We are suffering from high temp. inside the enclosure of Frame-6 GT. The Bently Nevada proximiters inside their junction box (located at the bottom of the C.C.) were found with their terminals melted. Only one cooling fan of the air suction is working as per their logic.

Can anyone advise me of the best solution for this problem? Does GE have special junction boxes?

 

Since how long this problem is being faced? How much is the temperature inside the chamber? Please check your inlet and outlet dampers in the air cooling circuits.

devraj @ nfl. co. in

 

Because GE-design Frame 6 heavy-duty gas turbines have only two bearings (one at the "cold end"--the inlet, and one at the "hot end"--the exhaust") there are not any vibration sensors required in the Turbine Compartment. The vibration sensors for the #1 Bearing would be mounted in the Accessory Compartment, and the vibration sensors for the #2 bearing are usually mounted OUTSIDE the Load Compartment (the area aft of the exhaust where the Load/Reduction Gear is located and the drive-end bearing of the generator is located; there is a shaft exiting the exhaust of the gas turbine driving the load gear (called the load coupling) and the shaft of the generator is connected to the output coupling of the Load Gear).

So, the melted cables you are describing are probably not in the Turbine Compartment, but more likely in the Load Compartment.

It is normally VERY hot (approx. 150-200 deg F) in the load compartment, but not generally hot enough to melt insulation. It is also fairly common for there to be exhaust leaks from the packing around the area where the load coupling exists (sometimes called the "load tunnel") and around the periphery of the exhaust where the exhaust T/Cs are located. So, the real problem is more likely exhaust leaks which are causing the Load Compartment temperature to be higher than normal.

Bently-Nevada makes high-temperature cables, so this might be another part of your problem--installation of the lower-temperature (lower cost) cables.

One thing which has been done many times is to relocate the JBs with the proximitors outside the Load Compartment (requires a longer prox extension cable and matching proximitor!), and this will usually resolve the issue--providing the JB isn't located under or near the exhaust joint--which has also been know to develop a leak!

But, you're fighting a symptom and unless you resolve the cause of the high temperature you're going to keep spending time (which is money) and money (to but parts).

Some Load Compartments have dampers which are latched in place by CO2 pressure-operated latches and these dampers have been known to come loose or to allow the dampers to close, which severely affects air flow through the compartment and increases the temperature. There are also sometimes gravity-operated dampers either on the fan suction or -discharge which can be non-operational, contributing to high compartment temperatures.

So, check for the temperature rating of the cables which are experiencing the failures; check for proper damper operation and adjustment; check for exhaust leaks (easier said than done--usually one uses a long wooden dowel or stick with a white rag tied to the end to pass around the exhaust area when the unit is running (but the compartment vent fan has to be shut down--so it has be a LONG stick)). Another way to check for exhaust leaks is to go into the exhaust when the unit has been shut down and is cool and look for daylight coming in through seals (this has to be done during daylight hours...one site actually tried to do this at night!!) so the Load Compartment doors have to be open and it helps to have a high-intensity work light shining on the load tunnel area.

markvguy

 

This may be flue gas having leaked through the flange or so. Since flue gas is also pressurised, any leak in the vicinity of junction box will lead temperature as high as 125 Deg. Centigrade. A small leak at the junction box may only damage the cable but it will not demand the cooling requirement.

Selecting high temperature leads & junction box is definately not a solution. This problem was faced by KOC, Kuwait with EVON gas turbine. The problem was the fibreglass insulation/coupling was damaged.

 

I might be able to help depending on how your ventilation is laid out. We experienced high temps in the gearbox where our Nevada probes are fitted. This was due to incorrect rebuild of the main plenum chamber after an outage. The main beam had not been re-welded back in place, allowing hot gas back into the compt.

 

Same with a question from HANY SALEH. In our Gas Turbine Enclosure GE 6FA is very hot. This picture was taken when we were commissioning the first time of the Engine. I think there is a miss in the engineering concepts for the enclosure. May be of airflow concept or something else. The wiring cable on JB under this engine is damage. In our country temperature of the air inlet is about 28 to 34 degrees C. I want to know the concept of GE 6FA that the airflow in the enclosure from the hot path to the cold path like in this picture and if any suggestions for us to solve these conditions.

 

What kind of alarm/trips have been annunciated ?

Did you check proper operation of vent flap /Damper ?

 

GE Belfort has the responsibility for the design and function of the GE-design Frame 6FA heavy duty gas turbine line. They have demonstrated time and time again they are willing to change tested, proven designs and try new configurations. You really have to ask GE Belfort if this result is what they had in mind.

Having said that, ControlsGuy25 is entirely correct in suggesting that you check any and all gravity-operated cooling air dampers, including those which are latched open and are supposed to close when a fire extinguishing agent is released into the compartment. It's common for these dampers to not be properly installed (to be installed backwards) and for the latched dampers to not be latched prior to a START or for the latch to vibrate loose, allowing the damper to close when it should be open. SOME GE-design heavy duty gas turbines have limit switches to alarm on incorrect positioning of cooling air dampers; some actually prevent a READY TO START indication/START when not in the proper position.

