Initial Torque Required for Gas Turbine

I

Thread Starter

Izhar

Dears

There is some problem with our Gas Turbine (Frame 9E). During the startup our gas turbine cannot start. Starting motor trips on overload and the alarm on Mark VI appears is " Starting means turbine shaft fail to break turbine away".

This problem haunts us always during the startup although sometimes the turbine starts normally. What we do is that we normally jack the turbine from coupling at the generator end(we have a special fixture for it).

I just want to calculate the initial torque actually required by the turbine. Regarding the torque convertor, I can have its maximum output torque from its performance curves. Any calculations that can give me the initial torque of the Turbine.

Looking forward to hear from you guys.

Thanks

--
Izhar
 
Dear,

Is the generator hydrogen-cooled or air-cooled?

Does the unit have a hydraulic ratchet for Cooldown? (If not, what kind of Cooldown method is used: Slowroll, or Turning Gear or ???)

Presuming someone can supply the required starting torque, how will you measure the actual starting torque being applied to the shaft from the torque converter? How will you know how efficient the torque converter is at transmitting the torque from the electric starting motor to the turbine-generator shaft? How will you measure the torque input to and the torque output from the torque converter during breakaway?

It's pretty certain that at during and after commissioning the starting motor and torque converter were capable of transmitting sufficient torque to the turbine-generator shaft to break it away from zero speed and accelerate it up to self-sustaining speed.

So, the question is: When did this problem start?

Torque converters do require maintenance. There was one thread on control.com where a Frame 9E's torque converter was replaced with a refurbished one and the site experienced similar problems starting, and the torque converter had to be replaced with another one because the refurbisher had not not properly re-worked the torque converter.

If the Frame 9E at your site uses the Voith torque converter with variable guide vanes to control the amount of torque being transmitted, are you certain the limit switches of the torque adjustor mechanism are adjusted/working properly? You say it starts sometimes, but not others. This is indicative of a problem with the torque adjustor limit switch operation/adjustment.

The electric induction starting motor should draw somewhere between 150-170% of nameplate rated current to break the shaft away from zero speed and accelerate the unit up to purge speed. Electric motor current draw is directly proportional to the amount of power being provided by the motor's output shaft.

This is the best way to determine if the torque converter is working properly: by measuring the amount of current being drawn by the starting motor. If the starting motor isn't drawing sufficient current (producing sufficient torque) then it's either a problem with the torque converter or the adjustment/operation of the torque adjustor limit switches.

There could also be a problem with the electric starting motor, but that's not common.

The usual cause of this kind of problem is that the torque adjustor mechanism (that positions the variable guide vanes of the torque converter) hasn't properly positioned the variable guide vanes to transmit sufficient torque from the starting motor to break the shaft away from zero speed. Especially if the turbine breaks away from zero speed sometimes, and doesn't others.

There is usually a hand-wheel on the torque adjustor mechanism, which is engaged by depressing a handle on the side of the mechanism. While depressing the handle you can turn the hand-wheel in the OPEN direction to increase the amount of torque that will be transmitted by the torque converter. You can even do this while the turbine is starting. <b>BUT, do NOT turn the hand-wheel in the CLOSE direction while torque is being transmitted. It can damage the torque adjustor mechanism.</b>

There is also usually a OPEN or INCREASE switch on the Torque Adjustor mechanism's motor starter which can also be used to increase the variable guide vane opening. Again, <b>DO NOT</b> CLOSE or DECREASE the the torque converter variable guide vane angle while torque is being transmitted; it can damage the torque adjustor mechanism.

The limit switches of the torque adjustor mechanism are not intuitive. It takes some study to understand how they work, and which switch does what. You will need to have the Torque Adjustor mechanism's MCC (Motor Control Center) starter schematic drawing to help in understanding how they work.

Again, even if you know the required starting torque I would be most interested to know how you would measure the actual torque to be able to know if the required torque was being supplied. Again, it's certain the turbine started reliably (if not, well, that's another issue) for some time before this problem started. It's possible that the torque adjustor limit switches were marginally adjusted during commissioning, but that shouldn't be the case--again, the breakaway and acceleration torque usually results in a starting motor current draw of approximately 150-170% of rated (which means that a 900 HP starting motor would be producing approximately 1300-1400 HP during breakaway/acceleration).

