This is meant to apply to the Rosemount 400, 400A, NGA-FID, NGA-HFID, and other Rosemount FIDs but it can also apply to other manufacturers.
When your analyzer had these symptoms:
- Was working before!
- shows that the flame is lit
- does NOT respond to process gas (sample) or span gas (upscale gas)
- flames out easily
- noisy readings at the low end of the scale
. . . then you probably have a false flame.
The Rosemount analyzers require the flame to be “seated” on the burner jet in order to measure the ion current generated by the burning of hydrocarbon molecules. If the flame is dancing above the jet then the ion current loop is broken (because one of the terminals of this current loop is the metal tube in the center of the burner jet).
This fix could be simple: extinguish the flame, and then relight it.
If the simple fix doesn’t work, or you experience this false flame regularly, then you probably have a defective burner jet (most likely caused by fouling at the point where the fuel and sample exit the jet tube). A fouled jet can swirl the fuel so violently that it can’t light properly.