Every plumber carries plumber's tape and uses it almost every time the occasion arises to join threaded water pipes and fittings, but there is one exception. Applying the tape to the threads of a compression fitting usually isn't necessary because the fitting is already watertight. If a compression fitting leaks, it won't be because the threads aren't sealed but because the compression ring isn't seated properly or has otherwise failed.
Plumbing Tape and Threaded Fittings
Threaded pipe ends and fittings are common on galvanized steel and iron pipes, and faucet hoses and valves often have threaded fittings as well. When they are tight, the threads lock and hold water inside the pipes, but even a small gap can be enough to allow the water, which is under pressure, to escape. To prevent this, plumbers seal the threads with pipe dope or plumbing tape, the latter being more common because of its ease of use. Wrapping the tape clockwise around the threads gives the metal a waterproof coating that seals gaps when the fitting is tightened.
A compression fitting is unlike a regular threaded fitting. It has a threaded seat large enough to accommodate the end of a pipe, but the pipe doesn't have threads and doesn't screw directly onto it. Instead, it fits inside, and a ring that fits tightly around it seals the gap between the pipe and fitting. When you tighten the compression nut, which also fits around the pipe, onto the seat, it compresses the ring into the gap to seal the water. The more you tighten the nut, the more the ring is forced into the gap and the better the seal.
Taping Compression Fittings
Because the ring produces the seal in a compression fitting, wrapping plumbing tape around the threads is unnecessary and even contraindicated. If the fitting is leaking, the plumber may hope to stop the leak with plumbing tape. After applying it, the fitting is more difficult to turn because the tape fills the threads, which may cause the plumber to over-tighten it, possibly damaging the ring, or worse, pushing the ring so tightly between the pipe and the seat that the fitting cracks. If a compression fitting leaks, replacing it is better than attempting to stop the leak with plumbing tape.
For compression fittings to work properly, the ring must be straight and the fitting must be left stationary once it is tightened. If pressure is applied to the pipe, the ring can dislodge and the fitting will leak. When this happens, you may need to unscrew the connection, adjust the ring and reassemble it. If this doesn't work, you usually have to replace the fitting. A compression fitting can be disassembled and put back together a number of times, but its reuse for a different application other than the original one isn't recommended.
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