The Mechanical Compression Joint is composed of an outer compression nut and an inner compression ring (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/olive but a flange with a ring of bolts that performs this task. The bolts have to be tightened down evenly.
Thread sealants such as joint compound 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 plumbers 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 over tightened the ferrule will deform improperly causing the joint to fail. Indeed, over tightening 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.
Mechanical fittings generally include a stiffener which is placed inside the pipe or fitting to ensure stability when the fitting is compressed. These stiffeners are also designed to withstand end loading where necessary. The fittings also include gaskets which are compressed to effect the seal at the rated pressure of the fitting.
