Abstract: The invention is a sealer unit useful for temporarily preventing water from flowing past a closed valve in a pipeline, such as a butterfly valve. This unit includes a cup-shaped seal member, and a solid plug member that fits inside the seal member. The seal is constructed of a material that swells on contact with water, and the solid plug is derived from a liquid material that can solidify at ambient temperature. The seal member is seated down onto the valve closure plate, and the plug member is formed inside the seal member. The seal member is then contacted with water, causing it to expand outwardly against the pipeline and inwardly against the solid plug member. In its expanded state, the seal member provides an effective seal to prevent water from flowing past the valve. In addition, the solid plug forms a coherent body that enables it to withstand the pressure exerted by the seal member, as it swells, but is capable of being broken into pieces if required.