issues concerning the reliability of cryogenic system

Chamonix XIV Jannuary 2005 M. Sanmarti/ AT-ACR 1

ISSUES CONCERNING THE RELIABILITY OF CRYOGENIC SYSTEM

Chamonix XIV Jannuary 2005 M. Sanmarti / AT-ACR2

Outline

From LEP2 to LHC

LHC Cryogenics system architecture (redundancy)

Reliability of sub-systems and components

Maintenance policy and shut-down strategy

Conclusions


Previous considerations

Most of the main cryogenic components have been extensively used at CERN

Considerations based on experience more than detailed failure risk analysis LEP2 and first LHC commissioning experience

Availability, failures & MTBF’s related to beam (LEP2) and beam commissioning (LHC)

Major or first order failures: ”something that breaks or something that does not work as expected“


LEP2 experience

Cryogenic system downtime rates from 1996 to 2000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

1996 1997 1998 1999 2000

Dow

ntim

e ra

te [%

]

Cryo failures rate

Utility failures rateDe-icing

LEP impact

Cry

o U

pgra

de

More than 120.000 h cumulated running hours

Cryo impact < 1%

Recovery after utility failures downtime <2%

De-icing: reduced cooling capacity, time used for MD

Sub-system or Hardware commissioning

Beam commissioning…


Learning from LEP2

Components (impact on machine): Main components failures during commissioning or restart after SD Cryogenics downtime (not including utilities): < 1,0%

MTBF Cryo system 0.1 years, MTBF Cryoplant 0.4 years, MTTR 1-2 hours Cold boxes (MTBF years): instrumentation and turbines very reliable Compressor stations:

Mainly aging problems on instrumentation/piping (MTBF 0.5 years) Controls: dedicated and robust control system was almost transparent Distribution & RF cavities:

mainly beam related issues (heat load) affecting cooling capacity Access needed although no urgent intervention required (key components

in RA) Impurities (De-icing):

Gaseous impurities at warm turbines level (120 K & 90 K) Predictable: time used for MD or interventions

Maintenance: Extensive preventive maintenance campaign during SD periods Corrective: MTTR < 1-2 hours but amplified impact on machine (x7)


LEP 1500 I/O channels, 8 compressors, 7 turbines per point (4 points)

LHC 9000 I/O channels, 16 compressors, 20 turbines per point (5 points)

From LEP2 to LHC cryogenic system

1

2

3

4

5

6

7

8

1

2

3

4

5

6

7

8

LHC

4 x 12/18 kW @ 4.5 K

288 SC RF cavities

2 km @ 4.5 K

8 x 18 kW @ 4.5 K

8 x 2,4 kW @ 1.9 K

1’800 SC magnets

24 km @ 1.9 K

36‘000 tons @ 1.9 K

LEP2

75 tons @ 4.5 K


The LHC cryogenic architecture per point

2

3

4 5

6

7

811.8

Built-in redundancy

Weak point: 2-3

Warm CompressorStation

Upper Cold Box

Interconnection Box

Cold Box


Lower Cold Box

Distribution Line Distribution Line

Magnet Cryostats Magnet Cryostats

Cold Compressorbox


Cold Compressorbox


Shaft

Sur

face

Cav

ern

Tun

nel

LHC Sector (3.3 km) LHC Sector (3.3 km)

1.8 K Refrigeration Unit New 4.5 K Refrigerator Ex-LEP 4.5 K refrigerator 1.8 K Refrigeration Unit

DFBA DFBA


Major (sub-systems) failures

One 4.5K Ref. or one 1.8K unit out of

order:=>Low intensity OK

(beam commissioning OK)

Common parts (QUI-QRL-DFB), loss of isolation vacuum : => Total stop of the machine

BUT transition:

≈ 12 to 24 hours

Unlikely to occur during life-cycle, but possible!

