Forensic Engineers and Consultants

Archive: Fires & Explosions

Moisture Intrusion into Structural Reinforced Concrete

Scenario:  The owners of Jones’ Marine, a marina with a wharf structure on a tidal saltwater river, cater to the operators of ships, barges, and large yachts.  There are both diesel and gasoline fuel pumps at the edge of the wharf.  In addition, a large mobile crane for lifting the large vessels out of the water for maintenance is in service at the wharf.  This crane is driven out to the edge of the wharf deck suspended over the water, subjecting this deck and the structure beneath it to very high loading.  They purchased the property from its former owners about 10 years ago and are conscientious about maintenance and upkeep of their thriving facility.  They decide it is time to have a survey performed of the condition of the precast reinforced concrete piles and pile caps beneath and supporting the concrete wharf platform/deck.  They are alarmed at what is discovered by this survey:  Significant spalling Read More

Conveyor Backstops: Sometimes One Isn’t Enough, Part 2

This is the second in a two-part blog series about conveying equipment that severely injured a worker at a mine. In case you missed it, click here to read Part 1 where I describe the incident and the mining equipment. In this part, I will discuss my engineering analysis of the incident and the machinery involved and share the conclusions I reached.

The injured miner was a front-end loader operator. He was not a maintenance worker. He simply responded to a radio request for help with the conveyor. Power to the electric conveyor motors was locked out, but none of the maintenance workers did anything to lock out or block the hazardous gravitational potential energy in the heavy load of stone on the belt.

Figure 1: A view of the head pulley showing the north and south side speed reducers. Only the north side speed reducer had a backstop. When the north side torque arm was disconnected, there was nothing to stop the belt and its load from running downhill due to gravity. The moving belt caused the partially disconnected north side speed reducer to unexpectedly rotate and repeatedly strike a miner.
Figure 2: A composite view of two speed reducers. The speed reducer on the left has a backstop as indicated by the red arrow. The speed reducer on the right has no backstop.

The mine relied on an outside company to properly design and install all components of the hill conveyor including the motors, speed reducers and the backstop function. The mine agreed that the backstop on the hill conveyor is a safety device but did not understand its importance. If the outside conveyor design company had recommended two backstops for the hill conveyor, one for each side, the mine would have purchased two backstops. The conveyor design company did not warn or instruct anyone at the mine that the hill conveyor and the north side gear reducer would move suddenly and unexpectedly if the turnbuckle on the north side was removed. The conveyor company did not train, warn or instruct anyone at the mine about how to prevent the conveyor from rolling backwards. The conveyor company did not warn or provide instruction that there was only one backstop on the hill conveyor drive. The conveyor company also confirmed that the backstop is a safety device to prevent backward rotation of the conveyor belt. The conveyor company agreed that two backstops on the hill conveyor would have made the hill conveyor safer.

The conveyor company agreed that it was technologically and economically feasible to add another backstop. The conveyor company agreed that the hill conveyor would not have released if there was a functional backstop on the other side of the hill conveyor. The cost of an additional backstop was negligible in comparison to the total cost of the conveyor company’s work.

As a result of my investigation of the incident, I concluded that:

  1. The hazard that injured the miner is a hill conveyor with two motors, two speed reducers and two turnbuckles but with only one backstop located on the north side speed reducer and no backstop on the south side speed reducer. When the turnbuckle on the south side is removed, the belt will not move because the north side backstop prevents the north side speed reducer from turning in reverse. If the turnbuckle on the north side is removed, the north side backstop becomes ineffective, and the south side gear reducer has nothing to prevent it from turning backward. As a result, the belt and its load run downhill, and the north side speed reducer rotates hazardously about the head pulley shaft.
  2. The single backstop on the hill conveyor with two motors and two speed reducers does not adequately control the hazard of unexpected movement of the belt, load, and machinery. The turnbuckle on the south side of the head pulley can be loosened or removed safely but removing the turnbuckle on the north side will produce disastrous consequences.
  3. It was technologically and economically feasible for the outside conveyor company to adequately control the hazard of unexpected movement by installing a backstop on each of the two speed reducers so that either turnbuckle could be disconnected without the conveyor moving due to gravity.
  4. It was or should have been foreseeable to the outside conveyor company that the hill conveyor would need to be serviced while the belt was loaded. The outside conveyor company could have and should have provided an adequate protective device to prevent the hill conveyor and speed reducers from moving during servicing operations.
  5. The hill conveyor is defective because:
    1. It contains an inadequately controlled hazard of unexpected movement due to gravity.
    2. There is a high and foreseeable risk of severe injury from uncontrolled movement of large, heavy power transmission components including the speed reducer and its turnbuckle.
    3. There is a high probability of occurrence of harm any time the large, heavy, uncontrolled rotating components strike a worker.
    4. It was technologically and economically feasible for the outside conveyor company to install a second backstop on the south side speed reducer that would have prevented the hill conveyor, north side speed reducer and turnbuckle from moving unexpectedly.
    5. The risks associated with the hill conveyor outweighed its benefit.
    6. Adequate warning or instruction was not provided.
  6. The defective condition of the hill conveyor was a cause of the miner’s injury.

