CMS THIP
Tracker Humidity Improvement Project
Document identifier:
Plans_for_Improving_Vapour_Barrier
s_and_Seals .docx
Institute Document No.
Created:
Page: 1 of 4
EDMS id
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Rev. No
DRAFT
Plans for Improving Vapour Barriers and Seals
in the CMS Tracker Bulkhead Area, from Beam-pipe up to the Vactank Ends
abstract
Prepared by:
Checked by:
Antti ONNELA, CERN-PH-DT
Riikka HÄSÄ, Helsinki Institute of
Physics
Domenico DATTOLA, INFN
Distribution List
Approved by:
Project Document No: 867594
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1
INTRODUCTION ................................................................................................................. 3
2
CURRENT HUMIDITY LEVELS, VAPOUR BARRIERS AND SEALS ................................ 3
3
HOW TO LOWER THE DEW POINT INSIDE THE TRACKER BULKHEAD? .................... 3
3.1
Dew point improvement method 1: Inject more dry gas ................................................................................................3
3.1.1
Dry nitrogen .................................................................................................................................................................3
3.1.2
Dry air ..........................................................................................................................................................................4
3.2
Dew point improvement method 2: Improve humidity sealing .....................................................................................4
3.2.1
Possible materials for the humidity sealing ..................................................................................................................5
4
DIFFUSION OF WATER THROUGH STAGNANT AIR ...................................................... 5
5
PROJECT PLANS ............................................................................................................... 8
5.1
Procurements and tests .....................................................................................................................................................8
5.2
Installation at CMS during LS1 .......................................................................................................................................9
Project Document No: 867594
1
Page: 3 of 9
Introduction
The target operating temperature for the CMS Tracker (TK) is -10C on the silicon sensors. This requires a coolant
temperature of -20C or even -25C for the innermost parts of the TK. To avoid condensation the surrounding gas
(air or nitrogen) needs to have a dew point lower than the temperature on the coldest surfaces. This is currently
not the case in the TK bulkheads, at the two ends of the TK volume, due which the TK is now operated with +5C
coolant.
The Long Shutdown 1, due from November 2012 until mid 2014, is the occasion for doing the needed dew point
improvements, and then allowing the lowering of the TK operating temperature. The following two measures
have been proposed:
1. Inject more dry gas
2. Improve vapour barriers and sealing
This document describes the plans concerning the second task, on improving the vapour barriers and seals in TK
bulkhead area and its direct vicinity from the beam-pipe up to the CMS vactank ends.
2
Current humidity levels, vapour barriers and seals
The CMS cavern dew point is guaranteed to be +13C or lower. This is too high for the TK and its cold cooling pipes.
Sufficiently dry local atmosphere needs to be created. Vapour barriers, thermal insulation and dry gas injection
are employed to achieve this.
According to the measurements taken until 2012 the dew point inside TK’s Silicon strip detector volume is low,
about -30C. This is fully satisfactory and there are no needs to change the system for this part.
For the Pixel detector volume the dew point has been measured to be higher than in the Silicon strip volume,
possibly due leaks at the end(s) of the Pixel detector volume. These leaks and repairing them shall be studied in
the beginning of the Long Shutdown 1, in 2013.
However, the dew point inside the TK bulkhead is significantly higher with measurements ranging from -20C up to
0C, varying in different sectors of the bulkhead. Some measurements show dew points For measurement
methods, data and other information see documents in https://espace.cern.ch/cms-trackerengineering/THIP/SitePages/Home.aspx and https://edms.cern.ch/document/1100541.
The dew points measured inside CMS, at the proximity of the TK bulkhead, are the same as measured inside the
cavern. So, currently CMS up to the TK bulkhead is not humidity tight, and also the TK bulkhead/cable channels
are currently leaky leading to the high dew point.
3
How to lower the dew point inside the Tracker bulkhead?
The next planned opening of CMS is planned for the long shut-down due in 2012-2013. This occasion must not be
missed, as the next occasion will be only several years later, while the TK operating temperature should be
lowered already as soon as possible to limit the radiation damage on the silicon sensors.
To execute the humidity improvement tasks in 2012-2013, the necessary procedures, materials and manpower
resources must prepared and ensured already within 2011.
3.1
Dew point improvement method 1: Inject more dry gas
3.1.1 Dry nitrogen
During normal operation dry nitrogen is injected into CMS for fire safety and humidity reasons. The current
amounts of dry nitrogen injected are the following:



Flow into the TK volume: 17.75 Nm3/h (design value 32 Nm3/h)
Total flow TK bulkheads + PP1: 17.4 Nm3/h (design value 24 Nm3/h)
o Of which into the TK bulkheads: ~ 3.6 Nm3/h per bulkhead
Flow into the CMS barrel/endcap gaps: [waiting for info]?
Project Document No: 867594
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The current available flow for dry nitrogen devoted to detector flushing is about 60 Nm3/h (not available during
magnet cool down). The ‘requested’ flow of dry nitrogen injection (engineering specification on-going) is 130
Nm3/h, out of which about 50 dedicated to the tracker bulkhead region.
To lower the dew point in the TK bulkheads more dry nitrogen could be injected there, either by increasing the
total nitrogen flow (if available) or by modifying the gas distribution.
3.1.2 Dry air
When CMS is opened for maintenance the dry gas injected needs to be air. The air flow into the TK and bulkhead
volume should be at least the same as what is the nitrogen flow during normal operation. Dry air flow could even
be rather big; a few hundred m3/h dryer units are not expensive as investments and are simple to operate.
The current dry air flow is limited by the compressor capacity available at CMS. Current capacity of the
compressors is 180 Nm3/h for the full “instrument air” circuit. Average consumption on this circuit (not taking
into account peaks nor detector flushing) is about 105 Nm3/h. A continuous monitoring of this system flow
consumption has been requested to EN/CV, as well as a study to increase the system capability.
The aim could be to have at least ~100 m3/h of dry air available for injection into the TK volume and bulkheads
during maintenance periods.
3.2
Dew point improvement method 2: Improve humidity sealing
Humidity can drift from the cavern air to the TK bulkheads via the following paths:
A. Through the gap between the CMS endcap and the beam-pipe
B. Through the CMS endcap that is not intrinsically ‘humidity tight’.
C. Through the gap between the CMS barrel and the endcap
As the exact paths of humidity within CMS are and will be impossible to know it seems that the correct approach
is to try to improve the humidity sealing where ever it is reasonably possible.
To improve humidity sealing the following locations within CMS can be considered (the list is ordered from beampipe radially outwards):
1. Add sealing between CMS endcap and the beam-pipe.
a. This option may be difficult to realize as the endcap needs to move with respect to the beam-pipe
and there is no/limited access to this area.
2. Wrap the nose of the CMS endcap with vapour barrier foils to prevent humidity drifting through the
endcap.
3. Verify and possibly improve the existing sealing between the low R corner of the CMS endcap and the
aluminium counter ring at the TK bulkhead.
a. Multiple silicon hoses make this sealing joint, however, there is currently no means of checking if
the joint is closed correctly. Adding e.g. indicator paint on one of the surface could allow checking
if the contact is made correctly.
4. Add vapour barrier foils on the face of the TK bulkhead.
5. Inject humidity sealing ‘paste’ in the TK cable channels at the start of the cable channels, next to the TK
outermost corner.
6. Add vapour barrier foils on the TK, barrel ECAL and HCAL cable channels inside the cryostat, and up to the
front face of the cryostat.
7. Add humidity sealing ‘paste’ at the outer end of the TK PP1’s where the cables and pipes leave the PP1’s
towards the balconies.
8. Add humidity sealing ‘paste’ in the cable channels at the front face of the cryostat, next to the inflatable
seal between CMS barrel and endcap.
9. Verify and possibly improve the inflatable seal between CMS barrel and endcap.
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3.2.1 Possible materials for the humidity sealing
The choice of the materials and methods should be done as much as possible before the intervention is due in
2012-2013. This preparatory work should therefore start now without delay. Here a first list of potential materials
to be used:
Use
Product name, supplier, other information
Vapour barrier foil
Armachek S by Armacell. Aluminium foil on a flexible glass-fibre
tissue. Typical roll size: 1040 mm x 25 m, which costs about 900
CHF. Local sales: Multijoint SA, 101, Route du Nant-d'Avril, 1217
Meyrin (CERN supplier code : MULT50, MA01). Used in CLOUD
experiment as chamber vapour barrier. Electrically “anti-static”.
https://edms.cern.ch/file/1100541/1/ARMACHEK-S.pdf, see
product code ACH-S25, which is the coating without the Armaflex
insulation.
Vapour barrier foil
DELTA-REFLEX by Dörken GmbH. Waterproof and non-corrosive
aluminium layer embedded between highly transparent, mesh
reinforced polyester film. Used in the walls and roof the building
186 upstairs clean-room. Electrically “anti-static”.
http://www.doerken.de/bvf-be/pdf/prospekt/Product-Range.pdf
Aluminium tape
Exist in many different variants, need to test and choose the best
suiting product for CMS use. Electrically “anti-static”.