[Before you, or your supervisors, go bonkers over this starting motor "overload"--be aware: This is normal operating procedure. These motors, and torque converters, are expected to do this only on a periodic basis (during starting) and don't run like this for extended periods of time. They are more than capable of surviving decades, with proper maintenance, while operating in this duty cycle.]

Please write back to let us know how the troubleshooting progresses. And, again, I would be most keen to know how to measure torque converter input- and output torque.
 
Dear,

Try searching control.com with the term:

"+torque +converter"

(with the double quotation marks) for more threads with similar issues.

Note that if the Frame 9E at your site uses jacking oil (lift oil) for the generator bearing, be certain that it is working correctly. Air-cooled generators typically used with GE-design Frame 9E heavy duty gas turbines usually have very heavy generator rotors, and some also use tilting pad bearings which require more torque than journal-type bearings.

Again, please write back with more details as you troubleshoot and resolve the problem.
 
Dear Izhar,

I have re-read the original post and am curious about the starting means trip alarm being experienced. As noted previously, the current drawn by the starting motor is much higher than nameplate rating (and that is normal and expected--though anything above approximately 170% would seem to be very excessive). A lot of sites I've been to have had to increase the starting motor overload relay setting to prevent tripping on starting when the torque adjustor limit switches are set properly.

So, another thing to check is the current actually being drawn by the starting motor during breakaway versus the setting of the starting motor overload/protective relay. It could just be that the overload/protective relay setting is marginally low and needs to be increased slightly.

And it could be a combination of a worn-out torque converter, and/or misadjusted torque adjustor limit switches.

Again, please write back with more details and to let us know how the troubleshooting progresses.
 
Dear Izhar,

It could also be that 20TU-1 (the torque converter "pressurizing" solenoid) is intermittently failing, though that wouldn't explain the tripping.

The tripping could be that somehow the torque adjustor limit switch for max torque is out of adjustment (set too high) and it's causing the starting motor overload/protective relay to trip before the shaft can be broken away from zero speed.

So, the most efficient thing to do would be (still) check the current drawn by the starting motor during breakaway and compare it to the overload/protective relay setting. If the current is much above the setting, and the setting is about 160-170% of rated motor nameplate current, then it's likely there is a problem with the torque adjustor max torque limit switch setting.

And, still--please write back to let us know how the problem resolution is progressing.
 
Dear

you should know each bearing load and then compute breakaway torque. normal force (load) is the amount of the journal load that the bearing experiences. and breakaway torque is the amount of torque necessary to turn the rotor after downtime. this calculation must be made to size turning gear motor or torque convertor.
 
Thanks guys for the feed back.

CSA

Please note a slight correction please. Starting motor didn't trip on overload, the starting motor runs for 30 s and if the turbine doesn’t starts from zero then the overall sequence finishes on the alarm i-e Starting means turbine shaft fail to break turbine away
Our generator is air-cooled with Turning gear cool down mechanism.
The problem started in 2011. But as I mentioned earlier sometimes the turbine starts normally. Recently we carried out CI and AA cooler modification, after that the turbine didn’t start normally. Our Frame 9E gas Turbine uses Voith torque converter with variable guide to control the amount of torque. As per electrical the limit switches are working fine however we will carry out all the checks again during next opportunity.

Regarding your suggestion to check the starting motor current, we checked out the history on DCS. The rated current rating of starting motor is 111 A.

I took the history of our recent startup and according to the trends following are some findings:

1) Starting motor starting current is 194 A (speed of turbine is zero and guide vane angle of T.C is 15 degrees which is the minimum angle)

2) After this the starting motor current becomes normal i-e 33 A while the guide vane angle is 15 degrees

3) When the guide vane angle increase to 67 degrees(maximum), the starting motor current increases to 66 A. After that alarm appears (Starting means turbine shaft fail to break turbine away). This current is not 150 % of the rated current of the motor.

4) Also the starting current of 194 A is on 11 kv feeder. Starting motor is 6.3kv and thus its current will be different (340 A, which is 300 %). Same will stand for the 66 A (on 11 KV feeder) it draws when the torque guide vanes is at maximum angle i.e. the actual current of the motor of 6.3 kV would be 115 A.

5) will share the overload settings (however it doesn’t trips on overload)

As soon as I have some findings, I will surely share them with you. Any thing else you want me to check?

As suggested by you I am studying the MCC of the Torque convertor actuator. Will come back to you soon. And am really sorry for my late replies.

