Cavitation behind a fast acting flap valve and prevention of water hammers

Fast shut-off experiments were carried out at a 200 m long experimental pipeline of the Fraunhofer Institute for Environmental, Safety, and Energy Technology (UMSICHT) in Oberhausen, Germany. The cavitation bubbles occurring at the fast acting valve were recorded by means of mesh sensors. Primarily these sensors record distributions of the electrical conductivity in the measuring cross section, which are converted into void fraction distributions, stored with a rate of 1024 frames per second. An excentrical butterfly valve was used to shut off the pipeline. The sensor was located at different positions up- and downstream the valve. In a pipe of 100 mm nominal diameter the cavitation bubble was recorded at distances of 0,15 m and 1,5 m downstream the valve. In the presentation, computer animations of the measured signals will be shown, in which the appearance and the collapse of the bubbles are visible in high temporal and spatial resolution. Furthermore, the influence of a pipe bridge located behind the valve as well as the damping effect of injected air to the amplitude of the pressure peaks will be discussed on the basis of such visualisations. Special cavitation patterns occurred during partial closing of the valve. In another experiment two sensors were directly flanged to a flap valve of 50 mm nominal diameter, located in the rear section of the pipeline. It was found that cavitation appeared also upstream the valve as well as at the edge of the flap of the valve. With the help of the obtained information it was possible to design measures for water hammer prevention. A back valve put at a certain distance downstream the shut-off valve closes in the moment of flow reverse. In this way, the cavitation bubble is captured and the cavitation collapse cannot occur. The closure of the back valve itself is not accompanied with a pressure peak. Water hammers upstream the main valve are prevented by a second measure: a disc brake acting at the shaft of the flap valve, which is interrupting the closing movement of the flap when the upstream pressure starts to grow. A combination of both methods allows to cut off the liquid flow without pressure peaks in the smallest possible time.