Problem with Velocity Sensor in GasTurbine

  • Thread starter sanjib_dibrugarh
  • Start date
S

Thread Starter

sanjib_dibrugarh

One velocity sensor out of two showing high vibration in the Exhaust End of Turbine after 1/2 hour of synchronizing of Frame 5 machine.

Velocity Model No: 5485C of Matrix. GT control System is Mark VIe.

Please give the reason
 
sanjib_dibrugarh,

What have you done to troubleshoot this problem?

When did this problem start? During commissioning of the Mark VIe? After some mechanical maintenance work was done on the #2 bearing or the reduction/load gear?

Are there any vibration sensors on the reduction/load gear? If so, do they register any increase in vibration at the same time as this sensor does?

Have you checked to make sure the two sensors are the same, and that the configuration of the two sensors in ToolboxST is the same (sensitivity)?

Have you checked the integrity of the cable between the input terminal board and the sensor by meggaring it (when it's disconnected from the Mark VIe and from the sensor)? If so, what reading did you record?

Is this a new gas turbine installation, or an upgrade to Mark VIe from an older turbine control system?

Does the other sensor of the pair show any vibration reading or increase at all? Many times the problem is not the sensor with the higher reading, it's the sensor with the lower reading.

Lastly, has anyone noticed an increase in vibration when standing on the grating near the turbine and/or load compartment?

 
S

sanjib_dibrugarh

Dear CSA
Thanks for your reply.

Your reply Point wise:
1. What have you done to troubleshoot this problem?
--We tighten all connection


2. When did this problem start? During commissioning of the
Mark VIe? After some mechanical maintenance work was done on
the #2 bearing or the reduction/load gear?
--During commissioning


3. Are there any vibration sensors on the reduction/load gear?
--Yes


4. If so, do they register any increase in vibration at the
same time as this sensor does?
--No

5. Have you checked to make sure the two sensors are the same,
and that the configuration of the two sensors in ToolboxST
is the same (sensitivity)?
--Same


6. Have you checked the integrity of the cable between the
input terminal board and the sensor by meggaring it (when
it's disconnected from the Mark VIe and from the sensor)? If
so, what reading did you record?
--We checked with multimeter. No Earth fall was observed with the cable of sensor side


7. Is this a new gas turbine installation, or an upgrade to
Mark VIe from an older turbine control system?
--New Gas Turbine


8. Does the other sensor of the pair show any vibration reading
or increase at all? Many times the problem is not the sensor
with the higher reading, it's the sensor with the lower
reading.
--No, Other is normal. This time this vibration showing high value


9. Lastly, has anyone noticed an increase in vibration when
standing on the grating near the turbine and/or load
compartment?
--Yes noticed. But no heavy vibration observed
 
Thanks for replying to all of the questions.

If this has been ongoing since commissioning, it's very likely the sensor has failed. Have you asked the turbine supplier to replace the sensor under warranty? I have seen vibration sensors permanently damaged by trying to "stimulate" them improperly--I'm NOT saying that's what happened at your site, but I am saying it has happened. Also, when the load coupling or load coupling guard between the turbine and reduction/load gear is installed, sometimes it swings a little more than intended. It can also be damaged during shipment.

Increased vibration at the load coupling is usually the result of alignment issues, and as the turbine is loaded and as it warms up vibration caused by alignment issues will increase. It's not unusual for there to be a small increase in vibration with load, but you have neglected to tell us exactly the difference in vibration signals from the two sensors at FSNL (prior to synchronization) and after the vibration on the one sensor begins to increase.

Is the unit equipped with proximity probes (Bently-Nevada or equivalent)? If so, what do these vibration sensors indicate on the #2 turbine bearing?

It's also entirely possible that both sensors were damaged either during shipment or during installation of the coupling/coupling guard and both are reading erroneously. Again, this would typically be considered a warranty issue and the turbine packager would normally replace the sensors if for no other reason than they are the devices they rely on the trip the turbine in the event of high vibration, preventing excessive damage to the machine should operators fail to take appropriate action in the event of increasing or high vibrations. The supplier should want to make sure the signals from all protective devices are working properly during the warranty period.

Lastly, you say you have checked and tightened all the "connections." Does that include the vibration pick-up mounting bolts? I have had to use a medium-strength thread locker on the mounting bolts on occasion as they can vibrate loose; some older machines even used stainless steel tie wire routed through holes in the mounting bolt heads to prevent the bolts from loosening, but I have also found that on more than one occasion the bolts were not fully tightened before the tie wire was installed. I've even found dirt and rust under the sensors (though not on new machines--but anything's possible).

If you're concerned that the problem might be the Mark VIe (not likely) you can try simply exchanging the vibration pick-up wires at the Mark VIe input terminal board (the sensors are redundant, but not specific to right/left). If the increasing reading occurs on the same Mark VIe input, then it might be a Mark VIe problem, and if the problem follows the sensor to the other Mark VIe input then it's likely the sensor and/or the wiring. I've seen a lot of poorly terminated vibration pick-up cables which had small nicks of wire insulation which were intermittently grounded, though this usually results in Diagnostic Alarms as well as suspect readings.

Please write back to let us know what you find.
 
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sanjib_dibrugarh

Dear CSA,
Vibration showing full range 40 mm/sec at full speed and rest velocity shows around 3 mm/sec. But low rpm it is under 3 mm/sec. We tight the bolt of vibration probe. At standstill it is showing 118 ohm at probe resistance.

Please advise what we can check now?
 
sanjib_dibrugarh,

Try exchanging the two sensors at the Mark VIe input terminal board and starting the turbine, and report back what happens to the readings for the two sensors.

And, do you have a spare vibration sensor? If not, why not?

 
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