Thanks
 
Izhar,

Thanks for the correction. And the feedback.

111 A on a 6.3 KV motor seems high, but with an air-cooled generator GE Belfort probably felt the extra HP/torque was warranted during breakaway and starting.

It's very ..., interesting that the unit was starting fine before the outage, and now it's not. Was anything done to the torque converter or torque adjustor mechanism during the outage?

In my personal opinion, the variable torque converter guide vanes should always be driven to maximum when the turbine is at zero speed (or on turning gear)--this in preparation for a START. If it's taking more than 30 seconds for the torque converter just to get from minimum (15 degrees) to about rated power output (which will likely NOT be sufficient to break the turbine away from zero speed) then I would say it seems there's something amiss with the slew (travel) rate of the torque adjustor mechanism.

In the past, when a START was initiated the starting motor was energized (with 20TU-1 de-energized) and after five seconds (to allow the starting motor to reach rated speed without any load--reducing the inrush current during the start-up of the electric motor) 20TU-1 was energized. If the variable torque converter guide vanes were not already at maximum, as soon as the START was initiated a RAISE was issued to the torque adjustor drive mechanism to increase the available torque when 20TU-1 was energized. On some machines, as was said, every time the unit went to zero speed or was on turning gear the torque adjustor was driven to maximum to make sure it was ready for a start.

So, when you say after 30 seconds the starting means SOMETIMES trips (by the Speedtronic because of a failure to breakaway) it would be helpful to know if that's 30 seconds after the START was initiated, or 30 seconds after the starting motor was started (L52CR goes to a logic "1" when the starting motor contactor is closed and that should start the breakaway timer).

And, it would be very helpful to know what the torque converter guide vane angle is when the breakaway time expires. Because, in 30 seconds after the starting motor is started the torque adjustor should be capable of reaching maximum (which should be about 150% of motor rated torque).

You should be consulting the Control Specification, I think Sect. 03.nn.nn (but I may be wrong about that) for more details on the starting sequence and how the torque adjustor limit switches should be set.

It's not likely that the torque converter isn't transmitting sufficient torque at rated power to break the shaft away, but if the torque converter is "mature" (old) and hasn't been refurbished in some time it could be, but it's not likely.

It's still odd that things were working fine before the outage, and then they were intermittent. So, my question is always: What changed? Sometimes it was something done intentionally; sometimes it was something done unintentionally.

The solenoids typically used for 20TU-1 are pretty robust, but they do fail--especially if the L.O. quality isn't top notch. (New L.O. formulations cause higher rates of varnishing, which lead to higher failures of solenoids through which oil passes, and to increases "failures" of servo-valves--but that's another story/thread).

Please do keep us informed!

[The inrush current drawn by an induction electric motor can be several times rated--that's why large H.V. motors shouldn't be started more than two-and-a-half times per hour. It's not important to the discussion here (the inrush starting current). The only current that matters is the current drawn when trying to break the shaft away from zero speed and during acceleration.]
 
Dear CSA

Thanks for your reply.

1) For general information can you please tell me why the motor has high rating due to the generator being air cooled?

2) What impact has the type of bearing on the initial torque requirement of Gas Turbine?

a) It's very ..., interesting that the unit was starting fine before the outage, and now it's not. Was anything done to the torque converter or torque adjustor mechanism during the outage?

Ans: No nothing was done on Torque converter during this outage. This problems haunts us but not in every shutdown. GT started normally after the HGPI in 2013. After that during this CI the problem started again. Before HGPI the same problem existed but not in every startup.

b) So, when you say after 30 seconds the starting means SOMETIMES trips (by the Speedtronic because of a failure to breakaway) it would be helpful to know if that's 30 seconds after the START was initiated, or 30 seconds after the starting motor was started (L52CR goes to a logic "1" when the starting motor contactor is closed and that should start the breakaway timer).

Ans: 30 seconds is the overall time when the starting motor starts till it trips on the mentioned alarm. The Torque convertor guide vane angles reaches maximum value in 20 seconds and then remains there for 10 more seconds, after that the alarm appears(failure to breakaway). This becomes a total of 30 seconds duration.

c) And, it would be very helpful to know what the torque converter guide vane angle is when the breakaway time expires. Because, in 30 seconds after the starting motor is started the torque adjustor should be capable of reaching maximum (which should be about 150% of motor rated torque).

