... prevents corrosive medium from etching the inner wall of the rubber joint in high temperature,acid and alkali and oil resistant pipeline ... metal parts and rubber balls/spheres bodyMetal can be chose:stainless or carbon steel flanges,galvanized malleable iron unionsRubber:NBR,NB,SBR ... ,EPDM etcStandards: BSPT,BSP,NPT,DIN,threads union.DIN2576,BS4504,JIS 5K,10K,ANSI B16.5 etc. universal rubber ...