At 11:04 a.m., hammering was noted during the start-up of a steam line in a biomass power plant associated with a chemical plant. The facility was shut down and the supply of steam was cut off. The operator isolated and depressurised the line. The piping, having a diameter of DN 300 and measuring 480 m in length, was damaged. The hammering had caused it to move 20 cm.

The phenomenon was associated with the presence of a few m³ of water in the piping in liquid state when the steam was let in. The operator’s analysis determined that the incident had several causes:

• the procedure for managing the 14 steam system purge panels was not adapted: failure to take into account a new condensate collection line from the chemical plant;
• the start-up parameters for the steam line were not adapted: poorly defined conditions and measuring point for the end of line preheating sequence;
• preheating of the line was too fast.

To avoid this type of incident, the operator:

• modified the procedure for managing the purge panels during steam line start-up, to take into account the new condensate drainage line. The purpose of this revision is to discharge condensate at atmospheric pressure as long as the steam network’s pressure is too low to send the condensate back into its collection circuit. It provides for a systematic check of the state of all the panels as well as marking on each valve, to guarantee that no liquid is in the line.
• changed the parameters of the control system for line start-up. This modification concerns the determination of new thresholds and measurement points: programming of progressive heating stages. Start-up messages for manual operations or checks were added.
• modified the installations by adding low flow by-pass valves on the steam line (ensuring low-speed circulation during preheating) and check valves on the condensate collection network.
• trained personnel in start-up operations.