For decades, GE used reverse tangential fans for compartment ventilation. These centrifugal fans rotate COUNTER to the intuitive rotation looking at the angle of the fan blades. If the rotate in the wrong direction, they will not move much--if any--air, and will actually draw excessive current. Simply reversing the fan motor's rotation (by switching any two of the three-phase motor leads) will greatly increase air flow AND also cause the motor current to drop considerably. Many of these reverse tangential fans were shipped with direction of rotation arrows painted on the fan housing--and often times the arrows were painted over during construction. So, often the fans were improperly rotating in the wrong direction. It's not known if GE Belfort still use reverse tangential fans, but it's definitely worth checking out. (It's a simple check--read the fan motor's nameplate to determine the rated current draw; run the fan and monitor the current drawn by the fan motor; if it is higher than rated, then it's very likely the fan is rotating in the wrong direction; also, the "suction" on the turbine compartment will not be very high when the fan is rotating in the wrong direction--it will be much easier to open a turbine compartment door when the fan is rotating in the wrong direction than when it's rotating in the correct direction.)

Some commissioning personnel are not well trained in proper check-out of the turbine and auxiliaries prior to STARTing--unfortunately, cooling air dampers are frequently overlooked. As are some manual valve positions....

From the graphic you posted, this particular turbine cooling air arrangement seems to create a slight negative pressure in the compartment. This negative pressure, "suction," will make it difficult to open the turbine compartment door when the fan is running. BUT, this negative pressure is necessary to evacuate any combustible gases which might leak into the turbine compartment.

It's very common for the air inlet dampers to be gravity operated--and these dampers must be installed to allow the negative pressure (suction) inside the turbine compartment created by the vent fan to draw air into the turbine compartment. It's often common that the dampers are installed backwards, as if the dampers should be opened by positive pressure in the turbine compartment.

Cool air is usually drawn in through vents along the lower edges of the compartment enclosure (through the gravity dampers) and circulates around the turbine and axial compressor gaining heat as it's drawn into the vent fan's suction. It's worth noting that GE DOES NOT usually use insulation blankets on their combustion turbines, and as such, it's much warmer in GE turbine compartments than in other manufacturers' turbine compartments.

When the turbine shuts down (or is tripped), and the vent fan shuts down the gravity-operated dampers on the air inlets close, and this helps to prevent excessive cooling of the turbine and compressor casings/shells, which can cause rubbing of rotating components during cooldown. This means that the opening and leaving open turbine compartment doors to hasten cooling of the turbine compartment IS NOT advised. [NOTE: It's also VERY common for the temperature in top of the turbine compartment to rise during cooldown when the turbine compartment vent fan isn't running--and it is also NOT advised to use the turbine compartment vent van to reduce this temperature as it will result in excessive cooling of the casings/shells and possibly lead to rubbing during cooldown.]

If there are junction boxes in the turbine compartment, the insulation of the wiring in the junction boxes and conduits should have a very high temperature rating, as should the components in the junction boxes (terminal boards, especially). USUALLY, junction boxes in turbine compartments are small and there aren't a lot of junction boxes in the turbine compartments (to prevent damage from high temperatures). So, it's possible that the junction boxes were improperly built or improperly placed (such that they don't experience properly cooling air flow), and/or that the wiring insulation was incorrect, or the internal components of the junction boxes weren't suitable for the expected temperatures in the compartment.

But, again, you need to review the damper operation and ensure it is correct, and then you need to work with GE Belfort to determine if the configuration and materials they provided (or that were used during construction) are suitable for the application at your site. Don't rely solely on commissioning personnel; often they are inexperienced and unsuitably trained. Be involved, as you seem to be trying to do, and make sure the installation and commissioning is correct (I know; you're supposed to be able to rely on commissioning personnel for this, but the world is changing... as is the climate...).

If this helps, please let us know. If not, please let us know what you determine to be the problem. Use the Operation & Maintenance Manuals provided with the equipment to start you investigations, and DO NOT be afraid to challenge GE Belfort. Their way is not always the best way.

 

It's also common for some locations in a turbine compartment to have lower cooling air flows than other locations--and if the junction boxes you wrote about are placed in one of those locations then it will experience higher temperatures than other locations.

In looking again at the graphic you posted, it looks like there is a large difference (62 deg C???) between the inlet of the fan suction and the roof of the turbine compartment where the highest temperature is indicated (at least on that graphic). This could be one of those areas that doesn't experience the highest air flows--and if it's directly above the turbine casing/shell it could, indeed, be indicating a higher temperature because of radiant heat from the shell/casing.

It's pretty common for GE to use CFD (Computational Fluid Dynamics) studies to model air flow in a particular compartment but it's just that--a model, and if the compartment isn't constructed according to the model OR the air flows aren't right because of installation problems or commissioning problems then it's not going to properly evacuated or cooled.

 

What kind of alarm/trips have been annunciated ?

No Alarm for this conditions, until now it's normal operation.

Did you check proper operation of vent flap /Damper ?

