Hi, we have 35 MW Frame 6B. Currently, we have 3 ea (2 continuous running, 1 standby) exhaust frame motor blowers (88TK-1,2,3). 88TK-1 and 88TK-2 are placed in the same enclosure. We are monitoring each motor's current and voltage as our predictive maintenance program. On February 2022, 88TK-1 current is 27.28A and 88TK-2 current is 27.38A. In my understanding, it means that each motor runs at 50% of its capacity because the FLA is around 52A. But on March 2022, suddenly the 88TK-1 current decreased to 20.76A and the 88TK-2 current increased to 34.8A. What are the possible causes of these current drastic changes for 88TK-1 and 88TK-2? All check valves, bearing, vibrations, and housing temperatures are in normal condition. There is also no unbalance in current and voltage for each motor. Thank you.
Turbine Exhaust Frame Cooling Blower & Motor GE Frame 6B
- Thread starter M Alif Fatullah
- Start date
"... There is also no unbalance in current and voltage for each motor."
Maybe the voltage is the same for each motor, but you are saying your problem is understanding why the currents of the two motors--which were roughly equal in February--are so different. Unless I'm misunderstanding something.
The current drawn by a motor is generally indicative of the work being performed by the motor. (There are some efficiency losses and such, but, the current drawn (consumed) by an electric motor is primarily a function of the work being performed by the motor.) So, it would seem that for some odd reason the work being performed by 88TK-1 is less than the work being performed by 88TK-2. I'm no electric motor expert but it's also possible that something has gone wrong with the electric motor itself (windings; rotor) and that's why it's not using so much current and not producing so much power.
The most obvious problem is the check valve at the discharge of 88TK-1 is not working properly. If it weren't fully open while the motor was running then the amount of air being moved by the fan (driven by the motor) would be decreased, resulting in less work requiring less current. BUT, if the motor was running at rated speed, and not moving as much air, then usually the fan housing will get hotter. Maybe there is enough radiation or conduction in the area where the fan is located that the change in housing temperature is not noticeable. Are you sure the check valve for 88TK-1 and 88TK-2 move freely and open and close fully when the fans are running and when they stop?
You mention three exhaust frame blowers. What about the state of the check valve at the discharge of 88TK-3--is it fully closed when it's not running?
What are the currents drawn when you run only 88TK-2 and 88TK-3?
I have seen couplings between the motor and the fan come loose making the fan not spin as fast is the motor shaft and that causes lower than normal current draw. Are you sure the 88TK-1 fan itself is running at rated speed?
Have you inspected the fan itself to see that it's fully intact and hasn't come apart?
Usually the inlets of the exhaust frame blowers have acoustic insulation to make them less noisy when running. Is it possible some of the insulation panels have separated or come loose inside 88TK-1 and are blocking air flow into the fan suction?
Again, the current drawn by an electric motor is indicative of the work being performed by the electric motor. That presumes all the bearing are good (and bad bearings would usually cause higher than normal current draw), and that the motor windings and rotor are all functioning properly (no shorted windings or rotor issues). If that's all good, then it's either the check valves, or the coupling between the motor and fan, or something is blocking the air flowing into the fan suction. There's not too much else it can be.
Please write back to let us know what you find. And if you require further assistance, please answer all the questions above. All of them, please.
Maybe the voltage is the same for each motor, but you are saying your problem is understanding why the currents of the two motors--which were roughly equal in February--are so different. Unless I'm misunderstanding something.
The current drawn by a motor is generally indicative of the work being performed by the motor. (There are some efficiency losses and such, but, the current drawn (consumed) by an electric motor is primarily a function of the work being performed by the motor.) So, it would seem that for some odd reason the work being performed by 88TK-1 is less than the work being performed by 88TK-2. I'm no electric motor expert but it's also possible that something has gone wrong with the electric motor itself (windings; rotor) and that's why it's not using so much current and not producing so much power.