Ans: The guide vane is at maximum when the breakaway time expires and the current of starting motor is 66 A on 11 kv feeder(115 A is the actual current of motor)

If the torque converter is not the issue than what can be? I am still trying to get the starting motor current values for previous successful startup and then I will compare both the trends i-e the trend of successful start with that in which we used the jacking system.

Regarding the Torque converter, voith provided me with the performance curves. I guess I can find the torque transmitted by the T.C (Initial torque). Its simple I think, graph is provided between output torque, input power, output speed and guide vanes turns. If the output speed is zero, input power is 1 MW (rating of starting motor) and the total turns are maximum i-e 238 turns then I can get the output torque which is around 20,000 Nm. But the problem is how do I know about the initial torque actually required by the Gas Turbine?

Thanks

--
Izhar
 
Izhar,

GE-design Frame 9E heavy duty gas turbines previously used hydrogen-cooled generators which were physically much smaller for the same rating. The rotors of hydrogen-cooled generators versus those of air-cooled generators for the same rating are much lighter and smaller, so the bearings don't have to carry as much load as for an air-cooled rotor of the same rating. Usually, hydrogen-cooled generators provided with GE-design heavy duty gas turbines have journal bearings, which have a low coefficient of friction compared to the tilting pad bearings usually provided with air-cooled generator rotors supplied with GE-design heavy duty gas turbines and which have a higher coefficient of friction and require more force during breakaway and while on turning gear.

Some air-cooled generators provided with GE-design Frame 9E heavy duty gas turbines require jacking, or lift, oil which is high pressure lubricating from an additional pump and helps to push the rotor up off the bearing making it slightly easier to rotate and break away from zero speed during starting and acceleration. This requires a separate pump and oil supply system, which slightly increases complexity and maintenance costs. If not working properly, a jacking, or lift, oil system may be contributing to the cause an intermittent ability to break away from zero speed during starting.

Your troubleshooting and problem resolution hinges on <b>one</b> thing at this point: Knowing what current is drawn by the starting motor when the variable torque converter guide vanes are at maximum (breakaway torque; purge torque; acceleration torque). Until you know that value you will NOT know if the torque converter is transmitting the amount of torque required to break the turbine away from zero speed (and I doubt very seriously if anyone here--with the exception of someone who currently works for GE Belfort or someone who recently worked for GE who might have had access to that information or still has access to that information) is going to be able to tell you with any precision what the breakaway torque requirement is.

You say it works sometimes and it doesn't work sometimes. You need to very closely analyze the data from those times when it does work to those times when it doesn't work to see what the difference is. And, I'm going to go on record as saying that the torque adjustor limits switches are not adjusted correctly and/or the dynamic brake of the torque adjustor mechanism isn't work correctly or the torque converter and/or the torque adjustor mechanism is damaged and isn't working correctly.

If the Speedtronic has a variable torque converter guide vane indication from the torque adjustor mechanism then I'm going to take this a step further and posit that it also has a starting motor current input from a transducer or shunt in the starting motor circuit to monitor actual starting motor current when the starting motor is running. I've seen this before--and I've see GE Belfort use the starting motor current feedback/input to adjust the variable torque converter guide vanes when the starting motor is running and 20TU-1 is energized to change the amount of torque being applied to the turbine-generator shaft. This is fine--as long as the Speedtronic only INCREASES/RAISES the amount of torque while 20TU-1 is energized and torque is being transmitted to the turbine-generator shaft!

If the Speedtronic DECREASES/LOWERS the amount of torque while 20TU-1 is energized and torque is being transmitted from the starting motor to the turbine-generator shaft then it's very likely that the torque adjustor shaft or components have been damaged over time. This is because closing the variable torque converter guide vanes to reduce the amount of torque while oil at very high pressure is flowing through the guide vanes requires a LOT of force--and the internal components of the torque adjustor mechanism isn't designed to apply that kind of force (to close the guide vanes when high-pressure oil is flowing through the guide vanes) without bending and/or damaging the internal components of the torque adjustor mechanism. This practice was supposedly stopped, but people in some parts of the world who want to unnecessarily increase the complexity of control systems and don't learn lessons the first or second or third or fourth time because they firmly believe their heritage endows them with special powers to keep making the same mistakes over and over and over without any repercussion don't re-think the "solution"--except to say, "It should work! On paper, it works fine!" But, in reality it <b>doesn't</b> work.

