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Water Treatment Plant Collapse

A water treatment plant in Southern California used a parallel arrangement of three boat clarifiers to separate sludge from the effluent. The clarifiers are so-named because they are shaped like the hull of a boat. The clarifiers were suspended in side-by-side oval-shaped oxidation ditches. During normal operation, the water level in the clarifiers is the same as that in the surrounding ditches. In June 1997 a contractor was flow testing the clarifiers after the ditch had been drained when the boat clarifier's support structure suddenly collapsed. Aptech was hired by the insurance adjuster to find the cause of this incident. Our initial examination and interviews clearly indicated that the collapse occurred as water was collecting in the bow of the clarifier while there was no water in the surrounding ditch. The operation manual contained warnings regarding the need to maintain equal water levels in the boat clarifier and its ditch. We analyzed the support structure and found that it could not support the clarifier without the buoyancy provided by water in the ditch. We also found metallurgical flaws in the welds in the support beams, which made the clarifier even more susceptible to collapse. The collapse occurred when water backed up into the clarifier with the empty ditch from an adjacent clarifier that was being filled. The contractor had improperly left a piping valve open during the flow testing. This accident could have been avoided if the plant designers had placed a check valve in the outlet piping of the clarifier. This would have prevented backflow from any of the boat clarifiers while they were being filled with water.

Satish Almaula		Plant failure and root cause analysis; process and design engineering analysis; process technology development and testing; plant engineering, operations, maintenance and safety management; plant and process control system management.

Michael Cronin, P.E.		Stress analysis, design evaluation, engineering mechanics

Kimble Clark, Ph.D.		Failure analysis, heat transfer & thermodynamics, process plant equipment failures & explosions, fuel science, combustion, industrial fires and explosions.

Geoffrey Egan, Ph.D.		Engineering mechanics, welding engineering, stress and fatigue analysis, risk analysis, nondestructive examination, project management.

Eric Sullivan, P.E.		Metallurgy, welding, failure analysis, fire and explosion investigations, equipment, piping.

Jeffrey Grover, P.E.		Mechanical engineering, failure analysis, civil engineering.