How they work
In small sizes, the compression fitting is composed of an outer compression nut and an inner compression ring or ferrule (sometimes referred to as an “olive”) that is typically made of brass or copper. Ferrules vary in shape and material but are most commonly in the shape of a ring with beveled edges. To work properly, the ferrule must be oriented correctly—usually the ferrule is fitted such that the longest sloping face of the ferrule faces away from the nut.
When the nut is tightened, the ferrule is compressed between the nut and the receiving fitting; the ends of the ferrule are clamped around the pipe, and the middle of the ferrule bows away from the pipe, making the ferrule effectively thicker. The result is that the ferrule seals the space between the pipe, nut, and receiving fitting, thereby forming a tight joint.
Larger sizes of compression fitting do not have a single nut to compress the ferrule but a flange with a ring of bolts that performs this task. The bolts have to be tightened evenly.
Thread sealants such as joint compound (pipe dope or thread seal tape such as PTFE tape) are unnecessary on compression fitting threads, as it is not the thread that seals the joint but rather the compression of the ferrule between the nut and pipe. However, a small amount of plumber’s grease or light oil applied to the threads will provide lubrication to help ensure a smooth, consistent tightening of the compression nut.
It is critical to avoid over-tightening the nut or else the integrity of the compression fitting will be compromised by the excessive force. If the nut is overtightened the ferrule will deform improperly causing the joint to fail. Indeed, overtightening is the most common cause of leaks in compression fittings. A good rule of thumb is to tighten the nut first by hand until it is too difficult to continue and then tighten the nut one half-turn more with the aid of a wrench; the actual amount varies with the size of the fitting, as a larger one requires less tightening. The fitting is then tested: if slight weeping is observed, the fitting is gradually tightened until the weeping stops.
The integrity of the compression fitting is determined by the ferrule, which is easily prone to damage. Thus care should be taken to when handling and tightening the fitting, although if the ferrule is damaged it is easily replaced.