ATLAS Muon Spectrometer
Thin Gap Chambers
Gas System Safety Report
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0
TGC-002-98
5 October 1998
ATLAS TGC Collaboration
ATLAS
Muon End-cap Trigger Chambers
Flammable Gas System Safety Report
5 October 1998
This document may be found:
on the TGC Web page as:
http://www.weizmann.ac.il/~fhatlas/TGCsafety.pdf
on the CERN mirror of the TGC Web page as:
http://wwwcn.cern.ch/~lellouch/TGCsafety.pdf
on the CERN AFS file system (access for any ATLAS userid) as:
/afs/cern.ch/atlas/project/TGC/TGCsafety.pdf
Revision record
Issue Revision Date
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Comments
1
0
15 August 1998
Version 1
2
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5 October 1998
Version 2 − DRAFT
ATLAS
Muon End-cap Trigger Chambers
Flammable Gas System Safety Report
5 October 1998
1 Introduction
The present report addresses the safety issues related to the use of a flammable gas in the TGC
detectors in ATLAS and in particular the way that any possible leaks can be kept at a safe,
controllable level. This is achieved by having a relatively small volume of flammable gas (4.2m3
of hydrogen equivalent in each ATLAS end-cap) that is monitored for losses by a set of 156 pairs
of input and output flowmeters, while the detectors themselves are surrounded by a volume of
circulating inert gas (CO2), which is monitored for the presence of n-pentane and oxygen.
Finally, the full system is surrounded with sniffers at various heights for the detection of
flammable mixtures. An overview of the gas system is shown in Figure 1-1.
The TGCs will be used in the ATLAS end-cap muon trigger due to their high granularity, good
time resolution, high rate capabilities and ageing characteristics. To attain this type of operation,
a highly quenching gas that avoids streamers, while permitting a saturated operation is needed.
This is achieved with a mixture of CO2–n-pentane (55%–45%), where the n-pentane plays the
role of the quencher. No other gas that is less flammable has been found that provides the above
performance. Slightly lower concentrations of n-pentane will be investigated. The intended gas
system for ATLAS is based on the experience obtained with the OPAL system, and whenever no
changes have been made, the OPAL system will be presented as the base solution.
Section 2 presents the proposed gas mixing system as used in the OPAL experiment. The gas
distribution system is presented in Section 3 and the gas circulation within the TGC modules is
given in Section 4. The safety controls are given in Section 5 and a summary is given in
Section 6.
SGX
CO2
supply
n-C 5H12
storage
nonflammable
gas
storage
flammable
gas
storage
purifier &
recovery
mixer
n-pentane+CO2
UXA15
distribution
rack
USA15
chamber
supply
2 supply pipes
2 return pipes
USA installation
Figure 1-1 Overview of the installation of the TGC gas system.
1
Introduction
1
ATLAS
Muon End-cap Trigger Chambers
Flammable Gas System Safety Report
5 October 1998
2 Gas mixing system
Figures 2-1 and 2-2 show the present arrangement of the OPAL n-pentane mixing and recycling
system. Figure 2-3 shows the OPAL gas rack. With minor modifications, a similar system is
intended to be used for the ATLAS TGCs. The system consists of four main elements: 1) the
n-pentane reservoir and pumping system, 2) the mass-flowmeters mixing system, 3) the control
system and 4) the n-pentane recycling system, which are described here:
The n-pentane reservoir and pumping system: The n-pentane reservoir is installed in a
concrete room with active ventilation. It consists of an 80 litre stainless steel tank. The liquid
n-pentane level is monitored with an automobile (Subaru) fuel gauge. n-pentane flows into the
tank from a mechanical pump for refill, the n-pentane recycler and the return line from the
n-pentane pump. The outputs include a line to the electric n-pentane pump and an exhaust line
to the outside that goes through a bubbler with 10 mb overpressure. Each of the elements of the
system is connected to the electrical ground of the building. In parallel to the main reservoir,
there is a small (20 l) reservoir at a pressure of 1.5 bar of CO2, in order to be activated in case the
n-pentane pump fails. The two containers have a screw cap for purging. The electric pump (εx)
that follows the reservoir, is located outside the concrete room. The pump operates via a
magnetic coupling and its output is split into two lines: one line goes to the mixing system
(joined by a line from the small reservoir) and the other goes (via a 0.5 bar overpressure valve)
to the main reservoir.
