"high exhaust spread"

F

Thread Starter

FSX

We are operating two GE frame V MS5001P gas turbines equipped with dual fuel nozzles. The turbines are currently being operated on Gaseous fuel.

On one of our turbines exhaust spread has increased to 37 degrees Celsius.

What problems can occur if turbine is kept operating with high exhaust spread?

Should we shutdown the turbine if exhaust spread exceeds allowable value?

This turbine has undergone combustion inspection in March. During that activity all fuel nozzles, combustion liners and crossfire tubes were replaced. Also on this turbine atomizing air compressor has been removed and fuel nozzles purge air and atomizing air inlets are seal welded.

Can this problem occur because of cracked fuel nozzle cap due to hot gas back flow into atomizing air passage?
 
Hai FSX,

Well, what is value of spread 2 and spread 3?

Whether one thermocouple is reading high or the region is having different readings? If one thermocouple is more or less then probably thermocouple fault.

take care
G.Rajesh
 
This is not the case of one thermocouple reading low or high. 2ndly the spread is gradually increasing. It looks as if it is not thermocouple fault.

Any experience sharing regarding whether we should operate the turbine with high exhaust spread (if so then up to what limit) will be very helpful
 
D
> What problems can occur if turbine is kept operating with high exhaust spread?

You ignore the origin of this problem, it can be a serious problem especially it occurs after a combustion inspection therefore you must pay a special attention to it and don't keep the turbine running with this alarm

> Should we shutdown the turbine if exhaust spread exceeds allowable value?

In the control system code certainly there is a threshold of exhaust temperatures spread which trip the unit once it is reached

Perform a boroscopic inspection on the nozzles and buckets as soon as possible and verify that all exhaust thermocouples are reading properly,
 
When did this problem start?

After the Main AA Compressor was removed and certain systems were seal-welded.

We don't know where the seal welding was done, nor what kind of back-flow problem might be possible or even has occurred.

One of the purposes of purge air is to cool the passages it passes through. Now, the cooling has been removed.

We don't know exactly what portion of the fuel nozzle is experiencing the problem.

But one thing we do know: The problem started after the Main AA Compressor was removed and some passages were blocked by seal welding.

One thing we can't tell you is that you can operate this machine indefinitely in this condition without major harm being done to hot gas path components. The purpose of monitoring exhaust thermocouple spreads, and the adjacency of high- and low exhaust T/Cs is to protect the rotating turbine buckets from experiencing rapid temperature changes each time the bucket passes a combustor which is either flowing too much fuel in relation to the combustors on either side of it or is not flowing as much fuel as the combustors on either side. This results in the turbine buckets being heated or cooled for a brief period each time they pass the area where there is an excess or fuel or deficiency of fuel.

(Spreads can also occur when a crack in a liner or transition piece or combustion liner hula skirt allows excess compressor discharge air into a combustor, causing a cooler than normal combustion gas flow.)

The rapid thermal cycling of turbine buckets (and wheels) which occurs when there are large spreads will eventually lead to premature turbine bucket failure, and subsequent catastrophic damage to other buckets, nozzles, and shroud blocks (and sooner than later if the spreads are large enough).

Someone, preferably someone with some experience, should come to site and review the changes site made when removing the AA Compressor and seal welding passages and make some determination about whether it was done properly or not. We can't do that without being able to examine marked-up Piping Schematics (P&IDs) and to see photographs of the damaged nozzles, and even then we still might not have enough information to make a proper recommendation.

If you think you will never run liquid fuel again, you should consider having a reputable firm provide you with gas fuel nozzles without AA- and pure air and liquid fuel components and passages.
 
Dear CSA thanks alot for your detailed reply. Fuel nozzles were seal welded at following points
Atomizing air inlet flange
Liquid fuel inlet connection
Purge air inlet connection

Regarding the back flow problem we also believed that seal welding fuel nozzles at above points will cause back flow of hot gases into atomizing air and liquid fuel passages due to total absence of atomizing air and purge air but we had to carry out this modification in emergency. However we did consult OEM before carrying out this modification. They said that with atomizing air inlet, liquid fuel inlet and purge air inlet passages seal welded, atomizing air and liquid fuel passages will become "death cavities filled with compressor discharge air." We are unable to understand this comment. Can any one help understand this?

