A first set of closers (18,21) at the same axial height close the line section (10,20) off under heat as against a second set (18',21') which are axially distanced from the first. First and second sets comprise segments which one or both are swung in closed by heat via a shape memory metal, bimetal or spring. In the two sets, only some of the distributed segments swing in to close the line off, leaving their respective interposed segments in their original position parallel to the line axis. The shape memory metal is preferably a coiled wire (23) which is run round the outside of the segments (18,18') so as to contract under heat and so force the segments inwards to closure.
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