Development and testing of a liner venting system

Prestressed Concrete Pressure Vessels are relatively thickwalled and are designed on the basis of internal pressure acting on a leak-tight liner. If a crack should develop in this liner, the pressurization of concrete could reduce the vessel's factor of safety against failure. Some existing PCRV codes for GCRs attempt to cope with this problem by designing the vessel for an assumed worst case. The Scandinavian criteria tries to limit the pressure load by venting away the leaking gas. For designing the venting system, an analysis was made of the potential leakage paths for gas through concrete. Based on the results of the analysis a design basis crack of 0.2 mm in the liner was postulated. The venting system itself consists of two layers (one vertical, one helical) of cored-out ducts in the heavily reinforced concrete adjacent to the liner. This layer of concrete functions as the catchment area for the venting system. A model with an I/D of 1.8 m and wall thickness of 0.8 m, was judged to be adequately large for testing the outward transport of gas through the vessel wall and the function of a system to vent this gas. Liner cracks were simulated by rows of 4 mm holes. The catchment area of the venting system was represented in full scale to simplify correlation of test results with a full-size vessel. After pilot tests on a 2.1 m O/D ring, the pressure vessel model was built during March-June 1974. Leakage flows and pressure profiles in concrete were studied at 40, 70 and 85 ato. Measurements were made without venting and compared with those with the leakage vented. The results presented in the paper show that the venting system is an effective method for relieving the pressure build-up in concrete due to large gas leaks through the liner. Conservatively estimated, at 70 bar vessel pressure the relative reduction of the forces due to gas pressurization in a vented vessel was between 50-70% of those in a nonvented one. Over 70% of the leaking flow through large leaks was vented through the venting channels. The system as designed at present seems to have an adequate margin of reserve capacity. (author)