Bellow Stem Seals Fractional Flow Control Valves Bellows stem seals eliminate fugitive emissions by surrounding the valve stem with a pressure-tight barrier, isolating the stem from the process fluid and preventing leakage to the atmosphere. For an extra measure of protection, the bellows stem seal option provides a back-up seal of Teflon or Graphite/Grafoil packing along with a threaded or NPT detection port for use in monitoring potential emissions leakage.