Markvguy, reur 18-Dec-06, 6:53 pm... thnx, it certainly cleared things up:

So, as I now understand it, the speed-amp data was that of the motor, while driving the torque converter, with the converter driving the gt rotor. Hence, during acceleration of the gt rotor, the motor is running at about nominal speed except at the higher slip-frequency caused by operating the motor in an overloaded condition for several minutes!

In answer to your specific question... everything I stated (re: rotor damage) is still applicable! Only now, the "growl" or noise I alluded to earlier will be heard at slip-frequency. The "growler" instrument you referred to has nothing to do with the "growling noise" I mentioned. The scope trace of motor current will still show the effect of rotor damage. In fact, if it can be heard, then it can be "observed!" A single-tube fluorescent lamp will flash at twice line frequency. When used to illuminate the motor shaft, then, the loudness of the "noise" emanating from the motor will be synchronized to the slowly rolling image of the motor shaft.

The instrument I alluded to was not the growler of old, but a modern version of the fluoresnt lamp technique. Instead, it uses a clamp-on current measurement to indicate what I described! The one I am best familiar with is manufactured by a Canadian Company... Iris Power Engineering Inc.

An aside, all of the torques developed by the motor, i.e., break-away, pull-up, pull-out, and max, together with their associated current magnitudes, have nothing to do with the driven-load! They are dependent only on the motor's electrical parameters!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Further to my 18-Dec-06:
pmmeeran... provide nameplate data for speed (actual, not synchronous), NEMA design-torque curve if USA design, or DIN design-torque curve if metric, ambient temp (actual and rated), and temp-rise over ambient. Finally, what is typical acceleration time (actual, not%) of gt rotor. With this information, it will be possible to determine the motor's performance, past and present!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

For pmmeeran and markvguy... still further to my 18-Dec-06:

While I discusssed a course-of-action involving calculation, there is yet another, even easier, technique you can use to aid your trouble-shooting efforts. I apologize for having forgotten it.

Search List Archives for thread # 1026165830, dated 24-Jan-03. The topic covered motor failures thought to be caused by "Radial Force on a Radial Bearing!" Only, there was nothing wrong with the bearing. Instead, it was caused by an open-circuited rotor-bar.

What you will find most interesting was the technique to find it!

Regards,
Phil Corso, PE {Boca Raton, FL, USA} [[email protected]] ([email protected])

 

That seems to be just a problem of dirt compressor. Don't mess with the IGVs or other settings unless you are absolutely sure on what you are doing.

The startup of a 6FA turbine is usually very critical.

A good water wash on the compressor will help for sure, because it will increase the overall machine efficiency and allow for a cooler startup.

Also look for spread, if by any chance you may have had a loss of flame.

 

For markvguy, Phil Corso and F Passetto,

This machine is running as Base Load machine and hence at present I am unable to check anything. HGPI is already programmed during february 2007. At that time we will check the Torque converter and Starting motor. Off line water wash also will be done at that time.

Mr. Passetto, can you please tell why you are suspecting the compressor?

Further to this, we are not changing any IGV settings because everybody said it may cause compressor stall.

Can anyone explain how a few degrees opening of IGV may cause compressor stall?

pmmeeran

 

First, on machines as complex as heavy-duty combustion turbines one should not make changes to control system parameters without fully understanding all the possible ramifications. You are wise to ask questions to try to understand the situation.

Second, axial compressor design is as close to rocket science as it gets. It is also fairly proprietary, and most heavy-duty gas turbine manufacturers are very protective of their designs.

Third, one of the limiting factors of heavy-duty combustion turbine output (efficiency/torque output) is axial compressor flow--in other words, the present state-of-the-art of heavy-duty gas turbine axial compressor design is at its limits. Most of the world's heavy-duty gas turbine manufacturers are developing new axial compressors in order to improve the efficiency/output of their existing turbine designs.

As such, most gas turbine axial compressors are currently being operated at the limits of their design capability so trying to increase the air flow through the compressor during starting and acceleration (which is already "problematic", as nearly every unit these days has some kind of "compressor bleed valve" or "pop-off" valve or something similar to protect the compressor against stalling/surging during part speed operation) would likely create additional problems--maybe not the day the change was made, but on a significantly cooler day or a significantly warmer day or after the inlter filters were replaced, etc.