QUI

QSRA

QSCA

QURC

QSCC

QRL

Sector

QURA

QSV

QSRB

QSCB

QURC

QSCC

QRL

Sector

DFB DFB DFB DFB


4.5 K & 1.8 K Refrigerators Instrumentation: high reliability and spares, MTTR~1-2h Warm compressors: no redundancy but spare capacity or connection

to adjacent refrigerators would allow degraded mode (low intensity): Oil piping: if spares, MTTR 1-2 days Motor/Compressor replacement??

Turbines: no spares at the moment, diagnosis + 5 h. intervention delay if spare available, otherwise degraded mode allows continuation of tests

Cold compressors: spares available, diagnosis + 5 hours delay, no degraded mode allowed

Impurities: Dryers (H2O), switchable adsorbers (Air, 80 K), single adsorber (H2, 20 K)

Vacuum (leaks): temporary solution until SD major intervention ACCESS constrains: underground and UX4, UX6, UX8 for QURC (1.8

K)

From the cooling capacity point of view such failures should not affect beam commissioning (spares, redundancy, adjacent refrigerator) but the operational constraints and the recovery time will increase

? Degraded modes could be a problem for scrubbing run


QUI (Interconnecting Box) Instrumentation:

Redundancy on control loop and QRL interfaces sensors Cryogenic valves (no redundancy): high MTBF Heater (warm up): redundancy but degraded mode, longer warm up

Vacuum (leaks): temporary solution until SD major intervention Impurities (Solid): possibility of clogging the QUI filter (line D) provoking a

stop of the cooling flow It would mainly happen during the cool down and the first few quenches It requires 1-2 days to replace the filter and reach again nominal

conditions

Filters in CFB (Magnets Test Bench) Shut Down 2004-2005

From the functionality point of view: The QUI assures the redundancy of the refrigerators Clogging of line D filter is the most likely failure to occur No redundancy for cryogenic valves of QRL interfaces Any intervention needs underground access: UX4, UX6 & UX8


QRL and Ring equipment I QRL

Instrumentation: Redundancy or degraded mode possible Most of Cryogenic valves are redundant (degraded mode):

• In situ exchange: up to 1 week intervention depending on valve position

Quench valves (cool-down/fill): no redundancy for filling (once/year), security redundancy

Beam screen: Clogging problems (small Ø pipe): beam screen temperature?? Loss of instrumentation (heater & temp., no redundancy):

No Temp. control, higher helium flow Problems during “scrubbing” run

DFB’s, Standalone magnets & DSL’s: Instrumentation:

HTS valves: easily repair HTS temperature: redundancy or other control options (valve

characteristics against current) Level gauges: redundancy or easily repairable (except for D2, D3) DFB: presentation by A. Perin this Workshop


QRL and Ring equipment II

Dipoles & Inner Triplets: Temperature sensors redundancy (needs electronics replacement) Other control options (opening valve characteristics, copy valve position of

adjacent cells) Level gauge bayonet heat exchanger: liquid in line B and possible magnet

temperature perturbation (operational issue not affecting pumping capacity but temperature control)

Isolation Vacuum: presentation by P. Cruikshank this Workshop

RF cavities: Instrumentation, valves as above Pressure stability and protection during quench/quench recovery (presentation by

S. Claudet)

From the functionality point of view: Reliability of primary components is high Replace (redundancy) possible or degraded modes: less control (temp.) and

higher helium consumption Any intervention needs access to the tunnel: radiation issues for IT??? (OK for BC)


LHC experience

New LHC 4.5 K cryoplant @ PM18 (2002-2004): No major gaseous impurities problems (solid impurities filters in

CFB, MTB) Availability about 99% (50% utilities/cryo) for 20000 cumulated

running hours with degraded modes or spare capacity

String2 experience (2002-2003): 98,5% availability over 4170 h (2002) & 98,6% availability over

1950 h (2003) Prototype/commissioning: mainly tuning, quench recuperation

and controls No major problems with instrumentation or beam screen circuit


0:00

6:00

12:00

18:00

24:00

30:00

36:00

42:00

48:00

0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

Utility failure [h]

