There is a class of bulk solids that do not seem to follow these simple assumptions and attempting to design for these materials cannot be successfully done using these equations and theory developed by Jenike and Johanson. These are bulk solids they have friction angle and possess bulk strength, but also have a viscous behavior to them. Oil sand, wax, biomass material, copolymer plastics and particles subject to glass transition, and materials that exhibit softening at elevated temperature all demonstrate some form of viscous component that changes the solids flow behavior. The following work presents test techniques that can be used to characterize these types of bulk materials so as to get data for the successful design of process vessels that must handle these materials. This work will present changes to the current theories that allow calculations of arching, rathole, mass flow velocities, and limiting flow rates – basic theories describing the flow of these viscous solids. The heart of the issue is that the unconfined yield strength is a function of both the consolidation stress values and the strain rate experienced by the bulk material during operation. This strain rate dependent bulk strength changes the mass flow characteristics as well as the incipient failure behavior of the bulk material in process equipment. This means hangups are coupled with velocity profiles in process equipment. The net result is that, under some situations, arches and even ratholes can form due to the strain history of the bulk material. The shape of hoppers capable of mass flow for these types of material then depends on the viscous behavior of the unconfined yield strength. New radial stress mass flow equations and velocity calculations will be presented and arching equations will be redefined to include the effect of these viscous solids.