The mass-flowmeters mixing system: The mixing rack contains two mass flowmeters (a gas
flowmeter for the CO2 and a liquid flowmeter for the n-pentane). Each one runs normally at
1000 l/hr of gas equivalent but is capable of reaching 3000 l/hr for flushing. The output of the
mass flowmeters goes to a heated (60°C) stainless steel container filled with glass beads, where
the mixing takes place. The output gas is then cooled to 17°C, through a condenser, in order to
ensure that gas has no excess of n-pentane (any liquid in the condenser will give an alarm). The
output mixture is then sent to the experiment via a heated stainless steel pipe. To protect against
overpressure should the gas valves be closed in the experiment, the output line has a bubbler
that goes to the general exhaust.
The control system: that monitors the various functions of the gas mixing can issue warnings,
alarms and take actions (i.e. stop the flow of CO2 and of n-pentane, as well as the return pump,
in case that an anomaly is found). The elements monitored are:
1. liquid level in the n-pentane reservoir,
2. pressures and flows in the CO2, n-pentane and mixed system,
3. presence of liquid n-pentane in the gas mixture,
4. overpressure exhaust (through a safety bubbler) in the output gas.
The n-pentane recycling system: The return flow through the heated stainless steel gas return
line is pumped by the gas return pump. A silica-gel container absorbs any water vapour from
the gas before it flows to two stainless steel tubes cooled to −15°C and −50°C. Here the
n-pentane condenses and flows back into the liquid n-pentane gas reservoir. The output gas is
then monitored for the presence of oxygen and exhausted into the atmosphere. If the oxygen
exceeds 1%, the gas mixing system, i.e. EV1 and the return pump, is stopped. The silicagel must
be regenerated every week.
The actions associated with the various sensors are listed in Table 2-1.
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2
Gas mixing system
ATLAS
Muon End-cap Trigger Chambers
Flammable Gas System Safety Report
5 October 1998
The various systems, as described above, have been used without major problems in the OPAL
experiment during the last four years. Nevertheless, a HAZOP study will be performed before
its implementation in the ATLAS experiment.
In parallel to the normal gas system, there will be a pure CO 2 system that will be used for inert
gas flushing around the detectors, as well as for flushing specific detectors. This system will
consist of a CO2 line followed by a pressure regulator.The return line consists of a return pump.
The gas will be exhausted into the atmosphere. The typical flow rate will be 1000 l/hr.
2
Gas mixing system
3
ATLAS
Muon End-cap Trigger Chambers
Figure 2-1 The n-pentane mixing system.
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2
Gas mixing system
Flammable Gas System Safety Report
5 October 1998
ATLAS
Muon End-cap Trigger Chambers
Flammable Gas System Safety Report
5 October 1998
Figure 2-2 The n-pentane recycling system.
2
Gas mixing system
5
ATLAS
Muon End-cap Trigger Chambers
Figure 2-3 OPAL gas rack.
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2
Gas mixing system
Flammable Gas System Safety Report
5 October 1998
ATLAS
Muon End-cap Trigger Chambers
Flammable Gas System Safety Report
5 October 1998
Table 2-1 Sensors and their associated actions in the mixing and recycling system.
Sensor
Measurement
Action
C0
CO2 mass flow
if out of range warning or alarm
C1
n-Pentane liquid mass flow
if out of range warning or alarm
C2
mixture mass flow
if bellow range: warning
if above range: alarm
C3
liquid in mixing reservoir
if out of range: warning or alarm
C4
temp. in mixing reservoir
if out of range: warning
C5, C10
temp. in n-pentane recuperator
if out of range: warning
C6
liquid n-pentane level
if out of range: warning or alarm
C7
liquid in n-pentane condenser
if out of range: warning or alarm
C8
ice in n-pentane recuperator
sends a warning; stops recuperators until Temp
C5, C10 are above 5°C and starts coolers again.
C9
temperature in condenser
if 16°C< T<16.5°C or 17.5°C< T<18°C warning,
otherwise alarm
C11
vacuum pump pressure
if out of range: warning or alarm
C12
no gas circulation to experiment
if out of range: warning or alarm
C13
pressure in liquid n-pentane
circuit, otherwise, alarm.
if 0.7<p<1.6 bar or 1.8<p<2.0 bar: warning;
otherwise: alarm
2
Gas mixing system
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ATLAS
Muon End-cap Trigger Chambers
Flammable Gas System Safety Report
5 October 1998
3 Gas distribution system
The two gas lines (CO2–n-pentane and pure CO2), with their corresponding return lines, are
split in USA to provide gas to the two end-caps. These lines are then split into four, to provide
the gas circulation to the four gas racks. These gas racks are located at three different heights, in
order to minimize the hydrostatic differences for each of the chamber groups that are being
supplied. From the gas racks, there are groups of seven (or two in the case of the EI chambers)
copper tubes to supply the gas mixture to the seven (two for EI) planes of chambers in groups of
gas modules. In parallel, there are three lines (one for the EI chambers), carrying CO2 for
flushing around the chamber modules (one triplet and two doublets of chambers). The way the
512 gas chamber modules and 224 CO2 flushing modules are arranged, is shown in Figure 3-1.
