The preferred method to fumigate grain cargoes originating from US Gulf ports is through the use of gas pellets (Aluminum Phosphide), which are distributed / scattered uniformly on top of the leveled stow of bulk grains by private companies which are certified applicators.
One of the benefits of the surface application of gas pellets is that there is an initial rapid build-up of gas in the void space on top of the cargo in the hatch square / coaming, with subsequent penetration downward into the cargo over a certain period of time. The pellets will react with atmospheric moisture and dissolve over time creating phosphine gas. The reaction process generates heat as the pellets dissolve. The speed of the reaction increases depending on higher temperatures and/or relative humidity of the air locked inside the void space. All that is left after the pellets are converted into gas is a gray-white powdery ash.
Aluminum phosphide by itself is not flammable, but when it becomes wet through contact with moisture in the air or water, hydrogen phosphide gas will be created, which can ignite spontaneously in air at concentrations above its lower flammable limit. Ignition of a high concentration of phosphine gas can result in explosions.
During the past several years there have been several cases where the phosphine gas on top of grain cargoes spontaneously ignited resulting in explosions inside the cargo holds. The power of these explosions resulted in the steel hatchcovers being lifted up out of their sealed position, resulting in deformed hatch panels, torn off cleats, etc.. Some vessels had to discharge their cargo at the load port in order to carry out required repairs prior to being allowed to reload their grain cargoes.
Investigations on these vessels revealed that the reason was uneven distribution of the gas pellets on top of the bulk stow. It was ascertained that the gas pellets had been piled in small heaps along the inside of the hatch coaming square. The piling of the pellets resulted in a concentrated / accelerated release of phosphine gas which filled the void space to the point that it reached its lower flammable limit and spontaneously combusted / exploded.
It is recommended that the crew keep a watchful eye on these cargo holds which are being fumigated to make sure that the gas pellets are evenly distributed / scattered across the top of the stow and that the pellets are not simply dumped in large piles, which could result in explosions during the sea voyage.
For vessel specific enquiries, members are asked to contact their usual Skuld business unit.
The Association is grateful to Frans C. Coppers of Coppers & Company for contributing to this article.