For frame VI gas turbine both Exhaust Frame cooling fans 88TK1 & 88TK2 are starting at 95 % speed (there is a time delay 10Sec for second one)

Each fan is 100% capacity, as per the logic one fan trip, it will give only alarm, if both trip GT will unload

Why two fans are required even if one is 100% capacity? What is logic behind it?

Is it advisable to run the GT with only one fan?

What will happen to the shell if both fans tripped

Thanks in advance for your comments & advice

Regards

 

The philosophy behind the logic/sequencing for a particular Frame 6 unit's exhaust frame cooling fans (blowers) varies with the packager: John Brown, Nuovo Pignone, BHEL, GE, etc. In general, GE's philosophy was to start and run both exhaust frame blowers any time the unit was above approximately 95% speed, to reduce load (automatic runback) to approximately 50% on the loss of one exhaust frame blower, and to reduce load to FSNL on the loss of both exhaust frame blowers. (The time-delayed start was to reduce the in-rush currents drawn if both blowers were stared simultaneously.) The capacity of Frame 6 exhaust frame blowers provided with GE-built packages varied, also, but generally a single fan was not capable of 100% capacity. It was never clear why a loss of both exhaust frame blowers didn't result in an automatic shutdown (i.e, an automatic stop), and just reduced load to FSNL....there's no reason to run a turbine if the generator is not producing electrical power.

The capacities of the Frame 6 exhaust frame blowers provided by the various packagers also varied. Many packagers resented being "told" by GE how to perform exact sequencing, and many packagers usually had a "better idea" when it came to auxiliaries and sequencing. (One packager's turbine compartment heater required 79 rungs of logic, three RTDs, four contact outputs, and four contact inputs to control a three-speed fan on the heating unit. There were also four process alarms associated with the heater.... And, the sequencing didn't work as required to prevent nuisance alarms and maintain compartment temperature! A better idea... possibly. A better implementation, not.)

The reason the sequencing in your unit requires both exhaust frame blowers to be running even though one fan is capable of providing 100% capacity is unclear. If each fan is truly capable of 100% capacity, they could have used a lead-lag control scheme.... But all the packagers built GE-design turbines under license from GE, and there were some "guidelines" which all had to follow....

It's not clear how you discerned that a single exhaust frame blower has 100% capacity--but it appears to be from the sequencing (because the loss of a single exhaust frame blower only results in a process alarm). There have been "mistakes" like this found in the logic and sequencing of units which have been running for more than 20 years. Many were only found because people tried to operate the units differently than they had been operated for years, and all of a sudden unusual problems started occurring.

If you discerned that each exhaust frame blower has 100% capacity from the unit's instruction manual, you should be cautious. There is always one great caveat when reading GE-design heavy-duty gas turbine-generator instruction manuals--regardless of packager: Read them for INTENT--not CONTENT. In other words, many of those instruction manual articles were never updated when changes were made to auxiliaries or control schemes. (We're not talking about vendor documents, just the system description articles written by the packagers.) When the article describing exhaust frame blower capacity was written, perhaps that packager was using blowers with 100% capacity. But, the fans provided subsequently for a different application might not have been so rated (fans with 80% capacity are less expensive...), and the instruction manual article may not have been updated to reflect the change. Unless the article states what the exhaust frame cooling air capacity requirement is and you can verify from the blower nameplate (or the blower manufacturer) that the blower's capacity matches the requirement, any statement made in the instruction manual should be suspect. (This author never saw any document which stated exactly what the exhaust frame cooling air flow requirement was for any particular turbine.)

How does one know what to trust in an instruction manual? Just read everything with a critical mind--can a statement be verified? In your case, can you find the exhaust frame cooling air supply requirement and confirm the blower capacity? Does any statement about unit operation match the logic in the CSP?

The unit should not be run without exhaust frame cooling; the exhaust frame can experience distortion leading to all manner of problems (increased/decreased internal clearances, high vibration, etc.).

markvguy

 

Well I just wanted to share my experience with this scenario. Our Frame 6 similarly has the 2 100% fans, we operate them on the lead/ lag config. One fan tripped and the Lag unit started but had a pressure sensor failure so even though the fan was runnin, Mark VI never saw it and automatically started to unload the machine. Until reading your coments I wasn't sure if it would stop at a predetermine MW or go all the way to FSNL. Now sure how to verify this. Anyway we got the fan that initially tripped to restart and the GT came back up to base.

 

Who is the packager of your unit (GE, GEEPE, ???)?

The usual logic signal name which unloads the unit on loss of exhaust frame cooling is L90TKL, which usually drives L70LX5, and L70L.

markvguy