We are working with GE Frame 5 Gas Turbine, 18 thermocouples are available and 13 for measurement. TT-XD-7 location G is going below the spread level. Is there anything with the Exhaust gas conveyor (the pipe with a Tee which holding the thermocouple) Tee orientation.
Appreciate your reply

 

"...Exhaust gas conveyor..."

Interesting translation of 'radiation shield'. Most GE-design heavy duty gas turbines use a radiation shield which is a length of pipe that protrudes axially into the exhaust area with a small "tee" which is open at both ends radially to the exhaust diffuser. There were some very old configurations that used some very "inventive" methods, but I would think most of them have been replaced with the more conventional radiation shield design over the decades.

So, the "body" of the radiation shield is perpendicular to the exhaust duct wall it is attached to, and the tee should be arranged perpendicular to the body of the shield, and the opening of one end of the tee should be pointed at the center of the exhaust (towards the turbine shaft). This allows the exhaust gas, from the diffuser, to pass through the tee, over the T/C tip, and out of the tee into the exhaust duct. This reduces some turbulence and has been found to be effective in providing a useful and meaningful exhaust temperature sensing method.

Presuming the unit uses radiation shields, the T/C tip should be visible in the radiation shield (looking through the "tee"). The tip of the T/C should be 1/8-inch to 1/4-inch away from the edge of the radiation shield, and should be centered in the opening (i.e., not touching either side of the pipe the tee is attached to). So, the T/C tip should be visible in the radiation shield and should not be touching metal anywhere in the radiation shield.

The T/C is contained in the tip of the sheath. It must be exposed to exhaust gases flowing through the open ends of the tee of the radiation shield ("exhaust gas conveyor"). All exhaust T/Cs should should be similarly installed and positioned. Many exhaust T/C spread problems can be traced to improper placement and positioning of the T/C tips in the radiation shields.

Many times when workers are working in the exhaust area during maintenance outages, they just love to poke their fingers into the radiation shields ("exhaust gas conveyors", which must be a French translation of radiation shield) and poke those little T/C tips. They don't know what they're touching, and what the effect of their touching does.

Many people installing exhaust T/Cs don't look at the Exhaust T/C Arrangement drawing in the Service Manual Parts List to see the Notes about installing them. The notes usually say to insert the T/C into the opening and push it in until it stops ("bottoms out"; meaning it touches the end of the tee of the radiation shield) and then draw the sheath back approximately 1/4-inch or so, and tighten the compression fitting. Most people just shove it in until it stops, and then tighten the compression fitting--which ain't correct.

Another problem with exhaust T/C feedback is the wiring terminations. T/C terminations to T/C extension wire are very "finicky" and critical. When people replace exhaust T/Cs, they generally just think that T/C wiring is just like all other wiring, and that's just not true. Special care must be taken when terminating T/Cs to T/C extension wiring to ensure that no unintended cold junctions are introduced into either lead of the circuit.

It's not likely the spread problem is related to the "exhaust gas conveyor", unless there is some physical or mechanical problem with the radiation shield that you haven't told us about. The existence of 13 monitoring T/Cs (out of 18 exhaust T/Cs total) indicates this is an older GE-design heavy duty Frame 5 gas turbine, which could mean the radiation shields are damaged or degraded (rusting out; missing sections; etc.).

If you're asking about a single radiation shield's orientation, they should ALL be oriented the same way, radially towards the center of the exhaust (pointed at the turbine shaft).

And, it's not clear what you mean when you say one T/C reading is "...going below the spread level...." The allowable spread (usually signal name TTXSPL1 in a digital Speedtronic turbine control system) is a calculated difference between the highest and lowest exhaust T/C values. So, it's not an exhaust T/C value, it's a differential temperature.

You seem to be on the right track for troubleshooting an exhaust T/C spread problem--ensuring that the T/Cs are installed correctly, and wired correctly. If you confirm that everything is physically okay with the installation and wiring, then an exhaust T/C spread problem will be caused by a mechanical problem in the turbine hot gas path, not a controls-related problem.