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For how long time we have to put the GE frame 9E DLN machine on turning gear if the GT trips and the turning gear is not engaged within 30 min before GT start?
> For how long time we have to put the GE frame 9E DLN machine on turning gear if
> the GT trips and the turning gear is not engaged within 30 min before GT start?
If I understand the question correctly, then there is no time which the unit must be put on turning gear before being started. <b>However,</b> read on.
So, what you seem to be saying is that a hot turbine and axial compressor reached zero speed and did not go on turning gear for approximately 30 minutes. And then the hot unit is re-started within approximately 30 minutes of reaching zero speed but not having been on turning gear. If the shaft will turn during the start sequence, then everything is probably "okay." If it won't turn, then things are not okay.
If the shaft turns during a start, it's pretty likely there will be high vibrations during the start sequence, possibly high enough to trip the unit.
If a hot shaft (turbine and axial compressor) reaches zero speed and does not go on turning gear within approximately 30 minutes then it's highly likely that some bowing/warping of the axial compressor will occur, and that depends on a lot of factors (how how the unit was when it was tripped or shut down; how cool is the ambient air drafting through the turbine during shutdown; etc.). Conventional wisdom says if a hot unit can't be re-started or put on turning gear within approximately 30 minutes of reaching zero speed that it's best NOT to try to start the machine or to put it on turning gear for fear of damaging one or more compressor blades.
The 30 minute period is just an estimate of how long it might take for the axial compressor to bow under its own weight when hot. It's not any kind of mathematically-derived value or any cast-in-stone number. If there were some instrumentation to measure eccentricity then there would be an absolute indication of when it would be safe to start the unit again. But, there is no eccentricity monitor on most GE-design heavy duty gas turbines.
What may happen if you try to put the unit on turning gear in such a condition is that either the rotor won't turn (worst case) because one or more compressor blades is in contact with the compressor casing, or (even worse case, actually) the rotor does turn after one or more compressor blades breaks off, which can cause quite a lot more damage to other blades (rotating and stationary).
In the best case, if the shaft turns on turning gear, then all will probably be alright, though there will likely be some warpage which must be "turned out". If you re-start the turbine and it rotates then it's very likely there will be a high vibration during some portion of the acceleration, and it might actually result in a turbine trip.
So, again, conventional wisdom says that if a hot unit can't be put on turning gear (or re-started) within approximately 30 minutes of reaching zero speed then it's best to just force the turning gear sequence OFF, leave the Aux. L.O. Pump running (to supply L.O. flow to the bearings for cooling), and to wait approximately 24 hours. During this time the axial compressor rotor will actually begin to straighten, and while it won't return to fully straight, it will eventually return to a condition where the unit can be put on turning gear, and the unit can eventually be re-started.
After a 24-hour period of sitting at zero speed with the Aux. L.O. pump running, if the unit is put on turning gear the longer it could be left on turning gear, the better. However, after the 24-hour period many turbines have been cranked (sometimes with an initial high vibration) and then FIREd for a few minutes and then accelerated to FSNL. Again, there may be a high vibration initially as the warpage is "driven" out by rotation at speed. Sometimes, the unit trips, and then is quickly re-started and the vibrations are much lower and the unit reaches FSNL without tripping.
It's a matter, again, of a lot of factors and variables. But, the 30 minute period is about the maximum that most turbines can remain at zero speed while "hot" without either being on turning gear or being cranked or started before the axial compressor bows sufficiently to cause one or more axial compressor blades to come into contact with the compressor casing. And, when that happens, if the shaft is turned before the shaft straightens enough to "lift" a compressor blade from contact with the compressor casing then bad damage can occur.
And, the 24-hour period is also just an estimate.
The problem with the 30-minute period estimate is that trying to turn the shaft when it's known to be much after 30 minutes at zero speed can result in damage, bad damage. And, the shaft may bow excessively in 24 minutes or 26 minutes or 29 minutes, too. So, 30 minutes is just an estimate, and without being able to measure eccentricity it's just a guideline (estimate).
The same can be said of the 24-hour period. The shaft may not straighten sufficiently in 24 hours. It may take 26 hours, or 26.5 hours, 04 24.7 hours. Again, without being able to measure eccentricity it's very difficult to say for sure what is the absolute minimum time, or maximum time.
