Movable plug for a fluid pipe, comprising: - two half-bodies (2, 3) arranged so as to form a fluid pipe, said half-bodies (2, 3) each being formed by a tube (21, 31) forming a pipe and at the ends of which tube are attached a flange (22, 32) and a plate (23, 33), respectively, each of said plates (23, 33) having a bearing surface (231, 331) and a gap between the two half-bodies (2, 3), - a seal in the form of a body delimited by two opposing planar longitudinal surfaces (111, 112) each arranged facing the bearing surface (231, 331) of the plates (23, 33), respectively, which seal has a solid part and a bored part through which an opening extends that opens into each of said longitudinal surfaces; the seal is mounted between the bearing surfaces (231, 331) in the gap between the two half-bodies (2, 3) so as to be movable between: • a closed position in which the solid part blocks the pipe so as to prevent fluid flow between the two half-bodies (2,3), • an open position in which the bored part is placed in the centre of the pipe so as to allow fluid flow between the two half-bodies (2, 3), - a locking mechanism for holding the plates (23, 33) in: • a spaced-apart position in which the bearing surface (231, 331) of the plates (23, 33) is spaced apart from the relevant longitudinal surface (111, 112) of the seal, so as to allow said seal to move between the closed and open positions, • a close position in which the bearing surfaces (231, 331) enclose the body of the seal in a sealed manner in order to lock said seal in the open position or the closed position, said bearing surfaces each coming into clamped contact with the relevant longitudinal surface (111, 112) of the seal, at least one projecting element (15a, 15b) made of a stiff material being provided on at least one bearing surface (231, 331) of at least one plate (23, 33) or on at least one of the longitudinal surfaces (111, 112) of the body of the seal, the projecting element (15a, 15b) being designed such that, in the close position, the gap between the two bearing surfaces (231, 331) of the plates (23, 33) is smaller than the combined thickness of said body and the projecting element (15a, 15b) such that, if the seal is not correctly positioned in the gap between the two half-bodies (2, 3) in the closed position or in the open position, the projecting element (15a, 15b) forms an obstruction which holds the bearing surface (231, 331) away from at least one plate (23, 33) and the relevant longitudinal surface (111, 112), and such that the locking mechanism cannot hold the plates (23, 33) in the close position in which they enclose the body in a sealed manner, characterised in that, at least when the plates (23, 33) are arranged in said close position, said projecting element (15a, 15b) extends symmetrically with respect to the transverse median plane of the body of the seal so as to be arranged equidistantly between the solid part and the bored part of said seal. - two half-bodies (2, 3) arranged so as to form a fluid pipe, said half-bodies (2, 3) each being formed by a tube (21, 31) forming a pipe and at the ends of which tube are attached a flange (22, 32) and a plate (23, 33), respectively, each of said plates (23, 33) having a bearing surface (231, 331) and a gap between the two half-bodies (2, 3), - a seal in the form of a body delimited by two opposing planar longitudinal surfaces (111, 112) each arranged facing the bearing surface (231, 331) of the plates (23, 33), respectively, which seal has a solid part and a bored part through which an opening extends that opens into each of said longitudinal surfaces; the seal is mounted between the bearing surfaces (231, 331) in the gap between the two half-bodies (2, 3) so as to be movable between: • a closed position in which the solid part blocks the pipe so as to prevent fluid flow between the two half-bodies (2,3), • an open position in which the bored part is placed in the centre of the pipe so as to allow fluid flow between the two half-bodies (2, 3), • a closed position in which the solid part blocks the pipe so as to prevent fluid flow between the two half-bodies (2,3), • an open position in which the bored part is placed in the centre of the pipe so as to allow fluid flow between the two half-bodies (2, 3), - a locking mechanism for holding the plates (23, 33) in: • a spaced-apart position in which the bearing surface (231, 331) of the plates (23, 33) is spaced apart from the relevant longitudinal surface (111, 112) of the seal, so as to allow said seal to move between the closed and open positions, • a close position in which the bearing surfaces (231, 331) enclose the body of the seal in a sealed manner in order to lock said seal in the open position or the closed position, said bearing surfaces each coming into clamped contact with the relevant longitudinal surface (111, 112) of the seal, • a spaced-apart position in which the bearing surface (231, 331) of the plates (23, 33) is spaced apart from the relevant longitudinal surface (111, 112) of the seal, so as to allow said seal to move between the closed and open positions, • a close position in which the bearing surfaces (231, 331) enclose the body of the seal in a sealed manner in order to lock said seal in the open position or the closed position, said bearing surfaces each coming into clamped contact with the relevant longitudinal surface (111, 112) of the seal,