A compression washer, better known as a compression fitting, is a plumbing device that connects one or two pieces of pipe to a valve or fixture of some sort. The compression fitting stops leakage without soldering or any sort of welding, making them popular with DIY plumbing enthusiasts.
Compression fittings solve the problem of connecting pipes that transport liquids. While connecting two pipes by use of a washer or some sort of screw-tight connection allows liquids to escape, causing leaks and rendering the pipes useless, a compression fitting uses physics to stop leaks. A compression fitting utilizes a ring made out of a malleable material inserted inside the connection, over the gaps left by the nut and washer that connect the two pipes. Liquid that would originally have escaped through these gaps presses against the compression ring, and the ring expands against the walls of the pipes and the fitting, stopping any sort of liquids from escaping. The same liquid pressure that renders non-compression fittings leaky, creates a seal against the walls of the fitting.
Compression filters must be reinstalled if taken apart for maintenance purposes. However, the reinstallation process takes less time than breaking and re-soldering a welded joint.