TL;DR
Zero or erratic AIG flow almost always means a stuck control valve, plugged nozzle, or cold vaporizer. Verify vaporizer temperature, check valve positioner output, and inspect the injection grid nozzles.
What you might see
- NOx outlet above permit limit despite adequate load
- ammonia slip analyzer reading near zero with AIG flow showing zero
- AIG flow-control valve stuck or hunting
- vaporizer outlet temperature below setpoint
Likely causes
AIG flow-control valve positioner fault or instrument air supply failure
Vaporizer heater element failure causing liquid ammonia carry-over
Plugged AIG nozzle(s) from ammonium bisulfate deposits
Loss of anhydrous or aqueous ammonia supply (tank low, pump off, block valve closed)
Required tools
- Portable ammonia gas detector
- 4-20 mA loop tester or signal generator
- Instrument air pressure gauge
- Non-sparking probe for nozzle clearing
- Full-face respirator with ammonia cartridge
Safety first
- Ammonia is a toxic gas. Any AIG or vaporizer work requires a calibrated ammonia gas detector and full-face respirator with appropriate cartridge.
- Lock out the AIG control valve actuator before performing mechanical work on the injection grid.
Procedure
- 1
Check the SCR DCS screen for AIG flow demand vs. actual; confirm the control valve is receiving a position signal and responding.
- 2
Verify the instrument air supply to the AIG control valve is at normal operating pressure (typically 80-120 psig). Confirm the positioner has no fault indicator.
- 3
Check the vaporizer outlet temperature against setpoint. If below setpoint, verify the heating element is energized and the temperature controller is in Auto.
Warning: Ammonia is a toxic irritant gas. Any work near the AIG, vaporizer, or ammonia storage requires a gas monitor and appropriate PPE including an air-purifying respirator rated for ammonia. - 4
Inspect the ammonia storage/supply system: confirm the tank level, delivery pump running status, and that no block valves have been left closed after maintenance.[1]
- 5
If the vaporizer and valve are functional but NOx stays high, suspect individual nozzle plugging. Schedule an offline inspection and clean nozzles with a non-sparking probe.
- 6
Return the AIG to automatic and monitor NOx outlet and slip readings for at least one hour.
- 7
Document the repair and log current NOx and slip values against permit limits.
Sources
Babcock & Wilcox B&W SCR SCR / Emission Control general technical documentation, Babcock & Wilcox
Selective catalytic reduction system operation and maintenance, AIG troubleshooting, general power-plant emission control guidance (general)
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