly and run by
trained and dedicated people. Some investment is needed in equipment but much useful work can be achieved
with simple instruments.
Lesson #1 Training is essential before starting CM work, followed by regular reinforcement
via courses, conferences. Consider getting certification to verify capability.
Lesson #1 Share your learning via on-line forums, conferences, articles in engineering
magazines.
Lesson #19 Make recommendations clear and concise: put the technical complexity in
appendices.
Lesson #20 THE MAJOR ONE. Condition monitoring is not an end in itself, and should be
applied along with other maintenance strategies as decide by an RCM or similar analysis.
Vol 24 No 2