BAL75 and BAL100 tapes by Multijoint SA in Meyrin. Thin,
easily applicable, but not very tear resistant. Used in
CLOUD. Adhesive material and its radiation hardness?
Aluminium tape with acrylic adhesive by 3M (product code
425). Thick 0.13 mm aluminium. Roll size 50 mm x 55 m.
Resistant but difficult to apply evenly. SCEM
04.95.20.250.9. Radiation hardness?
Kapton tape
KAPTON type H polyimide film with siliconised adhesive (SCEM:
04.94.70) have been tested to be radiation resistant, and very
reliable in use. Adhere well, but can also be removed from
surfaces without leaving traces.
Not electrically conductive. Could be used e.g. to reinforce and
ensure aluminium taped joints.
Humidity sealing ‘paste’
Paolo, what are the materials recommended here, what was the
silicone product used in the Tracker already?
Vapour barrier adhesive IWR701 from Insu-W-Rapid (Netherlands).
foil
(datasheet on this EDMS doc)
Other
4
Diffusion of water through stagnant air
CMS vactank’s inner diameter: 5945 mm.
Project Document No: 867594
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The cooling channel consists of … and it is estimated to have n leak channels. Air is fed into the cooling channel in
an environment where the dew point is at -50 °C. At the outlet the dew point is at +13 °C. The air at the outlet
has higher water content and the concentration gradient between the channel ends causes a diffusion water
vapour flow towards the inlet. The aim is to obtain approximate length of the cooling channel length and to
estimate the influence of diffusion.
First, the diffusive mass flux of water vapour in one leak channel is solved. The total diffusive mass flux from all
the leak channels along with the airflow from the inlet is then assumed to flow into a volume of stagnant air. The
diffusive mass flux increases the water content of the airflow, and a new dew point for air is obtained and
compared with the dew point at the inlet.
The mass flux of water vapour per leak channel is solved by using Fick’s first law of diffusion which is based on the
assumption of steady state. Fick’s first law is:
J  D
c
,
x
(1)
where J is the diffusion flux [g/m2s], D is the diffusion coefficient [m2/s], c is the concentration [g/m3] and x is the
position. The equation (1) can be written as:
J D
c
,
L
(2)
where Δc is the difference in concentration between the leak outlet and inlet and L is the length of the pipe.
Table 1. Variables.
D
22∙10-6 m2/s
co
11.276 g/m3
cI
0.038 g/m3
L
1m
A
1∙10-4 m2
The diffusive mass flux can now be obtained by multiplying the diffusion flux by the cross-sectional area of the
pipe:
m D  JA  DA
c
.
L
(3)
Using the values given in table 1, the mass flux in one leak channel becomes:
m D  2.472  10 8 g/s.
(4)
When the total mass flux of water vapour is mixed with the airflow from the inlet, the total concentration
becomes:
ctot 
Vc I  m D n
,
V
(5)
where V is the air inflow and n is the number of leak channels. The air inflow is 350 Nm3/s and n is estimated to
be 10 000. Thus, the total concentration becomes
ctot  0.0405 g/m3,
(6)
which approximately corresponds to a dew point at – 50 °C (table 2).
To compare the solution with the current installation at CMS, airflow is estimated to be 1/10 of the volumetric
flux and the number of leak channels was estimated to be 10 times higher than in the upgraded version. Using the
Project Document No: 867594
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values V = 9.722∙10-3 Nm/s, one can obtain a total concentration ctot = 0.0634 g/m3, corresponding to a dew point
at -46 °C.
When the number of leaks channels is then increased to n = 100 000, the total concentration becomes ctot = 0.292
g/m3, corresponding to a dew point at -31.5 °C.
Project Document No: 867594
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Figure 1: Dew point of air as a function of its water content. The second graph shows the lower end of the first.
5
Project plans
5.1
Procurements and tests


Procurement of best candidate products [Antti?, Paolo?]
Verification and validation of the safety matters of the products: fire, radiation, possible electrical
properties like requirements on “anti-static”. [Antti?, Paolo?]
Project Document No: 867594
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 Tests on working with the vapour barrier materials, their cutting, joining, etc. [“Technicians”]
 Tests on application of humidity sealing ‘paste’ [“Technicians”]
The “Technicians” should ideally be the persons who will be installing the materials during the long shutdown at CMS. Who could these persons be?



5.2
Eric Albert?
Some other CMS technicians?
DT technicians? How many should be requested from DT for this job? 2 persons?
Installation at CMS during LS1



Provide access and platforms to the work areas inside CMS from cryostat face up to TK bulkhead, and also
possibly on the CMS endcap nose. [CMS TC]
The work period and exact sequences need to be coordinated together with other tasks in the same area.
Technician team of 4(?) persons [from CMX and DT?], and 1 or 2 work supervisors [Paolo?, Antti?]
depending on the amount and type of work to be executed, shifts, etc.