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IEFCAugust 12th, 2011

Improvement to the TDC2 vacuum system

Paolo ChiggiatoTE-VSC group

Injectors and Vacuum Measurements

IEFC, 2011 August 12th2

Outline

1. The problem

2. Recent interventions

3. Proposed mitigations

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The Problem

o Localised corrosion in the TDC2’s vacuum chambers has been already reported and the associated risks have been pointed out since 2001.

o In summer 2003, a critical situation was tackled efficiently by protecting most of the beam pipes and replacing defective vacuum chambers (see J-M. Jimenez, Status of the Vacuum System in TDC2, July 2003)

o…..

o In 2009, a vacuum leak appeared in the main vacuum chamber of splitter 1 which was repaired by a resin patch.

o In June 2011, a leak opened suddenly in sector 2006; in July, in the nearby sector 2007.



The Problem

PVCDehydrochlorination is the major mechanism of PVC degradation by X and γ-rays.

PVC components were accidentally installed in TDC2: hoses, cables (fire alarm), mechanical protections for vacuum bellows (). The recent events suggest that PVC is still there…

MoistureIn the TDC2 there are several sources of moisture:• Leaking magnet cooling circuits. • Leaking water valves,• Infiltration from the tunnel ceiling.


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Condensed water droplets charged with Cl- ions can fall onto vacuum components, in particular welds and bellows (mechanically stressed areas) generating stress corrosion cracking in unprotected stainless steel components…

The Problem

IEFC, 2011 August 12th

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The Problem

… and previously protected by Al foils.

The last two leaks in sectors 2006 and 2007 appeared in ‘protected’ bellows.Their Al protections was not damaged at the time of the last survey in January 2011.

Few droplets, maybe a single one, are enough to generate corrosion and failure in ‘protected ‘ bellows.



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The ProblemA few ppm of Cl- ions are enough to develop a crack in austenitic stainless steels.

Ahmad Zaki , Principles of Corrosion Engineering and Corrosion Control, Elsevier

It is impossible to specify a Cl concentration threshold: even in small concentration (less than 10 ppm) Cl is concentrated to pitting site.

Vacuum leaks induced by stress corrosion cracking are characterised by a rapid, unexpected and huge pressure rise.


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Time

Pres

sure

[mba

r]

Leak

Pressure reading in sector 2006

On 9 May 1985, the suspended ceiling at Uster (CH) indoor swimming pool collapsed into the pool area

M. Faller and P. Richner, Materials and Corrosion 54(2003)331


9IEFC, 2011 August 12th

Recent Interventions

1. Ti-foil window valves for splitter 1- As suggested by this committee, the Spitter 1 is now equipped with 3 Ti window valves. The intervention was done during the last winter shutdown.

- The emergency rough pumping was reviewed: it is now based on dry pumps to be installed in TDC2 only in case of need.

- The Ti window valves can be operated only by TE-VSC specialists.

sect. 2005 sect. 2006

D. Forkel Wirth, EDMS 397107

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- The new valves are indicated with ‘W’ in PVSS.- Three locations for emergency rough pumping have been prepared.


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Recent Interventions2. Leaks in sector 2006 and 2007 (June and July 2011)- The sudden leaks appeared on both sides of the sector valve VVSB 220434.- The cause was the same: acid droplets moving along the cable tray and reaching

bellows after having perforated the Al protection.

Leak in June Leak in July

Splitter 2Collimateur


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Recent Interventions- The leak in sect. 2006 developed first.- The bellow replacement and the pressure recovery was fast because the sector is short.- The collective dose was shared amongst 8 TE-VSC staff members.

- At the same time a corrosion attach was observed in the bellow of the nearby sector. However the sector was not leaking.

- The corroded part was painted with VacSeal, the whole beam pipe covered with a thick stainless steel protection, and the production of the special vacuum chamber started. Hoping to keep the sector 2007 operational until the next winter shutdown…


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Stainless Steel 2-mm thick protections

Special chamber to be machined


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- But …with the usual signature, 19 days later, the expected leak appeared in sect 2007.- This intervention was heavier than the previous one and required the prompt

contribution of the mechanical (EN-MME) and chemical cleaning (TE-VSC) workshops.- The collective dose was shared amongst 12 TE-VSC staff members.- The pressure recovery was longer and it is still in progress.



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Next Leak (?)

An indication of corrosion was found in a welded junction in sector 2020


IEFC, 2011 August 12th 16

Proposed Mitigation

Based on the experience gained in the last 10 years, we should undertake:

1) A campaign of replacement of damaged or suspected vacuum components, as was done in 2003.

2) Protection of beam pipes by Kapton foils or/and 2-mm thick stainless steel sheets.

3) Replacement of ion pumps which are not or are poorly working at present. Change of ion pumps’ cables and connectors.

4) Increase of the number of pumps whenever possible and replacement of some ion pumps by new-generation NEG cartridges to accelerate recovery after air venting.

5) Replacement of at least 2 sector valves, namely VVSB 230102 and VVSA 240102

Vacuum system


IEFC, 2011 August 12th 17

Proposed Mitigation

1) Reduction of water leaks from hydraulic valves and water circuits.

2) Complete re-cabling of the TDC2 tunnel and PVC hunting.

3) Increase efficiency of air ventilation: at present the TDC2 tunnel is a cul-de-sac and no clear air flux is set in the extremity toward TT20.

4) Create a physical barrier made of acid resistant materials to protect the TDC2’s vacuum chambers.

