Hi all,

After giving a STOP command turbine is when trying to transfer from PREMIX to LEAN-LEAN with FAILURE TO REIGNITE PRIMARY ZONE. It happens only at Shutdown sequence. at Startup and Premix Transfer we have no warnings/alarms. Gas fuel pressure/Air pressure/Igniters are OK. We have some oscillation of MW and GCV just before the trip. Splitter valve is OK but setpoint is not changing to move it to 50/50 position. What are we missing and what else can be checked?

Thanks

 

Jokek,

The Process Alarm is exactly, 100% correct. When transferring from Premix to Lea-Lean during a shutdown the spark plugs have to fire to re-ignite the fuel in the Primary combustion zone. If they are not inserted at the proper depth <b>OR</b> they are worn (and the tips of the high-energy ignitors used in many DLN-I systems do wear over time) then they will not be capable of igniting the fuel in the Primary combustion zone (which, I believe there must be flame detected in the Primary combustion zone before the fuel is reduced).

The ignitors will work fine during normal starting if they are not inserted at the proper depth, and they will snap-crackle-and-pop during testing just fine--but they won't work when the air flows are very high as they are during Premix operation.

You may even have just replaced the ignitors during a recent maintenance outage and believe them to be incapable of premature failure--they're working just fine during starting, right? But if they weren't set to the proper depth during installation they won't work during shutdown.

The proper insertion depth is listed on the combustion liner drawing in the Service Manual. It's not easy to measure and set, and if they are set too deep into the combustion liner the tips will prematurely burn and erode.

Please write back to let us know what you determine!