Please note that we do not have any standby power source and therefore shutting down the turbine will cause us heavy production loss. It will be a great help if some one can advise any value of exhaust spread where turbine must be shut down
 
I don't believe they meant "death cavities"; more likely they meant "dead cavities pressurized with compressor discharge air".

To my mind, if all of the passages were seal-welded at the flanges on the fuel nozzle there should be no path for hot combustion gases to flow through. They air trapped in the passages will just be compressed to match compressor discharge pressure. Any path for hot combustion gases to back-flow through to an area of lower pressure would likely result in damage to the component/passage. Again, one of the primary purposes of purge air is also to cool the passages through which it is passing.

Without being able to see the damage to the fuel nozzles which you say has occurred and which seems to be causing the high exhaust temperature spreads it's very difficult to make any comment on what's causing that. I've seen some very "dirty" gas fuels (contaminated with various liquid products including lubricating oil (likely from the compressor(s) being used to compress the gas), hydraulic oil (likely from the hydraulic valve operators), and even gasoline (don't know how that got into the gas fuel pipeline but it was there!). These contaminants can cause coking of the nozzles which can lead to damage to fuel nozzle components. But, again, without being able to physically examine the nozzles it's very difficult to say what's causing the damage.

Now, with respect to what spread you can operate safely at, well, you need to monitor the allowable exhaust temperature spread (usually signal name TTXSPL) and your three exhaust temperature spreads (TTXSP1, TTXSP2, & TTXSP3). When the magnitudes of the spreads and adjacency reaches the allowable limit the unit is going to be tripped. That's the maximum allowable limit.

There are two calculated arrays used in determining when a GE-design heavy duty gas turbine will be tripped on high exhaust temperature spreads: TTXD2_nn and JXD_nn. The first is a sort of exhaust temperatures from highest value to lowest value (beginning at 00). The second is a sort of the location of exhaust temperatures from highest value to lowest value. So, in other words, if the highest exhaust temperature is 489 deg C at location 5, then TTXD_0 would be equal to 489 and JXD_0 would be equal to 5. If the second highest exhaust temperature value was 486 deg C at location 4 then TTXD_1 would be equal to 489 and JXD_1 would be equal to 4. Presuming a total of 13 exhaust T/Cs, if the lowest exhaust temperature value was 402 deg C at location 6, then TTXD2_12 would be equal to 402 and JXD_12 would be equal to 6.

By comparing the various exhaust temperatures and their locations to each other is how the "combustion monitor" determines when a turbine should be tripped. Generally, it requires the magnitude of the difference between the highest and the three lowest values AND for the locations to be adjacent (next to each other) for the turbine to be tripped. This has been covered in detail in other threads on control.com.

But, if you just run the unit until it trips, it won't be the best operating condition for the turbine. But, the combustion monitor function should protect the turbine--unless you keep re-starting the turbine without attempting to resolve the problem(s) which caused the spread and keep subjecting the turbine buckets to the cyclical (approximately 5100 RPM) heating/cooling and keep letting the combustion monitor trip the turbine then eventually the turbine buckets are going to fail. And then, you will have even bigger problems to deal with, and the loss of generation will last much longer.

So, it would seem that the OEM has told you there should be no problem to operate the unit without Atomizing/Purge Air. That would tend to indicate the problems are not related to the lack/loss of Atomizing/Purge Air. That would suggest the problem is caused by some other factor(s), possibly including contaminants in the gas fuel.

The OEM should have something called a "swirl angle chart" for the turbine you are operating. By knowing the IGV angle and the load at which the turbine is being operated and the location of the highest and coldest exhaust temperatures a swirl angle chart can be used to try to approximate the combustors/nozzles which are causing the high spread. You could use this information to try replacing those suspect nozzles when the turbine does trip on high exhaust temperature spread, thereby "buying" yourself more time before the next high spread trip.