They are serviced by 128 (× 2 for input + output) copper gas lines, carrying the CO2–n-pentane
mixture, and 56 copper tubes carrying CO2, for flushing around the detectors.
Movable EM chambers
Movable Forward
chambers
1
Inner station Forward
& Endcap chambers
4
2
3
Figure 3-1 Gas pipe layout on the detector, for one end-cap. Each connection line represents seven gas lines,
one per layer of trigger chambers, or, in the case of the Inner station chambers, two gas lines. There are 256 gas
modules per end-cap. The distribution is on three vertical levels to minimize differences in hydrostatic pressure.
(Gas Racks 2 and 4 are at the same vertical level.)
A schematic view of the gas distribution system contained in each gas rack is shown in
Figure 3-2. It includes two parts: 1) to provide the gas circulation of the CO2–n-pentane mixture
and 2) to provide the gas circulation of the inert gas around the chamber modules. The first part
consists of electric valves at the input and output of the system that permit stopping the gas
8
3
Gas distribution system
chambers
n-Pentane +
CO2 (45/55)
open if no
electricty
P
inert gas
chamber/channel
purge
measure
O2 content &
flammability
P
Gas distribution system
sampling
analysis
open if no
electricty
to recycling
on surface
emergency
backup in case
of static system
exhaust
to surface
emergency
backup in case
of static system
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Flammable Gas System Safety Report
5 October 1998
chamber
modules
back-pressure
regulator
P
P
over pressure
protection
P
monitor
ATLAS
Muon End-cap Trigger Chambers
3
Figure 3-2 Schematic view of the gas distribution system contained in each gas rack.
monitor
ATLAS
Muon End-cap Trigger Chambers
Flammable Gas System Safety Report
5 October 1998
circulation in case of gas alarms or power failure. The input valve is followed by a manual
valve, a mass flowmeter and a pressure regulator, to ensure a gas overpressure of 2mb with
respect to the local atmospheric pressure at the given height. Following is a manometer to read
the pressure value into the Detector Control System, DCS. In case of malfunction, an
overpressure bubbler ensures that the maximum overpressure on a detector is 2.5mb. The gas
line is split into two; one line goes to a test chamber, where 10% of the gas is sampled to monitor
the operation of the detector, and then joined together to provide the gas to the distribution
system. The distribution system consists of a tube with outputs to each of the lines, which are
connected with quick-release connections (in order to disconnect one line and move it to the
inert gas flow, in case of a leak). The return line from the chambers includes a mass flowmeter
followed by a fan-in of the various return lines. The combined return line is split into two; one
goes to a test chamber, where 10% of the gas is sampled to monitor the operation of the detector,
and then joined together. The combined line includes a manometer (read by the DCS), a
protection system for air pressure variations when the power is off, an overpressure protection
bubbler (at 1mb overpressure) and a back-pressure regulator which is set to zero pressure,
relative to the local atmospheric pressure. The return line goes through an electric valve before
going to the combined point of the various return lines at USA and then to the surface for
n-pentane recycling.
The inert gas line includes a pressure regulator, to ensure an input overpressure of 1.5mb,
followed by a mass flowmeter and a manometer (both of them to be read by DCS). The line has
a bubbler, used as an overpressure protection (set at 2mb) and then splits into two, one side
serves (after going through a manual flowmeter, as input for purging a given detector, while the
other side goes via an electric valve to a distribution system. The distribution system splits into
a number of lines that go to the various chamber modules, in order to flow gas around the
detectors and dilute any possible leak of flammable gas. The return lines are then sampled
before being combined into a single line. The various samples are transmitted to USA for a
measurement of the n-pentane and CO2 content. The combined line goes to a mass flowmeter, a
manometer (both of them read by DCS), and overpressure bubbler (at 0.5mb), a back-pressure
regulator at 0mb with respect to the local hydrostatic pressure and an electric valve, before
going to USA, where it is combined with the lines from the other racks and then vented at the
surface.
Each gas rack is sealed. It includes a flammable gas leak detector and an air extraction system.
Once the system is fully designed, it will also be the object of a HAZOP study.
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3
Gas distribution system
ATLAS
Muon End-cap Trigger Chambers
Flammable Gas System Safety Report
5 October 1998
4 Gas circulation within the TGC modules
Figure 4-1 shows a typical mounting structure of the TGC detectors. Such sub-structure carries
either 8 triplet modules or 10 doublet modules of TGCs. These structures will be mounted on a
large movable structure (the so called “big wheel”) which includes in its rim all the cables and
copper gas lines which supply the sub-structures. The lines around the “big wheels” will be
checked for leaks once mounted, while the lines in the sub-structures will be checked for leaks
together with the detectors, as a full unit. The detectors themselves will be checked for leaks
during three stages: after construction, before the cosmic ray testing and after arrival at CERN.
