LHC Cryogenics on the mend

On 29 September, repairs began on the LHC cryogenic distribution line, or QRL, to replace a faulty part that occurs in the hundreds of elements of the line that are already on-site. The Accelerator Technology Department is designing a work programme to finish the repairs as soon as possible and minimize delays to the rest of the LHC project.

The building, delivery, and installation of the LHC cryogenic distribution line falls under the responsibility of Air Liquide (France), who in 2002 won a CERN contract for this work. The company was one of three that built prototypes, which CERN tested and qualified before choosing Air Liquide as the single supplier. When QRL production started, they changed the material for a sliding table inside the QRL elements from that approved by CERN. With the new material, the sliding tables were more brittle, and many of them wound up breaking under the weight and dynamic load of the pipe bundles.

In order to avoid interfering with the ongoing production of new elements, CERN decided to repair those elements already on the CERN site, replacing the faulty tables with those built with the material originally planned. "This is a relatively simple repair," says Germana Riddone, QRL Project leader at CERN, "however, Air Liquide had already fabricated elements for two of the eight sectors and all these elements will have to be fixed. The contractor will repair the elements they've started but haven't yet completed, and all new elements will have the new tables installed."

The Technical Support Department will repair about 140 elements of the line, called service modules, vacuum barriers and fixed points. At a planned rate of two elements per week and then factoring in down time, this should take about a year and a half. The ICS consortium, which is presently in charge of the cryostating of cryo-magnets, has been asked to repair about 450 elements of the line, called pipe elements. They should be able to fix 10 elements per week, and during the next winter shutdown of the magnet test activities, 20 employees from that consortium will help on a "crash program" to quadruple the repair rate. In this way, the pipe element repair should be finished by June 2005. Several people from the AT-ACR, AT-CRI, AT-MAS, and AT-VAC are involved in the follow-up of the QRL repairs of the service modules and pipe elements.

The sliding tables are made from a composite of epoxy and glass fibres and are designed to support pipes inside the elements and allow them to slide past each other. In this way, during warm-up and cool-down, when the pipes expand and contract, they will not snap or buckle. However, the glass fibres in the faulty tables were only 0.5 mm long, instead of the 5 mm approved by CERN, and were not able to support the load of the pipes under shock conditions encountered during transport and handling.

The elements already fabricated had gone through several temperature and pressure tests and showed no sign of the broken tables. So it was serendipitous that in June 2004 a faulty sliding table was found while checking an unrelated problem with welds in one element. Endoscopic inspections showed that the tables were damaged.

At the beginning of July 2004 CERN began a systematic inspection of the elements that showed the problem with the tables was widespread, and then began a project to work with Air Liquide to design and manufacture new sliding tables, and repair the faulty elements. These repairs will delay the LHC magnet installation, but to minimize the delays the LHC group will do as much work as possible on them before installation and also increase the number of areas where the magnets are installed in parallel.

* Other members of the repair team for the service modules, vacuum barriers, and fixed points: Antoine Bouillot, Isabel Bejar Alonso, Stefano Sgobba, Marc Polini, Michel Caccioppoli, Noël Veillet, Didier Glaude, Nicolas Vauthier.