Compression fittings are also used extensively for hot and cold water systems and are well suited to this application, especially where working in confined spaces where copper pipe would be difficult to solder. The fittings are composed of an outer compression nut and an inner compression olive that is typically made of brass or copper. Olives vary in shape and material but are most commonly in the shape of a ring with beveled edges.
When the nut is tightened, the olive is compressed between the nut and the receiving fitting and clamped around the pipe: The middle of the olive bows away from the pipe, making the olive effectively thicker forming a tight joint. Thread sealants such as joint compound and 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 olive between the nut and pipe.
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 olive will deform improperly causing the joint to fail.
For an explanation on BSP threads, the meaning and importantly the sizing, please check our guide below