In a gas turbine PG9371 FB Simple-cycle single-shaft heavy-duty gas turbine at a speed of 1006 rpm for two measures the peaks and bb2 bb2X probes and vibrations occurring in the bearing 2 (bearing 1 GT) 0.18 mm/s, trip unit to the value of 0.22 mm/s. the second peak at 827 rpm with the flame off to 784 rpm.

At startup high vibrations also occur at 996 rpm, coinciding with the point of passage through the critical speed when starting and stopping the unit.

What could be causing this increase vibration values to the trip of the unit?

 

elomoon,

Your post is completely unclear. There is usually only one velocity ("seismic") vibration sensor mounted on the #2 bearing drain pipe (it's too hot to put sensors on the #2 bearing cap). Seismic sensors measure velocity (mm/sec). The typical device signal name for the #2 bearing seismic sensor is bb2 (or, BB2, depending on the version of Speedtronic turbine control system).

Some units have Bently-Nevada proximity sensors, which measure in displacement (mm), and sometimes have signal names like bb2X and bb2Y (because usually two proximity sensors are mounted on a bearing, separated by 90 degrees of rotation).

Has someone stood out next to the machine to feel the high vibration?

The overwhelming majority of vibration issues are the result of true vibration--and have nothing to do with controls or control devices (contrary to popular belief). When did the high vibration start? After a maintenance outage? After a trip from load?

Have you checked the seismic sensor to ensure its secure and hasn't become loose, and wasn't damaged during a recent maintenance outage?

Was the seismic sensor recently replaced--with a sensor of the identical output (volt per mm/sec)?

It's difficult to relate proximity sensor readings to seismic sensor readings (they don't have the same units of measurement, and proximity readings vary with speed).

But, in the end--unless the seismic sensor is failing--you will find the problem is not controls-related and you must have someone in to check and balance the machine--something we cannot help with here at control.com. It's most likely a true rotor imbalance, a failed or failing sensor, or a loose sensor, or one that was damaged during recent mechanical activity in the area, or a sensor with the wrong sensitivity was used to replace the original sensor.

It would be really great if you would write back to answer the questions above, and to let us know what you find!

Hope this helps!

 

Thanks for your reply.

The bearing is the # 1 GT but the number 2 single-shaft, and carries mounted seismic detectors (vibration of the housing relative to the shaft) and proximitors (bearing vibration relative to the shaft).

Maintenance does not check the sensors and to say they do not fail.
The unit will be left for boot priority as engineering GE wants to boroscopias.

When we find the cause I will send you my answer.

Forgive me if answer is not very clear, but my English is not very fluent.

Thank you

 

elomoon,

A colleague pointed out to me that I didn't properly read your post--I was mistakenly referring to a GE-design Frame 9E heavy duty gas turbine (which has three bearings) and your post was regarding a GE-design Frame 9FB heavy duty gas turbine. I apologize for the mistake; I endeavor to try to respond to the question by reading the post several times before responding. Somehow I completely missed the pertinent information.

Every instrument can fail--and seismic vibration sensors are no exception. The instance of failure is low, but they still do fail.

To understand how the GE-design F-class turbine (including the 9FB) bearings are numbered, you can refer to the documentation provided with the unit by the packager. I believe the bearing at the end of the turbine driving the generator is the #1 turbine bearing, and the bearing at the exhaust end of the turbine is the #2 turbine bearing. (I believe the driven end of the generator is also identified as the #1 generator bearing, and the collector end (the non-drive end, the one which also usually has the turning gear mechanism) is identified as the #2 generator bearing. (This numbering is different from non-F-class GE-design heavy duty gas turbines, where all the bearings in the unit (turbine and generator) are numbered sequentially from the axial compressor inlet to the non-drive end of the generator.

We can avoid confusion about numbering if you tell us if the bearing experiencing the high vibration is on the end of the turbine that drives the generator (closest to the axial compressor inlet, also called the "cold end" of the turbine), or if it's on the exhaust end of the turbine (called the "hot end" of the turbine). Seismic vibration sensors inside the exhaust where the hot end bearing is located have failed--usually more prematurely than the cold end vibration sensors. Ventilation (provided by an electric motor-driven fan) is very important to the hot end bearing. There are usually dampers in the inlet and exhaust duct work that can close and restrict the air flow--but there are usually limit switches to indicate their closure and lack of air flow to the bearing area.

I understand English is not your native language, but it would be most helpful if you would answer the questions asked when you post for help with a problem to the best of your ability. When a problem like this occurs, the most important question to ask is: When did the problem start? Such as after a maintenance outage, or after a trip from load, after a sensor was replaced, etc. I also neglected to ask what other alarms (Process and Diagnostic) were annunciated when the problem began or during starting/shutdown when the vibration is high.

If there are two seismic vibration sensors on the bearing in question, and only one of them is indicating a high vibration, then it's very likely that either the sensor or the wiring has failed. Mark* Speedtronic turbine control systems are pretty robust--and unless there is a serious wiring problem (short; ground; etc.) the inputs and outputs of the control system don't usually fail--and if they do, there are usually Diagnostic Alarms to indicate the failure.

Alarms (even those pesky Diagnostic Alarms) are very important to troubleshooting problems. Even if they don't indicate the real cause, they can be used to eliminate one or more causes.

We hope the GE field service people can resolve the problem--and we'll be very interested to learn about the resolution. Please do write back with the resolution.

 

These unit start twice a week and have not done any maintenance work. Today we made boroscopia R0 compressor and combustion chambers. We expect the test results.

Thank you