The ZSQF series pneumatic piston tank-bottom valve uses an integral straight-flow design with a smooth flow passage, low resistance and no dead zones. Soft-seated versions achieve zero leakage, while metal-seated versions withstand high temperature and particulate media. Downward-opening and upward-opening versions are available. In the downward-opening type, the plug moves into the valve body when opening and closes readily under tank vacuum. In the upward-opening type, medium pressure assists sealing; the plug moves into the vessel when opening, facilitating discharge of sediment.