Installation of Air Cannons in old storage hoppers
Whitacre Greer is a century old, family owned manufacturer of dry pressed clay pavers and firebrick. Currently we have one plant in Alliance Ohio which operates on a 24/7 schedule. In May 2011 we had a lost time accident involving our largest clay storage bin – Bin D. This accident occurred after we had gone over 1300 days and 500,000 hours since our last lost time accident, a record we were all disappointed to see broken. As a result, we were determined to do “something” to keep this from happening again.
Our processed clay and shale is stored in 2 large bins (C&D), 3 smaller bins, and two bunkers. The configuration of the bins, with their converging sides, depth, and live bottom belt, make them confined spaces with an engulfment and fall hazard. The opening at the top is approximately 10 feet and converges together to an opening at the belt of about a foot. The depth of the bins is 20 feet. Two situations occur that result in personnel going into the bins to manually knock down the clay. One is when the product tunnels out from the bottom leaving a gap between the belt and the remaining product or tunnels straight down causing a rat hole affect. The other is when the material drops down unevenly leaving material attached to the walls and not dropping to the belt.
The solution of both conditions requires two employees. One employee puts on a safety harness, equipped with a self-retracting line (SRL), and gets into the bin with a shovel. The second employee is a safety person who remains outside. The operator in the bins must go down the ladder and proceed to shovel thru the material attempting to break the bridging at the bottom or on the sides allowing the material to fall thru to the live belt. Depending on the moisture content, time of year and production rate this can happen half-dozen times per shift.
On the day of the accident an employee had been digging thru the material for some time and was hot and fatigued. Before making the climb out of the bin, the live belt being inactive, he removed the safety harness. As he made his climb out he “got dizzy and fell” into the bin landing on his feet. This resulted in a closed fracture of metatarsal in his right foot, closed dislocation tarsometata, ankle sprain and contusions which required surgery to repair. The total compensation loss was over $17,000 not including the salary continuation, or pain and suffering.
We considered redesigning the bottoms of the bins allowing for more flow but the cost and downtime required was prohibitive. Instead, our intervention involved the placement of air cannons along the sides of C and D bins. Video cameras were also installed to aid the supervisor in determining where a problem exists and as a safety measure to be sure the bins are not occupied when the cannons are discharged.
After determining the number of cannons required and their size and placement, they were mounted on the backside and through the wall of the bins, protruding toward the bottom of the bin at a depth where the problems generally occur. It was determined that 7 cannons would be required, 3 in C and 4 in D.
The programming for the activation of the cannons was incorporated into our existing mixing/blending control center. A TV Monitor, connected to the video cameras, was installed on the wall beside the control center allowing easy access on the production floor. When a low or no flow situation has been detected the supervisor must determine where the problem exists. Normally this can be determined from the monitor. If it is necessary for the supervisor to enter the storage area, not the bins themselves, the safety procedure is to push the E-stop before entering the area to deactivate the air cannons.
Once the problem area has been determined the supervisor decides which cannons to fire and in what sequence. When the cannons are engaged, a 15 second pulsing alarm is activated in the bin area. Within 10 seconds of the end of the alarm a cannon can be fired. A second cannon can then be fired within 10 seconds of the first. This sequence continues until all necessary cannons have been discharged. If more than 10 seconds elapses, the system shuts down and must be restarted. Each discharge of 34 cubic feet is complete in .22 seconds.
Testing and training of the cannons has just been completed in November and they worked as planned during testing. They have not, however, been needed for an actual event. The only problem encountered so far is that we found it necessary to reinforce the bin walls to eliminate the vibration caused during cannon discharge.
As we were in the planning stages for this project, we came across an additional benefit. The Ohio Bureau of Worker’s Compensation, Division of Safety, currently has a program to pay up to $40,000 for safety intervention projects at a funding rate of 3 dollars for each company dollar spent. We applied for and received the maximum funding available.