But, that's pretty hard on a turbine. Frame 5s are pretty robust machines, though.

This is not a recommendation to do or try anything, nor any kind of warranty or guarantee of suitability for recommendation for any particular purpose. You should really solve the <b>root cause</b>--which may or may not be related to the changes to the Atomizing/Purge Air systems.

Write back to let us know how you proceed.
 
High spread problem of GE frame 5 gas turbine. We are operating GE frame 5 gas turbines model MS-5001P. Dual fuel nozzles are installed in turbine but we are operating our turbine on gas fuel. Recently we have removed the atomizing air compressor due to excessive seal leakage and after OEM consent we have installed welded plugs on fuel nozzles on atomizing, purge and liquid fuel passages to avoid back flow of combustion gases.

In recent combustion inspection we observed burning and erosion on nozzle cap portion (atomizing air passage). We have changed all fuel nozzles the installed nozzles leak and flow test were carried out before installation and found ok.

After combustion inspection exhaust spread was 17 OC at load of 9.0 MW. But spread changed to 27 OC when load increased to 13 MW. Now a day's exhaust spread was in the range of 31-33 OC at same load of 13.0 MW. Following are the exhaust thermocouple readings at load of 12.8 MW

TT-XD-1 OC = 416
TT-XD-2 OC = 410
TT-XD-3 OC = 404
TT-XD-4 OC = 412
TT-XD-5 OC = 427
TT-XD-6 OC = 441
TT-XD-7 OC = 431
TT-XD-8 OC = 424
TT-XD-9OC = 413
TT-XD-10 OC = 415
TT-XD-11 OC = 412
TT-XD-12 OC = 428
Average exhaust temperature = 419
Change in spread = 32 OC
MARK-V allowable spread lie above 45 OC
Axial flow compressor discharge pressure = 7.51 kg/cm2
Axial flow compressor discharge temperature = 339 OC

Please suggest whether this change in spread is normal? If abnormal what the outcomes are if we will operate the turbine on this same spread of 31-33 OC?

As we are familiar that the by blinding of atomizing air passage cooling air was blocked in fuel nozzle but it can't change the gas flow from gas tip of fuel nozzles to cause change or improper combustion.
 
Dear CSA thanks for your help.

Can we connect atomizing air compressor suction and discharge lines with atomizing air compressor removed and use original dual fuel nozzles so that compressor discharge air will directly be used for cooling of fuel nozzles?
 
<b>From the information provided,</b> I'm not convinced the "burning" is caused by the lack of purge/cooling air. I believe that if you try what you've suggested you will most certainly create a path for the back-flow of hot combustion gases through the AA passages.

As an alternative suggestion, why don't you work with the supplier or refurbisher of your fuel nozzles to identify a gas fuel-only tip?

In my experience with lack of purge air the damage to the fuel nozzles caused deformation and warping of the AA "cone" tip which resulted in eventual loss of the tip which, very fortunately, did not result in damage to the first-stage turbine nozzles or rotating turbine buckets. The warped AA cone tips were not blackened, which is what seems to be reported at your site (correct us if we are wrong about that impression). If there is burning (which I have interpreted to mean blackening and/or black carbon deposits) of the fuel nozzle tips, then I would suggest the gas fuel has some carbonaceous liquid component that is condensing due to the temperature drop across the gas fuel nozzle orifices and is burning and leaving deposits that are causing uneven fuel flows and exhaust temperature spreads.

Or, the dew point of the natural gas is such that there is condensation of liquids which are burning , incompletely, and leaving black black deposits.

But, again, this has not been confirms and is only the interpretation I have made from the use of the word burning to describe the damage you have observed on the fuel nozzle tips. (What a difference one word can make!)