The most obvious problem is the check valve at the discharge of 88TK-1 is not working properly. If it weren't fully open while the motor was running then the amount of air being moved by the fan (driven by the motor) would be decreased, resulting in less work requiring less current. BUT, if the motor was running at rated speed, and not moving as much air, then usually the fan housing will get hotter. Maybe there is enough radiation or conduction in the area where the fan is located that the change in housing temperature is not noticeable. Are you sure the check valve for 88TK-1 and 88TK-2 move freely and open and close fully when the fans are running and when they stop?
You mention three exhaust frame blowers. What about the state of the check valve at the discharge of 88TK-3--is it fully closed when it's not running?
What are the currents drawn when you run only 88TK-2 and 88TK-3?
I have seen couplings between the motor and the fan come loose making the fan not spin as fast is the motor shaft and that causes lower than normal current draw. Are you sure the 88TK-1 fan itself is running at rated speed?
Have you inspected the fan itself to see that it's fully intact and hasn't come apart?
Usually the inlets of the exhaust frame blowers have acoustic insulation to make them less noisy when running. Is it possible some of the insulation panels have separated or come loose inside 88TK-1 and are blocking air flow into the fan suction?
Again, the current drawn by an electric motor is indicative of the work being performed by the electric motor. That presumes all the bearing are good (and bad bearings would usually cause higher than normal current draw), and that the motor windings and rotor are all functioning properly (no shorted windings or rotor issues). If that's all good, then it's either the check valves, or the coupling between the motor and fan, or something is blocking the air flowing into the fan suction. There's not too much else it can be.
Please write back to let us know what you find. And if you require further assistance, please answer all the questions above. All of them, please.
Thank you for the reply. I'll answer your questions:
1. "Are you sure the check valve for 88TK-1 and 88TK-2 move freely and open and close fully when the fans are running and when they stop?"
Yes, we did check valve inspection and all check valves are moving freely. They are fully opened when the fans are running and fully closed when the fans are stopped.
2. "What about the state of the check valve at the discharge of 88TK-3--is it fully closed when it's not running??"
The 88TK-3 check valve is fully closed when it's not running.
3. "What are the currents drawn when you run only 88TK-2 and 88TK-3?"
When I run 88TK-2 and 88TK-3, the currents are:
88TK-2 : 30.32 A
88TK-3 : 27.5 A
4. "Have you inspected the fan itself to see that it's fully intact and hasn't come apart?"
Yes, the fan is fully intact. No abnormalities were found.
5. "Is it possible some of the insulation panels have separated or come loose inside 88TK-1 and are blocking airflow into the fan suction?"
We did the inspection on fan suction 88TK-1 and we did not find loose or separated insulation panel
For the update, we also did run test 88TK-1 and 88TK-3. The current results are 25.27A for 88TK-1 and 30A for 88TK-3.
At this point there are two options. Have an electric motor company test 88TK-1 and -2 to see if there are any abnormalities in the two motors (while you're testing, also test 88TK-3 to have more data to compare). This may require removing the rotors of the motors for more detailed testing.
You could also use air flow-rate monitoring equipment on each of the three fans to try to determine if, in fact, the air flows are differ by enough to cause the imbalance in currents you are seeing--though, in my personal opinion, a difference of three amperes is negligible, while a difference of seven amperes is suspicious.
You've done all the easy stuff at this point; if you want to pursue this to a logical conclusion you're going to have to invest more time and money into more detailed data collection. Something changed, and it would seem it was a step change (which is always concerning), and the obvious causes seem to have been eliminated.
One last remote possibility which would be easy to check and eliminate is the state of the motor starter contacts and the motor starter assemblies of the three exhaust frame blower motors. A quick test would be to use a non-contact heat sensing "pistol" to see if there is any unusual heat differences of the various components of the motor starter assemblies of the three motors. Not knowing what kind of motor starter are in use at your site, if possible, it might be a relatively simple job to disassemble the three motor starters to have a visual inspection of the high voltage (440 VAC???) contacts of the three phases of each of the motor starters to see if they are similarly worn or deteriorated and if there might be a high resistance causing low current flow to one of the motors. It is presumed that you have checked the currents of all three phases of all three motors for these tests and are reporting the average or highest values of current for each motor....
