A flow-limiting device for a pipe carrying a fluid or gaseous medium, comprising a flap, means mounting the flap in the pipe for pivotal movement in a manner to vary the cross-sectional area of flow through the pipe, and spring means urging the flap in a direction to increase the cross-sectional area of flow and against the direction of flow through the pipe, so that the flap varies the cross-sectional area of flow through the pipe in dependence on the kinetic energy differential of the medium upstream and downstream of the flap. The spring means comprise a leaf spring one end of which is mounted in an upper part of a housing for insertion in the pipe and the other end of which carries the flap. There is a clearance beneath the lower edge of the flap to permit the passage of deposits in the medium past the flap in all positions thereof.
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