Machine Design

Up in flames: When engineers don’t think through product configuration

Two incidents illustrate what can happen when equipment incorporating reservoirs of flammable liquid is not designed with potential hazards in mind. The overriding principle is that liquids always seek a lower level. Failure to think through this reality can cause severe property damage and possible personal injury.

To understand these accidents, first consider the way industrial processes typically store liquids used as a force-amplification means (hydraulics), a fuel, or as a heat-transfer mechanism. Normally such equipment involves a reservoir of extra liquid. The reservoir tank often sits above the operating equipment so the height can supply fluid pressure and eliminate the need for a pump. For example, manufacturers of systems that use oil for heat transfer generally require the reservoir tank be placed above the equipment to assure oil is always available and so the setup can accommodate expansion of the oil at elevated temperatures.

The first incident took place when an oil heat-transfer unit caused a fire that nearly destroyed a small manufacturing facility. The heating system delivered oil at a specific temperature to a molding operation. The temperature controller in the heating system failed, drastically overheating the oil. The oil then expanded to completely fill the oil reservoir. Workers were horrified to observe oil flowing out the expansion tank onto the heater. The oil then immediately ignited. The fire burned rapidly, encompassing the small facility before the fire department arrived.

The second incident involved a waste-oil burner/heater in an automotive repair shop. The waste oil used to fuel the heater was placed higher than the heater, as the manufacturer required, so the system could gravity feed oil to the burner. The installer placed the waste-oil tank just above the heater. Over time, the fuel-line fitting loosened. It was the source of the leaked waste oil that hit the heater’s upper surface, causing the fire.

Guidelines for avoiding such difficulties may sound obvious, but only for designers who think about the consequences of failures in the components they specify. First, always avoid placing liquid reservoirs above sensitive equipment. That goes even for noncombustible liquids which can cause severe damage. For instance, leakage of brine from an expansion tank onto electrical equipment can bring down an electrical system. Also, designers should specify high-liquid-level alarms on liquid reservoirs to warn of impending overflow. Finally, reservoirs should incorporate an adequate overflow drain to another tank or containment area with sufficient capacity to handle the overflow.

Charles C. Roberts, Jr., Ph.D., P.E. is an engineering consultant in the areas of accident reconstruction, failure analysis, structural analysis, heat transfer, fire origin analysis, computer analysis, mechanics, and biomechanics. He can be reached at [email protected]

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