In general, we (and NSF and most recognized pool regulatory agencies) recommend no greater than 20 GPM/sq ft of filtration area in a sand filter. In some of our larger filters (TR100 and TR140) we recommend slightly less unless using our commercial versions, which have multiple diffusers to better distribute the water. When flow gets too high in a sand filter, it can create an uneven sand bed, or what you will sometimes hear referred to as channeling. You won’t so much see the pump “push dirt†through the filter as you will see it compromise the sand bed depth, and thus the filtration capability of that sand bed. It can also disrupt the integrity of that sand bed. If it is constantly agitated then the dirt could potentially work its way down through the sand bed as opposed to getting caught up in it. As to how big the pump would have to be, it of course depends on the system head and how it relates to the performance curve of the pump, but we do cite maximum flow rates on our filters which are generally based on that 20 GPM/sq ft rule. From the NSF 50 standard, here are the rules on sand bed flatness that generally assure effective filtration (but of course a lot depends on the type/quality of sand as well; we typically recommend #20 silica sand with a uniformity coefficient of 1.75 or less):
Way to go 