If you have a case involving an injury caused by conveying equipment, please give us a call. We at The Warren Group would be happy to help you determine the cause of the incident.

Jeffery H. Warren, PhD, PE, CSP, is the chief engineer and CEO at Warren specializing in mechanical, machine design and safety.  His deep expertise in machine design and safety analysis makes him a frequent presenter, trainer and expert witness. In addition to investigating more than 2000 claims involving property damage and injuries related to machinery and equipment since 1987, Jeff has an undergraduate degree in Mechanical Engineering from UNC Charlotte as well as a Master of Science and a Doctorate in Mechanical Engineering from Virginia Polytechnic Institute and State University — both with machine design emphasis.

 

Conveyor Backstops: Sometimes One Isn’t Enough, Part 1

This is the first of a two-part blog series describing an incident involving conveying machinery that seriously injured a miner. Part 1 describes the machinery and the incident. In Part 2 I will summarize my engineering analysis of the incident and share the conclusions I reached.

A loaded, inclined conveyor belt may contain hazardous levels of energy due to gravity. To protect workers, anti-reverse devices called backstops are installed on inclined conveyors to prevent unexpected downhill movement. The Conveyor Equipment Manufacturer’s Association (CEMA) defines a backstop as: Read More

Moisture Intrusion into Concrete Slabs-on-Grade

Scenario:  A young couple is excited about buying their first home.  They pick it out from a catalog of house plans from the developer of a new neighborhood in town.  It will be a brand new construction starter type bungalow home on a slab-on-grade foundation.  For a cost adder, which is considerable to them, they decide to upgrade the finished flooring from carpeting to hardwood laminate.  Unfortunately, after only a few weeks of residency, the hardwood planks begin to buckle and separate from the slab throughout the house.  They do not know what is causing it, or what to do to remedy it, Read More

Spontaneous Combustion…Is it hot in here or is it just me???

In the old-timey Fire Triangle, you have heat, fuel, and oxygen.  Get these three together in the right quantities, and you get fire.  What if the fuel provides its own heat?  That’s spontaneous combustion, or spontaneous ignition.  NFPA921 defines this as “initiation of combustion of a material by an internal chemical or biological reaction that has produced sufficient heat to ignite the material.” Read More

Issues with Moisture Intrusion through Flashing into Wood-Framed Structures

Flashing is a hot-button topic amongst residential designers, builders, and claims adjusters alike. In fact, currently one of the most prominent construction defect claims that we see involves moisture intrusion around improperly flashed window openings in wood-framed structures. Other trouble spots include the roof penetrations at chimneys and vent pipes. Read More

Heavy Machinery Fires Caused by Hydraulic Hose Failures

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Heavy machinery fires are often caused by hydraulic hose failures.  Pressurized hydraulic fluid escaping from a failed hose assembly can be atomized into a fine spray that can be ignited by heated engine surfaces such as the engine exhaust or turbocharger.

Hydraulic hoses near the engine compartment of an excavator that burned.

 

Hydraulic hoses often fail due to age and wear, requiring regular inspection and replacement of hydraulic hoses to prevent failures. Hoses may also fail if they are misrouted.  Misrouting can lead to the hose being pinched or causing it to chafe against a sharp metal surface. Read More

Installation of Structural Sheathing on Wood-Framed Structures

The facts presented in this blog lead to an interesting story.  During the heyday of the residential construction boom in coastal South Carolina circa 2005, many General Contractors were forced to go out-of-state to find framers and other subcontractors due to the demand creating a local labor shortage.  It turned out that the state of Texas had excess capacity and availability of framers and carpenters. Many Texas framing crews came to South Carolina to satisfy the shortage.  It soon became apparent that the Texas crews, many of which were from inland locations, were accustomed to installing 4’x8’ exterior wall sheathing with the long dimension vertical, i.e., parallel to the studs.  It is especially important and required that the long dimension be oriented perpendicular to the studs.  Laboratory testing has shown that Read More

Uplift and Shear Restraint Techniques for Residential Structures in Hurricane Wind Zones

Hurricane and Tropical Storm strength wind forces can wreak havoc on wood-framed residential structures.  One of the primary hazards is the negative pressures which can develop on the exterior building envelope when the structure is subjected to the high encircling winds.  These negative pressures act like the suction of a giant vacuum on a dwelling’s roof diaphragm, which produces enormous uplift forces throughout the entire structure.  The leeward walls are also subjected to negative pressures, while the windward walls take the brunt of the positive wind pressures.  The uplift on the roof is caused by what the author dubs “the airplane wing effect”.  In other words, Read More

Interpreting Industrial Incident Data – Lesson Learned

This is a case study about an incident I investigated involving a major upset in a distillation column.  This blog builds on the previous blogs about the Distributed Control System, DCS – Data is the Key.

Distillation is a method of separating mixtures of compounds with differing boiling points.  Uncle Bill with his still on the hill separates ethanol, that boils at 173°F, from water that boils at 212°F.  If the mixture is heated to above 173°F, but below 212°F, the ethanol will boil, the vapor will travel up out of the unit and then can be condensed and served over ice with an olive…   Any mixture of two or more chemicals with different boiling points can be separated in this way.  The distillation Read More

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