The key is patience. And not to try to force the shaft to rotate using jacks and other mechanical means. If it won't turn, just be patient; it will.
This happens all the time (failure to go on or maintain cooldown (turning gear; ratchet; slow-roll), and turbine survive with minimal effects--as long as people don't get too zealous trying to force the shaft to rotate.
Hope this helps!
> the GT trips and the turning gear is not engaged within 30 min before GT start?
If I understand the question correctly, then there is no time which the unit must be put on turning gear before being started. <b>However,</b> read on.
So, what you seem to be saying is that a hot turbine and axial compressor reached zero speed and did not go on turning gear for approximately 30 minutes. And then the hot unit is re-started within approximately 30 minutes of reaching zero speed but not having been on turning gear. If the shaft will turn during the start sequence, then everything is probably "okay." If it won't turn, then things are not okay.
If the shaft turns during a start, it's pretty likely there will be high vibrations during the start sequence, possibly high enough to trip the unit.
If a hot shaft (turbine and axial compressor) reaches zero speed and does not go on turning gear within approximately 30 minutes then it's highly likely that some bowing/warping of the axial compressor will occur, and that depends on a lot of factors (how how the unit was when it was tripped or shut down; how cool is the ambient air drafting through the turbine during shutdown; etc.). Conventional wisdom says if a hot unit can't be re-started or put on turning gear within approximately 30 minutes of reaching zero speed that it's best NOT to try to start the machine or to put it on turning gear for fear of damaging one or more compressor blades.
The 30 minute period is just an estimate of how long it might take for the axial compressor to bow under its own weight when hot. It's not any kind of mathematically-derived value or any cast-in-stone number. If there were some instrumentation to measure eccentricity then there would be an absolute indication of when it would be safe to start the unit again. But, there is no eccentricity monitor on most GE-design heavy duty gas turbines.
What may happen if you try to put the unit on turning gear in such a condition is that either the rotor won't turn (worst case) because one or more compressor blades is in contact with the compressor casing, or (even worse case, actually) the rotor does turn after one or more compressor blades breaks off, which can cause quite a lot more damage to other blades (rotating and stationary).
In the best case, if the shaft turns on turning gear, then all will probably be alright, though there will likely be some warpage which must be "turned out". If you re-start the turbine and it rotates then it's very likely there will be a high vibration during some portion of the acceleration, and it might actually result in a turbine trip.
So, again, conventional wisdom says that if a hot unit can't be put on turning gear (or re-started) within approximately 30 minutes of reaching zero speed then it's best to just force the turning gear sequence OFF, leave the Aux. L.O. Pump running (to supply L.O. flow to the bearings for cooling), and to wait approximately 24 hours. During this time the axial compressor rotor will actually begin to straighten, and while it won't return to fully straight, it will eventually return to a condition where the unit can be put on turning gear, and the unit can eventually be re-started.
After a 24-hour period of sitting at zero speed with the Aux. L.O. pump running, if the unit is put on turning gear the longer it could be left on turning gear, the better. However, after the 24-hour period many turbines have been cranked (sometimes with an initial high vibration) and then FIREd for a few minutes and then accelerated to FSNL. Again, there may be a high vibration initially as the warpage is "driven" out by rotation at speed. Sometimes, the unit trips, and then is quickly re-started and the vibrations are much lower and the unit reaches FSNL without tripping.
It's a matter, again, of a lot of factors and variables. But, the 30 minute period is about the maximum that most turbines can remain at zero speed while "hot" without either being on turning gear or being cranked or started before the axial compressor bows sufficiently to cause one or more axial compressor blades to come into contact with the compressor casing. And, when that happens, if the shaft is turned before the shaft straightens enough to "lift" a compressor blade from contact with the compressor casing then bad damage can occur.
And, the 24-hour period is also just an estimate.
The problem with the 30-minute period estimate is that trying to turn the shaft when it's known to be much after 30 minutes at zero speed can result in damage, bad damage. And, the shaft may bow excessively in 24 minutes or 26 minutes or 29 minutes, too. So, 30 minutes is just an estimate, and without being able to measure eccentricity it's just a guideline (estimate).
