Shifting of max vibration


Thread Starter


we have EGT make frame V, four nos gas turbine. in all the machines max vibration was seen at exciter end. recently, in Machine #2, the max vibration has shifted from exciter end to bearing #2 area. the value is steady at around 7 mm/sec and there is no visible sign of other problem.

can any one point out the real problem and suggest measures.
What have you done to try to troubleshoot the problem?

This is most certainly a controls problem, though, as the mechanical department can't conceive of a situation that would cause a high vibration to suddenly appear at a different place in a piece of equipment rotating at 5000+ RPM. Nope; controls problem for certain; no doubt. Those pesky controls sensors; one just can't trust them.

If you have redundant velocity ("seismic") vibration sensors on the #2 bearing, are they both indicating a high vibration? 'Cause if they both are reading high, then, it's a safe bet there's actually something mechanically wrong. Unless the mounting plate that holds the sensors to the bearing cap is loose. Yeah; THAT'S it! The control screws holding the mounting plate came loose. Pesky control screws.

If you also have Bently-Nevada proximity sensors on the #2 bearing, are they also indicating a higher than normal vibration reading?

Or is it just the seismic pick-ups and not the B-N pick-ups indicating the higher vibration reading?

Or is it just B-N pick-ups and not the seismic pick-ups indicating the higher vibration reading?

The most important question to ask when something has been running "correctly" for a long time, and then there's a sudden difference: What has changed? Was there a maintenance outage recently? Was there work done in the load tunnel? Is the temperature in the load compartment higher than normal? Is the load compartment vent fan working properly?

I believe the aft turbine support legs are water-cooled. If so, when was the last time the air was bleed from the cooling jackets?

How long has it been since the last major maintenance outage? Did the turbine trip recently from full load? Does it start and stop every day, and this just happened one day?

What are the wheelspace temperature readings doing? Are they increasing, higher than normal, decreasing, lower than normal, steady?

If the unit has bearing metal temperature T/Cs (or RTDs), what is the #2 bearing metal temperature now with respect to before the vibration increase?

If the unit has bearing drain temperature T/Cs (or RTDs), what is the difference of the #2 bearing between now and before the vibration increase?

You haven't provided enough information to make a considered and concise reply. Hopefully you have some things to consider. It's not likely we can help you with the problem since we just don't know enough about how the units are operated at your site, how they've been operated, and don't have any historical data to review. There can be lots of things which can cause a high vibration; LOTS of things. It might even be that a piece of one of the turbine buckets was liberated and caused a slight imbalance; it has been known to happen. An exhaust leak in the load tunnel can cause a higher than normal temperature which can adversely affect the bearing housing and/or the load coupling. A disruption of cooling & sealing air flow to the #2 bearing can cause a shift in the bearing position.

But, vibrations can suddenly increase because of any number of mechanical problems or conditions, and no matter what the mechanical department says, it's not always a controls problem. It can be something very subtle, but if redundant sensors are all indicating a higher than normal vibration it's a pretty safe presumption that there is some condition leading to increased vibration.

And it could be as simple as a loose sensor. Or a failing or intermittent sensor (if it's just one of a redundant pair that's indicating a high vibration).

Troubleshooting is logical process of elimination, and we don't know what you've done to eliminate anything.