Please use the emergency exit
In order to minimise the consequences of an incident like the one that occurred in 2008, the LHC has been fitted with safety valves. A total of 1,344 of these valves, whose function is to release the helium in the event of over-pressure, will be in place by the end of LS1.
The cutting operation is carried out using a circular saw designed at CERN. This photo was taken in Building 181 during training.
Let's go back five years. It’s 2008 and an electrical incident has just caused a helium leak in LHC Sector 3-4. 30-tonne magnets moved half a metre as a result of the over-pressure. What did the ensuing investigation find? A defective electrical connection between two magnets caused a short-circuit, provoking an electrical arc, which in turn led to a leakage of liquid helium into the insulation vacuum of the cryostats containing the magnet cold masses. While helium is liquid at a temperature of 1.9 K (inside the magnets), it becomes gaseous as soon as the temperature exceeds 4.2 K (outside the magnets), which means that its volume increases by a factor of more than 700 inside the cryostats, which, without a valve, are as airtight as a pressure cooker.
That’s where the new valves, the DN200s, come into play. In the event of a helium leak into the insulation vacuum, the gas will be able to escape through these "emergency exits," thereby reducing the over-pressure. “With one safety valve for each dipole, the LHC will eventually be equipped with 1,344 valves,” explains Anna Chrul, deputy leader of the Alfa & Omega team. "More than half of them were installed after the incident in 2008, and from 21 May this year, we will go on to install the remaining 612. To do this, we are working in collaboration with seven technicians from the JINR Institute in Dubna (Russia) who have come to lend us a hand.”
After the flange is in place, the welding points are cleaned with a stainless steel brush.
The teams begin the valve installation by opening the W sleeve – the external bellows that enclose the interconnections. The thermal shield just beneath the sleeve remains in place during this operation, thus protecting the electrical connections and the internal bellows. “Once the external sleeve is open, we insert a flame-retardant covering between the cryostat – in which we must bore a hole – and the layers underneath, which could ignite during the welding process,” adds Andrea Musso, Alfa & Omega team leader. “Then we slide a magnetic plate under the area to be drilled. This is to catch the metal swarf and dust produced during boring, ensuring that it doesn’t get into the machine.” Using a circular cutting machine specially designed at CERN, the technicians, who are trained and supervised by DN200 activity leader Manuel Gomes De Faria, then cut out a hole 20 cm in diameter, to a precision of one tenth of a millimetre. The valve, a stainless steel flange with a lid, is then welded in place. With 30 valves to install per week, Musso's team is going to have a heavy workload.