Now, I'm not saying this is what is happening at your site--I'm just saying that you are taking a very 'hands-off' approach to gathering data for this troubleshooting exercise. It's most likely possible for someone to stand in front of the starting motor starter cubicle and monitor the protective relay of the starting motor when it's transmitting torque and read the current being drawn by the motor. Another person can be stationed in front or near the torque adjustor mechanism to observe and report on the guide vane angles during starting. These two people, using radios, can relay their information to one person in the control room who is writing down their observations.

Further, it should be possible to station a third individual in front of the torque adjustor drive motor's motor starter to observe when the Speedtronic is issuing RAISES and LOWERS to the torque adjustor drive motor, in an effort to understand if the Speedtronic is trying to lower (decrease) the torque while 20TU-1 is energized.

The length of time for the failure to breakaway alarm could also be increased for troubleshooting purposes to see if just letting the starting motor have 10 or 15 more seconds would help the problem.

Also, it would be helpful to know if the calibration of the feedback from the torque adjustor position indicator is correct so that one knows of the indicated position on the HMI is equal to the actual position of the guide vanes.

Combine all of this with data gathered from the Speedtronic and you would have some pretty good data for analysis. And, I suggest someone have a look at the analog inputs to the Speedtronic to determine if there is a current signal from the starting motor as suspected, and add that to the signals being monitored.

I would even suggest having someone stationed at the torque converter during starting who could manually operate the torque adjustor mechanism to open the guide vanes while the turbine is trying to break away from zero speed to see if increasing the guide vane opening helps at all--while some is monitoring and recording the starting motor current which should be increasing as the guide vanes are being manually opened. Again, add 15-30 seconds to the failure to breakaway timer for this test.

If it works sometimes and it doesn't work others then the presumption has to be that something about the position of the variable torque converter guide vane positions isn't the same during the two starting attempts.

Another thing that helps with torque transmission is oil temperature. The colder the oil the more torque that's transmitted; the warmer the oil the less torque that's transmitted. So, can you look at your archival data and see what the oil temp's were when the turbine broke away versus when it didn't break away?

But, all of this calculation stuff isn't going to solve this problem any time soon. If it worked in the past, and it works intermittently now, knowing how much torque is required or even being able to estimate how much torque is being transmitted versus how much torque is required <i>is not going to solve the problem.</i> Because the problem is: Torque transmission is intermittent and something about the way the torque transmission is being controlled isn't uniform or the setting of the switches which control torque transmission is incorrect or borderline. And a calculation isn't going to pinpoint the source of the problem--it's only going to confirm there is a problem--which is not a point of discussion as it's not working properly and everyone can see that it's not working properly.

Or, the torque adjustor mechanism and/or the torque converter need refurbishment. Or, the 20TU-1 solenoid is in need of cleaning, repair or replacement because it's not properly pressurizing the torque converter cavity.

But, calculating required torque and estimating actual torque is only going to lead you to the same place: Something isn't allowing sufficient torque to be transmitted under every start attempt. Pure and simple. You can calculate it as much as you want and confirm what is already known (it's not working properly)--but at some point, someone is going to have to start eliminating possible causes until the actual cause is found and resolved.

And we would really like to hear about that resolution sooner rather than later.
 
Izhar, and others following this thread,

I need to qualify a statement I made in the third paragraph of the response above:


>... Until you know that value you will NOT know >if the torque converter is transmitting the
>amount of torque required to break the
>turbine away from zero speed ...

What I meant to say is: "You need to observe the amount of current required to actually break the shaft away from zero speed" instead of trying to calculate how much current is required.

To do this is probably going to require some manual steps, as well as at least one person strategically placed to observe current at the motor starter/protective relay--even if there is a starting motor current input to the Speedtronic panel (to compare against the starting motor current input to the Speedtronic panel--which I'm still presuming is present on this panel).

Someone familiar with engaging the manual handwheel of the torque converter adjustor mechanism and knows which direction to turn the handwheel to increase the torque output should be in position and at the ready to manually increase the torque during a test.

Someone should also be at the torque adjustor drive motor's motor starter (usually 88TM-1) to observe the signals coming from the Speedtronic <b>AND</b> to switch the HAND-OFF-AUTO switch to OFF when told.

Both of these people should have radios and be capable of hearing and responding (it's usually pretty noisy in the Accessory Compartment when the starting motor and Aux. L.O. and -Hydraulic pumps motors are running.

