December 6, 2012
New cooling pipes for CO2 operation of the
CMS Pixel detector
Introduction
The current cooling pipes for distributing the liquid coolant from the cooling
plant (based on conventional C6F14 liquid coolant) to the Pixel detector inside
the CMS experiment were installed in copper (14mm OD) along YB0, the central
piece of CMS (see figure 1), during the final installation phase of the experiment
in 2007. Since then they have been operated with C6F14 fluid at <10 bar
pressure and lowest temperature of -20C.
For the upgrade Pixel detector (to be installed during 2016-17 year-end
technical stop) it was decided to replace the current cooling plant moving to a 2phase evaporative CO2 cooling system as it allows for operation with much less
fluid and smaller pipes in the detector volume, hence reducing the overall mass
of the device resulting in a considerably better particle tracking performance.
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Figure 1: A photo of the YB0 piece of the CMS experiment taken from the +Z side
(the –Z end looks similar). Present copper pipes bundles are indicated by arrows.
The protruding cylinder is the CMS solenoid. To set the scale the outer diameter of
the solenoid measures about 8 meters.
In order to verify the suitability of the present copper distribution plant on YB0
for use with CO2 2-phase evaporative cooling system, an extensive set of tests
were carried out on identical copper pipes with similar distribution as in the
experiment, in a lab at CERN and showed that indeed the present pipes and
brazed joints can be safely operated at the higher pressure of ~70 bar required
by the new CO2 system.
Nonetheless it is still attractive the idea of being able to install a new set of pipes
for the following reasons:
a. It would allow for a quicker switch over from the old to the new Pixel
detector as the new pipes can be thoroughly tested “in situ” in advance.
b. It would allow for a full separation of the Pixel pipes from other pipes
serving other parts of the detector, which are, in principle, operated at
different (higher) temperature. A full separation allows for a true
operation of the Pixel detector at different temperature from the others
without warring about coupling effects.
c. Providing enough insulation, we could operate the new Pixel detector at
lower temperature (below ~ -25C which is the present limit imposed by
the installed amount of insulation). We would like pipe insulation not to
be the limiting factor.
d. It would allow for a more favorable design of inlet and outlet pipes (inlet
inside the outlet) and material (stainless steel) for operation with the CO2
2-phase evaporative system at high pressure (>100 bars)
Given these advantages associated to the installation of a new set of pipes, but
knowing that the present system can also do the job, the choice will be based on
ease of installation, time required, possibility of a better insulation (allowing for
lower temperature operation), cost and reliability.
Installation of new services can occur only during long shutdown 1 (LS1, about 2
years period from 11-02-2013 until 1-11-2014) during parts of which the YB0
piece of the CMS experiment will be fully available for refurbishment.
For what concern this specific task, two periods are currently foreseen:
o -Z end: verification of the 3D model on the YB0 solenoid, 1.5 working
days starting on May 29 2013.
Installation of the 4+4 concentric CO2 lines, 5 working days starting on
July 24 2013.
o +Z end: verification of the 3D model on the YB0 solenoid, 1.5 working
days starting on June 3 2013.
Installation of the 4+4 concentric CO2 lines, 5 working days starting on
November 21 2013.
These dates are subject to change (just slightly perhaps) and inputs for what
concerns the effective time needed for the verification and later installation
would be welcomed.
Proposal
We then studied the possibility of adding new independent pipes using the
following assumptions:
1. We need 16 inlet/outlet concentric pipes to reach the Pixel detector from
a distribution manifold located on a balcony next to the YB0 piece of the
CMS experiment.
2. They are organized in 4 bundles of 4 inlet/outlet concentric pipes each
and follow 4 different routes from the distribution manifold to the
detector.
a. Along the top of the +Z end of YB0 in order to reach the +top part
of the solenoid
b. Along the bottom of the +Z end of YB0 in order to reach the
+bottom part of the solenoid
c. Along the top of the -Z end of YB0 in order to reach the -top part of
the solenoid
d. Along the bottom of the -Z end of YB0 in order to reach the bottom part of the solenoid.
3. The outlet pipe diameter has ID = 10 mm
4. The inlet pipe diameter has ID = 4 mm and is placed inside the outlet pipe.
5. The insulation is achieved via vacuum jacketing on each individual outlet
pipe using 6 mm space around it. For conventional insulation material, a
space of at least 25 mm would be needed. It has to be noted that pipes are
NOT accessible, hence no intervention is possible to re-establish proper
vacuum in case of need (except during the rare occasion of a long shutdown and at the price of opening the endcap piece if no other provision is
established).
6. The system max operating pressure is 100 bars, design at 110 bars
(tested at 157 bars ?)
7. For vacuum jacketing, the system should be operable with CO2 flowing at
-40C without falling below +13C on the external surface in contact with
the surrounding environment at +18C.
As this new set of pipes need to be routed on top of the already laid services we
would like to have the possibility of disconnecting the pipes along the run on the
CMS solenoid. A proposal can be found in the attached 3D drawings. Other
connection points are possible provided that we agree on the number and
position.
+Z Top routing
In order to study the complexity of the job for evaluating its feasibility, we
developed a full 3D model of the +Z top routing of the new CO2 pipe bundle (4
concentric pipes) entering the YB0 piece from muon sector 4 (top) and reaching
the top position along PP1 +5. PP1 (Patch panel 1) is a large box located at the
entrance of the inside surface of the solenoid and used for connection of services
(pipes and cables). From there services continue further in toward the detector.
As we need to continue to operate the present detector with the present services
until year-end technical stop 2016-17, the new pipes will stop in the proximity of
PP1, awaiting for the swap to occur. At the time of the Pixel detector replacement,
the present pipes will be removed from the location of arrival of the new CO2
pipes all the way downstream toward the detector, creating the space needed for
a connection with the already installed new CO2 pipes and to further continue
installing new pipes toward the detector.
Figure 2: 3D view of the new CO2 pipes (dark blue) routing for the top +Z bundle.
Entering from muon sector +4 and reaching PP1 +5.
Figure 3: top view of the new CO2 pipes for the +Z top route. Note the “boxes” for
the VCR connection with conventional insulation.
Figure 4: Change of aspect ratio for new CO2 pipes for +Z top toward the edge of
the solenoid.
Figure 5: Pipes take a double 90 degree bend to end inside the solenoid in the
location shown in this figure.
Outlet
Conceptual connection
(a)
Vacuum jacket
Regular Insulation
Inlet
Conceptual connection:
Two layers of 2 connections
(b)
VCR connections
VCR connection
Regular Insulation
(d)
(c)
Figure 6: A conceptual design for the pipe-in-pipe connection using a VCR
connection. For a bundle of 4 pipes there will be a stack of 2 layers of connections,
each with 2 pipe-in-pipe. In the connection region regular insulation needs to be
applied.