Rec

over

y Ti

me

(CR

YO

OK

) [h]

LEP contractual performancesLEP 3.3kV Failures - 60'000 hours (98-00)LEP 400V Failures - 60'000 hours (98-00)LHC estimated performancesLHC Test String 2 mains failures - 5000 hours (01-02)LHC Test String 2 simulated utility stopLHC with degraded vacuum & leaking QRV

Utilities Failure Recovery (L. Serio @ Chamonix 2003)

Cryogenics is a recovery time amplifier

LEP contractual time recovery < 5.5 hours + 7*stop duration

LHC estimated time recovery < 6 hours + 3*stop duration

Controls :? Complete new control system (still design problems)? Ethernet dependent (control loops, PLC communication)

Recovery performances:× Recovery predictions have still to be validated for the

global system during hardware commissioning× Degraded modes will increase recovery time


Maintenance Policy

Existing maintenance plan to be upgraded (LEP), completed (new LHC installations) and everything to be implemented in CERN CAMMS

Based on preventive maintenance campaign during SD Baseline: 13 weeks for full maintenance campaign Issues arising: Safety valves (5000 u.) every 2 years inducing

corrective maintenance

Spare parts: first batch after commissioning using industrial method for criticity analysis, ~2,2% cryoplant cost (280kCHF for 4.5 K refrigerator) Assures MTTR of 1-2 hours No spare for turbines, warm compressors/motors…

Maintenance management: No CERN resources for execution Maintenance management needs to be reinforced and fully driven by

CERN Manpower management depending on SD scenarios

Presentation by T. Pettersson this Workshop


Shut Down Strategy (16 weeks from Chamonix’04)

Scenario 1: full maintenance & floating temperature (T~200 K) Keeps preventive/corrective ratio (LEP and present experience):

same availability rates Perturbations during cold check-out: corrective maintenance after SD Requires ELQA if T>80K (+ 5 weeks during MCO) Thermal cycling of components: helium leaks, welding stress…

Scenario 2: maintenance on 1 cryoplant/point keeping sectors “cold” Increases preventive/corrective ratio: reduce availability rates?? Lower risk of perturbations during cold check-out (after SD) No need of additional 5 weeks for ELQA No thermal cycling of components Not possible in sector 2-3

Utilities: driven by cooling water towers 4 weeks per LHC point (2 points in parallel): to be reviewed for

Scenario 2

In any case, warm up could be needed for ring components replacement (magnet, etc..)


Conclusions I In principle, the cryogenic system should have a very low impact

(except on sector 2-3) the beam commissioning because of: Redundancy of systems Available spare cooling capacity for low intensity beam Reliability of components and instrumentation

However, failure of sub-systems and components can not be ruled out completely and could result in few days delays to switch to redundant system or component and to adapt to new configuration

Worst failure would be the loss of insulation vacuum on the QUI or QRL as well as the refrigerator in point 2 or DFB’s for magnet powering

Most likely failure would be filters blockage on the QUI during or after the first cool down and magnets quench due to accumulation of impurities (consolidations under study)

Recovery time after major failure (utility or cryo) will be approximately 6 hours plus 3 times the stop length (15 times if bad vacuum/QRV leaks)


Conclusions II

The cryogenic sub-systems will be individually tested, but the overall cryogenics system will certainly require complex and extensive commissioning prior and during powering to validate the global and collective behavior and optimize operating modes

The availability or quench recovery performances of the cryogenic system:

could be reduced by additional heat loads or non conformities from commissioning

Depends on a correct Maintenance Management: it has already started and needs CERN dedicated resources!


Acknowledgements

Many thanks to: L. Serio S. Claudet G. Riddone R. Van Weelderen A. Perin P. Gomes Ph. Gayet N. Bangert

for their contribution to this presentation

issues concerning the reliability of cryogenic system

Documents

beam commissioning spares

beam lep2

order failures

beam commissioning lhcmajor

main components failures

degraded mode low intensity

mtbf cryo system

main cryogenic components