Compression Elbow Fittings This elbow fitting provides the ability to change the direction of the pipe within a system 90° in the shape of an arc, whilst still providing a strong and secure joint. The MDPE pipes to be joined must be of the same diameter. To maintain a rigid and reliable joint connection, it is highly advised that pipe liners (also known as stiffeners or inserts) are also used. Sizes available from 16mm x 16mm to 110mm x 110mm.