We have check dampers and everthing on the normal condition and position

 

We have check dampers and everthing on the normal condition and position

Thank you for your reply

Also as @WTF? Advised did you check fan/motor rotation...

 

abu_6FA,

Can you share a photo or photo's of the damaged JB and it's location in the turbine compartment? Was it just the wiring insulation that was damaged, or the terminal boards, or the wiring insulation AND the terminal boards? If crimp-on terminals were used on the wiring and they had plastic around the crimped area was the plastic melted?

Is there a long duct connected to the outlet of the two turbine compartment vent fans? Does the duct have elbows or changes in direction?

Is there anything outside the turbine compartment enclosure that is not allowing ambient air to enter easily through the gravity-operated cooling air flow dampers? Is there anything side the turbine compartment blocking cooling air from entering the gravity-operated dampers and impeding the flow of cooling air into the turbine compartment?

If you can provide photo(s) of the gravity-operated dampers it would be very helpful. If you can provide photos of the gravity-operated dampers being opened with one of the turbine compartment vent fans running AND all of the turbine compartment doors closed that would be most helpful. (I did work on one GE Belfort unit that had motor-operated dampers on the cooling air inlets to the turbine compartment. They failed relatively quickly, and were replaced with gravity-operated dampers--since if they failed in operation they wouldn't close or allow closure of the dampers to seal fire extinguishing agent in the turbine compartment in the event of a fire. Another GE Belfot idea gone unresearched and replaced with tried and proven equipment.)

When one of the turbine compartment vent fans is running (with or without the turbine running) how difficult is it to open one of the turbine compartment doors when they are all closed? Easy? Somewhat difficult? Do you have to brace your feet and pull significantly on the handle to get the door to begin to open? If it's easy to open a door with one vent fan running it's likely the fan is not rotating in the proper direction OR there is some restriction in the discharge of the vent fans OR there is something amiss with the gravity-operated cooling air inlet dampers. It should require some effort to open one of the doors of the turbine compartment when one of the vent fans is running; the negative pressure (suction) in the turbine compartment should be noticeable.

Again, I have to say that based on the temperatures in the graphic you posted the temperatures in the turbine compartment are not all that hot--even the one supposedly located at the roof of the turbine compartment most likely directly above the turbine casing/shell.

What kind of assistance is the turbine packager (GE Belfort or BHEL or ???) providing in resolving this problem?

 

Hi Can OP give us a quick update here I got a frale 6FA manual showing how to deal the vent fan turbine compartment and inlet compartment

 

Same with a question from HANY SALEH. In our Gas Turbine Enclosure GE 6FA is very hot. This picture was taken when we were commissioning the first time of the Engine. I think there is a miss in the engineering concepts for the enclosure. May be of airflow concept or something else. The wiring cable on JB under this engine is damage. In our country temperature of the air inlet is about 28 to 34 degrees C. I want to know the concept of GE 6FA that the airflow in the enclosure from the hot path to the cold path like in this picture and if any suggestions for us to solve these conditions.


View attachment 2606

Also one good thing would be to check proper operation on Flowmeter SPECIALLY 95BT-2 & 95BT-1 and others one

 

Same with a question from HANY SALEH. In our Gas Turbine Enclosure GE 6FA is very hot. This picture was taken when we were commissioning the first time of the Engine. I think there is a miss in the engineering concepts for the enclosure. May be of airflow concept or something else. The wiring cable on JB under this engine is damage. In our country temperature of the air inlet is about 28 to 34 degrees C. I want to know the concept of GE 6FA that the airflow in the enclosure from the hot path to the cold path like in this picture and if any suggestions for us to solve these conditions.


View attachment 2606

Can you share process/diagnostic alarms list

 

For high ambient temperature conditions, it will be better to relocate the JB's to outside the enclosure.
A relatively simple and easy solution to do.

 

Typically, the only JBs (Junction Boxes) in the turbine compartment of a GE-design 6FA heavy duty gas turbine are for the IGV servo & LVDTs (located at the "cold end" of the axial compressor section so not usually subject to a lot of heat), the fire detectors (usually connected with mineral-insulated wire for high temperature protection). Other devices like temperature sensors (switches; T/Cs (thermocouples), hazard gas detectors, flame detectors, combustion monitor devices, etc., are wired to JBs outside the turbine compartment. Typically. GE Belfort have been known to rearrange long-standing and proven wiring practices from time to time. Many F-class units (of which abu_6FA's unit is one) have solenoid-operated pneumatic valves for switching water injection- and liquid fuel purge systems on and off, and are usually located outside the turbine compartment and tubes run inside the compartment to shuttle valves/check-valves just to prevent locating the solenoids and associated wiring inside the turbine compartment.

As abu_6FA isn't providing any further information we are left to twiddle our thumbs and speculate. My speculation is that the turbine packager has resolved the problem as on re-inspection the gravity-operated cooling air dampers were found to be not working correctly or incorrectly installed AND/OR the turbine compartment cooling air fans were found to be operating in the wrong direction of rotation.