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NA58 flammable gas safety
Interim report.
M Davenport 31-01-01
Arising out of visit of safety delegation (M Davenport, J Gulley, G Mallot, C Nuttall, T Reynes, H
Taureg) to B888 on Thursday 18-01-01
1.
B908 primary gas supply
2.
Gas mixing zone
3.
Experimental area
Drift chambers and Micromegas
RICH tent
Muon Wall 2
4.
Noxal use in C4F10 recirculation system
5.
RICH chamber test area
1.
B908 PRIMARY GAS SUPPLY
Requirements:
The maximum flammable gas flow from B908 to B888 should be limited.
The CH4 lines for the RICH and MW2 should be separated and limited individually to 500l/h each (as
proposed by NA58).
The C2H6 line to be limited to 200 l/h.
Flow limiting orifices to be ordered by NA58 via M Bosteels (EST)
Flammable gas detection in B908.
Alarms and actions:
Level 2 (10% LEL):
Local Alarm
Alarm to TCR
Electrical contact to NA58 (for interlocks and slow control)
Level 3 (20% LEL):
Local Alarm
Alarm to TCR
Alarm Fire brigade
Switch off all flammable gas electromagnetic valves in B908
Ventilation on high speed
Electrical contact sent to NA58 (for interlocks and slow control)
Recommendations :
NA58 to cut flammable and inert gas inputs to flammable gas mixing racks in order
to maintain correct mixture in chambers - as inert gas flow is not cut by Level 3.
RICH chambers can be purged with N2
Ramp down HT on chambers affected.
2.
GAS MIXING ZONE B888
The NA58 gas list can be found on http://wwwcompass.cern.ch/compass/tech-board/gas/gas.html
Requirements:
Ventilation and gas detection around the flammable gas mixing racks is required.
The final details of ventilation and corresponding flammable gas detection system will be defined in
the coming weeks. (see Appendix 1).
The target for the installation of the system is the end of March 2001. The RICH photon detector
chambers will require a high CH4 flow near to the end of April.
It is essential that the design of the gas racks to be installed in the row of flammable gas racks should
be checked before construction and installation. Glimos to provide FGSO and TIS with:
A circuit diagram
A schematic sketch of the rack indicating electrical instrument positions, 220V supply points.
An equipment schedule – itemising components.
Flammable gas detection in gas mixing zone.
Provisional alarms and actions (awaiting final definition of ventilation and detection):
Level 2
Local Alarm
Alarm to TCR
Electrical contact to NA58 (for interlocks and slow control)
Level 3
Local Alarm
Alarm to TCR
Alarm Fire brigade
Switch off 220V in gas mixing zone.
Electrical contact to NA58 (for interlocks and slow control)
Off primary flammable gas at electromagnetic valves in B908
NA58 to cut flammable gas and inert gas inputs to flammable gas mixing racks [1]
Increase ventilation to high speed (if two-speed option is installed)
[1] RICH detector has special requirements. The detectors must normally be supplied with a constant
purge of clean gas to protect the CsI coated cathodes. The flow should not be interrupted for more than
a short time (RICH group to specify). RICH group to propose a solution. To be discussed with the
FGSO and TIS.
3.
EXPERIMENTAL AREA
The flammable gas usage in this area is divided into three distinct zones.
Zone 1:
Between polarised target and SM1 magnet
Drift and Micromegas chambers interleaved together tightly. Access to chambers by sliding out
on rails to each side of the beamline.
Micromegas
80% Ne/ 10% C2H6/ 10% CF4
Flammable mixture, relative density 0.95.
12 chambers, total volume 12 l, total flow 12l/h
Total volume <1 gram equivalent H2
Drift chambers:
45% Ar/ 45% C2H6/ 10% CF4
Flammable, heavy mixture, relative density 1.37.
2 chambers, volume 500l each, overpressure ~ +1mbar
Each chamber has ~ 120g H2 equivalent.
Flow per chamber 20-50 l/h
Zone 1 risk classification = Class 1
Requirements:
Construct a bund below chambers and the gas connection panel for flexibles.
