Wheel space temperature details and List of Alarms due to high temperature sensed by the wheel space thermocouples.

1. Wheel space temperature

2. Wheel space
Wheel space is the area between the turbine "wheels" and the stationary
turbine casing. The spaceis below the turbine buckets and nozzles, around the
shaft. The turbine buckets mount on the turbine wheels, and the diaphragms are
the stationary "separators" that the wheels fit in between.
Number of Wheel space:-
 There are two wheel spaces per turbine stage--a forward wheel spaceand an
aft wheel space.
 A two-stage gas turbine would have four wheel spaces.
 A three-stage gas turbine would have six wheels paces.
Fig shows: - 3 Stage Gas Turbine with 6 Wheel Spaces.
Wheel space of a turbine

3. Wheel space thermocouple
Wheel space T/Cs are used to measure temperature around wheel space
region of gas turbine which are inserted from the outside of the turbine
shell/combustion wrapper and one cannot see them as they are pushed through
guide tubes into the thermocouple well area . There are inner set of T/Cs and an
outer set of T/Cs (one set is closer to the shaft than the other set).
Fig shows: - Wheel Space Thermocouple.
Wheel Space Temperature
Wheel spaces temperature are measured because there are seals which
try to keep the hot combustion gases traveling axially through the nozzles and
buckets in an axial path in order to get the most energy from them. And, the
seals also keep the hot combustion gases from getting down into the turbine
wheel/diaphragm area where they could excessively heat them and cause
damage to the turbine shaft and casing/shell.
Mounted wheelspace
thermocouple

4. Wheel Space Over Temperature Alarms:-
If the wheel space temperature of a running turbine starts to increase or
decrease during normal steady-state operation (relatively constant load), then it's
an indication of either a problem with cooling and sealing air flow into the
wheel space or the wheel space seals are not working properly.(usually
combined with a change in turbine rotor axial position).
There are two dual element thermocouples (TC) in wheel space to
monitor the wheel space temperature. The average temperature measured by the
2 thermocouple is computed and compared to an alarm for each location, if the
average temperature exceeds then alarm is generated.
Fig shows: - Alarm wheel spaces Temperature values.
Fig shows: - High wheel space temperature reading by thermocouple.

5. WHEELSPACE THERMOCOUPLE PROBLEMS:-
Higher temperature difference than the normal temperature difference
between some of the thermocouple pair indicate thermocouple problems, or
possible seal leaks. However, these thermocouples rarely detect turbine
problems, but can often have sensor problems, such as: Bad insertion is a
common problem, particularly on the later turbine stages as the thermocouple
has to be pushed down a long and narrow tube with various turns in it. Often it
is not inserted far enough, so the thermocouple will partially detect hot air
inside other turbine spaces, such as the inside of the nozzles. This will give
incorrect and high readings. To get correct insertion, a note should be made of
the thermocouple length and how far it needs to be inserted. These numbers
should then match up after insertion. Other thermocouple problems can occur,
such as cold junction, shorts. If a seal leak is present, this should normally be
detected by both thermocouple pairs.
Recommendation:-
Carry out an inspection on the thermocouples and the wiring, and repair
and recalibrate as required. If the problems persist, check for seal leaks.