Transcripción del resumen del autor. Standpipes are an important element in Fluidized Catalytic Cracking (FCC) units. They supply the pressure buildup required to "drive" the solids through a controlling slide valve at the catalyst flux required. Most operators assume that the buildup in a fluidized underflow standpipe (the type used in FCC units) should be equivalent to the material's fluidized density multiplied by the length of the standpipe. Certainly, this is the maximum buildup possible in the standpipe, but standpipes do not always operate at this maximum pressure buildup condition. One reason that the standpipe does not always operate at its optimum pressure buildup condition is that the standpipe also serves another function in the pressure balance loop. The standpipe is the only element that can adjust itself to automatically balance the pressure drop in the circulation loop. Because of this, the standpipe does not always operate at optimum (maximum pressure buildup) conditions - it is constrained by the parameters of the pressure drop loop. If these constraints occur, standpipe pressure profiles and gas flows in the standpipe will be adjusted automatically to allow the pressure balance to occur, and negative standpipe buildups are even possible in certain limiting situations.