Abstract: A closure for pipelines carrying a fluid under pressure, the closure includes at one or more seals located on a vertical sealing face of the barrel collar along a leak pathway between a horizontal sealing face of the barrel collar and an ambient environment. In some embodiments, the horizontal sealing face includes a primary seal. The one or more seals are adapted to prevent the escape of fluids having a lower molecular weight than that of the main pipeline product or fluid contained by the closure, and is arranged to alter a path of a leak pathway to the ambient environment and to cause at a pressure drop across the one or more seals. The closure may also include a leak check valve for blowdown.