These fittings include a nut with a built-in sleeve (ferrule), so there’s no need to remove the nut for assembly. The sleeve compresses the tubing as you tighten the nut, creating a strong seal. Fittings are also known as flareless fittings. They are brass for good corrosion resistance.Tube supports are recommended if you want to use the fittings with soft polypropylene and polyethylene tubing to prevent the tube from collapsing.NPTF (Dryseal) threads are compatible with NPT threads.