A ball valve seat (10) is disclosed which provides a temporary seal for a plug (40). The valve seat comprises a substantially cylindrical body (30) of a first volume, which defines a seating surface (34). A pressure differential is developed across the valve seat when the plug sealingly engages the seating surface. The body is formed of an elastic material which compresses from a first volume to a smaller second volume by application of a force on the plug, to provide a clearance which is greater than a plugging dimension of the plug, thus allowing passage of the plug downstream. After passage of the plug, the body returns to the first volume.