In Electrostatic Precipitators (ESP) during unit light-up and hopper empty condition, the field rating (kV & mA) is set carefully to avoid damage and false alarms. The logic is as follows:

1. Unit Light-up Condition

• At boiler light-up, flue gas temperature is low and dust load is negligible.
• ESP should not be operated at full rating, otherwise back corona and spark rate will increase.
• Typical practice:
  • Keep ESP fields in “Auto” but with reduced TR setpoints (around 20–25 kV or ~30% of design rating).
  • Spark rate limit lowered (to ~20 sparks/minute) for protection.
  • As the load increases and flue gas stabilizes, gradually raise to normal operating level (40–50 kV as per design).

2. Hopper Empty Condition

• Hopper emptying causes rapping and sudden dust re-entrainment.
• If ESP fields remain at full rating, it may lead to excessive sparking and load tripping.
• Typical practice:
  • Put the concerned fields in “Voltage Limiting” mode or reduce to ~60–70% of rated kV before rapping.
  • Once dust settles, return gradually to Auto mode with full kV/mA rating.
  • Some stations even bypass fields for a few minutes during heavy rapping.

General Guideline

• Always follow OEM/plant-specific SOPs because design ratings vary (some ESPs operate at 50–60 kV, others up to 70 kV).
• During light-up/low load → keep ESP in safe mode with reduced rating.
• During hopper emptying/rapping → derate fields temporarily.
• Once stable → switch back to auto optimization (high efficiency mode).