High Pressure Valves and Fittings SD range of twin ferrule fittings work on the proven principle of two ferrules swagging to the tube when tightened. As the nut is tightened the front ferrule swages on to the tube taking a prominent leak-proof seal and the back ferrule makes a positive strong hold on the tube. Make and remark of the fittings is very much eased, as nuts are silver plated internally to prevent galling of threads. The identification between Metric and Inch fittings is made very simple with the help of stepped shoulders in metric fittings. Range of twin ferrule tube fittings manufactured are in between 1/16" to 1" and in metric tubing from 3 mm to 25 mm.