Investigations on strength of piping for non-condensable gas combustion (Part 3: Estimation of fracture strain of steel pipes subjected to hydrogen-oxygen detonation)

To assess the integrity of piping under the detonation pressure of accumulated hydrogen-oxygen, the fracture strain of the pipe material must be identified. In carbon steel pipe specimens that had been experimentally ruptured by hydrogen-oxygen detonation, the strain rate of the measured hoop strains was around 2000 s super(-1). To obtain fracture strains at such a high strain rate, tensile tests were conducted at strain rates of up to 5000 s super(-1) using specimens cut from carbon steel pipes, low alloy steel pipes, and weld metals. For all the different strain rates and materials, elongation was over 24 % and dimple patterns were found in the fracture surfaces. From this, it is apparent that the fracture mode of piping material subjected to a high-speed tensile load is ductile fracture. We had previously measured the strain history of the carbon steel pipe specimens ruptured by hydrogen-oxygen detonation, but we had not measured a strain higher than about 14 % because of the limitations of strain gauge measurement. FEM analyses of the ruptured pipe specimens were conducted to estimate the fracture strain at pipe rupture. We estimated that the strain at the pipe rupture was 20 - 22 %. The results of the material tensile tests and the estimated fracture strains of the pipe specimens revealed that steel pipes have sufficient ductility even if they are subjected to detonation pressure. Because pipe rupture is accompanied by large strains, we propose a method for assessing the integrity of a pipe subjected to detonation pressure by comparing the strain on a pipe estimated by FEM and the limiting strain of the pipe material. We propose a limiting strain of 8 %, which is reduced to one-third of the minimum elongation in material tensile tests in order to ensure a sufficient safety margin.