Lau Gatignon
21 June 2011
MINUTES OF CEDAR FLAMMABLE GAS SFETY MEETING
Present:
Agenda:
J.Gulley / DGS-SEE,
A.Sharma / PH-CMX (FGSO),
V.Falaleev / PH-UFT (GLIMOS-NA62),
J.Fry, C.Lazzeroni / NA62-UK,
R.Guida, S.Haider / PH-DT,
L.Gatignon / EN-MEF
1. Introduction
2. Gas System presentation
3. HAZOP
4. Zoning
1. Introduction
V.Falaleev presented the CEDAR counter and its environment in the experimental cavern
TCC8 with an emphasis on the gas system and potential leak sources, see slides.
The CEDAR has a length of approximately 6 m and contains a gas volume of 1.1 m 3. The
existing counter will be modified to have a shorter ‘nose’ and the light readout system (8
PMTs) will be replaced by a more performing and complex system with many more PMTs.
The old control electronics will be removed from the upstream foot to a location further away.
The downstream foot contains only two motors for adjustment of the horizontal and vertical
orientation of the counter. The Cedar used is of Cedar-West type, normally used with Nitrogen
gas, but for NA62 it will be operated with Hydrogen at an absolute pressure of up to 4 bars.
On each end the counter is separated from the beam vacuum by a hydro-formed Aluminium
window of 150 m thickness upstream (serves as fuse!) and 200 m downstream. Tests with
such windows will demonstrate that they withstand the gas pressure (maximum 5 bars in
operation) with the appropriate safety factors (tests to be defined with the help of P.Miguel
Santos Silva, HSE unit). If so required following the results of the tests, the windows will have
to be made thicker (the present windows are rated for 15 bars nominal working pressure and
are 400 m thick). In case of rupture, the Hydrogen gas will escape directly into the beam
vacuum and thus will not form a flammable mixture with air, although the shock wave may do
very serious damage to the NA62 detectors (Gigatracker and Straws in particular). A large (~5
m3) vacuum volume will be connected to the beam vacuum to serve as buffer volume.
The gas enters and leaves the detector from below on the upstream side. These are potential
leak sources. Other leak sources were listed: the entrance and exit windows towards the
beam vacuum, the quartz windows, the big O-rings above the two feet, the top vacuum and
safety valves, the diaphragm motor and its feed-through and the gas pipe for the pressure
gauge. Electrical equipment includes the diaphragm end-switches (inside the H2), the
diaphragm motor and potentiometer, the four PT100 temperature probes and the horizontal
and vertical motors for alignment with their switches and potentiometers.
A number of CEDAR safety discussions have taken place in the past, in particular one on 24
November 2008. The present discussions were building on these previous ones. The PMT’s
and their local electronics as well as the nose and the quartz windows will be housed in a
Nitrogen enclose. The diaphragm motor will be outside this inert volume and will be replaced
by a ATEX rated one. The same applies to the diaphragm end switches, which are inside the
pure H2 volume that could be contaminated with air in case of a wrong manipulation.
Finally it was pointed out that flammable/combustible materials shall be avoided in the
vicinity of potential Hydrogen leaks. This concerns in particular the insulating material, which
should be chosen mainly according to this criterion rather than solely optimum insulating
performance. It was noted that the temperature in TCC8 should be quite stable anyway. Of the
neighbouring beam equipment, only the FISCs were considered to merit further discussion in
terms of zoning.
2. The gas system
Roberto Guida explained the gas system in detail. Most of the system is installed in the surface
buildings (gas building and a mixing zone in B918), but the vacuum and N2 flushing related
parts are installed in the underground TCC8 area, not too far (~6 m) from the Cedar counter.
The operating principle is that the detector is emptied as well as reasonably possible (to 0.1
mbar or better) before filling it to the required pressure with pure Hydrogen from bottles
located in the gas building. All gas extracted (or escaping via the safety valve) from the
detector is evacuated to the outside via a 22 mm diameter H2-rated exhaust line. An auxiliary
system allows to flush the detector as well as the inlet and exhaust lines with inert gas,
Nitrogen, e.g. in case of interventions on the counter or the gas system. The vacuum pump
must be reasonably close, as it requires a large diameter vacuum tube (~25 mm diameter) to
the detector with minimal impedance.
For the filling two methods are available: a fast one for initial filling and a slow one for
topping up (miniscule) leaks. Apart from computer controlled valves there are necessarily a
number of manual valves. This has potential risks of bad manipulation, the consequences of
which must be mitigated. In both methods the filling stops automatically when the required
pressure has been reached, as measured by a pressure gauge. Emptying the counter is ‘just’
pumping out any gas present in the counter.
Basically there are 4 procedures (“nodes” in the HAZOP language) to be analysed in depth:
1. Purging the counter and gas lines with Nitrogen gas, by computerized system, or – if
necessary – manually,
2. Filling with Hydrogen – this requires manual intervention - and the automatic
procedure to top up leaks
3. The pressure scan, where the pressure is lowered in small steps, alternated with
efficiency measurements,
4. The emptying of the counter.
These four nodes have been discussed in depth in the framework of a HAZOP risk analysis, see
the next section.
Manual interventions with potential safety implications must be logged by computer. This
concerns e,g, manual valve or switch manipulations. Expert operators have the freedom to bypass certain parts of procedures. Such actions must be password-protected and logged.
3. The HAZOP
HAZOP stands for HAZard and OPerability analysis. It is a systematic approach to each of the
“nodes” analysed according to pre-defined keywords and parameters. Consequences and
required actions are defined as a result.
