Failure of pipe joints during hydrostatic testing

Large diameter butt-weld tee-joints (30-in. × 18-in.) on high-pressure gas-transmission pipeline exhibited leakage during the hydrostatic testing that was carried out to testify the integrity of the pipeline after its commissioning. The leakage-failure comprised the formation of small cracks at the neck-region of the tee-joints. A detailed examination showed that the cracking was preceded by plastic yielding, and that at these locations the wall-thickness was lower than even the minimum specified. A further examination of selected sections obtained from the failed-tees revealed that the tees had actually been ‘fabricated’ by butt-welding a small length of 18-in. diameter pipe onto a partially-formed neck on the 30-in. diameter pipe. And also, that the weld-filling of this butt-weld was then ground-off to smooth-out the curvature at the neck-region, or more likely, to conceal the weld and thereby the ‘faulty’ manufacturing procedure. An outcome of this procedure was that the wall-thickness at the neck-region which was lower (even to start with) than the minimum specified, had been further reduced by the grinding of the weld-filling. It was also concluded that a tee-joint of the required/specified design and dimensions could not be possibly produced by this method of ‘fabrication’.