This is seemingly an odd situation, but there is likely some explanation--it's just a matter now of how much more time and effort (money) you wish to invest in getting to the root cause. We don't know what the piping arrangement of the system is and it seems unlikely that the discharge of 88TK-1 is significantly blocked versus the other two motors. We don't know how the air flow from the motors is "divided" between the two sides of the exhaust frame. We don't know what happened just prior to this step change in current flows (was it shortly after some maintenance outage--if so, which outage and what was done to the exhaust frame cooling air system during the outage?).
That's about all I can think of without being on site. Three exhaust frame blowers was not typical when I was working as a commissioning engineer so I can't imagine what the piping arrangement looks like. Many people (mistakenly) believe that every GE-design Frame 6B is exactly like every other GE-design Frame 6B--and that's NOT true. The major differences are the auxiliaries--of which the exhaust frame cooling/blower arrangement is one. The exhaust frame blowers used to be mounted on the roof of the turbine compartment; they are not ground-mounted (usually). And, every packager seems to have a different idea of how to supply auxiliaries and what the configuration should be. So assuming that your site's configuration is exactly like every GE-design Frame 6B configuration is not correct (if you assumed that--most people do, especially the one's who write here for help).
Please keep us informed!
You could also use air flow-rate monitoring equipment on each of the three fans to try to determine if, in fact, the air flows are differ by enough to cause the imbalance in currents you are seeing--though, in my personal opinion, a difference of three amperes is negligible, while a difference of seven amperes is suspicious.
You've done all the easy stuff at this point; if you want to pursue this to a logical conclusion you're going to have to invest more time and money into more detailed data collection. Something changed, and it would seem it was a step change (which is always concerning), and the obvious causes seem to have been eliminated.
One last remote possibility which would be easy to check and eliminate is the state of the motor starter contacts and the motor starter assemblies of the three exhaust frame blower motors. A quick test would be to use a non-contact heat sensing "pistol" to see if there is any unusual heat differences of the various components of the motor starter assemblies of the three motors. Not knowing what kind of motor starter are in use at your site, if possible, it might be a relatively simple job to disassemble the three motor starters to have a visual inspection of the high voltage (440 VAC???) contacts of the three phases of each of the motor starters to see if they are similarly worn or deteriorated and if there might be a high resistance causing low current flow to one of the motors. It is presumed that you have checked the currents of all three phases of all three motors for these tests and are reporting the average or highest values of current for each motor....
This is seemingly an odd situation, but there is likely some explanation--it's just a matter now of how much more time and effort (money) you wish to invest in getting to the root cause. We don't know what the piping arrangement of the system is and it seems unlikely that the discharge of 88TK-1 is significantly blocked versus the other two motors. We don't know how the air flow from the motors is "divided" between the two sides of the exhaust frame. We don't know what happened just prior to this step change in current flows (was it shortly after some maintenance outage--if so, which outage and what was done to the exhaust frame cooling air system during the outage?).
That's about all I can think of without being on site. Three exhaust frame blowers was not typical when I was working as a commissioning engineer so I can't imagine what the piping arrangement looks like. Many people (mistakenly) believe that every GE-design Frame 6B is exactly like every other GE-design Frame 6B--and that's NOT true. The major differences are the auxiliaries--of which the exhaust frame cooling/blower arrangement is one. The exhaust frame blowers used to be mounted on the roof of the turbine compartment; they are not ground-mounted (usually). And, every packager seems to have a different idea of how to supply auxiliaries and what the configuration should be. So assuming that your site's configuration is exactly like every GE-design Frame 6B configuration is not correct (if you assumed that--most people do, especially the one's who write here for help).
Please keep us informed!
