Page 16 - Failure Analysis Case Studies II

Failure Analysis Case Studies II
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 3

BURSTING OF A SILO

R. KIESELBACH
Failure Analysis of Metals, Swiss Federal Laboratories for Materials Testing and Research,
Oberlandstrasse 129, CH-8600 Dlfbendorf, Switzerland
(Received30 Augmf 1996)

Abstract-This paper describes the bursting of a large silo on a farm, which caused considerable environmental
damage and cost. The cause was misuse of the silo for vegetable slurry instead of for feed for livestock, and
overfilling the silo. 0 1997 Elsevier Science Ltd. All rights reserved.
Key words: silo, failure, rupture, hydrostatic pressure

1. INTRODUCTION

Most of the accidents in connection with silos are due to suffocation or gas poisoning of the farmers
entering a silo. Some are also caused by the explosion of methane, which is produced by fermentation
of the forage. Cases of bursting or explosion are, nevertheless, rather rare.
In the present case, three identical silos had been built on a farm, each with a diameter of 6m
(20 ft) and a height of nearly 25 m (80 ft). The hull of the vessels was made of steel plates measuring
1.4 x 2.68 m, and the thickness of the sheets vaned from 5.7 mm at the bottom to 2.4 mm at the
top. All in all, the silo consisted of 16 rings and one base ring. The individual sheets had been
protected against corrosion by enamelling, and were joined by bolts and nuts. The joints were
protected against corrosion by a special kind of mastic. The total capacity of a silo was approximately
630 m3.
Since the farm no longer had any use for the silos, they were rented to a feed company for the
storage of feed for pigs. The slurry was delivered in tank cars and pumped into the silo. The silo
was filled up repeatedly in the following months. Finally, a few minutes after a delivery, when the
tank car had just left the site, the silo burst and spilled its contents, a slightly sour slurry. The
collapse of the top of the silo also damaged the next, still empty silo, which buckled and also
collapsed partially. The spilled slurry caused considerable environmental damage in addition to the
cost of the silos and the cost of the interruption to service. According to the lorry driver, the silo
had been, at that time, approximately three-quarters full, and the manhole lid had not been fastened,
but only laid loosely on its flange.

2. INVESTIGATIONS AND TESTS PERFORMED
2.1. Visual inspection
The site of the accident was visited and the following observations could be made (see Figs 1-3).
Silo 3 had failed and was severed above the seventh ring (counted from the bottom), where a
reinforcement ring was attached. A zone, four rings high, had been separated, and hung partially
on the silo, partially on the ground. The contents of the silo had spilled for approximately 30 m in
a semicircle uphill and 200 m downhill. The pasture had been destroyed, the slurry being slightly
sour after lactic acid fermentation.
The detached rings were separated into several pieces, and were in some places still immersed in
a pool of slurry, such that it was difficult to make out where the pieces had belonged. Failure had
Reprinted from Engineering Failure Analysis 4 (l), 49-55 (1997)