counting ring 97 and the locking ring 82 is free to be pushed in the upstream direction 54 by the wave spring 85 also illustrated in fig1 are an upper wiper seal 86 a lower seal 87 and a nut seal 88 according to the embodiment shown in fig1 both upper wiper seal 86 and lower seal 87 engage the upper piston 81 at the same diameter so there is no change in volume in annulus 89 when the upper piston 81 is actuated while not essential to the function of this embodiment of the fracture plug seat assembly this embodiment resists the accumulation of dirty fluid in the annulus 89 also the nut seal 88 guards against the incursion of debris into the space 91 expandable seat assembly 94 may be formed from any suitable material such as a segmented ring of drillable cast iron those of ordinary skill in the art will understand that the expandable seat assembly 94 may also be encapsulated in rubber so as to guard against the entry of contaminants into pocket 95 and to shield the cast iron from the abrasive fluid passing through the expandable seat assembly 94 it is to be understood that the means to.