A sealing system, sealing system components, and methods of manufacture and use for a vessel that holds corrosive liquid, with the sealing system having a port on the vessel, an insert, and a sealing element. The sealing element is corrosion-resistant, essentially impermeable to corrosive and critical fluids, essentially free of leachable impurities and is made with a material having a Shore A hardness between about 70 and about 95. A preferred material for the sealing ring is the class of materials known as polyolefins. The insert is operably engageable with the port and the sealing element to create a seal between the sealing element and the first and second sealing surfaces to prevent fluid flow through the port.