To the knowledge of the author there are not enough precise reports explaining the phenomenon of urea general caking stored in bulk (in onshore storage or in hatches of ocean going vessels). In the literature are found many studies and investigations reporting on the effect of various parameters to the caking (CRH, Particle Size and Hardness, Abrasion and Impact Resistance, Treatment and Conditioning, Storage Pressure and Temperature, Urea Moisture, Moisture Absorption, etc.), but only few explaining the mechanism of caking spreading from a relative limited initial area to the whole pile, observed often in stored urea even of very good quality, stored under more or less correct conditions in bulk.
The basic phenomenon governing the spreading of caking in a heap of urea in bulk, where initially a relative thin layer only, has absorbed humidity, (because hatch cover left open during a period of high atmospheric relative humidity), or where a relatively small amount of warm (above 55 oC) and of higher residual moisture urea is included into a heap of low residual moisture and relatively cold urea, is the so called humidity migration.
If this water pick-up, or this inclusion happened, and urea is subject to temperature cycling, which usually is the case during storage, or loading - shipping - unloading, then caking of the whole holding of the hatch may result. This caking may be severe resulting in hard to break lumps, or less severe (easy disintegration of lumps), depending on the quality of the urea and the number of cycles.