In a paper mill, a leak was detected on a pipe within the vapour network. The shutdown procedure targeting the boiler producing the vapour was activated. The pipe, operated at a pressure of 80 bar, connected the boiler outlet to a distribution barrel. The leak was located upstream of the barrel, at the level of the tie-in weld of a valve bypass tap. The barrel was stored in a seldom used utility room and the pipe’s heat insulation contained the leak, which therefore posed no hazard relative to individual safety.

Installation piping was composed of alloy steel with a grade of 13CrMo4 and a DN 175 specification; it was designed for a maximum allowable pressure of 110 bar. The tap served to balance pressure upstream and downstream of a 60-mm diameter valve. The leak had originated from mechanical and thermal fatigue cracking, caused by the high number of boiler start/stop cycles. The operator performed the tap repair by changing a 20-cm spool, in addition to modifying the boiler start-up procedure in order to limit both dilatation stresses around the bypass and pressure surges, by allowing for a very gradual division of this component. The pipe had not been inspected since its initial service start-up in 1970; hence the operator was motivated to implement a periodic inspection programme, in accordance with regulations specific to pressurised equipment.