Flexible lines to detector to be metallic - ensure good mechanical support at ‘quick’
connection to avoid undue force at the connections.
Establish procedure to ensure no magnetic tools are left in zone/ use of special tools for work
near to magnetic fields
No electrical equipment installed under the chamber platform.
Leak rates for each chamber to be measured at operational pressure.
Recommendation:
Incorporate differential flow measurement (in-out) on the two drift chambers. (Ask F
Hahn/TA1 Gas group for details)
Flammable gas detection in Zone 1
Place 2 catalytic detection heads on floor of bund (calibration C2H6 ) – one head should be under
connection point for gas lines
Glimos to contact ST/AA to ensure that the problem of spurious alarms from detection heads due to
ramping of magnetic fields has been resolved.
Alarms and actions:
Level 2 (10% LEL):
Local Alarm
Alarm to TCR
Electrical contact to NA58 (for interlocks and slow control)
Level 3 (20% LEL):
Local Alarm
Alarm to TCR
Alarm Firebrigade
Electrical contact to NA58 (for interlocks and slow control)
Switch off gas to Micromegas and Drift chambers at input to gas mixing racks.
Ramp-down HT on all chambers in Zone 1
Switch off Front End Electronics (FEE) on all chambers in Zone 1
Experimental area - Zone 2:
The interior of the RICH tent.
A tent is under design to enclose the region around the RICH detector. The tent is to be air-conditioned
(air to be partially re-circulated).
The interior of the tent contains a dense grouping of tracking chambers (non-flammable gas mixtures)
upstream of the RICH. The RICH front window has four photon detectors above and four photon
detectors below the beam line. The RICH contains a volume of ~ 100 m3 of C4F10 gas
RICH photon detectors:
CH4, relative density 0.55
(Note that the mixture, 80%CH4 / 20% iC4H10 remains an option- relative density 0.85)
Flow 300-800 l/h in closed circuit circulation.
8 chambers, volume of each chamber 40l.
Total volume 320l is equivalent to 96g H2
Risk classification = Class 1
Requirements:
The environment inside the tent however requires a special evaluation. The aspects of fire, smoke, CH4
leaks, C4F10 leaks, O2 deficiency must be considered together to define the detection system.
NA58 to supply full details of tent and air-conditioning as soon as possible to TIS and FGSO. Air
flowrate, air flow direction, volume of tent, fraction of fresh air, materials to be used for tent [2],
volume enclosed inside tent, type of electrical installations inside the tent, power dissipation.
Leak rates for each chamber or complete re-circulation system to be measured at operational pressure.
Provisional concept for the gas detection:
Considering only the CH4 leak, detection could be 4 catalytic heads, two above the photon detectors
below beam height and two for the upper detectors.
Considering a combined infra-red system which could detect low concentrations of both CH4
(iC4H10?) and C4F10 in the air conditioning exhaust flow. Sensitivities can only be defined when
details of the air-conditioning parameters are available. Smoke detection could be achieved using an
SDN system recuperated from LEP? (Consult J Nebout EST for availability)
Provisional Flammable gas alarms and actions:
Level 2 (?% LEL)
Local Alarm
Alarm to TCR
Electrical contact to NA58 (for interlocks and slow control)
Level 3 (?? % LEL):
Local Alarm
Alarm to TCR
Alarm Fire brigade
Electrical contact to NA58 (for interlocks and slow control)
Switch off gas flow to RICH chambers [3]
Ramp-down HT on all chambers in Zone 2
Off FEE to all chambers in Zone 2
[2] Tent construction material must be flame retarded– has conformity with CERN Safety Instruction
(SI 41) been checked with M Tavlet?
[3] The RICH detector has special requirements. The detectors must normally be supplied with a
constant purge of clean gas to protect the CsI coated cathodes. The flow should not be interrupted for
more than a short time. RICH group to propose a solution. To be discussed with the FGSO and TIS.
EXPERIMENTAL AREA (cont)
Zone 3:
The region surrounding the two modules of the Muon Wall 2 (MW2).