More general items


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Beam lines

Cable trays Cable trays

Beam lines

Proposed Mitigation

Glass fiber protection Glass fiber protection



o High radioactivity, moisture and chlorine contamination generate an adverse environment for the TDC2 vacuum system as already pointed out in 2001 and 2003.

o The main cause of fault is stress corrosion cracking of stainless steel bellow generating vacuum leaks.

o At present, the situation is under control thanks to rapid interventions of DG-RP, EN-MME, EN-EL and TE-VSC colleagues:

The two splitters can be isolated by Ti window valves 4 bellows and 2 vacuum chambers have been replaced Regular preventive inspections are planned

o However the radiation dose collected in TDC2 is increasing rapidly (about 2.5 mSv per intervention in the last 3).

o As in 2003, the replacement of damaged elements and an improved protection of the vacuum system should be planned in the next shutdowns.

o Nevertheless the vacuum system will never attain stable conditions unless radical changes are undertaken, namely eradication of water leaks, improvement of ventilation, complete replacement of cables.

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Conclusions


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How does it work in case of large vacuum leak in the splitters ?

1. The three corresponding sector valves would be automatically closed thus isolating the air leak.

2. A roughing pump would guarantee a pressure of about 10-1 mbar at best.3. The three Ti window valves are closed.4. Finally, the three sector valves are opened.

In this final configuration the proton beam could transit through the Ti foils without major losses.

In 2010 (IEFC, January 8th, 2010, G. Vandoni), it was decided to apply the same scheme to the vacuum system of splitter 1 (sector 2005)

Additional slides



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Additional slides

Courtesy J-M. Jimenez


TE-VSC Group for ALARA Committee on January 17th

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High Radioactivity + Moisture + PVC↓

Cl Dissolved in Water Droplets ↓

Stress Corrosion Cracking of Austenitic Stainless Steel ↓

Vacuum Leaks

Additional slides

IEFCAugust 12th, 2011

AD vacuum problem(s) and mitigation measure(s)

Paolo ChiggiatoTE-VSC group

Injectors and Vacuum Measurements

24

The Problem- On Friday July 15th the AD-CR called the VSC piquet: high pressure in sector 1A (VGI5303)

caused a reduction of antiproton extraction efficiency.

- The pressure was temporary reduced by sublimating Ti in 4 interventions during the weekend. This allowed AD working until Monday 18th in high-energy mode.

- Monday 18th, the pressure bump was correlated with a vacuum leak in the feedthroughof the ion pump VPI5309, which is directly connected to the septum magnet SMI5306.


Leak in ion pump’s feedthrough:

4x10-6 mbar l s-1

P> 1x10-8 mbar



The feedthrough of the ion pump VPI5309 was deeply corroded.The brazed joint between the ceramic insulator and the metal core was completely destroyed.

The Problem


- The sector was vented to atmospheric pressure by N2.- The ion pump’s feedthrough was changed in-situ; a continuous flux of N2 was set during

the time the sector was opened to air.- Despite the adopted precautions , the pressure decrease would have been too slow even

with Ti sublimation: bakeout was necessary to accelerate the pressure decrease.

The

Inte

rven

tion


The Intervention

-In agreement with the TE-ABT (kickers and septa) colleagues, the sector 1A was equipped for a partial bakeout on Tuesday 19th.

-The bakeout cycle started on Wednesday 20th and finished on Monday 25th.

- The vacuum intervention was concluded on Tuesday 26th in the morning. Beam was back again at 16:58.

- A leaking water circuit (not anymore in use) in the septum SMI 5306 is now continuously evacuated by a primary dry pump.

0

50

100

150

200

250

300

350

Kickers

SMI 5306

Penning gauges

VGI

RGA

Ion pumps

TE-VSC Group for ALARA Committee on January 17th

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The AD operation could have gained one day of beam if the bakeout had been started one day before.

The Intervention

Leak

Venting and repair

PumpdownPumpdown

IPs ON

Bakeoutstarted

IPs OFF

Bakeout cyclecompleted

IP ONTi sublimation

Dry pump in SMI’s leaking circuit


Preventing Actions- Leaks due to feethrough corrosion are rare events in AD.

- The last to be reported was in 2001 after a water leak in sector 2.

- Nevertheless we checked all ion pump feedthroughs before baking sector 1A.

- We found in two of them an early stage of corrosion (VPI 5503 & VPI 5609).

- They will be replaced during the next winter shutdown when the sector is opened for other interventions: SMI5306, kickers and Ti filaments.

- Permanent heating collars should be installed on the feedthroughs in the AD ring

VPI 4706 VPI 4803 VPI 4909

VPI 5007 VPI 5303 VPI 5309

VPI 5409

VPI 5503 VPI 5509 VPI 5603

VPI 5609

VPI 7015

Pictures of the 11 ion pump feedthroughs in sector 1A: small traces that could be ascribed to corrosion are visible only in VPI 5503 and 5609.


Conclusions

o The pressure increase recorded in AD sect 1A was due to a leak generated by a deep corrosion of a ion pump feedthrough (VPI 5309).

o The sector was given back to operation 8 days later: a bakeout had been necessary to speed up the pressure recovery.

o The other feedthroughs of the same sector were monitored. Anomalous spots could indicate an early stage of corrosion development in two ion pumps.

o The ‘incriminated’ feedthroughs are going to be replaced during the next winter shutdown.

o General comment: water leaks are very detrimental for bellows and feedthroughs.

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