Hi all,

We have 2 GE Frame 9E Gas turbines at our facility. The gas turbines are designed for a base load of 115 MW at an ambient temperature of 28 degree Celsius. The gas turbine has 3 bearings located before the inlet plenum, inside the combustion chamber, and after the exhaust frame. There are 2 more bearings on the generator & one on the exciter [which is overhung].

Recently, one of the gas turbines is showing a high shaft vibration on the exciter overhang bearing. The displacement value is in excess of 150 microns i.e. it is out of scale of the [Bently Nevada make] vibration pick-up. During the previous shutdown, since we had doubts as to whether the pickup might have been malfunctioning, we had it replaced. However, still it is showing the same value. If this value is correct, is it likely to cause any secondary damage? The vibration people are convinced that it is of no concern, since the seismic vibration velocity values are well within limits. Besides, the gas turbine trip logics are also based on seismic vibration velocity values i.e. 25.4 mm/s for at least 2 out of 3 gas turbine or generator-exciter values.

Any help from experienced vibration/operation personnel is most welcome.

 

My opinion its not actual vibration. try to clean the rotating diods on the excitor, as the ditrs which accumulate cause like an added weight and cause vibration.

good luck

 

Velocity--or "seismic"--vibration pick-ups are typically mounted on bearing housing caps (with the exception of the #2 bearing seismic pick-up on GE-design Frame 7E/EA and Fram 9E heavy duty gas turbines (which are mounted on the #2 bearing drain pipe which is securely bolted to the #2 bearing housing and will move when the bearing/shaft moves; this is done because it's too hot to put the pick-up in the #2 bearing area)). The shaft must move a LOT to actually make the seismic vibration pick-ups register vibration. This is fine for large, heavy shafts (such as are found on heavy duty gas turbines), and provides adequate protection for heavy duty gas turbines.

Proximity vibration sensors actually look at the distance between the shaft and the pick-up and so are more sensitive to shaft movement as a result of vibration than seismic vibration pick-ups are. They are usually a good indicator of an impending vibration problem.

However, most turbine bearings have two (2) Bently-Nevada proximity sensors, each 45 degrees from top dead center (or 90 degrees apart from each other). You only mentioned one of the Bently-Nevada vibration pick-ups is indicating a high vibration. If there is truly a high vibration problem developing both proximity probes will almost ALWAYS indicate increasing shaft movement (as a result of vibration).

If only one of the two B-N proximity sensors is indicating a high vibration, then it's likely some problem with the sensor (which you said was replaced), the proximitor, the wiring between the proximitor and the B-N monitor "rack", or the B-N monitor card/channel.

As ahdash says, if the turbine uses a brushless exciter it's very possible that dirt and humidity have combined to cause an imbalance somewhere on the diode wheel or elsewhere on the exciter. Brushless exciters require periodic maintenance and clean, dry air (as clean and dry as possible) for proper operation.

But, if the seismic (velocity) pick-up on the collector (NDE (Non-Drive End)) generator bearing is not indicating an elevated vibration, AND only one of the B-N proximity vibration pick-ups is indicating a high vibration, it's not likely a true vibration problem (unless the vibration is causing an extremely unusual orbital rotation resulting in only one proximity probe detecting abnormal shaft movement--which actually has occurred, but the circumstances and the occurrence are QUITE rare) it's likely the seismic vibration pick-up would see some elevated vibration as a result of the unusual shaft rotational orbit).

Please write back to let us know what you find!

 

KRManish

>The vibration people are convinced that it is of no concern, since the seismic
> vibration velocity values are well within limits. Besides, the gas turbine
> trip logics are also based on seismic vibration velocity values i.e. 25.4 mm/s
> for at least 2 out of 3 gas turbine or generator-exciter values.

Heavy Duty Gas Turbines are very smooth machines and the vibration level is usually very low about 2 to 5 mm/s. The alarm is usually 12,5 mm/s and trip is about 25mm/s.

In your case how much are you seismic readings and did they change when the displacement reading went out of scale?? If no change probably the trouble is with displacement probe, junction box, wiring.

You said you've got 2 GE F9E. If they are running under the same condition, you can use a vibration hand tool you can take measurements and compare them.

Each bearing has 2 displacement probes, regarding my experience, if the change happened <b>only on one probe..</b> Don't worry about it.
What you also need to check are the bearing temperature for any increase and the LO pressure for any change.

Hope this will help
Karim