Growing Pains
Heat expands and cold contracts: it’s a simple thermodynamic rule. But when temperatures swing from 300 K to near-absolute zero, this rule can mean a contraction of more than 80 metres across the LHC’s 27-km-long cryogenic system. Keeping this growth in check are compensators (a.k.a. bellows), which shrink and stretch in response to thermodynamic changes. Leak tests and X-rays now underway in the tunnel have revealed that these “joints” might be suffering from growing pains…
This 25-μm weld crack is thought to be the cause of the helium leaks.
Prior to the LS1 warm-up, CERN’s cryogenic experts knew of two points in the machine’s cryogenic distribution system that were leaking helium. Fortunately, these leaks were sufficiently small, confined to known sub-sectors of the cryogenic line and – with help from the vacuum team (TE-VSC) – could easily be compensated for.
But as the machine warmed up for the Long Shutdown, the cryogenic team began to suspect that a more systemic problem was at work. “As the machine grew warmer, five similar leaks were reported from across the system,” says Krzysztof Brodzinski, a member of the LHC Cryogenic Operation team in the Technology Department (TE-CRG). “In collaboration with our EN-MME partners we conducted X-rays of the sub-sectors where the original problems had been spotted. We discovered severe deformities in the compensators.”
The welded connection joins all plies and related flanges together. It is likely that helium leaks through this weld to enter the inter-ply volume.
With the cause of these deformities still unclear, experts from the Engineering Department were called in to perform a metallurgical analysis of the deformed bellows. These investigations revealed a faulty weld with a 25-μm crack, which is the likely source of the supercritical helium leak between the compensator plies (see image on the right). As the system warmed up, the helium expanded, forcing its way through the walls of the bellows. “Although the warm-up of the LHC complex takes weeks, this is a very short time compared to the years of LHC operation during which helium was seeping into the bellows' inter-ply volume,” explains Brodzinski. “The helium simply did not have enough time to escape back through the leak channel. While warming up, trapped helium expanded and damaged the remaining tight compensator plies.”
With this mystery solved, the cryogenic team is now examining the scope of the problem. While seven leaks were clearly visible in the system, X-rays had uncovered an eighth deformed bellow that had not yet started leaking. “It was this case that resulted in the decision to X-ray all the compensators of the main cryogenic supply line, fixing damaged bellows before they present a problem,” says Brodzinski. “While X-rays are still ongoing, the total count of faulty bellows remains at eight.”
The damaged bellows will be replaced and tested by the end of February 2014. These replacements will incorporate a new weld design and an adapted testing procedure (developed in collaboration with TE-VSC) that should avoid any further deformities.
