These corrosion-proof 18" PVC Butterfly Dampers are for the construction of exhaust ventilation systems. PVC material compounds used in the manufacture of these duct fittings conform to PVC 1120, ASTM D-1784, Type 1, Grade 1, Cell Class 12454-B. All internal parts are CPVC including the body, damper plate, and shaft. Locking hardware is stainless steel. Butterfly (balancing) dampers are furnished with a locking quadrant, to permanently position. Damper allows for less than 5% air flow restriction when open and offer an approximate 95% seal when closed. These dampers can be installed either vertically or horizontally. 18" damper is 17.75" long with a 2.5" socket depth and .25" thick damper plate. NOTES: Fittings are triple bead welded. Pictures are to represent style of fittings only; color is gray.