By opening the IGVs "...a few degrees..." during starting/acceleration the air flow through the compressor will increase. For most of the GE-design heavy-duty gas turbine axial compressors, the minimum operating IGV angle during starting and acceleration is 34 DGA (DeGrees Angle)--and you are stating that the Frame 6FA opening during starting/acceleration is less than 30 DGA.

Increasing the air flow will also cause the torque required to accelerate the unit...already somewhat problematic on your unit, eh? On many other non-F-class GE-design heavy-duty gas turbines, the starting means is not providing torque to the unit during starting/acceleration after approximately 60% speed, which means that there is sufficient air flow to prevent exhaust overtemperature during acceleration yet the unit is capable of operation/acceleration without a torque assist from the starting means.

Your Frame 6FA unit has a torque assist all the way up through 90+% speed--which says that the design of the unit is such that the fuel required to accelerate the unit without a torque assist would probably result in very high exhaust temperatures until the unit is very near rated speed.

We have no way of knowing the ambient conditions of your site--the humidity, the proximity of dust-producing sources, the prevailing winds which might blow that dust in the direction of the inlet filters. Filters can only remove particles down to a certain size--if the dust in your area is smaller, it can get through the filters. This author has also seen poorly constructed, sealed and maintained inlet ducts and filter houses which allowed unfiltered air to enter the unit.

One of the questions you did NOT answer was what the inlet air filter differential pressure was--a measure of the inlet air filter cleanliness. Depending on the nature of the particulates in the atmosphere and the amount of humidity (natural or unnatural, such as from an evaporative cooler or "fogger"), compressors can become very fouled. Can you say how much your power output has decreased from rated over the past few years of operation since commissioning?

This author has also seem inlet filters/elements rupture and allow unfiltered air (and particulates) to enter the unit--when, when combined with humidity will certainly cause compressor fowling and a loss of efficiency/output.

Mr. Rahul P. Sharma has just stated in a recent thread that output of their Frame 5 unit increased more than 10% after an off-line compressor water wash--which means that the efficiency of the axial compressor was greatly reduced simply because of the dirt which can accumulate in the compressor.

Stranger things have happened, but with the large reductions in acceleration rate reference, the increases in torque output reference, and the increases in fuel required to maintain even the lower acceleration rate references, it seems unlikely that a fouled compressor--unless it was severely fouled, indicating ruptured filters or worse--would cause the problems you are seeing.

It will be VERY interesting and informative for all if you will keep us informed of what happens after the scheduled maintenance outage, even though it's still several months away.

markvguy

 

The plant is situated 5 km away from sea shore and the surrounding is paddy fields. There is no sand storm and the environment is quite clear. The ambient temperatur during night is 23 to 25 and in day time it is between 35 to 38 deg c.

The inlet air filter differential pressure is normally at 20 mm water column. During heavy raining and fogging it reaches maximum of 60 mmwc.

The rated capacity of turbine is 62.5 MW. Even at minimum DP (20 mmwc) due to higher ampient temperature(38 deg c) the load reduced to 56MW. For the same DP and at 23 deg C the load is 61 MW.

I am working in this power station for the past one year only. So I don't have any past datas for the torque converter. During the last start up we observed the loacl pressure gauges. The filling pressure was 115 psi and housing pressure was 130 psi when the torque converter guide vanes are at 90%.

pmmeeran

 

The presumption is that it is plenty humid where the plant is located. Are there any other industrial areas located nearby (cement plant, refinery, etc.)? Are they upwind or downwind of the prevailing wind direction?

The differential pressures you cited don't seem high, actually they seem kind of low....

The dirt in sand storms would probably be trapped in the inlet filters; most installations in area prone to sand storms have additional equipment/methods to "knock out" the large dirt particles (sand) to prolong the life of the inlet filters (increase the period between replacement) including mechanical vibration, cyclone separators, and "self-cleaning" inlet air filter systems. Some of the more complicated filters have augers at the bottom to carry the larger dirt particles to a collection area for removal.

