A maintenance worker was replacing a part that forced him to reach into the head mechanism of a conveyor when the conveyor unexpectedly started, seriously injuring him.
The conveyor on the roof of a grain-elevator facility transported grain to and from arriving trucks. After the conveyor was installed, the company operating it cut a hole in the head-mechanism cover and replaced it with a hinged access door, secured by thumb screws.
The door provided access to a plug switch the operating company also installed. The switch detected the level of grain on the conveyor and switched off the conveyor if it plugged up. Workers could reset the switch once the blockage was cleared and grain levels returned to normal.
The worker and a partner were replacing the plug switch when the accident happened. Although a supervisor had instructed them to lock out the conveyor’s power supply in an electrical room elsewhere in the facility, conveyor operators said they needed the machine available to service arriving trucks.
The maintenance workers and conveyor operators reached a compromise in which maintenance work would wait until operators radioed from their control room that a 10-min shutdown was possible. The operators would radio again to warn the maintenance workers that they needed to restart the conveyor.
This shutdown-restart arrangement worked well through two cycles. After the third shutdown, however, the conveyor started without a radioed warning.
One worker had his arms inside the conveyor head cover when the mechanism started up. He was severely injured before his partner was able to trigger the plug switch and shut off the conveyor.
Investigators noted numerous lapses that contributed to or intensified the severity of the injury. First, the access door did not have an interlock to shut down the conveyor or prevent it from starting when the door was opened. The thumbscrews securing the panel indicated it was intended to be removed quickly, without tools, so it was reasonably foreseeable that someone removing the panel might not have gone through lockout/tagout (LO/TO) procedures.
Second, there was no LO/TO provision near the conveyor head where most of the maintenance took place. Because the power disconnect was in a separate location, it was less likely personnel would take time to perform proper LO/TO.
Third, neither of the workers received LO/TO training at this company, nor had they been specifically trained with regards to the conveyor.
Fourth, there was no warning that the conveyor was going to start. If the control-room operator was not able to visually verify no one was working near the conveyor, there should have been a provision to alert workers and delay the conveyor’s start to give any nearby workers time to reach safety.
Fifth, there was no emergency-stop provision near the conveyor head. If one had been available, the uninjured worker could have more quickly stopped the conveyor and minimized injury.
Further investigation revealed that the conveyor’s design was not the primary cause of the accident. The manufacturer, at the operating company’s behest, had installed the conveyor at a steeper angle than had been recommended. The steeper angle led to more-frequent plugging of the conveyor and necessitated the plug switch and easier access to it.
The operating company had added the access door to the head-mechanism cover without the manufacturer’s knowledge. Although the manufacturer made similar modifications for other customers, its access doors were always interlocked to stop the conveyor when opened and secured with fasteners that required tools to open.
This month’s safety violation comes from the files of Lanny Berke, a registered professional engineer and Certified Safety Professional involved in forensic engineering since 1972. Got a safety violation to share? Send your images and explanations to [email protected].
Edited by Jessica Shapiro