Compression fittings are generally used for plumbing and electrics to help join pipes and tubes. They are mostly used where the two joins are constructed from differing materials like copper or PVC. In terms of plumbing you will see compression fittings commonly used in sink faucets and toilet valves.
How Does it Work
A compression valve actually looks like a valve as you would understand it. It has a copper ‘wedding-style' compression ring, which fits inside the valve. This fits between the joints of two pipes, which then compress it together to eliminate the chance of leaks in the joint. The nuts on either side of the valve tighten up and close the valve in so that the ring is compressed inside.
Using a Compression Fitting
There is no requirement for plumber's tape (Teflon) or joint compound in the use of compression fittings. They work perfectly without it as long as they are fitted correctly. They are also extremely popular with plumbers because they don’t need soldering and they are quick and simple to fit. Despite their ease, however, they are slightly weaker than soldered joints and it is recommended that they only be used with joints which require or cause no movement.