As for the process, investigate: 1) Was the grain too wet or too hot? 2) Was there an upset condition due to trying to run the dryer slowly? 3) Did the silo get wet from condensation due to the temperature change or rain intrusion? 4) Was spilled material too heavy for the silo due to being wet or did it ferment and cause gas buildup (then why not vented?)? and 5) Was there a big burp?
Rolfe Hunt, manufacturing technical director
Oil-Dri Corporation of America, Chicago, Ill.
AVOID THE SILO NEXT TIME
This appears to be a materials handling problem. The failure of the old silo could be due to a load factor or a defective weld. Nondestructive testing of the welding, e.g., via x-ray, should be performed. The possible effect of weather on the structure and its contents is probably difficult to ascertain. As far as a similar shutdown, it is suggested that the dried brewer grain be conveyed from the dryer outlet to either bulk 20-ton railcars or truck hopper units to allow production at full capacity; railroad or truck scales can be used to obtain net weights. A diverter valve could be installed in chute work to permit switch over to 50-lb. bags when that system is ready to run.
Robert Drucker, consultant
East Northport, N.Y.
BLAME THE WELDS
It must be defective welding.
Nandan M. Bhandari, engineer
Maple Biotech Pvt. Ltd., Pune, India
WAS THE SILO ALTERED?
Assuming the silo is made of stainless steel, have they cut openings in it? This could cause stress concentration, hydrogen embrittlement and other problems. Inspect the silo to assure that there are proper stiffeners.
B. S. Angadi, engineer
Vijay Engineering Service, Mumbai, India
If there was an opening in the silo when the rain storm came through water could have entered the silo and caused the grain to expand. During June 2008 in Cedar Rapids, IA there was a 500 year flood. River water got into a soy bean silo and the beans swelled. The swelling created so much pressure that it caused the concrete silo to fail.
Len Riker, operations superintendent
Penford Products Co., Cedar Rapids, Iowa
There are several facets to this problem. First, is the age of the silo a factor? Second, did running the dryer at a low rate affect the product filling the silo? Did weather affect the silo — was it one storm too many? Was it the mere fact that the silo was run at maximum level? Was something done during the new construction that could have precipitated the collapse?
Clearly all of these questions need to be answered before risking another plant. As with all accident investigations, start with the history. Sometimes information is sketchy. Even with computerized records much is lost or unreliable. Talk to the operators. They'll tell you about the spot repairs, times when the silo creaked or about sudden surges and such; surging could put unusual loads on the thin plates used to construct silos. History may also tell you what products were kept in the silo in the past that might have caused corrosion or erosion damage; it could be that the silo is a retread, a used silo moved to the plant from a junk yard.
The product quality of the dryer at low throughput should be investigated. Introducing moisture could cause the brewers grain to expand. If material was bridged inside, a radial load could put undue loading on old walls.
Weather could be a factor. Perhaps water dripped into an open manway or through an undetected hole in the shell, causing the same effect as mentioned above. It's also possible that a sudden change in atmospheric pressure caused the walls to collapse. Silos are designed for 1–3 psig differential per ASME code requirements for an atmospheric vessel; vacuum is a far greater risk than pressure — 2.5 inches of water column can collapse a tank. Silos tend to be tall with large length-to-diameter ratios. Aerodynamics and weather could have conspired to do in old corroded walls.