However, as has been said before, inlet filters will NOT remove all contaminants from the air. And, if any filter element is ruptured or the inlet duct was not properly built/sealed, then unfiltered air can enter the axial compressor--and when combined with high humidity, greatly decreases the effectiveness of the axial compressor. Water washing, particularly off-line water washing, is the best method for cleaning the majority of typical contaminants from the compressor and restoring efficiency lost due to dirt/humidity contamination.

The torque converter manufacturer (Voith?) will be best able to analyze the performance of the equipment.

Again, please let us know the details of the troubleshooting/resolution. This author would be most interested to learn what the OEM/packager recommends regarding the acceleration rate reference should the torque converter be found to be at the root of the problem.

markvguy

 

Pmmeeran,

How about an update on your finding(s)?

Regards, Phil Corso, PE ([email protected])

 

Sorry for the late reply. HGPI for the machine have just completed due to non availability of spares. Off line water wash also done. Nothing done in torque converter side.

After the HGPI during the first few starts the machine tripped on flame failure during acceleration at around 40% of speed.( At firing and warmup condition all scannars sensed flame and during acceleration the machine tripped on loss of flame.)

Suspected less fuel and more air causing the problem. So increased the acceleration FSR rate of raise. Then the machine can reach FSNL. But during acceleration spread is very high and immediately after reaching FSNL machine tripped on Spread.(Because in combustion monitor the spread trip is enabled once the machine reached FSNL).

We tried several times and every time the low exhaust temperature thermocouples are not same. Some times it is 1,2,3,4 or 6,7,8,9 or 16,17,18,19 etc. Suspected the new nozzles have problem or something is blocking in fuel lines which is causing uneven fuel flow. Again checked all the nozzles, fuel lines and seals in combustion chamber. Nothing found abnormal but still the machine tripped on spread.

Finally we restored the acceleration constants and torque converter guide vane position to its original values. That is the acceleration FSR rate of raise increased and torque converter guide vane position decreased. (The reason and how much the constants previously reduced were already discussed in this forum). Then the machine can reach FSNL with out any problem.

After offline water wash the compressor throughput increased considereably. Hence it is concluded that as the fuel flow is less and air flow is very high the machine tripped on High exhaust spread due to uneven air or fuel distribution.

I am very much thankful to Mr. MarkVguy, Mr. Phil corso and Mr. F Passeto for their valuable comments and guidelines.

Thanks for all helps.

 

Thank you for the feedback. Since markvguy stopped posting it seems some individuals don't feel the need to provide feedback. Without it, the information provided is almost useless because we can't determine if the information provided was helpful or not. Anyone can write anything in response to a question or a problem, but without feedback no one else who reads the threads will know if the information was useful or correct.

So, thanks for letting us know what your issue was.

Unfortunately GE doesn't identify valves, except on hydrogen-cooled generators (what's up with that?). So, when people prepare to perform an off-line water wash they usually have to go and search for the valves which have to be opened or closed during the washing. And, then, when the wash and rinse is completed they have to go searching again for the valves to be closed or opened for normal running.

A maintenance technician taught me a good trick for this. Take some high-temperature paint, two colors, such as green and red, and paint the valves which are normally open during running one color and the valves which are normally closed during running another color. Then, when you get ready for an off-line water wash it's easy to spot the valves which must be operated; close the normally open valves and open the normally closed valves. And when the washing and rinsing are done open the normally open valves and close the normally closed valves. We modified our procedure to put numbered tags on the valves, and put spaces on the printed procedure form to check off that each valve had been put in the proper position for washing and then for running. Saves a *lot* of time and makes the whole process a lot more "friendly".

Another thing we learned is that we had to put valves on the low-point drains of the gas fuel manifold and the atomizing air manifold if there wasn't any there from the factory.

I've seen lots of flame-outs after water washes which were caused by water which had not been drained from or prevented from getting into some lines because valves weren't put into the proper positions before the water wash, or water wasn't drained properly after the water wash. As air- and fuel flow increases during starting, the water can be pushed along into the combustors extinguishing the flame and causing a loss of flame trip.

But, in any case, thanks for the feedback. Another important aspect of feedback is that we can all learn more if the problem is something other than what was presumed or presented.