If you believe there are hot combustion gases causing the "burning" and NOT contaminants or condensates in the gas fuel, then I would suggest sourcing and installing gas fuel-only tips on your fuel nozzles instead of the effort involved with re-piping and most likely creating back-flow passages which would most certainly result in fuel nozzle, or worse, damage. I've seen AA manifolds glow orange/red from the back-flow and circulation of hot combustion gases--and it lucky it was found because the manifold piping had already started to deform.

Unless I have misunderstood the problem, I don't believe I can add anything further to this thread--<b>EXCEPT</b> to ask that you update the thread with your progress and experience as you work to resolve the problem. it's this kind of feedback which makes these threads on control.com so valuable to readers--now and in the future. Thanks in advance for providing valuable feedback!
 
Dear CSA Thanks for your reply

regarding your reply of back flow of combustion gases we have inspected the seal welding of plugs by dye penetrant test and all the welded plugs of atomizing air passage were found ok.

secondly out of 10 fuel nozzles we have observed that 03 nozzles have heavy material loss of AA nozzle cap while the rest have minor burning on AA Nozzle cap.

while no damage was observed on gas tip/swirl tip of fuel nozzles however slight deposition of burned liquid was present inside the gas tip but it was only found on 02 nozzles of chamber#5 &6. but the deposition was not of such that it would blocked the gas tip holes and caused high spread.

can we improve the material of AA nozzle cap as existing nozzle cap material is SS 420 as i have visited on site where frame 9 turbine is in operation with same modification and there is no such problem as there material is HESTALLOY.

as gas fuel is flowing from separate passage from fuel nozzle and came out from gas tip then by burning of AA nozzle cap there should be no problem with spread.

can you give some opinion that why burning of AA nozzle cap causing high spread?
 
It would appear that the OEM who told you this was possible was at least partially incorrect.

The only reason I can offer for the damage you are seeing is that the temperatures in the area of the AA cone tip are just too high. Is it that there is no air ("purge") air flowing through the AA passages serving not only to cool the AA passage components and the AA cone tip, or that the flow-rate of the air serves to move the gas fuel flame "ball" a little further from the fuel nozzle and AA cone tip? I would guess it's a little of both.

There may be just enough of an opening and some condition that is being set up where hot combustion gases are circulating into and out of the AA passage of the nozzle tip. This doesn't seem likely, but somehow heat is causing warping which is causing more problems leading to high spreads. And, again, this problem seems to have started after the AA compressor was removed.

Unfortunately I don't have access to any fuel nozzle drawings, but if I remember correctly the gas fuel tip screws on over the AA cone. What I don't recall is how the gas fuel makes it way to the orifices of the gas fuel tip, if there is some interference fit of the gas fuel tip around the circumference of the AA tip. If the AA tip deforms and this interference fit opens then gas fuel would leak out of the gap and possible exacerbate the problem of warping, deformation, and uneven fuel flows causing the high exhaust temperature spreads. But, without being able to see the fuel nozzle assembly or the fuel nozzle assembly drawings, I'm just "thinking out loud (digitally!)."

It does seem hot combustion gases are finding their way into the AA passage of the fuel nozzle and causing elastic/plastic deformation of the AA cone tip. There must be some kind of flow path for this to happen, or as you suggest, there may be a material difference.

I'm not a materials engineer. I have no knowledge of the heat-resistant properties of either material or the suitability of either material for the desired application.

I would still say the cheapest and most cost-effective solution would be to obtain a gas fuel-only fuel nozzle "assembly." This could take one of two forms; the first being an assembly consisting of two parts, a "blind" AA cone without the hole in the end of it and a gas fuel nozzle tip. The liquid fuel "cartridge" would be completely removed, the blind AA tip screwed and staked in place, and the gas fuel nozzle tip screwed on and locked in place.

There might also be a single gas fuel nozzle tip which would replace both the liquid fuel cartridge and AA cone.

But, I suspect there may be some differences in gas fuel nozzle orifice angle and/or some difference in the gas fuel swirl tip that allows more combustion air to help push or keep the gas fuel flame ball a little further away from the nozzle tip in the absence of purge air flowing through the AA tip when running on gas fuel.

