Summary

Forage silos are enclosed areas where serious and even fatal accidents occur. Some of these accidents are caused by an atmosphere deficient in oxygen (O2) -- a deficiency resulting from the presence of carbon dioxide (CO2) -- or from the presence of toxic gases, including nitrogen dioxide (NO2). These gases are produced during the fermentation of forage.

At the request of the Commission de la santé et de la sécurité du travail (CSST) and the Union des producteurs agricoles (UPA), the IRSST conducted a study of the literature dealing with toxic gases produced during the fermentation of forage, and on mechanical ventilation by blowing, in vertical silos used for silage. The results of the study were published in Report R-672 - La prévention des intoxications dans les silos à fourrage (The Prevention of Poisoning in Forage Silos), available on the website of the Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST).

This initial study on the ventilation of silos was carried out using Computational Fluid Dynamics (CFD). This numerical method had been validated using comparisons with experimental data obtained in a reduced-scale silo designed by the IRSST. Given that forage fermentation usually occurs anaerobically, all interconnecting gates between the silo and the chute were considered closed during these simulations. Consequently, the general mathematical model and the simplified model that were obtained to estimate blowing time only took into account the free space in the silo, without considering the ventilation in the chute.

By contrast, since its calculation includes the chute, the present study complements the previous one. It proposes a new general mathematical model, taking into account the total volume of the space to be ventilated, i.e., the volume of the silo plus the volume of the chute. It has now been demonstrated that the estimated time, including the time that has elapsed in the chute, changes slightly compared to the estimated time not including the time in the chute. It is therefore suggested that the new equation be used, since it better reflects the real situation.

To determine whether the previously proposed simplified equation could still be used, it was compared with the new general mathematical model. The comparison found that the simplified model is still valid. A numerical simulation, CFD, was performed for extreme cases: when the volume in the chute is significant compared with the free volume in the silo, the results demonstrated that, with a ratio of chute/silo volumes greater than 0.4 (extreme case), the time required to secure the chute was greater than that of the silo by about 10%. This has no impact on the simplified equation since it is still more conservative than the time estimated by the general model and the numerical calculations. The simplified model remains valid even in extreme configurations.