I'd imagine one of two things is happening with markvguy. Either he's still reading the postings on control.com (though he can't post), or he's quit reading altogether. If he's still reading, he'd probably be happy to read some of the things which have recently been written, but he might be saddened by the lack of feedback on turbine-related posts. I know I am; I learn a lot from these posts, especially the replies.

 

Just in a humorous note, MarkVguy might be posting as MarkVIguy or MarkVIeguy these days...

 

We have 6FA Gas turbine advance gas path blade. During start up,6FA Gas turbine frequent tripped on Exhaust temperature High. During that time primary to Lean lean mode transfer all thermocouple temperature suddenly raise from 640 to than 670 and hence Gas turbine frequent tripped on Exhaust temperature High. Before AGP blade upgradation, the above problem not observed. We suggested to OEM, they simply reply Offline water wash to be carried that the above Exhaust temperature High temperature trip not occurs due to compressor fouling. So, Every startup off line water wash was carried out by us. This is not permanent solution. If any torque converter maximum opening set point constant and IGV min opening 27% to 30%., Please be correct solution given to us. Thank you

 

prakash1974, First of all this problem of yours should be opened as a new thread, instead of tagging onto an existing, similar, but not identical thread. I say similar, but not identical because you have not identified the type of combustion system being used on the 6FA at your site nor have you identified the type of control system in use on the 6FA at your site. If you have a digital Mark* turbine control system, can you provide the Trip Log (or Trip History) files when an exhaust over-temperature trip has occurred?

Second, what Process- and Diagnostic Alarms are present during the starting and when the exhaust over-temperature trip occurs? Is the gas fuel supply pressure stable during the combustion mode transfer? Is the P2 pressure stable during the combustion mode transfer? Are the IGVs stable or are they oscillating when the exhaust over-temperature trip occurs? Is the ambient temperature higher or lower at this time of year than normal? Does the unit have IBH (Inlet Bleed Heating)--and if so, is it enabled, working and stable when the exhaust over-temperature trip occurs?

Third, you wrote that performing off-line water washes is not an acceptable long-term solution--which implies that it works to prevent the problem you are experiencing with exhaust over-temperature trips during starting. Is that correct--if an off-line water wash is performed prior to a START the unit does NOT experience an exhaust over-temperature trip during starting?

Fourth, PRECISELY WHEN during the START does the unit trip on exhaust over-temperature? I have very little experience with DLN-2 combustion systems (they are the least common of the GE DLN combustion systems) and what I do recall about it is that they do transition combustion modes at about 80% speed (TNH) BEFORE reaching FSNL. Is this when the exhaust over-temperature trip is occurring? Or, is it after synchronization when the unit is loaded when the exhaust over-temperature trip is occurring?

You would be a VERY lucky person if someone could just read the post you made and be able to precisely tell you what is causing the problem and what you can do to resolve it once and for all. VERY lucky; because you didn't provide very much in the way of information that is useful. Sure; this problem started AFTER the advance gas path turbine upgrade, but that may or not be the root cause of the problem. For all we know, the inlet air filter system at your site doesn't work very well, or the ductwork has gaps and allows unfiltered air and contaminants into the air stream, and the ambient air is full of dust (road dust; cement dust; ???) and/or hydrocarbons , and the unit may use evaporative cooling and there may be a lot of carryover--both of which will contribute to quick fouling of the axial compressor, which will require more frequent off-line water washes to resolve (presuming the inlet air filter system can't be upgraded or repaired and/or the evap cooler carryover problem can't be resolved--which would be the root of this particular problem).

You have been asked several questions, and a couple of them have sub-questions (in particular see the second question)--and they all need to be answered if you want help with this problem. In the future, please open a new thread for a problem like this, and include as much information as you can (combustion system; fuel being burned; type of turbine control system; speed at which the trip occurs; etc., etc., etc.). Help the good people here help you by providing useful data. As much as people want to believe it--every GE-design heavy duty gas turbine IS NOT like every other GE-design heavy duty gas turbine. Even in the same Frame size; there can be different auxiliaries; different combustion systems; different compressors; different IGVs; different hot gas paths (such as your which has been upgraded). No one reading this forum is standing or sitting next to you and knows what you know about the problem you are (so poorly) describing. You want help? We need information.

It's as simple as that.