MW2
75% Ar/ 25% CH4
Flammable, slightly heavier than air, relative density 1.17.
2 chambers, volume 900l each. Each chamber ~ 70g H2 equivalent.
MW2 is divided into 12 modules for gas distribution.
Gas flow per chamber (gas module) maximum 500 ( 80) l/h
Gas flow per chamber (gas module) typically 150 (25) l/h
Zone 1 risk classification = Class 1
Chambers are made of stacks of metal tubes. The gas and HT connections are made at the ends of the
tubes and are enclosed in metal protection boxes. The proposed internal gas distribution system is made
with plastic pipes, interconnections are not made with standard gas connections.
Requirements:
The gas distribution system and connections inside the protective boxes must be checked before use on
the final detector.
NA58 to send details of the construction of the MW2 modules to TIS. A meeting will be organised
with TIS and the detector expert to discuss the detector, materials used, leak-tightness and a decision
made on flammable gas detection, inerting the protection boxes, installating differential flow
measurements (in-out) on the 12 gas channels.
Leak rates for each chamber to be measured at operational pressure.
4. NOXAL USE IN C4F10 RECIRCULATION SYSTEM
Located in a RICH radiator gas zone ~ 20 m from the flammable gas mixing zone in B888.
Noxal 2 (2% H2/ 98% Ar) is non-flammable (Carbagas specification sheet). No precautions necessary
with this mixture other than exhaust at exterior of B888 and metal pipework for this permanent
installation.
M Bosteels EST to test feasibility of regeneration of C4F10 gas filters with Noxal 2.
5. RICH CHAMBER TEST AREA
Located outside the experimental area. To be used to test up to 4 RICH chambers (160 litres CH4)
Area to be constructed by ~ end of January 2001.
Flammable gas detection required, catalytic head calibrated for CH4.
The alarms and actions could be separated from those of the main experiment.
Requirements:
NA58 to provide FGSO and TIS with:
A circuit diagram of proposed gas circuit, location of gas distribution rack.
Description of electrical apparatus to be installed in the test area.
Provisional Flammable gas alarms and actions:
Level 2 (10% LEL)
Local Alarm
Alarm to TCR
Electrical contact to NA58 (for interlocks and slow control)
Level 3 (20% LEL):
Local Alarm
Alarm to TCR
Alarm Fire brigade
Electrical contact to NA58 (for interlocks and slow control)
Off CH4 to RICH chamber test area
Off electricity to test area. (Ramp down HT and off FEE if supplied remotely).
APPENDIX 1: Ventilation system for gas mixing enclosure
The conceptual design has converged on the idea of enclosing the racks completely in a ‘soft
enclosure’. Above the enclosure would be a series of ventilation hoods linked to a common extraction
ventilation system. Within the enclosure, the racks would be separated into compartments served by a
ventilation hood. The enclosure would be made of curtains around 3 sides and for the front side:
Either, a transparent plastic curtain,
or, a series of plastic-windowed sliding doors.
In this case the compartments could be considered as approaching to a rigid enclosed volume. This
enables the ventilation airflow to be reduced from an open sided structure. It must be stressed that the
enclosure must be well made to permit correct ventilation of the racks. Materials used must be flame
retarded–conformity with CERN Safety Instruction (SI 41) to be checked with M Tavlet.
Number/type/sensitivity of detection head-depends on ventilation and level of leak detection required.
A Level 3 alarm for a 50l/h leak of CH4 is the working design sensitivity.
As an example:
An extraction of 6 volume changes per hour (~ 22m3/h per compartment of 2 racks) would produce 2%
(5%) LEL for a 20l/h (50l/h) CH4 leak.
ICARE have promised to test the reliability to noise and drift of a catalytic head being operated in the
range 0-15% LEL (0 to full scale deflection). If this development is not feasible then a more sensitive
detector will be required (e.g. an infra-red device positioned in the common extraction trunking).
ST/CV and ST/AA to be commissioned to make a study.