Compression Coupling Fitting Pipe couplings are used to join two lengths of MDPE pipe together. These couplings require the pipes to be joined to be the same diameter. These pipe couplings are simple to install and provide a strong compression fitting joint. To maintain a rigid and reliable joint connection, it is highly advised that pipe liners (also known as stiffeners or inserts) are also used. Material: polypropylene Compression fitting Simple to install