I've been trying to get a Frame 5 up and running after a fuel nozzle replacement with no luck!

The turbine in question has mechanically atomizing nozzles, with purge gas.

The turbine will run up to around 33/34% speed then trip on loss of flame. We have achieved 95% speed on 2 occasions, but when the IGVs open, we're blowing the flame out. Before this speed, the flame scanners (4 off) are flickering about.

We have no gas to try a start on that, only liquid fuel.

Liquid fuel stop valve is opening, we have good flow to and across the flow divider, even pressures at the nozzles. Check valves all tested for opening pressure and good. IGVs closed at start up, bleed valves open etc.

I'm suggesting we have a rogue set of nozzles from the repair shop, but my boss is having none of it. The turbine was running absolutely fine before it was shut down to replace the nozzles, and given it's the only thing we've changed, surely it has to be them!?

Is there anything else I should be checking before I take the chamber doors off and stick a boroscope in to check liner louvers, TPs etc??

Cheers.

 

Jimbo76,

Are these the "traditional" dual fuel nozzles with the liquid fuel "cartridge" screwed into the fuel nozzle body in the center of the nozzle assembly, with the AA tip around that, and the gas fuel tip around the outside of those two?

If so, are you sure the internals of the liquid fuel cartridge were properly assembled and staked to prevent loosening, including the liquid fuel cartridge to the fuel nozzle body? I've seen the internals come apart because they weren't properly tightened AND staked.

Are you sure the liquid fuel purge air is OFF? If purge air were enabled, say, at 95% speed, it could be putting the flame out.

Does the unit have a Booster AA Compressor? If so, is it working correctly? Is the check valve free to operate? Are the solenoid-operated valves working correctly?

Are you sure the AA Bypass valve is closed during liquid fuel firing?

If this is a dual-fuel unit, it's important to ensure the proper amount of AA is getting to the fuel nozzles during firing and acceleration. Most dual-fuel units have AA Bypass valves to reduce the AA pressure/flow during gas fuel operation. It must be closed during liquid fuel firing; I can't recall if that valve is normally open or normally closed--sorry. Have a look at the Piping Schematic (P&ID) for the Liquid Fuel and/or AA System(s), then check the valve and it's solenoid operator.

I would expect that if there was insufficient AA then the exhaust would have a lot of white vapours--which should clear up after initial light-off presuming light-off occurred fairly soon after firing started.

I presume you've checked the Tell-Tale Leakoff to be sure there's no fuel coming out of it (which would indicate poor atomization and/or dribbling of the liquid fuel into the combustors).

I would agree--if the nozzles were the only thing changed, then something is amiss with either the nozzles or the assembly of the nozzles. Of course, it's not uncommon for solenoid-operated valves to fail--and at the most inopportune time.

Finally, air in the liquid fuel supply piping is always a potential problem. Were the liquid fuel filters changed during the fuel nozzle change-out? If so, are you certain they were properly bled of all air? Is the liquid fuel supply pressure stable (upstream of the Liquid Fuel Stop Valve and the High-pressure Liquid Fuel Pump)?

Was the unit running on liquid fuel prior to this outage? Frequently, or just periodically? If periodically, did the unit successfully start on liquid fuel, or was it transferred from gas to liquid and back to gas without much problem (low or no load swings; no loss of flame trips)?

I doubt there's a problem with the liners/transition pieces--unless the new nozzles caused a problem--but it doesn't sound like the unit has run long enough for that to occur (though stranger things have happened).

If the fuel nozzles were changed out, is it possible that rags were not removed from the AA pigtails/piping/flex hoses and could have caused a problem?

Are the exhaust temperature spreads abnormally high during warm-up and acceleration? You say the flame detectors are flickering--that's either low fuel flow-rate, poor atomization (which would be the "mechanical atomization" or lack of AA flow) or incorrectly sized fuel nozzles.

Please write back to let us know what you find!!

