Experience with Turbomolecular Pumps

1
Experience with Turbomolecular Pumps

• for LHC Insulation Vacuum Systems
• D. Convers, P. Cruikshank, R. Gavaggio, B.
  Heywang, N. Hilleret, A. Grimaud, W. Maan,
  G. Moranne, L. Mourier

• Adixen by Alcatel Vacuum Technology, Annecy (FR)
• 40-30, Seyssinet-Pariset (FR)
• Air Liquide DTA, Sassenage (FR)

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Contents

• LHC insulation vacuum system.
• Technical specification for turbomolecular pump
  supplies procurement.
• Integration of the turbomolecular pump into
  different pumping group systems.
• Failures and some remedies.

3
LHC Insulation Vacuum Systems

• QRL Magnet insulation vacuum system
• Continuous arc cryostats subdivided into
  insulation vacuum subsectors or volumes
• 112 magnet and 72 QRL insulation vacuum
  subsectors
• 80 m3 (214 m magnets and 428 m QRL)
• 200 m2/m Multi Layer Insulation
• 128 permanently installed and from distance
  controlled Turbomolecular 300l/s pumping systems
• 400 positions to attach a mobile pumping system
  (rough and turbomolecular)
• Stand alone cryostats (one insulation vacuum
  volume or subsector)
• 50 volumes/sectors
• Average of 4 m3
• 66 permanently installed and from distance
  controlled Turbomolecular 60l/s pumping systems
• High Vacuum system
• 10-3 mbar at room temperature before cooldown,
  10-7 mbar at cold
• Internal process line leak tightness requirement
  lt10-8 mbarl/s He at warm
• Important quantities of water vapour during
  initial pumpdown
• Not very clean vacuum (variety of materials,
  dust, particles, swarfs)

4
Technical specification for turbomolecular pump
supplies

• Invitation to tender ( March 2002) for the supply
  and maintenance of Turbomolecular Pumps and their
  power supplies
• High pumping speed (gt200 l/s N2 eq and gt180 l/s
  He eq)
• High compression required to pump possible He
  leaks from cryo systems (gt108 for N2 and 104 for
  He)
• Radiation resistant up to 200 Gy (20 Gy per year)
• Forced air cooling to ensure operation up to 35ºC
  ambient air temperature (lower temperature limit
  not specified)
• Remotely controlled (on/off, rotation, nominal
  speed, failures)
• Maintenance for permanent installed
  turbomolecular pumps included in offer
• Preventive (executed by CERN but materials to be
  foreseen within contract or executed fully by
  contractor on the CERN premises)
• Corrective (all defective pumps should be
  replaced immediately to ensure the functionality
  of the LHC i.e. replacement pumps and power
  supplies to foreseen. Repair of damaged pumps can
  be done at CERN two times a year)
• Operation according to specification up to 43
  khours during the LHC life

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Procurement

• The order was placed for
• 300 l/s Hybrid Turbomolecular Pump (up to the
  total of 168 units)
• Sealed bearings for minor maintenance
  constraints
• Special developed power supply (more than 170
  units)
• High radiation resistance due to simple
  traditional electronics
• High power output to drive turbo with extended
  cables (gt100m)
• Maintenance contract to guarantee the performance

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Integration of turbomolecular pumps into pumping
systems

• Several Turbo Molecular pumps have been stored
  (gt6 months) before being integrated into a
  pumping system and put into service.
• 128 permanently installed and remotely
  controlled.
• 40 mobile turbo pumping systems (installation,
  commissioning and interventions)

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Failures and remedies on the Turbomolecular
pumping systems

• Turbo powering difficulties over long distance
  (gt100m) deported power supply.
• CERN developed a cabling configuration where the
  power to run the motor of the turbo is
  transferred via one cable and all the control
  signals (rotation speed, temperature) in a
  separate second cable.
• Charge independent speed oscillations (Conceptual
  error in pump temperature treatment).
• CERN started to by-pass the temperature readings.
  
• On request of the supplier, CERN has to implement
  additional logic on our pumping group control
  level (using rotation speed signal) to protect
  the turbomolecular pump
• Bearing failures on both mobile and permanently
  installed turbomolecular pumps.

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Bearing failures on both mobile and permanently
installed turbomolecular pumps.

• 168 pumps (128 permanently installed and 40
  mobile pumping systems)
• 72 pumps (43) failed and have been returned for
  revision
• 6 pumps (8) failed a second time after revision
• 56 of the failures appears before 4000 hours of
  operation (10 of the required number of running
  hours for the LHC)

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What caused the bearing failures?

• Pumps?
• Usage?
• Storage?
• Environment?
• Actual status
• The LHC is starting up and we encounter about 6
  bearing failures a month.
• The maintenance contract with the supplier needs
  to be revised.
• 43 pumps awaiting bearing change to re-enter the
  guarantee and maintenance contract with the
  supplier (due to storage).