The turbine should be in CRANK mode and at zero speed. I would recommend increasing the 'Failure to Breakaway' alarm timer to 60 or even 90 seconds for this test.

We don't know what version of Mark* Speedtronic is in use, but it should be possible to trend some data during this test. L4TML and L4TMR are the usual discrete output signal names being used to drive the torque adjustor drive motor to lower and increase torque (respectively). These should be trended during the START.

You say there is some indication of the torque converter's variable guide vane position; this signal should also be trended during the START.

If there is a starting motor current input to the Speedtronic, that should also be trended.

The starting motor RUN/STOP signal (Master Control Signal) should also be trended; the signal name is usually L4CR or L4CR1X or something similar. The cranking motor starter status should also be monitored; the signal name is usually L52CR.

The status of the signal for the torque converter "pressurizing/fill" solenoid, 20TU-1, should also be trended. The signal name is usually something like L20TU1X.

The turbine shaft speed should also be trended; TNH and TNH_RPM (percent of speed and RPM).

When the START (in CRANK mode) is initiated the person observing the 88TM-1 motor starter needs to indicate what's happening; the RAISE or LOWER ind. light should illuminate--hopefully the RAISE light will be lit for some time and the LOWER will NOT be lit at all, but .... Record the sequence of lights. The person observing the cranking motor amps at the motor's starter/protective relay should be recording or reporting the current draw--even if it's increasing. Any decrease in current draw should be noted during the START test.

The person at the torque converter's manual handwheel should be ready to engage the handwheel, and if after 30 seconds or so the shaft hasn't started to move then the person at the 88TM-1 motor starter should be directed to put the starter H-O-A switch into OFF AND the person at the torque converter should be directed to start opening the torque converter's variable guide vanes with the manual handwheel. The person monitoring the starting motor current draw should observe an increase in current flow, and report/record the same.

The person at the torque converter should be opening the variable guide vanes at a reasonable rate (not too fast; not too slow) and at the instant the shaft breaks away from rest (zero speed) he should stop turning the handwheel.

At this point the turbine-generator shaft should be accelerating in speed, and I would recommend letting it continue to accelerate until the speed settles (levels) out, and then recording that speed. At this point, the test is complete, trending can be discontinued and all field personnel can be recalled with their data and observations for analysis.

If the turbine doesn't break away from zero speed and the handwheel reaches the full open mechanical stop, the person turning the handwheel should indicate such and NOT apply excessive force to the handwheel--when it stops turning the indicator on the torque adjustor mechanism should be at about maximum open and it should be recorded and reported. At this point, the person observing the starting motor current should also be asked to report the current value. And, then a STOP should be initiated (if the unit hasn't already annunciated a 'Failure to Break-away' alarm).

Please assemble the data from the test in a simple table form and paste it to this thread for us to view and analyze and comment on. It should be very helpful.

Also, if you would please tell us what version of Speedtronic the turbine is using that would be helpful.

Lastly, if you would tell us if, in fact, there is a starting motor current input to the Speedtronic that would be very helpful.

Thanks--and it's hoped this will be helpful in getting actionable data (hard numbers instead of anecdotal observations from the HMI) to solve this problem.
 
CSA...

"At this point, the person observing the starting motor current should also be asked to report the current value..."

A motor's starting or "breakaway" torque is determined by the motor's parameters! Hence starting-current does not vary for most of the runup duration!
 
Dear all

Thanks alot for the feedback! CSA thank you very much for your responses. I was away from office for quite some time therefore wasn't able to reply. I have well noted all the points and will carry out the checks you mentioned sooner as we have HGPI planned in November. I also had conversation with the vendor and they have also given some recommendations. I will carry out all the checks sooner, will definitely let you know about the result.

Thanks again and please accept my apologies for late reply.

Thanks in advance

--
Izhar
 
Why do you use a special fixture to jack the turbine at the generator coupling? Are turbine starts usually successful after you do this?

Do you have lift oil? Have you measured the lift. You can use your BN proxies for this.

Our 9E cranking motors draw 160amps during start/purge. If 88CR draws this current and the shaft fails to breaker away we look straight at the lift oil system.
 
Dear 309Guy,
I think you are confusing this jacking with the bearing jacking. its not same.

We inserted pipe in flanged coupling holes of generator coupling and jacked it upward, which helped in rotation of turbine. We jack turbine in this manner and then we give start command and turbine start after this. This means we aid breakaway of turbine with extra torque provided by jack.

