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 has these symptoms:
1. was working before
2. shows that the flame is lit
3. does NOT respond to process gas (sample) or span gas (upscale gas)
4. flames out easily
. . . 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 burner jet then the ion (measuring) 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 (version #1): extinguish the flame, and then relight it.
This fix could be simple (version #2): measure the flame temperature; it should be between 30 and 150mVDC; a false flame reveals itself at something more than 200mV. Extinguish the flame and then relight it.
► On older analyzers (120vAC version) the flame signal can be read at TP13 (use TP8 as ground reference).
► On newer analyzers (120vAC version but with an internal 24 vDC power supply) the flame signal can be read at TP3 (use TP15 as ground reference).
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-sample gas so violently that it can’t light properly. Replacing the burner jet will be required; RIGAS has new burner jets.
(Diagrams later).
Keywords: TP-13 TP-8 TP-3 TP-15