Audible Combustion dynamics
You have probably been out at the unit during a startup and heard the low-frequency rumble when the combustion system lights off. It might remind you of a passing freight train. But what about during normal operation, maybe even at base load? If you hear a similar sound then, it may be a sign of trouble.
What to do?
The low-frequency (~16 Hz) chug, or “lean-blow-out” (actually a misnomer) combustion dynamics is audible if it approaches 1 psi peak-to-peak amplitude. This resonance often occurs when the DLN 2.6 outer fuel nozzles (PM2 and PM3) are flowing very similar amounts of fuel gas. This is sometimes called “even splits” and is the PM3 split value that produces the lowest NOx. However, once these tones are excited, they can easily get amplified and at some point (when the walls of the PEECC begin to shake), the sound will literally blow out your flame, even at base load.
With a typical PM3 split schedule, reducing load will usually back you away from this problem, but if you are sitting at the controls and have access to the PM split schedule, ramp up the PM3 splits +1% or 2% as fast as possible. If you have a good autotune program, this is what should happen automatically. This will break up the uniformity of the outside diameter of the flame and defuse that resonant 16 Hz acoustic dynamics. You might then be stuck with NOx that is higher than you like, but that is a different story and a job for a tuner.
