We have 2 dual fuel GE 6B DLN1 CT's (peakers) that were commissioned in 2001. MARK V. On a hot restart after racheting all evening we will trip on excessive vibration at the #2 bearing on both units. If we start the units in 2-3 days after a run, no problem. We see elevated levels but no trips. There has been no work done on the machines. We borescoped them in Sept. and found nothing abnormal. No alarms. Any direction in which to head?

 

1) Have the machines done this since new or when did the problem start?

2)Frame six machines have a common habit off doing this at #2 Brg. if any work has been done on the load coupling or any re-alignment work. Has any of this been done.

3) What happens on a cold start?

 

We have not done any work on either unit, both experience same issue. One more than other but still trip. On a cold restart (roughly 48 hours later) minor vibration with no trip. It something that has been developing with in the past 2-3 years. We are called on more lately so it has surfaced as an issue.

 

I'm with Bob Johnston on this one; sounds like an alignment issue, particularly if the same thing is happening to both machines albeit at slightly different rates/levels. And if the vibration has been increasing over time on both machines then it certainly seems like some kind of alignment issue.

You haven't said at what speed the excessive vibration occurs during the start-up.

Also, does the Load Compartment (where the Reduction/Load Gear is located) have an AC motor-driven ventilation fan? Does it run during cooldown operation (on ratchet)?

Does the Cooling Water Pump run during cooldown (ratchet operation)? If so, does it run continuously, or does it cycle based on L.O. header temperature? What is the L.O. temperature during a hot re-start attempt versus a "cold" start attempt?

What about turbine compartment ventilation during cooldown (on ratchet)? Does the turbine compartment vent fan run continuously or at all during cooldown operation?

But, after asking all of the above, it still seems like the issue is related to alignment. You might try replacing one of the two #2 Brg. vibration pick-ups on one or both of the machines and record the vibration feedback from both pick-ups during starting. While it's not likely that heat has damaged the pick-ups (they're designed for the intense heat in the #2 Brg. Area) nothing lasts forever. Heat and cycling take a toll on everything over time.

Please write back to let us know how this progresses and is resolved.

 

Thanks for the help. Our cooling water pump is driven by the accy drive. Turbine fan shuts off after lower temp is established. Load compartment fan I will check on. I will leave both in hand for extended cooling. 30-45 % speed is where we see the vibration. I will also do the replacement of a detector. One of the suggestions at a time. Strongly leaning to an alignment issue as we see elevated levels at the #1 bearing also. I will update as we progress. Thanks again.

 

Now you have really changed the whole issue by saying that you see an elevated vibration at Brg. #1 as well, this starts to point to imbalance rather than alignment.

Firstly, I would suggest a quick vibration analysis at Brgs #1 & 2 to try and establish imbalance vs. alignment. If it is imbalance, it is possible to reduce these start-up vibrations by doing a trim balance at either the Accessory coupling or Load coupling bolts at the bearing end of the coupling (only one coupling, not both). Any decent vibration analyst should be able to do both jobs for you. This is not GEs officially recognized practice but I know they have done it on "sensitive" rotors like yours (In fact GE did ship a series of "sensitive" rotors which exhibited this problem)

How many fired hours and fired starts do you have on your machines? Do you know which 2nd. stg. bucket part No. you have installed?
I'm kind of surprised, even if they are peakers, that no maintenance work has been done on 11 year old machines, are you sure?