The same can be said of the 24-hour period. The shaft may not straighten sufficiently in 24 hours. It may take 26 hours, or 26.5 hours, 04 24.7 hours. Again, without being able to measure eccentricity it's very difficult to say for sure what is the absolute minimum time, or maximum time.
The key is patience. And not to try to force the shaft to rotate using jacks and other mechanical means. If it won't turn, just be patient; it will.
This happens all the time (failure to go on or maintain cooldown (turning gear; ratchet; slow-roll), and turbine survive with minimal effects--as long as people don't get too zealous trying to force the shaft to rotate.
Hope this helps!
G
GTG
CSA,
That was a great explanation.
I have one doubt, some people say that if gas turbine is under shut down for an extended period like a month or so, it is advisable to crank the machine or bring it to FSNL once a week to prevent shaft bowing. So my doubt is, <i>'will a shaft which is at ambient temperature bow so much, that it should be cranked/ or brought to FSNL once a week??'</i>
That was a great explanation.
I have one doubt, some people say that if gas turbine is under shut down for an extended period like a month or so, it is advisable to crank the machine or bring it to FSNL once a week to prevent shaft bowing. So my doubt is, <i>'will a shaft which is at ambient temperature bow so much, that it should be cranked/ or brought to FSNL once a week??'</i>
Doubt is such a ... nasty word. It implies distrust, and is not a synonym for 'question.'
A turbine shaft will bow under its own weight if left long enough in a stationary position.
The axial compressor is composed of many discs which are all bolted together with long through-bolts (have a look in the Service/Maintenance Manual). And the longest distance between between bearings is the length of the axial compressor; comparatively, she length of the turbine shaft is much shorter, as it's only two or three stages, as compared to sixteen or seventeen stages for most axial compressors. So, the weight of the axial compressor, if left stationary for long periods of time, will eventually cause the through-bolts to stretch due to its own weight (gravity) and cause the axial compressor to bow.
Rotation, even if just at cooldown speed, will help to prevent long-term damage, as well as high vibrations during starting and acceleration.
It's not necessary to crank the unit or run it up to FSNL to reduce the effects of long term, stationary position. One just needs to put the unit on cooldown (ratchet; turning gear; slow-roll--whatever method the packager provided) prior to operation, or periodically. It's not a good idea to start the machine (fired operation) if it's been sitting stationary for days/weeks. Usually there will be high vibration during starting/acceleration as the shaft straightens.
The best practice for a machine that's not operated very often and sits stationary for days/weeks is to institute a periodic plan of cooldown operation, say, once per week for four hours, or every third day for two hours, or something similar.
A turbine shaft will bow under its own weight if left long enough in a stationary position.
The axial compressor is composed of many discs which are all bolted together with long through-bolts (have a look in the Service/Maintenance Manual). And the longest distance between between bearings is the length of the axial compressor; comparatively, she length of the turbine shaft is much shorter, as it's only two or three stages, as compared to sixteen or seventeen stages for most axial compressors. So, the weight of the axial compressor, if left stationary for long periods of time, will eventually cause the through-bolts to stretch due to its own weight (gravity) and cause the axial compressor to bow.
Rotation, even if just at cooldown speed, will help to prevent long-term damage, as well as high vibrations during starting and acceleration.
It's not necessary to crank the unit or run it up to FSNL to reduce the effects of long term, stationary position. One just needs to put the unit on cooldown (ratchet; turning gear; slow-roll--whatever method the packager provided) prior to operation, or periodically. It's not a good idea to start the machine (fired operation) if it's been sitting stationary for days/weeks. Usually there will be high vibration during starting/acceleration as the shaft straightens.
The best practice for a machine that's not operated very often and sits stationary for days/weeks is to institute a periodic plan of cooldown operation, say, once per week for four hours, or every third day for two hours, or something similar.
Take a look in GER 3620 Heavy Duty Gas Turbine Operating and Maintenance considerations. There is a table there in Appendix F that gives minimum Ratchet/Turning Gear durations for turbines in different states (e.g. Warm Rotor, Cold Rotor,etc)
If you don't have a copy, drop me an Email to [email protected]
If you don't have a copy, drop me an Email to [email protected]
S
sardar9
S
