p 1 equalizes with relatively minor pressure drops with the pressure p 2 in the bonnet cavity 108 the bonnet cavity 108 is fluidly coupled to the right side of the gate above the seat pocket 18 b so that the pressure p 2 on the right side of the gate in fig2 is equal to the pressure p 2 on the left side the mirror image orientation of the seat 20 b in the seat pocket 18 b and the reverse orientation of the seals compared to the seat 20 a creates a seal so that the fluid at pressure p 2 is prevented from leaking downstream of the seat 20 b into the downstream portion of the flow passage 13 at pressure p 3 thus the upstream seat 20 a leaks intentionally and the downstream seat 20 b seals intentionally in the orientations of the fluid flow shown if the flow was reversed so that seat 20 b became the upstream seat and seat 20 a became the downstream seat the result would be the mirror image where the upstream seat 20 b would leak and the downstream seat 20 a would seal the simplicity of this design and yet the ability to seal in such fashion is caused by astute orientation and selection of the various components described here more specifically the pressure p 1 in conjunction with the action of the wave spring 96 forces the seat 20 a toward the gate 30 the metal to metal contact of the metal radial sealing surface 90 or the metal radial sealing surface 92 or both is not effectively engaged to seal against the.