Cryogenics system: strategy to achieve nominal performance and reliable operation

During the LHC operation in 2010 and 2011, the cryogenic system has achieved an availability level fulfilling the overall requirement. To reach this level, the cryogenic system has profited like many other beamdependent systems from the reduced beam parameters. Therefore, impacts of some failures occurred during the LHC operation were mitigated by using the overcapacity margin, the existing built-in redundancy in between adjacent sector cryogenic plants and the “cannibalization” of spares on two idle cryogenic plants. These two first years of operation were also crucial to identify the weaknesses of the present cryogenic maintenance plan and new issues like SEUs. After the LS1, nominal beam parameters are expected and the mitigated measures will be less effective or not applicable at all. Consequently, a consolidation plan to improve the MTBF and the MTTR of the LHC cryogenic system is under definition. Concerning shutdown periods, the present cryogenic sectorization imposes some restrictions in the type of interventions (e.g. cryo-magnet removal) which can be done without affecting the operating conditions of the adjacent sector. This creates additional constrains and possible extra down-time in the schedule of the shutdowns including the hardware commissioning. This presentation focuses on the consolidation plan foreseen during the LS1 [1] to improve the performance of the LHC cryogenic system in terms of availability and sectorization [2]. INTRODUCTION During the LS1, the main constraint will be the available resources to perform the identified consolidations. Consequently the proposed strategy is based on first to recover the nominal performance of the cryogenic system including the safety aspects for personnel, hardware and helium inventory; and secondly to improve the reliability and the availability of the system. RECOVERY OF NOMINAL PERFORMANCE Safety first Some safety issues were identified during the first operation period and mitigation measures have been taken to allow safe LHC operation. During LS1, consolidations are needed to recover the required level of safety compatible with the LHC operation at nominal condition. Concerning safety related to personnel, collectors have to be added on the safety valves protecting the inner triplet vacuum enclosure. In case of helium release in the vacuum enclosure, the discharge flow is collected and discharged after the evacuation door allowing personnel evacuation without crossing a cold helium jam. Some deflectors must also be added on DFB access-door safety valves to avoid direct helium jet in the tunnel transport area. Personnel access to equipment located behind the LHC machine (QRL, cable trays...) is presently difficult and risky both for personnel and hardware. Access platforms are consequently required. The sectorization of the helium ring line is also highly recommended to reduce the risks of oxygen deficiency hazard and helium inventory loss. Following the 2008 incident, the periodic monitoring of the tunnel ventilation conditions (temperature, pressure and air speed) is recommended. Concerning safety related to equipment, the completion of the insulation vacuum enclosure protection must be performed on 3.5 sectors; this includes DN200 safety valves on the cryo-magnets as well as DN240 safety valves on the DFB access-doors. In addition, temporary safety devices must be replaced by final design safety valves. The protection of the beam vacuum pipes must also be consolidated by adding periodic rupture disks. To complete the safety chapter, the quench lines at the odd points must be consolidated. Today, these lines are not conformed and are consigned with a risk of large helium losses in case of massive magnet quenches. Table 1: Hardware consolidations for nominal performance recovery Consolidations Present limitations Steadystate Transient DFBA Splices X DFBA flexible hoses X DFBX current-lead controls X Inner-triplet copper braids X X Y lines X X Leaks (in S3-4, S4-5, P8...) X X Beam screen circuit (Q6R5) X Instrumentation NC X Standalone magnet and DBF cooling limitation X X Leaks in LN2 pre-coolers X X Proceedings of Chamonix 2012 workshop on LHC Performance