Gas Turbine Tripping on Coupling Compartment Temperature Switch Actuation

Dear Community Member,

We've recently faced an outage of our GE Frame V Gas Turbine due to Coupling/Gear Compartment Temperature Switch Actuation. Below are the initial findings:
1. 02 Temperature switches are installed inside coupling compartment, 01 of them operated leading to CO2 release and tripping the turbine.
2. We replaced the temperature switch preventively, and tested the removed switch multiple times but its operation was found normal i.e. it kept on operating at 316 deg.C (its preset set-point)
3. We also found 02 very minor hairline cracks on the supporting ribs of casing between coupling compartment and turbine compartment. Preventively applied belzona to avoid any actual heat ingress from turbine compartment.
4. Turbine was taken in service and we kept on measuring the temperatures on coupling compartment using thermography as there was no temperature measurement instrumentation installed in coupling compartment.
5. After 4-5 hrs. of operation, we found that temperatures in coupling compartment are again on the higher side i.e. ~337 deg.C and we also compared this value with other Gas Turbine running on similar load and that was running on much lower temperature than this (~257 deg.C).
6. We shutdown the turbine again and found that 01 of the exhaust damper operated after CO2 release in the coupling compartment was closed
7. There are 02 manual dampers installed on exhaust of coupling compartment. Operations team only check the 01 damper whose limit switch is in start permissive of Gas Turbine. However, 2nd shutter type manual damper installed underneath the main exhaust damper is never checked by operations team.
8. So, turbine was taken back in service after opening this 2nd exhaust damper.
9. Meanwhile, our instrument team installed RTD just above the temperature switch to check actual temperatures at the location instead of performing thermography again and again.
10. Now, temperatures are well within range.

My query here is; what should be the cause of 1st actuation? As I stated, temperature switch seems fine we have 02 options left, 01 option is to consider the actual heat ingress inside coupling compartment from minor cracks observed in supporting ribs of casing between coupling and turbine compartment and 2nd option is to consider that manual damper was closed even before CO2 release that day.
Can turbine remain operational for long period if 01 of the exhaust dampers of coupling compartment remain close for long duration, like a year or so?

Please share your opinions.

Regards, Arslan
 
@Arslan,

It is presumed that BOTH dampers have CO2 pressure-operated latches to hold the dampers open (this type of latch--operated by CO2 pressure when a fire was detected and CO2 was discharged; the pressure of the CO2 would open the latch and the weight of the damper would close it to prevent air from entering the compartment AND to build up and maintain a concentration of CO2 to both extinguish the fire AND keep it from igniting again because of the presence of air (oxygen). The CO2 both displaces air to remove one of the key elements of combustion and then, by maintaining a high concentration of CO2 over a period of time (20-45 minutes, usually) the CO2 concentration would prevent re-ignition of the fire hopefully removing the temperatures required to re-ignite the fire as well as maintaining a very low level of air (oxygen) and a high level of CO2.

Based on the information provided it would appear the cause of the initial trip and discharge of CO2 was likely caused by one of the exhaust dampers being closed prior to the actuation of the CO2 system. The latches mentioned above were very prone to loosen--especially if not properly latched--allowing a damper to close during normal turbine operation and prevent the normal flow of cooling air in and through the Load Compartment (or the turbine or accessory compartment if one or more of the dampers in those compartments were also latched open with CO2 pressure-operated latches). Often the metal chains attached to the dampers and held in the "hook" of the latch would rust, or the hooks would not be properly latched prior to re-starting the turbine and normal turbine vibration (and high air flows) would cause the latches (which were usually made of brass to prevent them from rusting).

You mentioned two temperature switches. What is the function of the other temperature switch? Should it alert the operator(s) of a high temperature BEFORE the second switch initiates a trip and CO2 discharge? Typical GE fire detection/protection systems usually had redundant switches--which means two (or more) switches (temperature switches--sometimes switches designed for detection of fires in high temperature compartments/enclosures) would have to indicate a fire/high temperature before the machine would be tripped and CO2 would be discharged. GE licensed different companies to package GE-designed turbines and those companies would often make "subtle" changes to typical GE control and protection schemes--such are using non-redundant devices (temperature switches; vibration sensors; pressure switches; etc.) which would not provide the reliability of redundant devices in preventing shutdowns and/or trips. BUT, if there's a second switch was it tested to see if it is working properly and is its function known and has it been tested recently? (There is a very large--and, in my personal opinion, dangerous--change in operation and maintenance practices that has severely reduced the verification of the operation of switches and transmitters in order to reduce maintenance costs and outage times. It's usually called "calibration"--but what it really is periodic testing of the switches and transmitters--AND the wiring to the turbine control system, or the fire detection/protection system--to ensure everything is working properly and is at specification (the "calibration" portion of what is really verification testing).)

Anyway, now you have a method of monitoring the temperature in the compartment--at least in the area of the switch which is known to have tripped the machine and discharged CO2. (Newer GE-design machines mostly have these kinds of sensors (thermocouples, usually) which work well in the beginning but don't get the proper attention when they've been removed for a maintenance outage and are pooly re-installed (leading some sites to abandon them because of "false" indications....)

That's about all that can be said based on the information provided (which was actually pretty good). Pictures and drawings can be attached to posts here on Control.com and can be very helpful in troubleshooting.
 
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