Dear WTF,
Thank you so much for your efforts and support.
I would like to share an issue is repeated in our plant and if you can please see whether it was possible to get a further assessment.
The running motor 88TK-2_GT-2 current was checked and the replacement of the GT-2 88TK-1 motor during the recent GT-2 outage. It was found that it was drawing a high current, Referring to the nameplate current rating shown bechanged over the motor from 88TK-2 to 88TK-1, and the 88TK-1 motor's running current was measured. Compared with the nameplate current rating shown below, the running current of the 88TK-1 motor was also found high. 88TK-1 running current was monitored for three days, and the current readings remained the same.
Thank you so much for your efforts and support.
I would like to share an issue is repeated in our plant and if you can please see whether it was possible to get a further assessment.
The running motor 88TK-2_GT-2 current was checked and the replacement of the GT-2 88TK-1 motor during the recent GT-2 outage. It was found that it was drawing a high current, Referring to the nameplate current rating shown bechanged over the motor from 88TK-2 to 88TK-1, and the 88TK-1 motor's running current was measured. Compared with the nameplate current rating shown below, the running current of the 88TK-1 motor was also found high. 88TK-1 running current was monitored for three days, and the current readings remained the same.
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@Murtadha AlHajji,
Really not much to work with; we don't even know what Frame size machines you have at your site.
About the only thing I can think of--and I should have mentioned it above--is that the OEM has used many reverse-tangential fans for cooling fans. When they are rotating in the correct direction, they usually draw slightly less than current than motor nameplate rating. However when they rotate in the wrong direction, they draw more current than motor nameplate rating (usually resulting in a Aux. Motor Overcurrent alarm).
The fans, when they arrive on site for installation during original construction/commissioning have a direction of rotation arrow painted on the outside of the fan housing. BUT very often, either during original construction or at some time during years of operation the fans housings and motors get painted, and the direction of rotation arrow gets painted over and forgotten. (Sometimes, there is a small metal tag with an arrow stamped or painted on it--but it usually gets painted over at some point.)
The thing about reverse-tangential fans is that if one looks at the fan impeller and tries to discern proper direction of rotation from the angle of the impeller fan blades they actually rotate in the opposite direction one would think they would rotate in (based on the angle of the fan impeller blades). And if they are wired to rotate in the wrong direction they will draw excessive current--and move less air than they should if they were rotating in the correct direction.
Based on the information provided, that's my best guess. It's always worrying--to me--when I see a direction of rotation arrow painted on a three-phase AC motor driving a fan (or even a pump), because a direction of rotation SHOULD BE indicated on the driven device, NOT the driving device. And when a reverse-tangential fan is the driven device--it will draw more current that the driving motor nameplate.
Let us know what you find out, please--and thank you.
Really not much to work with; we don't even know what Frame size machines you have at your site.
About the only thing I can think of--and I should have mentioned it above--is that the OEM has used many reverse-tangential fans for cooling fans. When they are rotating in the correct direction, they usually draw slightly less than current than motor nameplate rating. However when they rotate in the wrong direction, they draw more current than motor nameplate rating (usually resulting in a Aux. Motor Overcurrent alarm).
The fans, when they arrive on site for installation during original construction/commissioning have a direction of rotation arrow painted on the outside of the fan housing. BUT very often, either during original construction or at some time during years of operation the fans housings and motors get painted, and the direction of rotation arrow gets painted over and forgotten. (Sometimes, there is a small metal tag with an arrow stamped or painted on it--but it usually gets painted over at some point.)
The thing about reverse-tangential fans is that if one looks at the fan impeller and tries to discern proper direction of rotation from the angle of the impeller fan blades they actually rotate in the opposite direction one would think they would rotate in (based on the angle of the fan impeller blades). And if they are wired to rotate in the wrong direction they will draw excessive current--and move less air than they should if they were rotating in the correct direction.
