Queries regarding GE Gas Turbines Frame VI MarkVIe

GE Frame VI & Mark VIe Gas Turbine Generator (GTG) spreads was found on the higher sides during loading (approx. 21 MW+).

As recorded,GTG spreads at 22.4 MW load were:
 Spread-1 = 45.4 deg C
 Spread-2 = 32.1 deg C
 Spread-3 = 31.1 deg C.
M/C was Shutdown for GTG-2 High spread problem rectification.

Requested for solution/rectification for high exhaust spread problem frequently.

You have a GE-design Frame<b>6</b> gas turbine and generator with a Mark VIe turbine control system.

Exhaust temperature spreads have been covered MANY times before on control.com. There is a cleverly hidden 'Search' field at the top right corner of the Menu bar of every desktop version of control.com, and if you're using the mobile version of control.com the search feature is hidden under the 'Control.com' tab. It is recommended you use the Search Help the first few times you use control.com's Search feature as the syntax of search commands is not like most World Wide Web search engines; it's different but it's just as fast and returns relevant results just as quickly.

You will find in your research that exhaust temperature spreads are almost always (99.7489% of the time) the result of problems with either fuel nozzles or cracks or holes in the combustion liners, problems with the hula skirt seals of transition pieces, or problems with the assembly of the transition piece side seals on the first stage turbine nozzles.

Most GE-design heavy duty gas turbines use unique terminal boards in the exhaust/load compartment for exhaust thermocouple terminations. That, too, has been covered before in several control.com threads about high exhaust temperature spreads. Incorrectly terminating the thermocouples and thermocouple extension wires can cause cold junctions which can add to any real exhaust temperature spread that already exists making them worse.

These terminal boards DO NOT have Chromel and Alumel (Type K) terminals; they are simply one screw per terminal, with a capture plate under which the thermocouple and thermocouple extension wires must be terminated--properly. EITHER both the Chromel and Alumel thermocouple wires must be touching on one side of the screw under the capture plate, OR both the Chromel and Alumel thermocouple and thermocouple extension wires must be NOT touching and terminated on opposite sides of the screws under the capture plates. This will either NOT create a cold junction or will create two cold junctions, which will cancel each other and negate the cold junction differentials.

The latter is the preferred method of termination: with the thermocouple and thermocouple extension wires on opposite sides of the screw under the capture plate for BOTH the Chromel and the Alumel wires. This creates two cold junctions for each pair of Chromel and Alumel wires--but the two cold junctions cancel each other out (because they are at the same ambient temperature in the junction box!) and therefore negate each other.

If one of the pairs is touching and one of the pairs is not touching (say the Chromel pair is touching under the capture plate and the Alumel pair is NOT touching under the capture plate), then a single cold junction will be created and will cause the reading at the Mark* to be off by a few degrees--sometimes enough to cause a high spread problem. Most people do not know how to properly terminate thermocouples and thermocouple extension wires on the terminal boards provided by the OEM, so they do it incorrectly leading to unintended problems. It's a relatively easy thing to check, and it should ALWAYS be checked during or after any maintenance outage, especially if thermocouples were removed and replaced.

If the unit is running on liquid fuel, it's rather easy to find the most likely combustors and/or fuel nozzles and/or check valves which are causing the problems. You need to send two people, one of them with pencil and paper, to the Accessory Compartment next to the Liquid Fuel Flow Divider. While one person rotates the selector valve at the discharge of the flow divider and calls out the pressure readings, the person with the pencil and paper should be writing down the pressure readings. The general rule is: Any pressure which is more than 10% different than the average of the fuel pressures (excluding the high-pressure liquid fuel pump inlet and outlet pressures) is considered to be suspect and deserving of an inspection (when the unit is shut down).

Also, look at the Liquid Fuel P&ID or the Liquid Fuel Purge P&ID and find the Tell-Tale Leak-off. Then, go find the Tell-tale Leak-off on the unit (HINT: It's usually on the right side of the turbine compartment, underneath the walkway grating.) If there is fuel coming out of the Tell-tale Leak-off then one or more of the liquid fuel purge check valves is leaking, and the pressure readings for one or more of the combustors will be low--and those with the lowest pressure reading are suspect and the liquid fuel purge check valves of those combustors should be removed, inspected, tested and replaced, if necessary.

Exhaust temperature spreads are, again, usually the result of mechanical problems with fuel nozzles or check valves (if the unit has liquid fuel). Sometimes it's caused by improper insertion of exhaust thermocouples in the radiation shields in the exhaust. Sometimes it's made worse by improper terminations in the exhaust thermocouple junction boxes.

Lastly, when writing for help with operational problems (such as exhaust T/C spreads), please tell us what fuel is being burned when the problem is occurring, and if the unit has conventional (diffusion flame) combustors or DLN combustors (with premix flame), and any other pertinent information (such as if the unit has water- or steam injection for emissions control; etc.). The more information you can provide the better and more concise our answers will be.

Please write back to let us know how you fare in resolving the problem!