Preventing Entrapment & Suffocation
The entrapment and subsequent suffocation of workers in stored grain and other materials is preventable, and managers, supervisors, and workers all have a role in creating and maintaining in achieving the goal of total safety.
Collapsing material has entrapped and suffocated workers while they were working on or under the unstable surfaces of stored grain or other materials such as sand, gravel, and coal. In many cases entrapment results in death because workers cannot free themselves and their coworkers’ attempts to save them fail.
Entrapment and suffocation are hazards associated with the storage and hoppers where loose materials such as grain, sand, or gravel are stored, handled, or transferred. This type of material is unstable and its behavior is unpredictable. Entrapment and burial can occur in a matter of seconds. Fatalities most frequently occur when suspended materials or crusted surfaces of stored material suddenly break loose and engulf workers who are unable to free themselves.
Sometimes, material being drawn from the bottom of storage bins can cause the surface to act like quicksand. When you empty a storage bin from the bottom, the flow of material forms a funnel-shaped path over the outlet. The rate of material flow increases toward the center of the funnel. During a typical unloading operation, the flow rate can become so great that once its momentum draws a worker into the flow path, escape becomes impossible.
Bridging is a condition that can create hazardous situations. It occurs when grain or other loose material clings to the sides of a silo or bin that workers are emptying from below. A bridge of material may collapse without warning, entrapping workers who are standing below or on top of the bridge and who are unaware that the surface is unstable.
Bridging can occur in storage bins, silos, and hoppers that contain ground grains or meal such as soybean meal or other loose materials such as cement, limestone, coal, or sawdust. Grains such as barley, oats, and corn, that have not been ground, are less likely to form bridges because the individual kernels do not stick to the sides of the storage bin. In addition, the diameter of the storage vessel and the moisture content of the stored materials are factors that contribute to bridging.
The following cases studies of fatalities come from the National Institute for Occupational Safety and Health (NIOSH):
Two men were working outside a bin that was being emptied of grain. Believing that the bin was completely empty, one of the workers entered it through the bottom access door. After he entered the bin, material suddenly broke loose from the sides of the bin buried him. The worker was unable to find the access door, and the other worker could not locate him in time to save him [NSC 1977].
A self-employed truck driver pulled his truck underneath an auger that dispensed sawdust. When the driver saw that very little sawdust was coming out of the auger, he turned the auger off and entered the top of the bin to dislodge sawdust from the sides using a long metal pole. While the worker was standing on the bridged sawdust, the surface collapsed beneath him, burying him with sawdust and suffocating him [NIOSH 1986a].
A worker received fatal injuries while trying to free a blockage of sand when the bridged or caked surface of the sand pile on which he was standing collapsed. He was engulfed and suffocated [MSHA 1986a].
No specific requirements cover work on or around grain storage areas. However, NIOSH believes that there are existing standards whose requirements are appropriate for similar applications in general industry. These are:
- The OSHA construction safety standards [29 CFR*250 (b) (2), General Requirements for Storage] require workers to use safety belts while working on stored materials in bins or similar storage areas.
- The Mine Safety and Health Administration (MSHA) has requirements for storage of materials in the mining industry [39 CFR 56]. These requirements address the storage of loose, unconsolidated materials [39 CFR 56.16002], safe access [39 CFR 56.11001], and the use of safety belts and lines [39 CFR 56.15005].
The facts of the incidents reported here suggest that the following factors may have contributed to the fatalities:
- Materials became lodged in storage areas, prompting workers to enter the areas to dislodge them.
- When workers entered storage areas, they were unaware that the stored material was unstable or bridged.
- Workers were operating below or on top of unstable stored grain or other materials.
- Properly equipped standby personnel were not present.
- Safety belts or harnesses were not used.
- Supply and discharge equipment for moving grain or other material was not de-energized or locked out.
- Proper emergency procedures were not in use.
NIOSH recommends the following to address the seven factors noted above:
- Train workers to assume that all stored materials are bridged and that the potential for entrapment and suffocation associated with stored grain or other loose materials is constant. The training should include information on safe work practices and rescue.
- Never allow workers to enter a storage area from the bottom when material is adhering to the sides or is bridged overhead.
- When workers must enter storage areas, they should stay above the material at all times and should never stand on top of stored material.
- Post safety signs to warn workers of the hazards of working with stored grains and other loose materials. Safety signs alone are not sufficient to provide the information needed to prevent fatalities; such signs should be only one component of a comprehensive safety program.
- Equip bins, hoppers, silos, tanks, transport vehicles, and surge piles where loose materials are stored, handled, or transferred with mechanical devices or other means of handling materials so that workers do not need to enter such storage areas. You can usually prevent bridging by mechanical agitation or vibration of stored materials.
- Stop the supply and discharge of materials, and lock out the supply and discharge equipment any time a worker enters a storage area (bin, tanks, etc.) (NIOSH 1983].
- Workers should wear safety belts or harnesses equipped with properly fastened life lines when entering storage areas. A similarly equipped standby person should remain outside the area [NIOSH 1979, 1986b].