During the initial firing period cans 7&8 are failing to ignite. All other cans ignite and crossfire well in both directions but stop at cans 7&8. The firing and warm-up values have been adjusted several times but with no success. All check valves, fuel nozzles and liners have been changed but the same problem still occurs. The flow divider has been changed and the fuel distribution lines blown through and still no change. Flow divider speed is good at around 200rpm and the fuel pressures to the cans are also good at around 250/260psi. On a couple of occasions can 7 fired allowing us to continue past the 60 second firing period but as can 7 lit can 10 flamed out. At around 2000rpm turbine speed all cans ignite but at this point the turbine trips on over temperature (this is being put down to the unburnt fuel from the unlit cans igniting and inducing a sudden spike in exhaust temperature). The liquid fuel servo valve has also been replaced.

Has anyone experienced this problem before or does anyone have any ideas?

 

When did this problem begin? After a maintenance outage? A combustion inspection or hot gas path inspection or major inspection?

Is the liquid fuel supply pressure (upstream of the stop valve) stable and within specification (usually about 4.0 to 4.5 barg)? This should be stable during firing, warm-up, and acceleration.

I'm presuming that flame detectors are present in cans 7 & -8 and that's how you know there is no flame in them. This would be pretty unusual as the flame travels through cross-fire tubes on both side of each combustion can around the machine to establish flame from the combustors where the ignitors (spark plugs) are located. And for *ONLY* the two combustors with flame detectors not to "fire" consistently is pretty unusual.

(It would be interesting to know how you know there is or is not flame in the cans without flame detectors. Older units used to have sight glasses in the combustor end-covers; is that what you're using?)

Spreads at light-off (during firing, and during warm-up and acceleration) are not unusual when firing on liquid fuel, in fact, they are almost unavoidable. To reduce spreads and try to ensure flame gets established in every combustor one can increase the firing flow-rate reference and even the warm-up value but that usually results in very high exhaust temperature spikes, and that isn't good for the hot gas path parts.

I would suspect something is wrong with the cross-fire tubes on the outside edges of can 7 and can 8, possibly even the one between cans 7 & 8.

Or, some problem with the flame detectors--dirty lenses, low voltage, ground, rusty or occluded sight tubes, isolation valves not fully open, bent sight tubes, etc. Have you tested the flame detectors individually, by removing them temporarily and holding a flame in front of them? The older Honeywells needs a stronger flame; newer Reuter-Stokes will sense bright sunlight!

I've even seen combustion liners not installed in the right combustors. Usually the liners for the combustors with flame detectors have holes for the flame detectors to look through and the other liners do not. More than one site has installed liners without the flame detector holes in the combustors where flame detectors are present, only to have this problem. Have you pulled the flame detector flange off the side of the combustor and looked into the combustor to see if you can see the face of the nozzle or the inside of the liner?

While you have the flange/sight tube off, be sure to inspect it to see that it's not obstructed and is relatively clean and not rusted. If it's badly rusted, you should probably run a drill motor-driven bottle brush through the tube several times to clean it up.

You didn't say how much you've increased firing rates from the start of this episode. It would also be helpful for us to know what kind of control system the unit has. And the type of liquid fuel control valve (bypass valve or variable displacement pump or ???).

Also, how long is it taking to establish flame--and to clarify that, I mean, how long is it taking to see exhaust temperature increases? (Pretty fast increases when flame is established in a cool or cold machine; they shouldn't be high spikes, but they should "jump" when flame is established.)

 

The control system is a MKII speedtronic panel. There are sight glasses in all cans so you visibly see there is no flame in the cans, this is backed up by the high exhaust spread alarm being annunciated.

Fuel supply pressure is steady at around 4.5 bar. The pressure at the flow divider selector valve varies for each can between 220 and 280 psi, but is steady at each nozzle.

The firing and warm-up values have been varied from as high as 4.27vdc to as low as 3.7vdc, although 4.03vdc/200rpm floedivider speed give the best light off. Flame is established within 5 to 10 seconds and cross fires in both directions instantly but stops at cans 7&8. You see a flash in can 7 & 8 but then goes out straight away. The crossfire seems to be working but the cans will not light off. A borescope has been carried out and no faults found. All nozzles and check valves were replaced with the same results. Liners were pulled and inspected but were all ok. New liners were fitted in cans 6,7&8 and still no change.

The liquid fuel system uses a LFBV.

This problem did not occur after any outage. The unit was down for a period of months and then ran on liquid fuel for a few days. There were issues with the diesel supply at the time and i think high spreads were present during operation. When the GT was taken offline the fuel nozzles were changed as a result of the poor diesel supply/high exhaust spreads and then these problems occurred.

 

Liquid fuel firing issue

How many flame detectors do you have in site. is it more than 2. you seem to be sure that all other cans fire , do you have a flame detector confirmation for that , or you are just saying that from the increase in the exhaust temp.

I have seen a very similar condition in one of the turbines which just came out of maintenance.

all fuel fired gas turbines have a manual purge valves . they are present between the flow divider and the burner. thus for a given skid (on base or off base ) you have 10 manual purge drain valves. this must be closed fully.

