Silo Holds Mysteries

Readers ponder why an old tank failed.

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THIS MONTH'S PUZZLER
We used an old welded stainless steel silo at our Missouri site to store dried brewers grain. During a September shutdown, we installed a new bagger and weigh scale. This prevented us from loading railcars for several days. So, to keep the dryer running during the outage, we cut its output to a rate that would only fill up the silo. The weather was hot but a rainstorm swept through with a cold front. After the storm, we noticed a seam on the silo had split apart, dumping several hundred tons of dried brewers grain. Now, corporate managers, concerned because another plant is planning a similar shutdown and bagger installation soon, want to know why the silo failed. What do you think caused the accident? What should we tell management?


AGE IS AN ISSUE
Evidently, no one bothered to check the outlet temperature of the brewers grain and just slowed the line down without adjusting the airflow and temperature. The grain probably absorbed more energy in the dryer and entered the steel silo hotter than usual.

It was an old silo. The weld was probably stress cracked enough that it would have likely split anyway within the next 6–12 months. The combination of hot grain on the inside (expanding the metal) and cold air/rain on the outside (contracting the metal) probably caused an existing stress fracture to grow until it could no longer handle the load and split.

X-ray the welds on the silos that are going to used during the conversion at the other site. The problem is likely the age of the silo, not the fact that a slight process change sent it over the edge to failure.
John G. Tiessen, applications engineer
Sun Chemical, Northlake, Ill.


FIND THE CAUSE OF STRESS
There are eight forms of corrosion: 1) uniform or general attack; 2) galvanic; 3) crevice; 4) pitting; 5) inter-granular; 6) selective leaching; 7) erosion; and 8) stress. Because of the dry environment in the silo and the lack of erosive forces, the only applicable form of corrosion to consider is stress corrosion.

Stress corrosion is the result of a combination of tensile stress and a specific corrosive medium. A classic example is the failure of brass ammunition cartridges in the tropics after heavy rainfall. There is a parallel in this story of the stainless silo involving high summer temperatures and a sudden temperature change induced by rain. In the case of the silo, any corrosive media must be very slight and the predominant mode of failure is simply mechanical stress.

I would tell corporate management that the failure was most likely due to stress and the cause of the stress will be investigated. I would also speculate the causes of the stress that will be investigated include 1) rapid temperature change in a low strength enclosure and/or 2) stress induced from "flow of solids" rheology phenomenon involving the full hopper and the temperature change.

In regard to "flow of solids" phenomenon, it is common for hopper angles to be improperly designed (not sloped enough) when compared to the angle of internal friction of the solids. The addition of aerator devices to partially fluidize the bed of solids and reduce the angle of internal friction can give far superior results to external vibrators with much less mechanical stress. The sudden temperature change shrinking the metal onto a bed of solids that refused to flow may thus have been the root cause of the mechanical stress.
Mike Gentilcore, chemical process engineering manager
Covidien Imaging Solutions, Hazelwood, Mo.


CHECK THE SILO AND THE PROCESS
We need information to decide if the cause was the silo or the process. For the silo, let's ask several types of questions: 1) How does the failure look? Is there corrosion or embrittlement? Does the weld appear normal? Did failure occur in the weld, at the intersection of weld and welded material, or in the tank wall not impacted by the weld? How do other welds (those not failed) look? 2) Is wall thickness adequate for the density and depth of material stored? 3) Is the silo adequately vented against over-pressure or under-pressure? Does it appear bulged or collapsed? and 4) What is the history of this "old" silo? How many years has it been used? How many fill/empty cycles has it gone through? Has it been overloaded anytime in the past? Is it subject to vibration from attached or other piping or equipment? Has it been left idle for years? When idle, was it kept clean and dry?

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