As a general rule, gate valves are divided into one of two designs: parallel and wedge shaped. The parallel gate valve uses a flat disk gate as the closure element that fits between two parallel seats an upstream seat and a downstream sear. To achieve the required shut off either the seats or the disk gate are free floating, allowing the upstream pressure to seal the seat and disk against any unwanted seat leakage, in some designs, the seat is spring energized by an elastomeric that applies constant pressure to the disk seating surface. The knife gate valve often has a sharp edge on the bottom of the gate to shear particulars or other entrained solids as well as to separate slurries.
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