KPIs and measurements
Opacity (stack)
Also known as stack opacity, opacity excursion.
Opacity is the percentage of light obscured by particulate matter in stack flue gas, measured continuously by a transmissometer (opacity monitor) installed in the stack. Opacity is the headline visual KPI for ESP performance and is permit-limited in most jurisdictions — typically 20–40% on a 6-minute rolling average, with absolute peaks limited to 60% for shorter periods.
Opacity excursions
Opacity excursions are typically driven by:
- ESP rapping re-entrainment puffs
- Baghouse bag failure (sudden particulate breakthrough)
- ESP back-corona collapse
- Combustion upsets producing unusually high inlet particulate
- Soot-blower-triggered re-entrainment
How sonic horns reduce opacity
Sonic horns deliver continuous gentle dust release rather than periodic aggressive rapping puffs. Plants retrofitting horns on opacity-limited ESPs commonly see 20–40% opacity-peak reduction without other changes — the headline business case for many sonic-horn ESP installations.
Related terms
Related terms
- Continuous Emissions Monitoring SystemCEMS instruments measure stack emissions in real time — opacity, PM, NOx, SOx, CO, O2, moisture — providing the data on which environmental compliance is judged.
- Electrostatic precipitatorAn ESP removes particulate from flue gas by charging dust and collecting it on plate electrodes. Sonic horns are widely used to dislodge ash from plates and to keep hoppers from bridging.
- Re-entrainmentRe-entrainment is the recapture of just-rapped dust by the flue-gas stream before it falls into the hopper. It causes opacity spikes and is the main reason continuous sonic cleaning is preferred.
- Sonic hornA sonic horn is a pneumatically-driven low-frequency sound emitter (typically 60–400 Hz at 140–180 dB SPL) used to dislodge particulate fouling from boilers, ESPs, baghouses and process vessels.