Measurements of capillary suction potential in drainage processes have developed a capillary bundle model, which makes it reasonably possible to deduce a dimensionless capillary suction potential by considering the dependency of tortuosity. The capillary suction potential has been found to depend strongly on the tortuosity, especially for the compressible filtered cakes. The tortuosity determined empirically using the capillary model provides the dimensionless capillary suction potential which can correlate to the reduced saturation for both incompressible particulate beds and compressible filtered cakes. The final average residual saturation can be accurately estimated from calculation of a newly defined capillary number, which takes into account the final equilibrium saturation profile. The calculated results agree well with experiments observed for incompressible particulate beds drained under gravity.