23 / Jan / 2019
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  • Wastewater Pump Station in a port area receiving industrial / domestic effluent
  • Wet-well / Dry-well, duty / standby configuration, built 1959/60
  • Wet-well approx. 4.9m deep

What happened sequence 16/10/2018

  • Dry-well flooded alarm indicated
  • Two operators attended to find dry well flooded to approx. 1.5m deep
  • Dry-well pumped out with a vacuum truck
  • Effluent entering dry-well via failed gland seal to pump drive shaft
  • Operators shut one isolation valve
  • Exited dry-well, then re-entered to close other isolation valve
  • Detected strong smell / taste in air and made their eyes water and irritated their throats
  • Gas detectors showed nothing unusual
  • Fire Brigade called in to assist with BA equipment
  • Odour is thought to be chemical based and investigations are ongoing as to source

What can be Learned?

  • Just because a space has been entered previously, don’t assume it will be OK to repeat after such an incident
  • Gas detectors are gas specific, and don’t cover all gases
  • Ventilate well before entry
  • Ventilation was in place but from just above dry-well floor level which once drowned doesn’t work. Need air extraction from both high and low levels
  • Dry-wells are not preferred, use submersible pump chambers instead
  • Next item to get wet would have been electrical equipment (also below grade) on the pump motor floor only 700mm above level to which flooding had risen
  • Electrical cabinets below grade not preferred
  • This pump station has no gravity overflow
  • Dry-wells do get flooded