We do have lift oil pump. they were running but we didn't checked value of lift with BN proxies. Next time we will.

If 88 CR draws 160 Amps current this means 1450KW power is provided by motor which is even greater than rated power of torque converter. Rated power of torque converter is 1200KW at 0 rpm.
 
Farhan,

No; 309EGuy is not confusing this coupling jacking with jacking oil (bearing lift oil); he realizes the two are very distinct things. He's just curious why it was necessary to use a jack on the coupling to help the turbine break away from zero speed--and why the torque from the starting means (via the torque converter) wasn't enough. It's dangerous to do this hydraulic jacking on the coupling bolts.

And, as was written above and noted by 309EGuy: If the jacking oil (bearing lift oil) pressure isn't correct then the force required to break the shaft away from zero speed will be higher than normal. That's the whole purpose of jacking oil (bearing lift oil)--to provide a "lift" using oil pressure to get the shaft up off the bearing so that it will turn easier and not damage the bearing or the shaft.

Most of the GE-design heavy duty gas turbines I have worked on with jacking oil (bearing lift oil) have had locking (keyed locks) pressure regulators to adjust the jacking oil (bearing lift oil) pressure. They are meant to be unlocked with the key, adjusted, and then locked in place once the pressure has been set. (This is done because vibration has a tendency to cause the pressure setting to change if not secured in some manner). Sometimes the B-N proximity probes are used to measure the lift instead of monitoring pressure--it's essentially the same thing, so much pressure over so much area results in so much lift. (The lift is usually determined during commissioning when the pressure is monitored when setting the pressure regulators before locking them in place.)

And, Farhan, are you saying that someone has actually monitored starting motor current during a start and found it to be 160A?

Before doing any of the recommended checks of the torque converter <b>the jacking oil </b>(bearing lift oil<b> pressure should be checked and adjusted, if necessary.</b> That's a critical part of the starting scheme and the torque converter could be damaged if the torque adjustor were incorrectly set to compensate for the lack of jacking oil (bearing lift oil) pressure. Not to mention that the starting motor overcurrent relay might operate if the torque adjustor were not set correctly.
 
Dear all

Thank you all for your support. I have been successful in overcoming the problem.

What we did was that we checked the lift of Gen Brgs, we observed that the lift was not in the recommended range i-e it was 0.01 mm at the turbine side and 0.065 at the non drive side. We lifted the turbine by increasing the flow of jacking oil and the turbine started successfully. Now this fact is established that the problem is with lift of the generator. What we will do now is that we will increase the jacking oil pressure from 19 Mpa to 24 Mpa to get the desired lift to avoid any unsuccessful start up in the future.

CSA, special thanks to you. Looking forward for your support in the future as well.

Thanks

--
Izhar
 
izhar/Farhan,

Thanks very much for the feedback! "Feedback is the most important contribution!"(c) here at control.com; it's what let's others who read these threads know if the information provided was helpful--or not. It really is what sets this forum apart from so many others, getting the feedback so others know how problems were resolved. These threads are (or should be) read by many people over time, and so we're not just helping the original poster, but we're helping many others solve similar problems or just increase their knowledge and understanding of the equipment and concepts.

It was 309EGuy who specifically suggested checking the bearing lift oil pressure/lift and the method (using the proximity probes (we presumed you used the prox probes; maybe you used dial indicators?); he deserves the accolades for helping to resolve this issue. <b>Thanks to 309EGuy</b> for his contribution/suggestion!

I'm hoping you can tell us about how the lift oil pressure is adjusted on the generator bearings at your site--that is, are "keyed" pressure regulators used at each bearing, or at the supply to the lift oil manifold? Are there pressure gauges on each bearing for lift oil pressure, or are there taps available for installing lift oil pressure gauges on each bearing?

Also, is the lift oil pressure specified in the Device Summary drawing provided with the unit?

Thanks again for the feedback!
 
Yes special thanks to 309EGuy. Pressure regulators are present on the pump body, I guess we can easily increase the pressure by adjusting the relief valve. Will let you know about the outcome.

309EGuy, can you please tell me what pump model do you have at your site and what are its current setting? How much do you increase the setting? How much is the total lift in each bearing of the generator?

CSA -- We don't have separator regulators at each bearing instead we have the regulators at the pump body. And we do have pressure gauges at the discharge.

Thanks

Izhar
 
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