Which gasket material is appropriate for a flange union in cold water piping?

Rubber is the appropriate gasket material for a flange union in cold water piping due to its excellent sealing properties, flexibility, and resistance to water. Rubber gaskets can effectively accommodate the slight variations in flange surfaces, ensuring a tight seal that prevents leaks. They also perform well under low-temperature conditions, making them ideal for cold water applications. Rubber's durability and ability to maintain its sealing characteristics over time under varying pressures contribute to the overall integrity of the piping system.

The other materials mentioned may not provide the same level of suitability for this specific application: metallic gaskets may be too rigid and not form a proper seal under low-pressure conditions typically found in cold water systems. Glass is generally not used as a gasket material due to its brittleness and potential failure under pressure. Silicone, while versatile, is not most common for cold water piping where rubber is specifically designed for such applications.