Gas Turbine Handbook : Principles and Practices

214 Gas Turbine Handbook: Principles and Practices
• For gas turbines with an annular design it is sufficient to remove
one fuel nozzle from each side of the gas turbine in order
to view all of the first stage turbine nozzles and blades.
Figure 13-8 is a boroscope view of the internal annular combustor
liner and fuel nozzles. This photograph was taken by inserting
a flexible boroscope through one of the boroscope ports provided in
the combustor section. This procedure would have confirmed the
increase in turbine temperature spread and the decrease in combustor
efficiency associated with the events leading up to the complete
disintegration of the combustor shown in Figure 13-9, the hole in the
combustion liner shown in Figure 13-10, and the hole in the transition
piece shown in Figure 13-11. Liquid carryover in the fuel gas was
the primary cause of the problems depicted in Figures 13-9 and 13-10.
Identifying the problem early is preferable to finding it after extensive
damage has been done. The combustion process is often accompanied
by a high frequency vibration, which can excite the combustor
and transition ducts. High frequency movement of the mating metal
parts, can and often does, result in fretting and subsequent failure.
Detecting the initial fretting is difficult. Once a crack starts it grows
rapidly. Therefore, monitoring the turbine temperature or exhaust
temperature profile is the best indicator of an internal duct failure.
Turbine temperature profile distortions are also caused by
burnt, plugged or “coked-up” fuel nozzles as shown in Figures 13-12
and 13-13. Where boroscope ports are available, this condition (once
highlighted by a temperature profile change) can be verified quickly
and easily. Another alternative is to calculate the swirl effect and
remove the suspected fuel nozzles. Frequent use of a boroscope will
detect problems such as the small holes in a combustion liner (as
shown in Figure 13-14) before the problem becomes serious enough
to affect burner efficiency, temperature profile, or unit operation.
Turbine Blades And Nozzles
Boroscoping through the combustor boroscope ports or the fuel
nozzle port, provides a broad view of the first stage turbine nozzles as
shown in Figure 13-15. Moving the boroscope closer makes possible a
good view of the turbine nozzle airfoil ID and OD platforms as shown
in Figure 13-16 and 13-17. This view is sufficiently close to have
detected the corrosion, erosion, and burning ( oxidation) shown in