In the site , 3 of the valves was passing ; in the ones going to compartment 5,6 and 7. when the machine was started the flame established in can 8 , no flame in can 7 . all of a sudden there was a flame spike in can 7 , then in went out , seconds later the machine tripped on exhaust spread high.

we suspected a problem with the fuel check valve , replaced the the check valves and started the machine . again the same thing happened , and this time the machine tripped on exhaust temp high.

after this one of the operators noticed that one of the drain valves was not properly closed. just to be sure of we reduced the pressure of HSD and did a pressure check of the drain valves. 3 were found to be passing. one of them heavily and two of them moderately. due to this , no fuel was ported to the combustion chamber. we replaced all the drain valves with new one. checked for leaks and started. gt came online like a song.

seems that something very similar is ha penning here. check out this possibility.

 

Well, this is very unusual. Again, it's even more unusual because the two cans that aren't lighting are the two cans with flame detectors (or at least it seems that way). That's a very strange coincidence, but stranger things have happened.

In my estimation, the problem is most likely "mechanical" and something wrong with the fuel nozzles or check valves. If the unit is a dual fuel unit, and it has Purge Air, then the Purge Air check valves at cans 7 & -8 might be leaking and so not allowing sufficient fuel to get to the nozzles.

I've also seen the internals of some pressure-atomized nozzles loosen and cause poor atomization of the fuel. I know you said you've changed nozzles and check valves, but things can come loose very quickly.

That doesn't explain why it's ONLY happening on the same two cans. It really seems to be something physically wrong, not a controls issue.

You mentioned in your first post that the unit had mechanical atomization. There is no Main Atomizing Air Compressor, or is there no Booster Atomizing Air Compressor which is usually used for providing atomizing air during starting?

Quite often, units with a warm-up function that comes in shortly after flame is detected (by the flame detectors) will cut back the liquid fuel too much and actually cause flame to be lost in one or more cans during starting. As you have noted, flame will usually be established later during acceleration, but in your case the flame must be detected in one or more of the cans with flame detectors to get past the firing time and get into warm-up and acceleration.

Excessive liquid fuel flows can also cause problems, though usually with wetting the ignitors/plugs and causing them to not fire. Also, very high exhaust temp spikes can trip the turbine (as you have noted, though yours seem to be occurring when unburnt fuel suddenly ignites).

Which leads to the question, are the false start drains working? There should be a false start drain on the exhaust plenum for unburnt liquid fuel to drain out of. There should also be false start drains on the lower combustion cans, and sometimes on the bottom of the combustion wrapper/turbine shell. If those are allowing unburnt liquid fuel to drain out, then that could the source of the sudden high exhaust temps which are tripping the turbine.

The last thing I can think of is, is the unit firing at the proper speed? Sometimes, if there is too much air flowing into the combustors because the speed is too high (or the IGVs are excessively open, or maybe because the compressor bleed valves aren't fully open.?.?.?) then it can be very difficult to establish flame reliably in all the combustors.

But I keep going back to the inability to reliably establish flame in the same two cans. To me, it just keeps pointing to something physically, mechanically wrong.

 

>all [liquid] fuel fired gas turbines have a
>manual purge valves .

ProcessValue, that's not true. It's only true for units which have liquid fuels which may need to be purged if the unit trips while operating on that fuel and can't be quickly re-started, or units that can't be started on the fuel which was being burned. Examples are units which must start and stop on distillate (HSD; diesel fuel) and can also shift to naphtha or heavy fuel oil (bunker or crude oil) some other liquid fuel, but can't be started or stopped on naphtha or heavy fuel. Some turbines can be started on naphtha, but they are not typical of most of the machines which can burn naphtha. Most units which can burn bunker or crude cannot be started on bunker or crude, but rather can only shift to bunker or crude at rated speed or above a minimum fuel flow-rate while under load.

The unit <b>may have</b> manual drain valves between the liquid fuel flow divider, but <b>every</b> liquid fuel-fired unit does <b>not</b> have such valves.

It's a good thing to note, though, again, not all liquid fuel-fired machines have these manual drain valves.

 

CSA , well , i do not have your experience nor have i seen all the gas turbine designs out there. but i Have seen manual drain valves on all duel fuel machines light distillate/ heavy distillate ; light distillate/gas etc . HSD is a preferred start up fuel as it is not volatile and has a higher vapour point and flash point. I have not seen a machine which was started on "natpha".naptha is volatile and easily forms fumes and it has low flash point. yes the purge timer during the starup is especially given to purge / flush out any unburnt fuel and combustible vaporous. which is quite enough.

if at all there is a machine which is designed to be started with naptha it would be a rare thing. primary reason for starting with HSD is safety. leaks in fuel lines predominantly come during start up , especially after maintenance , i will always start with hsd , because even if there is a leak the chances of flare up is reduced. It is a standard operating procedure in refineries to load the machine which has just come out of maintenance till base load to check for leaks in the fuel system. only then a fuel transfer is made.

manual drain valves are not only used for purging , they are also used to flush the lines off combustible vaporous. this is necessary when doing hot works inside the accessory compartment. Manual drain is more or less a necessity as per my view.

but of course i do not know if this particular site has manual drain valves. if it has then what i have said above has a great chance of being the problem at the site.