Based on the information provided, that's my best guess. It's always worrying--to me--when I see a direction of rotation arrow painted on a three-phase AC motor driving a fan (or even a pump), because a direction of rotation SHOULD BE indicated on the driven device, NOT the driving device. And when a reverse-tangential fan is the driven device--it will draw more current that the driving motor nameplate.
Let us know what you find out, please--and thank you.
@WTF?
Thank you for your quick response.
We have GE Frame 7FA machines, and we have checked the rotation of 88TK-1 motor is in service....DOR anti clock wise looking from NDE, and 88TK-2 is standby....DOR anti clock wise looking from NDE as well which is the correct rotation.
Thank you for your quick response.
We have GE Frame 7FA machines, and we have checked the rotation of 88TK-1 motor is in service....DOR anti clock wise looking from NDE, and 88TK-2 is standby....DOR anti clock wise looking from NDE as well which is the correct rotation.
The impeller that we have for 88TK can be run in any direction. The one that dictates the actual direction is the housing of the fan.
All our drives rotational direction is seen from drive to driven. This is the normal convention. And direction of rotation is painted on the casing not on the motor.
All our drives rotational direction is seen from drive to driven. This is the normal convention. And direction of rotation is painted on the casing not on the motor.
@Murtadha AlHajji,
Thank you for the information. The fan you describe is something I haven't encountered.
Use the Cooling & Sealing Air P&ID to check all of the mechanical components of the exhaust frame cooling air system.
You didn't say when the problem started--after a maintenance outage? After a trip from load?
What does the turbine-generator packager say--have you asked them for assistance?
Have you considered asking the blower manufacturer for assistance?
Have the AC induction motors been tested by a competent electric motor repair shop?
Is it dusty where these machines are located? Is there any kind of "filter" on the inlet duct of the exhaust frame blower fan?
What was the most recent maintenance outage (CI, HGPI, MI)--and was anything done to the exhaust frame of the turbine during the maintenance outage?
My experience with centrifugal blowers is that the driving motor current draw is a function of the amount air being moved by the fan when running, assuming the piping/pathways are unobstructed as per original design. If more air than usual is being moved by the fan, then the current draw increases. (The exception to this is the reverse-tangential fans previously described.) Blockages--either in the suction of the blower or the discharge, usually, result in lower than normal current draw and excessive heating of the impeller housing (caused by low air movement).
GE did have some kind of performance modification that had some ducts that blew air from a blower on the exhaust shell to prevent irregular expansion during operation; I only ever saw pictures of that modification and didn't have an personal experience with it. Is this what you are referring to?
Anyway, that's all I got. Again, please let us know what you discover as you resolve this problem.
Thank you for the information. The fan you describe is something I haven't encountered.
Use the Cooling & Sealing Air P&ID to check all of the mechanical components of the exhaust frame cooling air system.
You didn't say when the problem started--after a maintenance outage? After a trip from load?
What does the turbine-generator packager say--have you asked them for assistance?
Have you considered asking the blower manufacturer for assistance?
Have the AC induction motors been tested by a competent electric motor repair shop?
Is it dusty where these machines are located? Is there any kind of "filter" on the inlet duct of the exhaust frame blower fan?
What was the most recent maintenance outage (CI, HGPI, MI)--and was anything done to the exhaust frame of the turbine during the maintenance outage?
My experience with centrifugal blowers is that the driving motor current draw is a function of the amount air being moved by the fan when running, assuming the piping/pathways are unobstructed as per original design. If more air than usual is being moved by the fan, then the current draw increases. (The exception to this is the reverse-tangential fans previously described.) Blockages--either in the suction of the blower or the discharge, usually, result in lower than normal current draw and excessive heating of the impeller housing (caused by low air movement).
GE did have some kind of performance modification that had some ducts that blew air from a blower on the exhaust shell to prevent irregular expansion during operation; I only ever saw pictures of that modification and didn't have an personal experience with it. Is this what you are referring to?
Anyway, that's all I got. Again, please let us know what you discover as you resolve this problem.