All the above tests are performed at a factor of two overpressure, above their normal operating
overpressure. The test results for each module are stored in the construction data base.
Figure 4-1 shows a general design drawing of a typical module. It can be seen that it consists of
three separated gas volumes (each 2.8mm thick) with their separated connections pipes (details
C and E) made out of FR-4 machined pieces, on which a short flexible plastic pipe is mounted
with a metal pressure ring around it. The gas volumes are contained between two (1.6 and
1.8mm thick) FR-4 plates. The inner separation between the FR-4 plates is provided by 20mm
thick honeycomb, while the outer 1.8mm thick FR-4 plates are protected by a 5mm thick
honeycomb covered by a 0.5mm thick wall of FR-4, that gives needed mechanical strength. This
arrangement has been tested (1) for possible damage by flying objects (due to metal objects left
in the magnetic field). In the test, a 1kg hammer was dropped from a height of four meters and
a 350g screw driver was dropped from a height of seven meters. Neither test resulted in any
damage to the internal gas volume of the chamber.
The edges of the detector are contained in a second gas volume, through which the inert gas
flows. The connection points for the inert gas circulation are shown in details A and B. The inert
gas circulation dilutes any gas leak from the chamber volume. The inert gas also circulates
around the points where the HV elements are located (detail J for HV resistors and detail K for
HV capacitors). All HV elements (capacitors and resistors) are potted in epoxy. The front end
electronics boxes (see detail K) are located outside the gas volumes. The electronic cards have a
total power dissipation of 1W per card. They have direct thermal contact to the copper cage via
an electrically isolating thermally conductive sponge material. The power is provided in the
same twisted pair cables that include the read-out lines. The LV lines have a 0.5A fuse on each
±3V line at the adapter board (located 1 to 4m away). The fuses are reachable during any short
access, and the currents are checked continuously by the DCS.
1. G. Benincasa, Report on Mechanical Test of TGC, forthcoming ATLAS Safety Note.
4
Gas circulation within the TGC modules
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ATLAS
Muon End-cap Trigger Chambers
Figure 4-1 A typical mounting structure of the TGC detectors.
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4
Gas circulation within the TGC modules
Flammable Gas System Safety Report
5 October 1998
ATLAS
Muon End-cap Trigger Chambers
Flammable Gas System Safety Report
5 October 1998
5 Safety controls
There will be two independent control systems for the gas circulation: The first is located in the
mixing room and is described in Section 2; the second is located in the experimental area and is
described here. The safety controls in the experimental area include the following levels:
1. Monitoring of the flammable gas flow in each individual circuit. This will be done by
using the mass flowmeters in each of the return gas lines. If the costs are not prohibitive,
there will be an input and output mass flowmeter, otherwise, it will be done by
comparing the input flow in the gas rack to the sum of the output flows. The difference
should not exceed 10% of the flow. Any channel with a potential leak will be identified, its
flow will be stopped and its corresponding HV will be turned off (as part of the DCS
interlock).
2. Monitoring the independent inert gas system that flows around the detector modules, in
order to dilute any possible leak. This is done by sampling the return lines of the inert gas
every 11min (10sec/line) for O2 and n-pentane contents. If the O2 content exceeds 5% by
volume or if the mixture exceeds 20% of the Lower Explosive Limit (LEL), the
corresponding flammable gas lines and the corresponding HV lines will be shut off. A
warning will be given at 10% of the LEL.
3. Flammable gas detectors will be mounted in the trench. Furthermore, sniffing tubes
leading to a flammable gas monitor (e.g. an infra-red monitor) will be installed in every
octant of the detector. If at any point, any of these detectors shows a mixture exceeding
20% of the Lower Explosive Limit, the flammable gas circulation to the TGCs will be
stopped, as well as all HV and LV for all the detectors in the trench (the MDT and TGC
EM layer). A warning will be given at 10% of the LEL.
6 Summary
The proposed gas safety system for the ATLAS TGCs is based on the experience gained with the
same detectors operating in the OPAL detector. A major change has been made by including a
circulating volume of inert gas (CO 2) which allows to dilute any possible leak of flammable gas.
This change permits, furthermore, to have three independent systems controlling the safety of
the area.
5
Safety controls
13
ATLAS
Muon End-cap Trigger Chambers
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6
Summary
Flammable Gas System Safety Report
5 October 1998