 

Thanks for the reply.

The turbine I'm working on doesn't have atomizing air at all. No compressor, booster, pipework, so can rule that out. These units are odd in that they also have purge 'gas', rather than purge air.

I've removed what I deemed to be the offending nozzles, and found no plastic covers on any of the swirl caps, and no blockages in any fuel gas or distillate supply lines.

The nozzle caps were all tight and well staked, and appeared to be free of any debris, coking, or anything else obvious for that matter.

The liquid fuel stop valve is operating fine, we're seeing good fuel flow through the bypass valve and across the flow divider, which is operating nicely and making expected speed. All fuel pressures at the selector block are even, and within 0.5 bar of each other eliminating check valves.

The temperature spread was excessively high around a certain portion of the machine, and only started to even out as we got closer to FSNL, but once the IGVs opened the air blew the flame out, due to it being a poor burn.

The unit did run on liquid fuel just a couple of weeks ago with no issue. I'm on an oil platform just now, and we lost produce gas, so the machine swung over without hesitation, then back to gas, then back to liquid fuel, but no start. The non starting issue was why we decided to change the nozzles, and it's worse! Haha.

There are too many folk over analysing things now, and are doubting my diagnosis of the nozzles being at fault, but I can't help but think that given that's all we've changed.

We're receiving another replacement set tomorrow, so all I can really think is that I install them and try again!! I just wish we had gas availabe to try a start on that to eliminate that side of things.

Cheers, Jimbo76.

 

Jimbo76,

This is an interesting new piece of information:

>The unit did run on liquid fuel just a couple of weeks ago
>with no issue. I'm on an oil platform just now, and we lost
>produce gas, so the machine swung over without hesitation,
>then back to gas, then back to liquid fuel, but no start.
>The non starting issue was why we decided to change the
>nozzles, ....

You seem to be saying the unit transferred to liquid fuel because of a loss of gas fuel, then went back to gas fuel (presumably because the operator initiated a transfer back to gas fuel when it became available again (??)), and then on a subsequent loss of gas fuel the unit transferred back to liquid fuel. The unit then was either shut down or tripped (??) and on a re-start was unable to fire, so the nozzles were changed out. Is that a correct statement of events and time-line of events?

I've never worked on a unit without AA, so I'm unfamiliar with mechanical atomization. You've checked/verified the check-valves, so they shouldn't be an issue. I would presume that mechanical atomization requires a minimum pressure which would be a function of proper liquid fuel check-valve operation.

I'm also not familiar with "purge gas"--are you saying that gas fuel is used to purge liquid fuel from the nozzles after a transfer from liquid to gas? Is this only a temporary process--or does "gas" flow continuously through the liquid fuel purge check valves and the liquid fuel nozzles when running on gas fuel?

If there's no gas fuel it would seem that since gas fuel is not available this would not be a problem--especially since the unit is running on liquid fuel.

So, I'm kind of confused about the source of "purge gas" and if it flows at all times during gas fuel operation.

For me, without being able to see the Piping Schematic drawings (P&IDs) for the fuel and purge systems I'm at a pretty significant disadvantage since I've not encountered liquid fuel with only mechanical atomization.

If the unit ran successfully on liquid fuel when at rated speed, I would think there's no problem with the liquid fuel pump clutch (slipping; intermittent operation).

Sorry I can't be more help.

Please let us know how this works out!

 

To CSA & Jimbo

Yes, I think this is an old machine.

Yes, they did make with mechanically atomized nozzles, generally called Group 12 nozzles and, Yes, they had gas purge when on gas fuel.

To be honest, these nozzles were a pain in the a*s, but mainly to get them ignited, I haven't seen this blow out phenomena when the IGVs open.

I tend to think Jimbo is right and there is something dodgy about these new nozzles. I know it is hard, but could you try and go back to the old nozzles and that would be a near final solution if the old ones get you to FSNL?

Good luck and please report back, this is interesting.