The Case of the Martyred Mulcher
Myron J. Boyajian, President, Engineering Consultants -- Design News, April 9, 2006
In one southwestern state, recycling, including the recovery of materials and by-products, has become a fine art and science. Plastic and recyclable metals are manually isolated for further processing or conversion. The balance, typical municipal waste, is spread over hundreds of acres of a dumpsite. Colossal trash compactors with giant steel wheels, studded with scores of heavy knobs, crushed and mulched waste and used their front-mounted blades to spread the crushed waste evenly over the fields. At day's end, regular crawler-type dozers spread layers of dirt over the mulch. Breakdown of the mulch, hastened by bacterial and chemical action, yielded methane gas. Tapped by a network of pipes, methane provided energy for a 5-MW, steam turbo-generator plant that provided all on-site electrical needs with a surplus of power pumped and sold into the grid.
Scene of the Crime
I was retained to inspect a mulcher at that site that caught fire and was so heavily damaged it was to be scrapped. The manufacturer refused to pay for the damaged compactor claiming negligence and poor maintenance caused the fire. I was called on the case soon after the fire, but the manufacturer already tore it down and scrapped all parts. Even with a legal issue of evidence spoliation, my inspection still had to proceed. An exemplar compactor, a sequentially serial-numbered unit, was made available for inspection.
The Investigation
After a walk-around, I took photos and videos of the exemplar and other makes of compactors. Guided by engineering drawings and photos of the burned unit, I inspected the interior spaces and components of the exemplar. Built like an articulated wheel loader, this compactor had vertically-pinned joints in a center space containing a nest of hydraulic hoses. The compactor was of robust design, using beefy structural members and turbo-diesel power. A dry-chemical, fire extinguisher system, with various nozzle locations, was triggered by heat sensors and a driver's central control.
The fire marshal's report quoted witnesses who said the fire originated in the center space and not externally. Perched up high and unaware of the fire, the driver bailed out only after a radio alert. The fire spread and the engine compartment, driver's cab, and center compartment were damaged beyond repair. With that report in mind, I focused on the center compartment. I found debris that got past the compartment doors. Oily mulch lay in deep piles on every horizontal surface, on, around and under the hoses, and under the two steering cylinder piston rods. Landfill personnel said, even with regular cleaning, debris always accumulated there. I collected samples for later chemical analysis.
I then searched for a possible ignition source. I identified connections from a 24-V battery to a wiring panel in the engine bay. A heavy-gauge wire went forward through a long, hollow, heavy-wall, rectangular tubular frame member, then out into the center compartment to a solenoid contactor on a motor that drove a hydraulic fluid transfer pump.
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The cable lacked a protective sleeve or conduit specified in the diagrams, but had rubbed on rough-cut tube edges. I could see and feel the eroded cable insulation at the point of tube exit. There were no fire extinguisher nozzles or fire sensors in the center compartment to protect this space. I concluded my inspection and alerted the landfill operators to have the critical cable protected.
The Culprit
The lab report showed the presence of hydrocarbons, typical of diesel fuel and hydraulic fluid, likely from fuel spillage and hydraulic fluid weeping from the steer cylinders. I opined that on the burned compactor, flammable debris was ignited by electrical sparks from the unprotected cable that had likely rubbed through on the sharp edges of the frame tube just as I saw on the exemplar compactor. Also, the manufacturer knew flammable materials accumulated in the center compartment, but failed to effectively seal the space. The failure to install fire sensors and extinguisher nozzles allowed fire to rage in the center compartment, without detection and suppression. A settlement was reached before trial.
To view a block diagram of the Mulcher's center compartment, go to http://rbi.ims.ca/4919-510.
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Yes, superb!
- 2006-24-10 06:45:14 EDT -
Great article
- 2006-24-10 06:44:42 EDT
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