Engulfment means to be swallowed up in or be immersed by material, which may result in asphyxiation.
Being inside a tank that contains liquid, and that tank then fills with that liquid while you are in there, you drown by asphyxiation. By isolating the valves leading into the tank, you are admitting that there is a risk of engulfment.
Examples of materials that may pose a risk of engulfment include plastics, sand, liquids, fertiliser, grain, coal, coal products, fly ash, animal feed and sewage.
Stored materials such as sand and grain can form a crust or bridge when a container is emptied from below, leaving the top layer in place. Workers walking on the bridge or working below the bridge on the floor of the container may be engulfed if a bridge collapses.