In our case we have defined four nodes. The detailed outcome of the HAZOP analysis is
reported in the attached HAZOP sheets. Below we summarise some additional remarks that
came up in the discussions for each node.
1. Purging the lines and detector, including the exhaust line
The aim is to have a Nitrogen purity of at least 99%, as a contamination below 1% is
considered harmless whatever the nature of the contaminant. It was pointed out that
this is a possible action although not necessarily one that is used routinely for normal
operation. It is mainly useful before interventions on the system or detector. The 1% is
not measured but derived from the time of flushing. It was pointed out that for this
node, like for the other ones, the proper function of the Cedar safety valve is the
principle safety against overpressure in the Cedar volume. It was also pointed out that
certain problems can occur in case the computer blocks. In that case a watchdog will
diagnose this and stop all inlet and outlet processes. The gas will be left in the system
as it is. Any alternative action could imply risks that are potentially worse than leaving
the limited amount (< 200 mg) of Hydrogen in place.
2. Emptying the counter
This is a rather simple procedure where all valves are closed, except the one between
the counter and the vacuum pump. The beam must be evacuated to avoid rupture of
the Cedar windows.
3. Hydrogen filling
This is rather similar to the first node, except the fact that this time the Hydrogen inlet
valve is opened instead of the Nitrogen one.
4. Pressure scan
It is considered better to scan from high to lower pressure, as the opposite process
could have a larger impact on the temperature in the detector and thus on the
refractive index and detector performance.
In view of the limited quantity of Hydrogen in the system, it was agreed that no “emergency
dump” procedure for H2 evacuation is required. But this implies also that some zoning is
necessary.
4. Zoning
We discussed in detailed the need for some declared ATEX zones. As a result of our
discussions and analyses eight zones were defined and they are described in Table 1. All other
regions are not considered part of the zoning system:
The diaphragm motor and feed-through would fall in a Zone 2 (declared around feed-through
and safety valve in vicinity of motor), and therefore will have to be replaced by an ATEX rated
motor. ATEX end switches (inside H2 volume, declared as a Zone 2) will also be required.
The safety valve must be connected to the exhaust line, the vent of which wil also be a
declared Zone 2.
All ATEX Zoning referred to in Table 1 (apart from the gas storage building) to be recorded by
FGSO for PH (A.Sharma) using standard forms.
The alignment motors below the downstream end of the CEDAR will be outside the declared
zone. These motors and switches need therefore not be ATEX rated. For the same reason the
FISCS are considered to be outside the zone.
It was discussed to avoid zone #7 by having fixed welded connections between detector and
gas rack, but this was finally considered too impractical.
5. Final remarks
The slides and some additional information is available from the Indico agenda of the meeting
at http://indico.cern.ch/conferenceDisplay.py?confId=144135.
The (ambitious) plan is to operate the modified Cedar with Hydrogen gas in TCC8 in the NA62
technical run in September or October 2012.
TABLE (2 pages)
Zone
Description
Comments
1
The gas storage building
2
The mixing zone inside B918
3
The underground gas system
4
Quartz windows
This is where the Hydrogen bottles are stored. It is
an existing gas building, but the detection equipment
must be suitable for Hydrogen (light gas that goes
upward, sensitivity to be calibrated for H2, etc).
Needs to be /installed F.Schmitt (GS/ASE). As FGSO
for the gas building he must also perform and record
the Hazardous Area Classification (ATEX Zoning)
using the standard form.
In a narrow (~2 m wide) corridor behind the control
rooms. This zone is relative well equipped, but the
state of the roof must be verified and it must be
emptied of temporary storage (cupboards,
combustible materials, etc).
For the Hydrogen rack which will be under Local
exhaust ventilation:
- if the rack is closed the zone (Zone 2) can be
confined to the dimensions of the rack,
- if not closed the zone may extend beyond the
dimensions of the rack (e.g. 0.5 to 1 m).
For the Hydrogen rack which will be under Local
exhaust ventilation:
- if the rack is closed the zone (Zone 2) can be
confined to the dimensions of the rack,
if not closed the zone may extend beyond the
dimensions of the rack (e.g. 0.5 to 1 m).
In practice this implies that the electronics rack of
the gas system can be rather close (0.5 to 1 m
distance from the gas rack).
The gas lines from the rack to the Cedar may run
over the floor, provided they are properly protected
and separated from electrical cables.
Only the central part of the Nitrogen enclosure that
encapsulates the quartz windows are considered
“Zone 2 NE, of Negligible Extent” due to the tightness
of the seal. The outer part of the N2 enclosure that
houses the PMTs and local electronics can be
considered the safety area around this zone. A H2
detector must be installed in the top part of this
outer part of the N2 enclosure.
The N2 flow must be interlocked with the power to
the electronics.
Zone
5.
6.
7.
8.
Description
Comments
Feedthroughs and safety relief The diaphragm motor would fall in a Zone 2
valve
(declared around feedthrough and safety valve in
vivinity of motor), and therefore it will have to be
replace by an ATEX rated motor.
Inside H2 volume
ATEX end switches Iinside H2 volume, which will be
declared as a Zone 2) will also be required.
The gas connections
The gas connections below the upstream part of the
Cedar form a Zone 2 with dimension ±0.5 m. Its
presence and boundaries must be clearly indicated
and ignition sources are forbidden in this zone.
Exhaust from vent lines
An ATEX Zone (1 or 2) will be declared around the
exhaust from the vent line (at a safe place outside),
Table 1: The definition of the zoning