Again, as I said before, there's really nothing I can add to this thread.

The problem started when the AA compressor was removed. The problem appears to be completely related to the absence of purge air in the AA passage when running on gas fuel. Simply connecting the AA compressor inlet to the AA compressor outlet will likely not result in a sufficient flow of air through the AA system when running on gas fuel. In fact, all of the losses of the piping (angles, bends, flanges, etc.) will also contribute to an inability to flow sufficient air without the AA compressor to prevent backflow of hot combustion gases into the AA system. You could certainly try it, but I would suggest having a Plan B in the works if more fuel nozzles are damaged in the process, and to carefully and constantly monitor the AA manifold piping around the compressor case while running with this workaround. You might consider using one or two T/Cs attached to the AA manifold piping to monitor manifold temperature.

But, this problem seems to be primarily related to the loss/lack of air flowing through the AA system when running on gas fuel. The black deposits you mentioned at Can 5 could just be the remnants of oil from the failed AA compressor seals.

There is one last thing you might try since you have welders there. Try welding a disc to the back of the AA cone to cover the opening in the AA cone. Make the disc large enough to completely cover the interior diameter of the AA cone tip (not just the hole in the tip), and seal weld it entirely around the edge. Then also seal weld the outside of the AA cone tip to the disc, so that hot gases can't get between the tip and the disc.

You would have to remove the liquid fuel "cartridge" to install this home-made gas fuel-only AA cone, then install the gas fuel tip over that.

The problem with this is that there is still no flow of air through the AA passage when running on gas fuel which might be helping to push or keep the flame ball a little further away from the nozzle tip. But, since there wouldn't be any opening for hot combustion gases to enter because of the disc welded in place, then it might be just enough to work.

And I'm wondering if part of the problem might also be incorrect assembly technique, such as not torquing the AA tip sufficiently before staking it in place, and then not sufficiently torquing the gas fuel tip before deforming the locking ring.

I would suggest a good plan would be to work with the OEM or your supplier of new or refurbished fuel nozzle components to find a gas fuel only tip, and to examine the tip next to one for a dual fuel machine to note the differences.

All of my fuel nozzle suggestions are based on limited experience working with dual fuel nozzles on machines built since the early 1980s. If your nozzles aren't like the ones I've had experience with (and there are LOTS of fuel nozzles I haven't had experience with!) then you will need to use the drawings you have along with the site experience you have to decide how to proceed.

This is not a controls-related problem. I've already spent more time on this than I should have.

Please write back to let us know how you proceed and how you resolve the problem, even if it's a couple of months or more from now.
 
thanks CSA for your comments

i am agree with your comment of fuel flame "ball" that without the absence of purge air the flame "ball" is somewhat near to the AA nozzle cap and this will be the cause of burning of AA nozzle cap. secondly we have also removed the AA Compressor from accessory gear box and we after seal welding of fuel nozzle atomizng,liquid and purge air passage no temperature or sign of deformation has been observed on the AA piping.

regarding gas fuel way to gas tip

i want to add that there is no interface fit between the AA CAP and gas tip/swirl tip. but the gas moves form outside diameter of AA cap and then enter to the gas tip holes. although there is some clearance between the two components after installation them on the fuel nozzle. i think when burning and erosion of AA cap increased its hole will also increased so the hot gases then will interept <b><i>(moderator's note:</b> interrupt or intercept)</i> the gas fuel flow and this will cause the high spread.

secondly we have suggested a change in design and material of AA cap in which we have machined the cap without hole (passage for Atomizing air) but this is under consultation with OEM. This modification will prevent the combustion gases to back flow but also by using high temperature material it will prevent burning of AA cap. As i have visited a site in which frame 5 gas turbine is operating with by changing the material of AA cap with high temp. material.

while all the parts during reclamation of nozzles including gas tip AA CAP are torqued as per OEM provided torque.
 
N
Why you decided to remove the air compressor?

Why you decided not to rectify the air compressor leaking seal(s)? Since the oil leaks to air side and being carry-over to the combustors?
 
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