This week LS1 successfully passed an important milestone: the first pressure test of a complete sector, sector 6-7. The objective of this test was to check the mechanical integrity and overall leak-tightness of this section of the LHC by injecting it with pressurised helium.
"Given the scale of the work and of the operations carried out during 2013, particularly in the framework of the SMACC project and of the repair of the compensators of the cryogenic distribution line (QRL), we need to revalidate the integrity of the systems before the accelerator starts up again," says Olivier Pirotte, who is in charge of the pressure tests (TE-CRG).
The pressure tests are performed over a single day after two weeks of intensive activity to prepare and specially configure the cryogenic instrumentation in the tunnel, and the pressure within a sector is increased in stages, as Olivier Pirotte explains: "We gradually increase the pressure by 5 bar every hour with a 10-minute pause before each new increase until we reach a pressure of 25 bar, which is 5 bar above the maximum operating pressure of the LHC. This generates substantial mechanical stress but it enables us to be sure that the sector is really robust. We monitor the operation from the cryogenic control room of each sector via pressure transmitters mounted on the accelerator’s cryogenic circuits. They tell us whether the pressure is stable or not: if the pressure does not decrease, it means there are no leaks and that everything is working properly."
After the last pressure increase to 25 bar, which takes one hour, the pressure is gradually lowered to 10 bar. "At this intermediate pressure, the vacuum teams take over to perform overall leak tests to check that the cryogenic circuits and the sector’s insulating vacuum are leak-tight."
The team in charge of the operation feeds helium into the sector from three different sources depending on the requisite gas pressure: for the pressure increases from 0 to 10 bar, gaseous helium stocks kept at the surface are used; between 10 and 20 bar, the gas is distributed via the helium compressors; from 20 to 25 bar, the helium is delivered by a specially customised lorry fitted with helium batteries at a pressure of 200 bar (the gas is then depressurised to 25 bar). Naturally, once used, the helium is recovered and stored. It will later be liquefied to cool down the magnets.
The tests and activities scheduled during the end-of-year closure all went well, in particular: maintenance of the electrical substation of the Meyrin site, the emergency stop tests on the Meyrin and Prévessin sites and evacuation system tests.
The first wagon of the SMACC consolidation project train, which is responsible for opening the W sleeves, reached its destination. The train has now completed its work in sector 6-7 (which explains why it was possible to conduct pressure tests) and is about to wind up operations on sector 7-8.
As announced in the previous LS1 Report, the final operations on the injectors are currently going ahead according to schedule.