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WESP Explosion


Operational mistakes can cause an explosion to occur in the wet electrostatic precipitator (WESP) of an incineration system.  Combustible gases build up in the sludge lines when the valves are closed to the incinerator and when opened quickly these gases enter the reactor. Starting the fluidizing air blower then pushes these gases through the heat exchanger, scrubber, and into the WESP. There, the gases are introduced to an induced electrostatic charge, creating an ignition source causing an explosion. As shown, this type of explosion can severely damage tray scrubbers along with ancillary equipment.


Initial and observed symptoms:
  1. Loud bang and rumbling, followed by a large amount of smoke.
  2. WESP imploding and expanding with rumbles.
  3. Fuel oil smell throughout plant
  4. Fuel oil in the lagoon
  5. Automated high temperature shutdowns were observed in the bed and at the primary heat exchanger inlet. These shutdowns were directly proceeded by a dip in oxygen, a spike in CO and erratic scrubber d/p behavior.
  6. Also witnessed white smoke leaking from the scrubber expansion joint, accompanied by a strong smell of fuel oil.

Analysis

Numerous log entries concerning problems with the fuel oil system combined with strip chart evidence of poor/incomplete combustion lead to the ultimate conclusion that an excessive amount of fuel oil was being injected into the incinerator and/or its supporting air systems. Furthermore, a large portion of the fluidizing air was found to be bypassing the furnace and going straight to the stack resulting in higher windbox temperatures and poor fluidization.  

The main indications of the problem were those of poor or incomplete combustion. There was commonly an extreme depression in the measured levels of oxygen with a corresponding spike in carbon monoxide associated with each alarm or “explosion” event. Indications of the actual explosion type phenomenon are evidenced by a spike in the measured scrubber and venturi differential pressure. These key indications were commonly found during the restart following a brief shutdown period.
WESP Explosion Damage
Damage resulting from a WESP explosion due to operational error.

Conclusions

  • Replace all differential pressure and pressure sensing lines with larger diameter stainless steel tubing.
  • Replace the fuel oil solenoids, ensuring that adequate safety measures are in place to prevent fuel oil from inadvertently entering the furnace.
  • Internal inspection of the Primary heat exchanger is needed to determine the extent of the damage and recommend repairs.
  • Revise startup/shutdown procedures to prevent the unnecessary injection of fuel oil and the ignition of combustible gases inside the WESP during startup.