Discussion of flammable gas safety strategy for NA58 in 2001
22/12/00 10.00 B 153
M Davenport, J Gulley, C Nuttall, H Taureg
Gas zone:
Risk classification
Normally not done for a gas zone-industrial standard equipment. Classification depends not only on the volumes involved but also the flow potential of gas leaks from pressurised lines.
Flow limiting in B908
Flows should be limited in B908
There should be 2 separate CH4 lines (as suggested by NA58). In this case, each can be limited to 500l/h. Flow limiting orifices to be calculated and ordered – Glimos to ask M Bosteels.
C2H6 limit to be set to ~ 250 l/h
Scrutiny of gas rack design before construction and delivery to CERN
For all flammable gas racks to be installed the following items should be sent to FGSO as soon as possible (to avoid where possible delays if modifications are deemed necessary ):
A circuit diagram
A schematic sketch of the rack indicating electrical instrument positions, 220V supply points.
An equipment schedule – itemising components.
Technician responsible to organise installation, operation and maintenance.
The appointment of a qualified technician to be responsible for the gas systems is highly recommended. Such a technician is essential during the key period of installation and commissioning in 2001.
MD/HT to send memo.
Alarm actions for Level 3 in gas mixing zone
Cut 220 V in gas mixing zone.
(Cut 220V to experimental zone was not I think deemed not necessary??)
Stop flammable gas at Electrovalves in B908
Interlock to cut all gas flow (inert as well as flammable components) to detectors using a mixture containing a flammable component.
RICH to propose a solution in view of need for a near continuous purge the CsI coated photocathodes with clean gas (at the moment, if CH4 is cut off, N2 takes over automatically).
Ventilation to high speed
Use of Noxal (6% H2/94% Ar) in a nearby inert gas zone used to re-circulate and purify the RICH C4F10 Cherenkov radiator gas.
This use could be considered as used in an ‘industrial system’. It may be possible to accept its usage without full weight of safety systems. An inspection of the installation with M Bosteels is needed.
Risk classification
Can be specified after defining the zones.
Information on gas volumes and flowrates contained in the Gas ISIEC form supplied by R Joosten 13-07-00
Definition of zones
Gas retention bunds/ hoods
Number /type of detection heads
Positioning of electrical racks
Can be defined after a visit to B 888 by a safety delegation early in the New Year to evaluate.
Detector construction leak tightness
Glimos to give estimated leak rates for each detector containing flammable gas and then update with measured values when available.
MW2 (75% Ar / 25% CH4) small prototype was inerted in 2000 test run in the region on the electrical and gas connection box.
Glimos to give details of the final construction and leak rates of gas distribution system on the detector. N.B. no PVC pipes allowed on detector.
Inertion on the full scale detector may be necessary.
Influences of magnetic field on heads – have ST/AA solved this problem
Glimos to pursue this with ST/AA group.
RICH tent (Fire, flammable gas detection, oxygen deficiency, electrical racks ?)
Memo to Glimos to be sent by MD/HT requesting all information on this installation. Copies to MD/HT/TIS.
Alarm actions (In particular 220V cuts/general interlock system for gas racks)
MD to request to D Hay - can the detection system differentiate between different gas detection heads?
Alarm actions to be defined after safety delegation visit to B 888.
New RICH chamber test zone in B 888. Requirements ?
Detection: Yes
Alarms: Yes, but could be considered as a separate zone- not interlocked with the rest of experiment.
(Discussion took place after C Nuttall had left).
Ventilation- several meetings with ST/CV autumn 2000. System not yet defined.
Problem:
If the racks are enclosed on only three sides - very large flows are needed (~13000 m3/h) to ensure adequate air velocity to capture leaking gas (worst case 500l/h). No detection of small leaks would then be possible except with a very sensitive IREX detection system.
The conceptual design moved towards separating each two racks into compartments.
To reduce extraction air flow – enclose racks completely in ‘soft enclosure’. Curtains around 3 sides and
Either, a transparent plastic curtain in front , or, a series of plastic-windowed sliding doors.
In this case the compartments could be considered as similar to a rigid enclosed volume.
Number/type/sensitivity of detection head-depends on ventilation and level of leak detection required.
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).
A new iteration with ST/CV and possibly a test with ST/AA and TIS needed as soon as possible.
Signalling
Labelling of gas pipes
To be defined after safety delegation visit to B 888.
Does the Polarised target region have specific dangers?(Near to Drift and Micromegas chambers).
Ask Glimos. To be defined after safety delegation visit to B 888.