Gas Turbine Handbook : Principles and Practices

Detectable Problems 187
tion, the power is determined by the heat balance across the power
turbine.
P PT
= W g
(h i
– h o
) – Q r
– Q m
(12-3)
where
P PT
= power input to the load device, Btu/sec
W g
= weight flow of gas entering the power turbine, lb/sec
h i
= enthalpy of gas entering the power turbine, Btu/lb
h o
= enthalpy of gas exiting the power turbine, Btu/lb
Q r
= radiation and convection heat loss from the power turbine
casing Btu/sec
= mechanical loss of bearing of power turbine, Btu/sec
Q m
In the above cases Q r
and Q m
can be reduced to functions of the
air (or oil) flow and temperature differential.
TURBINE BLADE DISTRESS
(Erosion/Corrosion/Impact Damage)
Indications and Corrective Action
Turbine blade failures account for 25.5% of gas turbine failures 3 .
Turbine blade oxidation, corrosion and erosion is normally a longtime
process with material losses occurring slowly over a period of
time. However, damage resulting from impact by a foreign object is
usually sudden. Impact damage to the turbine blades and vanes will
result in parameter changes similar to severe erosion or corrosion.
Corrosion, erosion, oxidation or impact damage increases the area
size of the turbine nozzle. The increased area size is seen as a decrease
in turbine efficiency, which is a gas path measurement. It also
causes an increase in N 1
stall margin; however, to detect this requires
sophisticated tests and measurements. Parameter changes that can
readily be seen by the operator are (at constant power): an increase
in fuel flow, exhaust gas temperature, and compressor discharge pressure
(Figure 12-1). While decreases in N 1
rotor speed and air flow also
occur, these changes are not easily detectable, especially with small
changes in nozzle area.
Considering the high pressure turbine on dual-spool machines,
turbine blade and vane erosion results in a decrease in